advances in membrane technologies for water treatment – materials, processes and...
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Woodhead Publishing Series in EnergyNumber 75
Advances in MembraneTechnologies for WaterTreatmentMaterials Processes and Applications
Edited by
Angelo Basile Alfredo Cassanoand Navin K Rastogi
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CONTENTS
List of contributors
WL Ang Research Centre for Sustainable Process Technology UniversitiKebangsaan Malaysia UKM Bangi Selangor Malaysia
A Antony The University of New South Wales Sydney NSW Australia
P Argurio Universita della Calabria Rende (CS) Italy
P Arribas Campus of International Excellence Moncloa Campus (UCM-UPM)Madrid Spain Department of Applied Physics I University Complutense of MadridAvda Complutense Madrid Spain
N Bajraktari Technical University of Denmark Lyngby Denmark
A Basile Institute on Membrane Technology ITM-CNR University of CalabriaRende (CS) Italy Ast-Engineering srl Rome Italy
M Bodzek Institute of Environmental Engineering of the Polish Academy ofSciences Zabrze Poland Silesian University of Technology Institute of Water andWastewater Engineering Gliwice Poland
SI Bouhadjar Institute of Applied Research (IAF) Karlsruhe University ofApplied Sciences Karlsruhe Germany
A Cassano Institute on Membrane Technology ITM-CNR University of CalabriaRende (CS) Italy
E Curcio University of Calabria (DIATIC-UNICAL) Cosenza Italy Institute onMembrane TechnologyndashNational Research Council of Italy (ITM-CNR) CosenzaItaly
L Daal DNV GL Energy Arnhem The Netherlands
SA Deowan Institute of Applied Research (IAF) Karlsruhe University of AppliedSciences Karlsruhe Germany
F de Vos DNV GL Energy Arnhem The Netherlands
T de Vries GDF SUEZ Energie Nederland Zwolle The Netherlands
G Di Profio Institute on Membrane TechnologyndashNational Research Council of Italy(ITM-CNR) Cosenza Italy
E Drioli University of Calabria (DIATIC-UNICAL) Cosenza Italy Institute onMembrane TechnologyndashNational Research Council of Italy (ITM-CNR) CosenzaItaly
E Fontananova Institute on Membrane TechnologyndashNational Research Council ofItaly (ITM-CNR) Cosenza Italy
VS Frenkel San Francisco CA USA
MC Garciacutea-Payo Department of Applied Physics I University Complutense ofMadrid Avda Complutense Madrid Spain
N Ghaemi Department of Chemical Engineering Kermanshah University ofTechnology Kermanshah Iran
L Gil School of Forest Engineering University Polytechnic of Madrid AvdaComplutense Madrid Spain
S Guclu Istanbul Technical University Civil Engineering Faculty EnvironmentalEngineering Department Istanbul Turkey National Research Center on MembraneTechnologies (MEM-TEK) Istanbul Turkey
C Heacutelix-Nielsen University of Maribor Maribor Slovenia Technical University ofDenmark Lyngby Denmark
J Hoinkis Institute of Applied Research (IAF) Karlsruhe University of AppliedSciences Karlsruhe Germany
Y Ji School of Resources and Environmental Engineering East China University ofScience and Technology Shanghai PR China
M Kallioinen Lappeenranta University of Technology Laboratory of SeparationTechnology Skinnarilankatu Lappeenranta Finland
M Khayet Department of Applied Physics I University Complutense of MadridAvda Complutense Madrid Spain Madrid Institute for Advanced Studies of Water(IMDEA Water Institute) Alcala de Henares Madrid Spain
I Koyuncu Istanbul Technical University Civil Engineering FacultyEnvironmental Engineering Department Istanbul Turkey National Research Centeron Membrane Technologies (MEM-TEK) Istanbul Turkey
M Lee Imperial College London London UK
GL Leslie The University of New South Wales Sydney NSW Australia
K Li Imperial College London London UK
SS Madaeni Membrane Research Centre Department of Chemical EngineeringRazi University Kermanshah Iran
M Meuroantteuroari Lappeenranta University of Technology Laboratory of SeparationTechnology Skinnarilankatu Lappeenranta Finland
AW Mohammad Research Centre for Sustainable Process Technology UniversitiKebangsaan Malaysia UKM Bangi Selangor Malaysia
R Molinari Universita della Calabria Rende (CS) Italy
xiv List of contributors
M Nystreuroom Lappeenranta University of Technology Laboratory of SeparationTechnology Skinnarilankatu Lappeenranta Finland
L Palmisano Universita di Palermo Palermo Italy
ME Pasaoglu Istanbul Technical University Civil Engineering FacultyEnvironmental Engineering Department Istanbul Turkey National Research Centeron Membrane Technologies (MEM-TEK) Istanbul Turkey
I Petrinic University of Maribor Maribor Slovenia
H Rajabi Membrane Research Centre Department of Chemical Engineering RaziUniversity Kermanshah Iran Department of Civil Engineering Razi UniversityKermanshah Iran
NK Rastogi Department of Food Engineering Central Food TechnologicalResearch Institute Council of Scientific and Industrial Research Mysore India
Seyed MK Sadr Centre for Environmental and Health Engineering (CEHE)Department of Civil and Environmental Engineering University of Surrey GuildfordSurrey United Kingdom
Devendra P Saroj Centre for Environmental and Health Engineering (CEHE)Department of Civil and Environmental Engineering University of Surrey GuildfordSurrey United Kingdom
R Sengur National Research Center on Membrane Technologies (MEM-TEK)Istanbul Turkey Istanbul Technical University Nanoscience and NanoengineeringDepartment Istanbul Turkey
J Soons PWN Waterleidingbedrijf Noord Holland Velserbroek The Netherlands
KH Tng The University of New South Wales Sydney NSW Australia
T Turken Istanbul Technical University Civil Engineering Faculty EnvironmentalEngineering Department Istanbul Turkey National Research Center on MembraneTechnologies (MEM-TEK) Istanbul Turkey
B Van der Bruggen KU Leuven Leuven Belgium
Y Wang The University of New South Wales Sydney NSW Australia
Z Wu Imperial College London London UK
List of contributors xv
Preface
Water is the most abundant and renewable resource in the world Unfortunately only asmall quantity is fit for use to sustain human life In addition population growthcoupled with industrialization and urbanization has led to the increased pollution ofexisting freshwater resources resulting in an increased demand for fresh water Atthe same time challenges related to water systems are expected to increase in thenear future requiring further investment and technological innovation to meet globalneeds
Currently water recycling is widely accepted as a sustainable option in response tothe general increase in the demand for fresh water and to water shortages and environ-mental protection According to this view industrial companies are increasingly inter-ested in recycling wastewater to reach an ideal zero-discharge condition
Membrane technology has become a significant separation technology in the fieldof water filtration over the past two decades providing effective alternatives to relatedtechnologies such as adsorption extraction distillation ion exchangers and sand fil-ters It enables desalination or obtaining drinking water from saltwater as well as puri-fication of groundwater or wastewater
Low-energy consumption continuous separation easy scale-up modularityremote control and no phase separation are well-recognized key advantages of mem-brane processes over conventional separation technologies
The growth of the global membranes market is mainly the result of the impressivedevelopment of materials used for membrane fabrication and modification improve-ments in membrane modules and the evolution of different related systems plantsand equipment
This book covers the most recent and applicable achievements regarding materialsprocesses and applications of membranes for water treatment
The book is split into three parts The first is related to both novel membrane mate-rials and advances in membrane operations The second part considers how to improvemembrane performance and the last part illustrates selected applications in water treat-ment In the following section each chapter is briefly introduced
Chapter 1 (Madeani Ghaemi and Rajabi) illustrates the recent development of newmaterials and methods for the fabrication and modification of polymeric membranesfor water treatment Chapter 2 (Lee Wu and Li) provides an extensive analysis con-cerning recent progress in ceramic membranes for drinking water production and in thetreatment of municipal and industrial wastewater produced water (waste stream gen-erated from oil and gas operations) and wastewater generated in the food and beverage
industry In Chapter 3 (Koyuncu Sengur Turken Guclu and Pasaoglu) after a gen-eral overview of the global membrane market and various membrane fabrication tech-niques advances in water treatment by membrane processes such as ultrafiltrationmicrofiltration and nanofiltration are extensively illustrated Water treatment byboth reverse and forward osmosis for the desalination of water is described in Chapter 4(Rastogi Cassano and Basile) Differences among various membrane processesand the fundamentals of water treatment by reverse osmosis and forward osmosisare also considered Chapter 5 (Deowan Bouhadjar and Hoinkis) introduces mem-brane bioreactor technology with particular attention to water treatment Both aero-bic and anaerobic reactors are described The various factors affecting membraneperformance in both reactors (membrane fouling hydraulic residence time waterflux decline and so on in aerobic reactors and temperature organic loadingrate membrane properties and so on in anaerobic reactors) are seen as importantissues in this application of technology For each reactor a case study is also pro-posed The state of the art in the use of electrodialysis and electrodialysis with bipo-lar membranes for water treatment is reported in Chapter 6 (Van der Bruggen) Inparticular after a general description of the operational mechanisms of electrodial-ysis progress in anion and cation exchange membranes and new developments inmodule configurations and process integration are described Finally a brief over-view of applications of electrodialysis for water treatment is given Chapter 7 (Moli-nari Argurio Palmisano and Grillone) discusses both the basic principles ofphotocatalysis and advantages related to its coupling with membrane separationin so-called photocatalytic membrane reactors (PMRs) Important aspects for appro-priate large-scale implementation are the types of membranes used their criteria ofselection PMR configuration and membrane operation Some case studies in watertreatment are also discussed evidencing possibilities drawbacks and future trendsChapter 8 (Arribas Khayet Garciacutea-Payo and Gil) describes pressure-driven mem-brane processes and significant progress achieved over the past few years regardingthe fabrication of novel membranes and their modification In particular the chapterfocuses on novel flat-sheet and hollow-fiber membranes made with innovative mate-rials and with improved properties suitable for specific applications In Chapter 9(Arribas Khayet Garciacutea-Payo and Gil) the authors of the previous chapter extendconsideration to the electric potential and concentration gradient membrane pro-cesses In particular in this chapter the authors focus on water treatment by electro-dialysis with forward osmosis Also special attention is dedicated to alternativetechnologies of emerging interests used to produce power such as reverse electro-dialysis and pressure-retarded osmosis able to generate electricity from salinity gra-dients Some critical challenges (eg concentration polarization membrane foulingreverse solute diffusion and draw solute design) are also discussed
Chapter 10 (Frenkel) introduces the importance of membrane technologies whenthe water supply is considered for the community industry or agricultural user Infact membrane technologies have entered every aspect of water and wastewater treat-ment such as municipal and industrial water advanced wastewater treatment andreuse and seawater and brackish water desalination This chapter pays attention tothe design of both low- and high-pressure membrane systems for water treatment
xxii Preface
Finally integrated membrane systems and a combination of membrane treatment andother technological processes are briefly considered After distinguishing betweenmembrane ageing and failure Chapter 11 (Tng Antony Wang and Leslie) describesmembrane ageing during water treatment with particular attention to the modes andmechanisms of failure The authors review possible monitoring and control techniquesnecessary for the early detection of membrane ageing They also suggest some meas-ures for mitigating membrane failure caused by ageing Chapter 12 shows the impor-tance of mathematical modeling in membrane operations for water treatment (Ang andMohammad) In particular this chapter provides useful data on designing the plant andhelpful prediction of the performance of the membrane water treatment plant Differenttransport mechanisms are involved in pressure-driven membrane operations Thus themathematical models necessary to predict their performance are also different Theauthors show that the selection of a particular model of membrane water treatmentplant is crucial because it leads to a better understanding of its long-term performanceChapter 13 (Curcio Di Profio Fontananova and Drioli) provides a general overviewof membrane operations currently in use in seawater and brackish water desalinationfor potable water production In this chapter reverse osmosis novel salinity gradientpower technologies (with emphasis on pressure-retarded osmosis and reverse electro-dialysis) and membrane distillation are treated as viable options in the logic of zeroliquid discharge After introducing a section on drivers and barriers to membrane tech-nologies for municipal wastewater treatment Chapter 14 (Sadr and Saroj) describesvarious membrane-assisted processes and technologies in wastewater treatmentAnother important part of the chapter is related to both applications of nanofiltra-tionreverse osmosis after biological treatment and the design operation and controlof membrane processes in municipal wastewater treatment The optimization of mem-brane processes in municipal wastewater treatment is also well described taking intoconsideration cost assessment energy efficiencies and operational costs In Chapter15 (Bodzek) an analysis of the main organic and inorganic micropollutants of watersources is presented Membrane processes such as reverse osmosis and nanofiltrationultrafiltration and microfiltration in integrated systems Donnan dialysis and electro-dialysis as well as membrane bioreactors and liquid membranes are presented as alter-native technologies for producing high-quality drinking water as well as purifiedwastewater that can be drained off into natural water sources Chapter 16 (Yang) high-lights key advantages of pressure-driven membrane operations and their integrationinto existing systems for water treatment and reuse in the gas and petrochemical indus-tries Methods to reduce concentration polarization and membrane fouling phenom-ena which mainly affect membrane system performance and directly determine thecapital and operational costs of membrane systems in this field are also presentedand discussed Membrane technologies for water treatment and reuse in the textileindustry are described in Chapter 17 (Petrinic Bajraktari and Heacutelix-Nielsen) Theauthors present various examples of reverse osmosisnanofiltrationultrafiltration-based systems and membrane bioreactor technology that have been investigated fortextile wastewater remediation Forward osmosis used to concentrate textile dyes isalso presented Chapter 18 (Cassano Rastogi and Basile) presents an overview ofmembrane-based processes for water reuse and the environmental control in the
Preface xxiii
treatment of wastewaters from food-processing industries Various applicationsinvolving the use of pressure-driven membrane operations electrodialysis membranebioreactors and integrated membrane systems are shown and discussed In particularthe authors illustrate typical applications for the treatment of process waters from thefruit and vegetables dairy fish meat soya and wine industries highlighting theiradvantages and drawbacks with respect to conventional technologies Chapter 19(Meuroantteuroari Kallioinen and Nystreuroom) focuses on the use of membrane technologiesto purify raw water and wastewaters circulate process waters and recover valuablematerials in the pulp and paper industry The driving forces to use membranes and bar-riers as well as their challenges are discussed in the introduction Chapter 20 (Daal)focuses on water treatment required for the production of demineralized wateremployed for the watersteam cycle in industrial plants Water purification technolo-gies including membrane technologies and operational experiences with membranesare presented and discussed
The editors wish to take this opportunity to thank all of the authors of the chaptersfor preparing developing and improving their text Special thanks to all of the staff ofWoodhead for their excellent help to both the authors and the editors
Angelo BasileAlfredo CassanoNavin K Rastogi
xxiv Preface
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Woodhead Publishing Series in Energy
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2 Advanced separation techniques for nuclear fuel reprocessing and radioactive wastetreatmentEdited by Kenneth L Nash and Gregg J Lumetta
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4 Understanding and mitigating ageing in nuclear power plants Materials andoperational aspects of plant life management (PLiM)Edited by Philip G Tipping
5 Advanced power plant materials design and technologyEdited by Dermot Roddy
6 Stand-alone and hybrid wind energy systems Technology energy storage andapplicationsEdited by John K Kaldellis
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12 Handbook of advanced radioactive waste conditioning technologiesEdited by Michael I Ojovan
13 Membranes for clean and renewable power applicationsEdited by Annarosa Gugliuzza and Angelo Basile
14 Materials for energy efficiency and thermal comfort in buildingsEdited by Matthew R Hall
15 Handbook of biofuels production Processes and technologiesEdited by Rafael Luque Juan Campelo and James Clark
16 Developments and innovation in carbon dioxide (CO2) capture and storagetechnology Volume 2 Carbon dioxide (CO2) storage and utilisationEdited by M Mercedes Maroto-Valer
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25 Advanced membrane science and technology for sustainable energy and environ-mental applicationsEdited by Angelo Basile and Suzana Pereira Nunes
26 Irradiation embrittlement of reactor pressure vessels (RPVs) in nuclear power plantsEdited by Naoki Soneda
27 High temperature superconductors (HTS) for energy applicationsEdited by Ziad Melhem
28 Infrastructure and methodologies for the justification of nuclear power programmesEdited by Agustiacuten Alonso
29 Waste to energy conversion technologyEdited by Naomi B Klinghoffer and Marco J Castaldi
30 Polymer electrolyte membrane and direct methanol fuel cell technology Volume 1Fundamentals and performance of low temperature fuel cellsEdited by Christoph Hartnig and Christina Roth
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32 Combined cycle systems for near-zero emission power generationEdited by Ashok D Rao
33 Modern earth buildings Materials engineering construction and applicationsEdited by Matthew R Hall Rick Lindsay and Meror Krayenhoff
34 Metropolitan sustainability Understanding and improving the urban environmentEdited by Frank Zeman
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xviii Woodhead Publishing Series in Energy
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75 Advances in membrane technologies for water treatment Materials processes andapplicationsEdited by Angelo Basile Alfredo Cassano and Navin K Rastogi
76 Membrane reactors for energy applications and basic chemical productionEdited by Angelo Basile Luisa Di Paola Faisal Hai and Vincenzo Piemonte
xx Woodhead Publishing Series in Energy
Advances in polymericmembranes for water treatment 1SS Madaeni1 N Ghaemi2 H Rajabi131Membrane Research Centre Department of Chemical Engineering Razi UniversityKermanshah Iran 2Department of Chemical Engineering Kermanshah University ofTechnology Kermanshah Iran 3Department of Civil Engineering Razi UniversityKermanshah Iran
11 Introduction
As themost precious and renewable resource in the world water is an important aspect oflife Theworldrsquos population tripled in the twentieth century and it will increase by another40 to 50 within the next 50 years Population growth coupled with industrializationand urbanization has resulted in the rapidly increasing demand for fresh water Further-more some existing fresh water resources have gradually become polluted because ofhuman or industrial activities Problems with water are expected to grow in the comingdecades with water scarcity occurring globally even in regions currently consideredwater-rich Therefore many researchers have focused on suitablemethods to obtain freshwater by purifying and reusing water to support future generations Water purification isthe process of removing unpleasant agents such as chemicals organic and biologicalcontaminants and suspended solids from water to obtain satisfactory water
Owing to its low cost and high efficiency membrane technology has dominatedwater purification technologies Compared with the other types of membranes poly-meric membranes lead the membrane separation industries and markets because theyare economically and practically beneficial However limited chemical mechanicaland thermal resistance restricts their application Extensive efforts have been imple-mented to improve both flux and selectivity and reduce membrane fouling as the mostimportant problem in application of membranes To eliminate obstacles and decreasethe problems in membrane use much research and numerous studies have been conduct-ed to develop newmaterials and methods for fabricating and modifying polymeric mem-branes This chapter covers the most recent and applicable achievements regarding thepreparationmodification and performance of polymericmembranes for water treatment
12 Advances in polymeric membranes
121 Composite (mixed matrix) membranes
Sometimes when fabricating membranes structural modification is necessary toenhance overall performance as well as the mechanical chemical and thermal stability
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400001-0Copyright copy 2015 Elsevier Ltd All rights reserved
of the membrane Methods to overcome the challenges of membrane modification havebeen well established for polymeric membranes Much research on membrane scienceand technology has focused on the development of new membrane preparation tech-niques and material An alternate way to improve membrane performance includingpermeability and selectivity involves introducing second phases into the membranematrix Polymers and some nanofillers were mostly introduced as a second phaseinto polymeric membranes to prepare different kinds of composite membranes
1211 Composite membranes prepared via blendingof polymers
Blending of additives into the casting solution is one of the most applicable methods tochange the phase separation process and consequently membrane characteristicsPolymeric materials are applied considerably as additives into the dope solution(Musale Kumar amp Pleizier 1999 Ochoa et al 2001) Although certain polymerssuch as polyethersulfone (PES) and poly(vinylidenefluoride) (PVDF) possess excel-lent thermal and mechanical stability as well as acceptable film-forming propertiesthat make them ideal materials for membrane preparation their application is oftenrestricted because of their hydrophobic nature which results in low water permeationand high fouling On the other hand the membranes made of hydrophilic polymerssuch as poly(vinyl alcohol) (PVA) cellulose acetate (CAc) and polyacrylonitrile(PAN) are also applied for the fabrication of liquid separation membranes howeverthe thermal and mechanical resistance and chemical stability of these membrane arelow Blending of polymers in membrane fabrication has been widely investigatedbecause of the simplicity of procedure and high efficiency in developing newmembranes with elevated properties and performance
The main focus in blending polymers is to increase hydrophilicity and decreasemembrane fouling Rahimpour and Madaeni (2007) prepared PES membrane byblending different concentrations of CAP (20 30 and 40 wt) and PVP (2 4 and8 wt) as a pore former Contact angle measurements showed that the hydrophilicityof membranes was enhanced for all compositions because of numerous acidic andcarbonyl functional groups in the CAP structure Pure water flux milk water perme-ation and protein rejection also increased with an increase in PESCAP compositionsup to 8020 Moreover the antifouling property of PES membranes was improved byadding even small amounts of CAP into the casting solution In another study(Masuelli Marchese amp Ochoa 2009) a PVDF ultrafiltration membrane was modifiedby blending PVDF with PVP and sulfonated polycarbonate to enhance permeation fluxand reduce fouling Surprisingly the hydraulic permeability of the membrane wasreduced and fouling increased for composite membranes (Masuelli et al 2009)Composite polysulfone (PSf) membranes were synthesized by Adams NxumaloKrause Hoek and Mamba (2012) by blending PSf with different concentrations ofb-CPU (0e10) using a phase inversion technique Addition of 5 b-CPU to PSfmembrane greatly improved the hydrophilicity of composite membranes comparedwith higher amounts because fewer pores were created on the membrane surfaceand owing to the chemical interaction between OHNH and the sulfonyl backbone
4 Advances in Membrane Technologies for Water Treatment
of PSf Moreover it was revealed that a low concentration of b-CPU was effective inimproving flux without compromising membrane rejection (Adams et al 2012)Table 11 summarizes some studies on composite membranes and the main propertiesof blending polymers
Table 11 Studies on composite polymeric membranes
Materials Feed solution
Properties ofcomposite versusnascentmembrane References
CASPSf mdash Larger pore size MalaisamyMahendran andMohan (2002)
PVDFPMMA Effluent fromengine factory
Largermacrovoideshigherhydrophilicitylower fouling
Ochoa Masuelliand Marchese(2003)
PSPA Solution of CNeZn2thorn ZnethCN2
4 THORNGood chemicalstability lessmechanicalresistance goodselectivity
Amado GondranFerreiraRodrigues andFerreira (2004)
PESCAPPVP Milk Higherhydrophilicitymore pure waterflux milk waterpermeation andprotein rejection
Rahimpour andMadaeni (2007)
PESPIPVP Salt solution Higherhydrophilicitymore pure waterflux more saltrejection
MansourpanahMadaeni Adeliand Rahimpour(2009)
SPCPVDF Oilewateremulsion
Lower fouling Masuelli et al(2009)
PSfPAN Aqueous solution ofheavy metals
Decrease in meansurface-pore sizeoverall porosityand permeabilityhigh leadcadmium andchromiumrejection
Mbareck NguyenAlaoui andBarillier (2009)
Continued
Advances in polymeric membranes for water treatment 5
1212 Thin-film composite membranes
Dense membranes generally have low flux but high selectivity whereas porousmembranes have low selectivity but high permeability To increase flux through adense membrane with high selectivity the thickness of the membrane should bereduced as much as practically possible However the membrane should be
Table 11 Continued
Materials Feed solution
Properties ofcomposite versusnascentmembrane References
CAPU Aqueous solutionscontaining textiledye
Lower thermalstability
Zavastin et al(2010)
HPAAPSf BSA solutionaqueous solutioncontaining heavymetal
Higher water fluxhigher rejectionof BSA and Cd(II)
Han Yu et al(2012)
CNCPSf Papermakingeffluent
Decreasingmechanicalstrength byincreasing CNCcontent higherpermeation goodability inpapermakingeffluent filtration
Zhou Zhao BaiZhang and Tang(2012)
PVDFSPPO Ionic solution Higher watercontent and ion-exchangecapacitysuperiorperformance by60 SPPO and40 PVDF
Khodabakhshi et al(2012)
PSfb-CPU Solution of heavymetal
Higherhydrophilicitymore water fluxand rejectionlower porosityless mechanicalstrength
Adams et al (2012)
6 Advances in Membrane Technologies for Water Treatment
defect-free and possess adequate mechanical strength This may be achieved with theuse of thin-film composite (TFC) membranes
Such composite membranes often consist of two layers a thin dense selective toplayer supported by a porous substrate The main role of the porous support is to providemechanical strength whereas the thin skin layer is responsible for permselectivityA TFC membrane is usually formed by a two-step process formation of a thickporous and nonselective substrate followed by coating with an ultrathin skin layerAmong different methods for forming the top layer dip-coating (Lang SourirajanMatsuura amp Chowdhury 1996 Peng Huang Jawor amp Hoek 2010 Susanto ampUlbricht 2008) and interfacial polymerization (Kim Kim Yu amp Deng 2009 PrakashRao Desai amp Rangarajan 1997 Rahimpour Jahanshahi Mortazavian Madaeni ampMansourpanah 2010 Reddy et al 2005 Shao et al 2013 Verissimo Peinemann ampBordado 2005 Wu et al 2006) were widely used Of these interfacial polymerizationis particularly interesting taking into account the considerable amount of research inthis field
TFC membranes have had remarkable development since the concept of interfa-cial polymerization was introduced by Mogan in 1965 (Lau Ismail Misdan ampKassim 2012) In this technique polymerization reaction takes place between tworeactive monomers at the interface of two immiscible solvents First the pores ofa membrane used as the support are filled with liquid A then the support isimmersed in a bath containing a reactant for liquid A As the result of interfacialpolymerization reaction a dense highly cross-linked polymer layer is formed onthe surface of the membrane at the interface of two solutions Heat ultraviolet(UV) and plasma treatment are often applied to control and complete interfacialpolymerization The top skin layer is extremely thin (01 mm or less) so TFCmembrane permeability is high Also membrane selectivity is high as a result ofthe high cross-linking of polymers
Commonly used reactive monomers are aliphaticaromatic diamines such as piper-azine (PIP) m-phenylenediamine (MPDA) and p-phenylenediamine (PPD) and acidchloride monomers such as trimesoyl chloride (TMC) isophthaloyl chloride (IPC)and 5-isocyanatoisophthaloyl chloride (ICIC) Among these materials and preparedTFC membranes polyaniline (PA) membranes prepared by interfacial polymerizationof multifunctional amine and acyl chloride monomers (Han Chung et al 2012 KimHwang Gamal El-Din amp Liu 2012 Kim et al 2009 Maurya Parashuram SinghRay amp Reddy 2012 Rahimpour et al 2010 Shao et al 2013 Verissimo et al2005 Yu Liu Liu and Gao 2009 Wu et al 2006) are the most typical and successfulTFC membranes Rahimpour et al (2010) prepared a TFC PA nanofiltration mem-brane using interfacial polymerization of PDA with TMC The TFC polyamide mem-branes displayed a higher ability to soften water The composite membrane exhibitedwater permeability of 7 and 21 kgm2 h for salt solution containing NaCl (1 gl) andMgSO4 (1 gl) at 5 and 10 bar respectively Also rejection of the divalent saltMgSO4 (85 and 90) was high compared with the monovalent salt NaCl (64and 67) at 5 and 10 bar respectively In other research Han chang et al (2012)demonstrated that blending a certain amount of sulphonated poly(ether ketone)(SPEK) material into the PSf substrate of TFC forward osmosis (TFC-FO) membranes
Advances in polymeric membranes for water treatment 7
not only has a key role in forming a fully spongelike structure but also enhances mem-brane hydrophilicity and reduces structure parameters In this study MPDA and135-TMC were employed as monomers for the interfacial polymerization reactionto form a thin aromatic polyamide selective layer The TFC-FO membranes withthe most hydrophilic SPEK (50 wt) in the substrates exhibited the lowest membranethickness a fully spongelike structure and the highest water flux of 50 and 35 lm2 hwhen deionized water and 2 M NaCl were used as the feed and draw solutionrespectively
New synthesized monomers were also employed to prepare TFC membranes withnotable properties Li and Kim (2008) synthesized a series of isomeric tetra-functionalbiphenyl acid chloride (mm-BTEC om-BTEC and op-BTEC) to prepare TFC reverseosmosis membranes through an interfacial polymerization technique with MPDA asthe amine monomer in aqueous solution According to this study the organic phasereactants had a considerable impact on membrane performance compared with theaqueous phase reactant Experimental results showed that the membrane preparedfrom op-BTEC revealed the highest permeability (542 lm2 h) followed by mem-branes prepared from om-BTEC (500 lm2 h) and mm-BTEC (317 lm2 h) whentested using a 2000 ppm NaCl solution at 2 MPa The rougher and larger surfacearea of the op-BTEC membrane which led to greater contact with water moleculeswas reported as the reason for the flux enhancement of this membrane (Li ZhangZhang amp Zheng 2008) In another study Chen Li Zhang and Zhang (2008) synthe-sized a new class of polymeric amine named SPES-NH2 which was applied for thepreparation of TFC reverse osmosis membranes Polysulfone was used as substrateand TFC membrane was prepared through interfacial polymerization of TMC solu-tions and amine solutions containing SPES-NH2 and m-Phenylenediamine (MPDA)The salt rejection and water flux of the composite membrane prepared under the opti-mum condition (acyl chloride monomer frac14 10 ratio of MPDA to SPES-NH2 frac14 214 min contact time with organic solution) reached 973 and 512 lm2 h respectivelyThe improvement in water flux was due to the incorporation of hydrophilic SPES-NH2
to polyamides On the other hand the high salt rejection was related to the chain stiff-ness of the copolymer and the high degree of cross-linking A novel amine monomerb-cyclodextrin polyurethane (DABA) with three amino groups was synthesized andapplied along with MPDA in TFC membrane preparation by Wang Li Zhang andZhang (2010) to increase the hydrophilicity of a PAN nanofibrous substrate Withan increase in the DABA content in the aqueous phase from 0 to 025 (wv)the TFC membranes showed an increment in water flux from 375 to 554 lm2 h whilemaintaining high salt rejection (w98) in the filtration of a salt solution containing2000 ppm NaCl at 2 MPa
With respect to the fouling resistance Abu Seman Khayet and Hilal (2010 2011)reported on the preparation of TFC membranes with improved antifouling tendency bymeans of interfacial polymerization between BPA andor TMBPA with TMCIrreversible fouling of modified polyester TFC PES membranes decreased duringfiltration of solutions containing humic acid at different pH values The highly uniformtop polyester layer coupled with the negative charge of the composite membranewas introduced as the main reason for the lower fouling of the TFC membrane
8 Advances in Membrane Technologies for Water Treatment
Furthermore An Li Ji and Chen (2011) incorporated PVA into composite nanofiltra-tion membranes by adding different amounts of PVA into PIP during interfacialpolymerization with TMC to improve the antifouling performance of compositemembranes The incorporation of a hydrophilic PVA chain into the PA active layerhad beneficial effects on developing a smoother surface and increasing membranehydrophilicity that were effective in diminishing the fouling of protein over thelong run
The application of additives such as synthesized hydrophilic surface modifyingmacromolecules (Abu Tarboush Rana Matsuura Arafat amp Narbaitz 2008 RanaKim Matsuura amp Arafat 2011) was shown to be effective in altering TFC mem-brane performance Abu Tarboush et al (2008) prepared a TFC membrane with amore hydrophilic surface by incorporating the synthesized additive hydrophilicsurface modifying macromolecules (LSMM) into the active PA thin top layer Itwas observed that during in situ polymerization LSSMs could migrate toward thetop airepolymer interface rendering the membrane hydrophilic and producing acomposite membrane with improved flux stability over an extended operationalperiod compared with an LSMM-free composite membrane Another importantdevelopment in TFC membrane technology was the incorporation of nanoparticlesinto the TFC membrane structure (Kim Kwak Sohn amp Park 2003 Lee ImKim Kim amp Min 2008) Kim et al (2003) prepared a hybrid TFC reverse osmosismembrane by the self-assembly of anatase TiO2 nanoparticles through coordinationand H-bonding interaction with the COOH functional groups of an aromatic poly-amide thin-film layer to reduce membrane biofouling The photocatalytic bactericidalability of the hybrid TFC membrane was examined by determining the survival ratiosof Escherichia coli cells with and without UV light illumination Photocatalyticbactericidal efficiency was remarkably higher for the hybrid TFC membrane underUV light illumination than that without illumination and the neat TFC membranesLee et al (2008) also reported on a preparation of PA nanocomposite membranecontaining TiO2 nanoparticles synthesized via in situ interfacial polymerization un-der various curing temperatures and curing times The interfacial reaction occurredbetween the aqueous phase of MPDA and the organic phase of TMC in whichTiO2 nanoparticles were homogeneously dispersed The incorporation of TiO2 nano-particles into PA resulted in an increase in water flux owing to the enhancement ofhydrophilicity in the membranes When the concentration of TiO2 nanoparticleswas 50 wt the optimum membrane performance was obtained together withstrong mechanical properties In more concentration of TiO2 the permeation fluxincreased and salt rejection decreased significantly Moreover the decrease inmechanical strength of the membranes resulted in easier peeling-off of thePA-TiO2 layer from PES substrate after the filtration experiment These resultswere attributed to the fact that a lower degree of polymerization of PA occurs athigh TiO2 concentrations because of an increase in the interference of interfacialpolymerization by TiO2 nanoparticles The incorporation of zeolite-A nanoparticles(Jeong et al 2007) and multi-walled nanotubes (MWNTs) (Wu Tang amp Wu 2010)throughout the thin-film layer were also explored as a facile approach to producesuperior hydrophilic membranes with improved performance
Advances in polymeric membranes for water treatment 9
The addition of co-solvent into the organic phase is another alternative to improveTFC membrane performance (Kim Kwak amp Suzuki 2005 Kong KanezashiYamomoto Shintani amp Tsuru 2010) Kong et al (2010) applied this method andadded acetone as the co-solvent into the organic phase to synthesize TFC nanofiltra-tion membranes with controllable active layer thickness and effective nanopores Thepresence of acetone could eliminate the large immiscibility gap between waterand hexane therefore it caused the interfacial polymerization reaction zone tobe controllable Consequently the permeation flux increased noticeably from21 1012 m3m2 Pa s for membranes prepared by conventional interfacial polymer-ization to 80 1012 m3m2 Pa s for TFC membranes prepared by adding co-solvent
122 Nanocomposite membranes (nano-enhanced membranes)
In another approach to membrane development processes researchers focused onincorporating inorganic nanoparticles into polymeric materials which resulted in theformation of nanocomposite (nano-enhanced) membranes with improved mechanicaland physicochemical properties Various nanomaterials have been employed as fillersuch as carbon nanotubes (CNTs) nanoclay nanosilver and nanosized TiO2 ZnOAl2O3 Fe3O4 SiO2 and ZrO2 More details about developments in nanocompositemembranes and their performance in water treatment are presented in the followingsections
1221 Carbon nanotubes
Among the various nanomaterials studied CNTs have received a great deal of atten-tion With high hydrophilicity good chemical stability and high surface area CNTsinduce highly antibacterial properties and increase the porosity of nanocompositemembranes They are ideal for reinforcing membranes because of their high aspectratio and high axis strength (Daraei Madaeni Ghaemi et al 2013) The wide rangeof possible applications for CNTs has been a driving force for the production of high-quality single-walled (SW) double-walled (DW) and MW CNTs for incorporationinto CNTepolymer composites (Vatanpour Madaeni Moradian et al 2012)
Several methods have been applied to produce inorganic-polymer membranessuch as filtration of the solvent containing dispersed CNTs through the membraneand then in situ polymerization onto the membrane surface (Madaeni Zinadini ampVatanpour 2011a) dispersion of CNTs in the monomer solution and then interfacialpolymerization onto a membrane as substrate (Roy Addo Ntim Mitra amp Sirkar2011) and blending nanoparticles into the casting solution followed by immersioninto the coagulation bath (Celik Park Choi amp Choi 2011 Choi Jegal amp Kimb2006 Mansourpanah et al 2011) or evaporation (Tang Zhang Wang Fu andZhang 2009)
Blending has some advantages over other methods The method is simple and hasmild conditions reproducibility and capability for industrialization Also preparationthrough polymer solution casting permits the use of polymers which were previouslyfound not to be suitable for in situ polymerization (Vatanpour Madaeni Moradian
10 Advances in Membrane Technologies for Water Treatment
et al 2012) Previous studies by Choi et al (2006) showed that MWCNT-blended PSfmicrofiltration membranes had slightly higher flux and rejection rates than nascent PSfmembrane Also those authors found that MWCNT-blended membranes were morehydrophilic than PSf ones In a related work Qui et al (2009) showed thatMWCNT-blended PSf ultrafiltration membranes had higher flux and lower rejectionas well as lower protein adsorption compared with nascent PSf membrane
Although CNTs have excellent separation characteristics and electrical and me-chanical properties there are problems in preparing mixed matrix membranes usingthis material such as no suitable dispersion of synthesized CNTs in organic solventsand different polymers (Celik et al 2011 Shirazi Ahmadzadeh Tofighy andMohammadi 2011 Vatanpour Madaeni Moradian et al 2012) Thus surfacemodification of this material is necessary to fabricate a homogeneous nanocompositemembrane Recent studies showed that surface modification can increase thedispersing properties of CNTs Chemical modification and functionalization ofCNTs surfactant treatment and polymer wrapping are several methods that havebeen widely used for uniform dispersion of nanotubes in a polymer matrix and forenhancing CNT adhesion to the polymer (Choi et al 2006 Eitan Jiang DukesAndrews amp Schadler 2003 Islam Rojas Bergey Johnson amp Yodh 2003 OrsquoConnellet al 2001 Qu et al 2004 Yang Chen et al 2007) In this regard ShawkyChae Lin and Wiesner (2011) added different concentrations of MWCNTs tothe PA membrane casting solution to form a nanocomposite structure improvethe mechanical properties of these membranes and increase their ability to rejectsalts The MWCNTs were dispersed into a mixture of dimethylacetamide(DMAc) (solvent) and lithium chloride salts Benzoyl peroxide (BPO) initiatorwas added into the casting solution with the goal of forming free-radicals onboth CNTs and PA This process resulted in polymer-grafted nanotubes thatdispersed better throughout the casting solution and a more homogeneousMWCNTsePA composite membrane was formed Investigation of the hydrophilic-ity and mechanical strength of nanocomposite membranes revealed a decrease inhydrophilicity owing to the hydrophobic nature of nanomaterials and an increasein the mechanical strength of the membranes because of the strong interactionsbetween the PA matrix and MWCNTs and homogeneous dispersion of CNTs
The decrease in permeability from 32 to 28 lm2 h and increase in salt rejection from24 to 76 respectively for the PA and MWCNTsPA (15 mgg) composite mem-brane were attributed to this network structure In addition humic acid removal byMWCNT composite membranes increased from 54 to 90 as the MWCNT loadingincreased from 0 to 10 mgg (Shawky Chae Lin and Wiesner 2011)
The introduction of hydrophilic functional groups onto the surface of the CNTsusing chemical modification especially acid is one of the simplest most widelyused and least expensive methods for uniform dispersion of nanotubes in the polymermatrix (Balasubramanian amp Burghard 2005 Celik et al 2011 Eitan et al 2003Shirazi et al 2011 Vatanpour Madaeni Moradian Zinadini amp Astinchap 2011)Vatanpour et al (2011) fabricated nanocomposite membranes composed of PES andacid-treated functionalized MWCNTs using a solution of 3 M HNO3H2SO4 (13vv) Functionalized MWCNTs were embedded in PES matrix using the phase
Advances in polymeric membranes for water treatment 11
inversion method The treated MWCNTs exhibited good dispersion and compatibilitywith the polymer matrix The blending of MWCNTs into PES matrix resulted in anincrease in hydrophilicity and water flux along with a decrease in fouling of the nano-composite membrane caused by Bovine Serum Albomin (BSA) filtration
In another work the same author (Vatanpour Madaeni Moradian et al 2012)fabricated an anti-bifouling nanofiltration membrane by mixing PES with TiO2-coated functionalized MWCNTs They coated MWCNTs with TiO2 nanoparticlesto enhance the dispersion of CNTs in organic solvent and polymer and improvethe interaction between the CNTs and the polymer matrix To purify and functional-ize CNTs the MWCNTs were dispersed into a mixture of H2SO4 and HNO3 with aratio of 31 The functionalized MWCNTs could participate in a reaction with otherreagents as a result of the formation of carboxylic acid groups on the surface ofMWCNTs (Vatanpour Madaeni Moradian et al 2012) To better deposit TiO2
nanoparticles onto the surface of oxidized MWCNTs dendrimer polycitric acidwas grafted onto functionalized CNTs At the final step MWCNTs were coatedwith TiO2 nanoparticles A schematic of the preparation of TiO2-coated MWCNTsis presented in Figure 11
The TiO2-coated MWCNTs showed suitable compatibility with polymeric matrixresulting in a low agglomeration of MWCNTs Addition of the modified MWCNTsinfluenced the surface mean pore size and porosity of the nanocomposite membranes
31H2SO4HNO3
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
COOH
COOHCOOH
COOH
COOHCOOHCOOH
COOH
COOHCOOHCOOH
COOH
HOOCCOOH
COOH
OH
COOHHOOC
HO
TiOC2H5
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5C2H5OC2H5OC2H5O
C2H5OC2H5OC2H5O
Oxidized MWCNT
Monohydrate citric acid
Heated in vaccumfor 3h150 0C
Heated at 500 0C in N2 mediumfor 100 min
Polycitric acid-grafted MWCNTs
1) TiCI41 M HCI2) 5 M NH3stiring
for 3 h
Ti
O-Ti(OC2H5)(C2H5O)Ti-O
Ti
Anatase TiO2
TiO2-coated MWCNTS
Ti COOH
MWCNT
12 h reflux
Figure 11 Schematic showing coating of multi-walled carbon nanotubes (MWCNTs) withTiO2 nanoparticlesReprinted from Vatanpour Madaeni Moradian et al (2012) with permission from Elsevier
12 Advances in Membrane Technologies for Water Treatment
The prepared mixed matrix membranes showed higher hydrophilicity compared withnascent PES membrane which resulted in an increase in pure water flux Also theanti-biofouling characteristics of the nanoparticle-embedded membranes improvedas a result of increased hydrophilicity and decreased membrane surface roughness(Vatanpour Madaeni Moradian et al 2012) Daraei Madaeni Ghaemi Ahmadiet al (2013) and Daraei Madaeni Ghaemi Khadivi et al (2013) studied the effectsof blending polymer-modified MWCNTs and PES membranes and compared the re-sults with acid-functionalized MWCNTs They modified acid functionalizedMWCNTs using three hydrophilic polymers through an in situ polymerization reac-tion Citric acid acrylic acid and acryl amide were polymerized on acid-functionalized MWCNTs to achieve a higher number of functional groups onMWCNTs (Daraei Madaeni Ghaemi Khadivi et al 2013) The CNT-mixed mem-branes were fabricated using the phase inversion precipitation method Although thistreatment process caused a higher pure water flux with unchanged protein retention byadding a constant amount of PAA and PAAm-modified MWCNT into the membranematrix the best antifouling performance was shown by the membrane containingPCA-modified MWCNT The hyperbranched PCA caused an efficient dispersion ofMWCNTs because of good compatibility with PES This membrane also showedacceptable reusability and durability during three cycles of foulingewashing steps
1222 Clay nanoparticles
Clay nanoparticles are another inorganic additive for enhancing membrane perfor-mance Unmodified montmorillonites (NathornMMT) and Cloisite grades (modifiedMMT) are widely used nanofillers in the preparation of polymer nanocompositesMontmorillonite contains phyllosilicate groups and possesses an octahedral sheetsandwiched between two tetrahedral sheets forming a plate-shaped structure (withthe average diameter around 1 mm) (Abdollahi Rahmatpour Aalaie amp Khanbabae2008 Xie Tan Liao and Liu 2010) The layered structure of silicate has attractivehydrophilic properties and good thermal stability at high temperature (BebinCaravanier amp Galiano 2006) The most common methods used to prepare polymerclay nanocomposite technology are in situ polymerization melt intercalation andsolution dispersion In the latter method the clay mineral is exfoliated in single layersin the solvent medium and polymer chains are intercalated into these clay minerallayers The clay mineral platelets are joined by weak van der Waals forces and caneasily be dispersed in the solvent because of the increase in entropy caused by theirdisorganization Then polymer is adsorbed onto the delaminated clay mineral layersand the layers are reassembled after evaporation of the solvent and filled with polymerchains forming an intercalated nanocomposite (Anad~ao Sato Wiebeck Rolando ampDiaz 2010 Ghaemi Madaeni Alizadeh Rajabi amp Daraei 2011)
The clays most applied in polymer clay nanocomposite materials are generallythose containing clay minerals of the smectic group especially those containingMMT which are often employed for several innovative uses for example asabsorbers in the retention of pollutant gases in acid catalysis in sensors and as nano-fillers in nanocomposite preparation
Advances in polymeric membranes for water treatment 13
Many researchers have investigated the effects of adding clay particles on themorphology and thermal mechanical and hydrophilic properties as well as on theperformance of composite polymericmembranes Bebin et al (2006) prepared a compos-ite membrane containing clay particles Membranes were prepared by a recasting proce-dure using a Nafion solution mixed with Laponite particles comprising sulfonic acidgroups bonded to its surface The surface of clay particles was modified using plasmaactivation to bond styrenesulfonic moieties at their surface The dispersion of modifiedclay particles had a significant effect on the behavior of the Nafion membrane andenhanced both thewater retention and proton conductivity of nanocompositemembranesMonticelli Bottino ScandaleCapannelli andRusso (2007) reported on improvingwaterpermeability for PSf membranes blended with cationically modified clays The cationicclayePSf membranes showed the most flux and the least water contact angle (mosthydrophilicity) resulting in the enhancement of membrane performance Anad~ao et al(2010) also used modified clay as a PSf dopant and reported significant changes to themembrane morphology and thermal mechanical and hydrophilic properties
Another typical polymer used to prepare polymer clay composites is PVDFLi and Kim (2008) investigated the effect of adding modified clay (Closite Nathorn) onthe thermal properties of microporous PVDF membranes The authors reportedthat the thermal properties of PVDF nanocomposite membrane diminished afteradding modified clay nanoparticles In another work various kinds of clay particles(Closite Nathorn Closite 15A Closite 20A and Closite 30B) were incorporated intoPVDF membrane by a solution mixing method and PVDFeclay nanocomposite mem-branes were prepared by the phase inversion method (Hwang Kim Kim Hong ampNam 2011) Improved mechanical properties and thermal stability against shrinkagewere obtained by incorporating clay particles into the PVDF matrix In contrastKoh et al (2010) did not find considerable improvement in the mechanical propertiesof PVDFehexafluoropropylene (PVDFeHFP)eclay nanocomposite membranesbecause of the increased porosity of the nanocomposite membranes
Ghaemi et al (2011) synthesized PESorganically modified montmorillonite(OMMT) nanocomposites using Closite 15A and reported significant changes to themembrane skin layer and sublayer with increasing clay concentration in the casting so-lution They declared that the addition of OMMT can be effective in considerablyimproving membrane hydrophilicity and thermal and mechanical resistance as wellas pesticide retention capability at different solution conditions (acidic and neutral)In similar work Mierzwa Arieta Verlage Carvalho and Vecitis (2013) investigatedthe effect of incorporating unmodified clay nanoparticles (single platelet MMT) withand without sodium hexametaphosphate as a clay nanoparticle dispersant into the cast-ing solution on the morphology and performance of PES ultrafiltration membranesThey showed that clay additives have the ability to increase membrane permeabilityas a result of changes induced on the structure of the internal and surface pores ofthe membrane The addition of clay nanoparticles resulted in a reduction in hydrophi-licity and the negative charge of the membrane surface Surprisingly it was revealedthat although the clay membranes were more prone to fouling membrane permeabilityin the filtration of alginate solution was still greater than the nascent membrane(Mierzwa et al 2013)
14 Advances in Membrane Technologies for Water Treatment
Wan Ngah Teong and Hanafiah (2011) used chitosan-based composites containingMMT clay bentonite etc for dye removal from effluents Improvement of chitosanproperties and performance was considered the main achievement of fabricating suchnanocomposites An increase in mechanical and thermal strength as well as higher plas-ticity of chitosaneOMMT nanocomposite was also reported by researchers (Abdollahiet al 2008 Casariego et al 2009 Han Lee Choi amp Park 2010) Moreover manystudies were conducted on the adsorption of pollutants onto the chitosaneclay compos-ites in which the increment of the adsorption capacity of chitosan membranes wasproved as a result of adding clay nanoparticles (An ampDultz 2007 Bleiman ampMishael2010 Celis Adelino Hermosin amp Cornejo 2012 Chang amp Juang 2004 NesicVelickovic amp Antonovic 2012 Pandey amp Mishra 2011) For example Nesic et al(2012) tested the adsorption capability of chitosanMMT (Closite Nathorn) with BezactivOrange V-3RBO (BO) solution The effect of different amounts ofMMT on the adsorp-tion of BO was evaluated under different pHs and temperatures It was revealed thatchitosan in combination with clays produced a reliable adsorbent The main advantageof these membranes compared with similar systems is that these membranes possessedsignificantly higher adsorption capacity at low acidic conditions (pH above 6) There-fore the pretreatment of wastewater is not needed in real applications Despite relativelylong adsorption tests thesemembraneswere applicable as the result of a high adsorptioncapacity Thin-film composite membranes using chitosanenanoclay (Closite 15A and30B) coated onto the PVDF microfiltration membrane was fabricated by Daraei et al(2013) Insertion of organoclays with various percentages into the chitosan made itmore efficient for fabricating TFC membranes with the valuable ability to removedye (methylene blue and acid orange 7) from effluents
1223 Silver nanoparticles
Compared with other nanoparticles silver (Ag) is one of the first nanomaterials thatgained attention for fabricating composite membranes The bactericidal nature of silvernanoparticles brought them into the field of mixed matrix membranes Silver nanopar-ticles have a large surface-to-volume ratio thus they are used as a sustained local supplyfor Agthorn ions and provide a prolonged prevention of bacterial adhesion (Cao Tang LiuNie amp Zhao 2010 Martinez-Gutierrez et al 2012 Mollahosseini RahimpourJahamshahi Peyravi amp Khavarpour 2012) Silver is believed by some researchers(Danilczuk Lund Saldo Yamada amp Michalik 2006) to be nonallergic nontoxicand environmentally friendly Other researchers believe that silver is less harmful tohuman cells compared with bacteria (Taurozzi et al 2008) Research has shown thatincorporation of Ag nanoparticles onto the surface or in the matrix of membranes iseffective against the bacteria E coli Pseudomonas mendocina KR1 P aeruginosaand Staphylococcus aureus (Li Shao Zhou Li amp Zhang 2013 Liu Zhang HeZhao amp Bai 2010 Liu Rosenfield Hu ampMi 2013 Mollahosseini et al 2012 ZhangZhang Gusseme amp Verstraete 2012 Zodrow et al 2009) Also it was proved that theaddition of Ag nanoparticles into the membrane matrix not only makes the membranesbactericidal but also results in a significant improvement in virus removal (Zhang et al2012 Zodrow et al 2009) Chamakura Perez-Ballestero Luo Bashir and Liu (2011)
Advances in polymeric membranes for water treatment 15
described in detail the steps of the antibacterial mechanism including (1) generation ofreactive oxygen species indirectly (2) direct interaction of Ag with proteins and lipids inthe cellwall and alsoproteins in the cytoplasmicmembrane and (3) interactionwith deox-yribonucleic acid Some investigators (Koseoglu-Imer Kose Altinbas amp Koyuncu2013) suggested that Ag nanoparticles might pass through cell barriers and then releaseAgthorn ions Then Agthorn ions interact with the thiol groups of enzymes and cell proteinsdamage bacterial respiration and transport systems across the cell membrane
Silver ions were employed in membrane fabrication because of their low toxicity to-ward humans and their antibacterial ability In this regard researchers prepared mem-branes with silver ions to obtain a superior antifouling performance (Basri et al2010 Chou Yu amp Yang 2005) In contrast with silver ions silver nanoparticlesinduced long-lasting antibacterial and anti-adhesive effects and consequently consider-able resistance to biofouling in the membranes (Li et al 2013 Zodrow et al 2009) Totake advantage of Ag nanoparticles Ag does not necessarily need to be embedded intothe membrane structure during preparation It can be added to a commercial membraneand regenerated if needed However loss of antimicrobial and antiviral activity ofAg-coated membranes might occur as a result of depleting silver particles from themembrane surface and ineffectiveness against silver-resistant bacterial strains (Zodrowet al 2009) For this reason silver nanoparticles have received more attention in less-ening biofouling through embedding into the membrane backbone so far most studieson antifouling nanocomposite membranes have focused on incorporating silver nano-particles inside the membranes (Gusseme et al 2011 Li et al 2008 Sawada et al2012 Thomas et al 2009 Travan et al 2009 Zhang et al 2012 Zodrow et al2009) This way silver nanoparticles were added into the PAN (Yu et al 2003) poly(2-ethyl-2-oxazoline) (Kang Kim Char Won amp Kang 2006) PA (Lee et al 2007)polyimide (PI) (Deng Dang Zhou Rao amp Chen 2008) PES (Basri Ismail amp Aziz2011 Basri et al 2010 Cao et al 2010 Huang Arthanareeswaran et al 2012) CA(Barud et al 2011 Chou et al 2005) chitosan (Liu et al 2010 Zhu Bai Wee Liuamp Tang 2010) PVDF (Li et al 2013) and PSf (Zodrow et al 2009) compositemembranes Antibacterial experiments confirmed that bacterial growth was effectivelyinhibited and anti-biofouling performance rose in these kind of nanocompositemembranes
An increase in the hydrophilicity and selectivity of membranes blended with Agnanoparticles was also reported apart from the antibactericidal and antifouling proper-ties of silver (Basri et al 2011 Koseoglu-Imer et al 2013 Yong et al 2010 Zodrowet al 2009) Silver nanoparticles can be used as a suitable candidate for the simulta-neously improvement of membrane surface hydrophilicity and antifouling perfor-mance under proper conditions This phenomenon occurs because Ag particlesdiminish the surface tension of a pristine membrane and so water can easily spreadonto membrane surfaces (Li et al 2013)
1224 Titanium dioxide nanoparticles
Titanium dioxide (TiO2) is a special semiconductor that has been the focus ofnumerous investigations because of its excellent photocatalytic and hydrophilicity
16 Advances in Membrane Technologies for Water Treatment
properties in absorbing UV rays and its stability cheapness and commercial availabil-ity Therefore many researchers have prepared TiO2-mixed matrix membranes toimprove membrane performance by increasing hydrophilicity and reducing fouling(Madaeni amp Ghaemi 2007 Vatanpour Madaeni Khataee et al 2012 Yang ampWang 2006 Yang Zhang et al 2007)
Ebert Fritsch Koll and Tjahjawiguna (2004) investigated the influence of incorpo-rating TiO2 particles on the performance of PVDF and PAI membranes at elevatedpressures and temperatures Membranes fabricated by the phase inversion methodrevealed considerable thermal and mechanical stability In another work Yang andWang (2006) prepared a PSTiO2 organiceinorganic hybrid ultrafiltration membranewith high hydrophilicity and permeability and mechanical and thermal stability using asol-gel and phase inversion process The hybrid membranes exhibited extraordinaryhydrophilicity increased porosity and superior permeability without changingretention
The main benefit of using TiO2 nanoparticles in a mixture with polymeric mem-branes is the reduction of membrane fouling Much work has been conducted inblending TiO2 nanoparticles with different polymeric membranes to prepare compos-ite membranes with antifouling properties Table 12 lists some prepared polymericTiO2 composite membranes with antifouling properties
Regarding the photocatalytic bactericidal ability of TiO2 nanoparticles in recentyears heterogeneous photocatalysis UVeTiO2 systems have been also applied tosolve a variety of important environmental problems such as detoxifying water andproducing self-cleaning materials Damodar et al (2009) prepared PVDFeTiO2 com-posite membranes via the phase inversion method by mixing different amounts of TiO2nanoparticles (04 wt) into the PVDF casting solution The photocatalytic bacteri-cidal ability of composite PVDFeTiO2 membranes was tested using E coli (initialconcentration frac14 66 10thorn07 colony-forming unitsml) with and without the presenceof UV light (Figure 12)
These results were compared with a reference E coli solution that was kept indarkness Result showed that almost all E coli cells survived in the dark whereasabout 58 of cells survived on pristine PVDF membrane exposed to UV light for1 min The number of surviving E coli cells decreased by adding TiO2 particles andincreasing their content in the PVDFeTiO2 composite membrane and almost completeremoval of E coli occurred with a 4 TiO2ePVDF membrane during 1 min (Damodaret al 2009) In similar work with similar results Rahimpour Jahanshahi Rajaeian andRahimnejad (2012) induced bactericidal ability into a PVDFeSPES-blend membraneby incorporating TiO2 nanoparticles into the membrane to remove E coli
To increase homogeneous dispersion reduce agglomeration enhance nanofillerepolymer interaction and improve the stability of TiO2 nanoparticles chemicalapproaches have been applied to modify TiO2 nanoparticles (Li Zhu amp Zheng2007 Madaeni Zinadini amp Vatanpour 2011b Razmjou et al 2011) Li et al(2007) organically modified TiO2 nanoparticles with silane couple to overcomeaggregation and improve the dispersion of TiO2 nanoparticles in organic solventThe morphology and structure of PPESK ultrafiltration membrane indicated thatthe addition of TiO2 nanoparticles increased the thickness and porosity of the skin
Advances in polymeric membranes for water treatment 17
layer as well as the surface wettability and hydrophilicity of the membrane Thetensile mechanical strength of the membrane was also enhanced after the additionof TiO2 Moreover permeability and rejection tests showed that the pure waterflux and solute rejection of composite membranes were remarkably elevated whena small amount of TiO2 nanoparticles was added into the membrane structure
In other research Razmjou et al (2011) employed mechanical and chemical mod-ifications of TiO2 nanoparticles to improve their dispersion into the PES membranematrix For mechanical modification TiO2 nanoparticles with a volume of 50 cm3
Table 12 Some prepared polymericeTiO2 composite membraneswith antifouling properties
Polymer TiO2 (type) TiO2 (size nm) Reference
PSf-PVDF-PAN Degussa P25 20 Bae and Tak (2005)
PVDF Rutile 26e30 Cao et al (2010)
PVB Anatase 180 Fu Matsuyama and Nagai(2008)
PES Degussa P25 20 Rahimpour MadaeniTaheri and Mansourpanah(2008)
PES Rutile 30 Wu Gan Cui and Xu (2008)
PVDF Degussa P25 20 Damodar You and Chou(2009)
PVDF Degussa P25 20 Oh Kimb and Lee (2009)
PES Degussa P25 20 Li et al (2009)
PVDF Degussa P25 20 Yu Shen et al (2009)
PES Degussa P25 20 Hamid et al (2011)
CA Anatase 62 Abedini Mousavi andAminzadeh (2011)
PVDFSPES Degussa P25 20 Rahimpour et al (2011)
PVDF Degussa P25 25 Wei et al (2011)
PVDF Degussa(80 anataseand 20 rutile)
20 Madaeni Zinadini andVatanpour (2011b)
PES Degussa P25anatase
20 8 15e25 Vatanpour MadaeniKhataee et al (2012)
PVDF Anatase 20 Shi Ma Ma Wang and Sun(2012)
Reprinted from Vatanpour Madaeni Khataee et al (2012) with permission from Elsevier
18 Advances in Membrane Technologies for Water Treatment
were ground into fine powder until the volume decreased to 125 cm3 Afterward thepowder was dispersed in DMAc by sonication in a bath for 15 min followed by 10 minsonication by probe at an amplitude of 20 kHz The chemical modification of TiO2 wasalso carried out by surface modification of TiO2 nanoparticles with aminopropyltrie-thoxysilane (APTES) as silane coupling agent First 25 g of mechanically modifiedTiO2 nanoparticles was added into pure ethanol under nitrogen purging this was fol-lowed by 30 and 10 min sonication in bath and by probe respectively Differentamounts of APTES (2 20 50 and 80 wt) were added drop-wise to the mixture underan N2 atmosphere After stirring for 2 h at 65 C the particles were separated from thesolution by centrifuging at 10000 RPM for 10 min Finally the TiO2 particles weredried in an oven for 24 h at 50 C and then were ground into fine powder The com-bination of chemical and mechanical modifications was significantly effective for thefree energy and roughness of the surface pore size and protein absorption resistanceof the membrane surface as well as improved hydrophilicity of the membraneAlthough most researchers reported that an increase in hydrophilicity is the most likelyreason for decreased fouling Razmjou et al (2011) believed that the other parameterssuch as changes in the membrane morphology and local surface modifications mighthave a similar effect on the higher fouling resistance of TiO2-embedded membranes
1225 Zinc oxide nanoparticles
Another applicable nanoparticle used in modifying membranes is nano-zinc oxide(ZnO) As a multifunctional inorganic nanoparticle ZnO nanoparticles have recentlydrawn increasing attention owing to their prominent physical and chemical propertiessuch as high catalytic activity and effective antibacterial and bactericide capabilities(Balta et al 2012 Hong amp He 2012) Moreover ZnO nanoparticles are moreeconomical than some other nanoparticles such as TiO2 and Al2O3 nanoparticles
CFU0 = 66E+07 CFUml
Figure 12 Removal efficiency of Escherichia coli in water on the surface of differentpoly(vinylidenefluoride) (PVDF)TiO2 membranes with and without the radiation of UV lightReprinted from Damodar You and Chou (2009) with permission from Elsevier
Advances in polymeric membranes for water treatment 19
On the other hand ZnO nanoparticles are easily able to absorb hydrophilic hydroxylgroups (OH) (Shen et al 2012) Thus the supplementation of ZnO nanoparticlescan improve the hydrophilicity and mechanical and chemical properties of the polymermatrix (Hong amp He 2012 Lin et al 2009 Shen et al 2012)
Balta et al (2012) significantly reduced fouling of PES nanofiltration membranesafter blending with different concentrations of ZnO nanoparticles (0035 0070085 0125 0250 0375 0500 0750 1 2 and 4 wt) The ZnO-blended mem-branes showed a lower decline in flux and better permeability compared with pristinemembrane owing to the significantly higher hydrophilicity of nanocomposite mem-branes even at ultra-low concentrations of ZnO nanoparticles Also raising the nano-particle concentration to 0125 wt increased the relative flux of nanocompositemembranes but at higher concentrations no further increment was observed Further-more the addition of ZnO nanoparticles considerably improved the fouling resistanceof composite membranes during filtration of solutions containing humic acid This wasattributed to the reduction of adsorption of organic pollutants within the membranestructure as a result of increments of membrane hydrophilicity after the addition ofZnO nanoparticles Shen et al (2012) obtained similar results in fabricating aPESeZnO composite membrane Hydrophilicity thermal resistance porosity waterflux and antifouling capability of the nanocomposite membrane improved after addingZnO nanoparticles (Shen et al 2012)
In other work (Leo Cathie Lee Ahmad amp Mohammad 2012) ZnO nanoparticleswere added into a PSf ultrafiltration membrane to create antifouling properties not onlyon the membrane surface but also inside the pores Based on the results the addition ofZnO nanoparticles significantly increased the membrane hydrophilicity As a result ofincreasing the mean pore size and membrane hydrophilicity an increase as much as100 was observed in permeability The PSf membranes blended with ZnO nanopar-ticles also exhibited less fouling compared with nascent PSf membranes during filtra-tion of an aqueous solution containing oleic acid Moreover the composite membranesshowed improved thermal stability
In a study conducted by Liang Xiao Mo and Huang (2012) ZnO nanoparticleswith different concentrations were blended with PVDF polymer and applied for thefabrication of a composite PVDFeZnO membrane using the wet phase separationmethod Multi-cycle filtration tests demonstrated significant anti-irreversible foulingof the modified PVDF membrane Specifically all modified membranes reachedalmost 100 water flux recovery and maintained the initial fluxes constant in multi-cycle tests Similar to much other research this change was attributed to an increasein the surface hydrophilicity of the membrane
1226 Aluminum oxide nanoparticles
The stability availability hydrophilicity and suitable mechanical strength ofaluminum oxide (Al2O3) nanoparticles make this inorganic material another optionfor preparing of nanocomposite membranes By the aid of Al2O3 blending the mem-brane can combine the basic properties of these materials and display specific bene-fits with respect to thermal and chemical resistance separation performance and
20 Advances in Membrane Technologies for Water Treatment
adaptability to the severe water and wastewater conditions Much research has beendone in applying Al2O3 nanoparticles blending with a PVDF membrane to determinethe effect of Al2O3 particles on the hydrophilicity permeation flux morphologymechanical properties and antifouling performance of membranes
Various mixed matrix membranes of alumina (Al2O3) with CAc (Wara Francisand Velamakkani 1995) CAP (Mukherjee amp De 2013) PA (Saleh amp Gupt 2012)and PVDF (Yan Li amp Xiang 2005 Yan Li Xiang amp Xianda 2006 YanHong Li amp Li 2009) were prepared to change membrane properties such as hydro-philicity permeability and fouling resistance Yan et al (2005 2006) prepared aPVDFeAl2O3 nanocomposite membrane with different concentrations of Al2O3
(1 nm) using the phase inversion method The addition of nanosized Al2O3 particlesincreased membrane permeability with no change in the membrane pore size andnumber just by improving the surface hydrophilicity of the membrane The improve-ment in the hydrophilicity of the nanocomposite membrane also increased its anti-fouling performance in filtering oilewastewater solution from the oil field (Yanet al 2005) In another work the same authors prepared the phase inversion methodusing a nanocomposite tubular ultrafiltration PVDFeAl2O3 membrane (Yan et al2009) The flux of the modified membranes was always higher (about twice) thanthat of the unmodified membranes owing to the improvement of membrane hydro-philicity caused by adding nanosized alumina particles into the PVDF Interestinglythe permeation performance of the modified membrane increased significantlywithout sacrificing its retention properties All of these studies indicate that thepermeability and antifouling performance of the modified PVDF membranes wereconsiderably changed by incorporating Al2O3 nanoparticles into the membranematrix (Yan et al 2009 Yi et al 2011)
On the other hand Liu Moghareh Abed and Li (2011) used g-Al2O3 nanoparticlesThe authors tried to modify PVDF chains and impede segregation of nanoparticles by agrafting reaction between g-Al2O3 nanoparticles (containing a substantial amount ofhydroxyl groups [OH]) and the PVDF polymer catalyzed by some acid (H2SO4)The authors presented a mechanism for the reaction between g-Al2O3 and PVDFshown in Figure 13 The PVDFeg-Al2O3 nanocomposite membranes revealedhigh hydrophilicity and fouling resistance owing to their surface activity high adsorp-tive ability and surface enrichment of reactive functional hydroxyl groups
CF2 CF2
CF2
CH2 CH2
CH2
n
n
nn
OH-
CF CH
H+ FCH
FCH
CHCHγ -Al2O3-OH
γ -Al2O3-O
Basic γ -alumina
y
ndash
+
x
Figure 13 Schematic of the reaction between g-Al2O3 and poly(vinylidenefluoride) polymerReprinted from Liu Moghareh Abed and Li (2011) with permission from Elsevier
Advances in polymeric membranes for water treatment 21
In studies conducted by Maximous Nakhla Wan and Wong (2009) andMaximous Nakhla Wan et al (2010) PES membranes were modified by insertingdifferent concentrations of nanosized alumina (48 nm) particles inside the polymermatrix The increase in porosity and decrease in the hydrophobic interaction betweenmembrane surface and foulants changed the performance of the PES membrane afteradding Al2O3 nanoparticles into the casting solution As a result of the inducedchanges on the membrane after adding Al2O3 nanoparticles the nanocomposite mem-brane showed less flux decline compared with a pristine PES polymeric membrane(Maximous Nakhla Wan et al 2010 Maximous et al 2009)
1227 Iron oxide nanoparticles
Magnetic nanoparticles have gained much attention by researchers in recent yearsBecause of their inherent super paramagnetic properties magnetic nanoparticleshave been studied for various applications including drug and gene targeting cell sep-aration and hyperthermia (Ye Liu Wang Huang amp Xu 2009) Iron oxide (Fe3O4)has excellent thermal and chemical stability and good magnetic performance butalso good biocompatibility and biodegradation ability (Dudek et al 2012 Huanget al 2008) In this regard a PSfeFe3O4 composite ultrafiltration membrane wasfabricated by Fe3O4 nanoparticles (8e12 nm in size) wrapped in the oleic acid usingthe phase inversion method and the membrane performance in retaining lysozyme wasinspected under the magnetic field (Jian Yahui Yang amp Linlin 2006) Although thethermal stability and porosity of the PSf membrane increased after adding Fe3O4membrane hydrophilicity declined as the result of the hydrophobicity of Fe3O4 nano-particles which were wrapped in the oleic acid The rejection of lysozyme using PSfmembrane remained constant within and without the presence of the magnetic fieldHowever the rejection of the nanocomposite membrane was clearly reduced underthe magnetic field the retention recovered quickly when the magnetic field wasremoved On the other hand with the increase in magnetic intensity the decline inrejection was more noticeable The authors concluded that it is possible to separatesome substances using a PSfeFe3O4 nanocomposite membrane by altering themagnetic intensity
In various studies Huang Guo Guo and Zhang (2006) Huang Chen ChenZhang and Xu (2010) and Huang Zheng et al (2012) investigated the effects ofincorporating Fe3O4 nanoparticles on the performance of polymeric membranesThe authors showed that magnetization of a PAN membrane using Fe3O4 nanopar-ticles led to an increment of membrane permeation and antifouling performance inthe ultrafiltration of pig blood (Huang et al 2006) Moreover Fe3O4 nanoparticleswith different concentrations were applied in fabricating PSfeFe3O4 (Huang et al2010) and PVDFe Fe3O4 (Huang Zheng et al 2012) nanocomposite membranesThe composite membranes showed the best performance in terms of pure waterflux rejection and fouling resistance
Apart from improvements in membrane characteristics obvious benefits of ironoxide nanoparticles in the removal of arsenic (Sabbatini et al 2010 Sabbatini RossiThern amp Marajofsky 2009 Park amp Choi 2011) and adsorption of heavy metals
22 Advances in Membrane Technologies for Water Treatment
(Daraei et al 2012 Gholami Moghadassi Hosseini Shabani amp Gholami 2013Mansour Ossman amp Farag 2011) were proved in some research Daraei et al(2012) prepared Fe3O4ePA nanoparticles as adsorbent and used them in the PESmatrix to obtain a nanocomposite membrane with enhanced affinity for the removalof copper ions According to the experiments nanocomposite membranes indicatedhigher Cu(II) ion removal but lower pure water flux compared with pristine PESmembrane This was due to the surface pore blockage as a result of the presence ofnanoparticles in the superficial pores of the membrane during preparation (Figure 14)In other research conducted by Gholami et al (2013) iron oxide nanoparticles wereblended with polyvinyl chloride-blend cellulose membrane to apply the adsorbentproperties of Fe3O4 nanoparticles in the removal of lead from aqueous solution As ex-pected the permeability and rejection of lead rose in nanocomposite membranescompared with pristine membrane
1228 Silicon dioxide nanoparticles
Silicon dioxide (SiO2) is another convenient nanoparticle widely used in fabricatingnanocomposite membranes because of the mild reactivity and distinguished abilityin increasing hydrophilicity and improving the thermal and mechanical stability andfouling resistance of membranes (Jin Shi et al 2012 Jin Yu et al 2012 Ogoshi ampChujo 2005 Wu Xu amp Yang 2003) Silica was widely used with polymericmembranes such as PMMA (Zulfikar Mohammad amp Hilal 2006) PES (ShenRuan Wu amp Gao 2011 Wu et al 2003) PSf (Zhang Liu Lu Zhao amp Song2013) PA (Jin Shi et al 2012 Kim amp Lee 2001) PVA (Nagarale Shahi ampRangarajan 2005 Thanganathan Nishina Kimura Hayakawa amp Bobba 2012Yang James Li amp Liou 2011) and PVDF (Cho amp Sul 2001 Cui Liu Xiao amp
Figure 14 Field emission scanning electron microscopic image of cross-section of membraneprepared with 1 wt polyacrylonitrile (PAN)Fe3O4 nanoparticlesReprinted from Daraei et al (2012) with permission from Elsevier
Advances in polymeric membranes for water treatment 23
Zhang 2010 Liao Zhao Yu Tong amp Luo 2012 Yu Xu et al 2009 Zhang MaCao Li amp Zhu 2014) using sol-gel (Thanganathan et al 2012 Wu et al 2003)phase inversion (Cui Liu Xiao amp Zhang 2010 Liao Zhao Yu Tong amp Luo2012 Shen et al 2011 Zhang et al 2013) and interfacial polymerization (JinShi et al 2012 Namvar-Mahboub amp Pakizeh 2013) The sol-gel technique allowsthe formation of an inorganic framework under mild conditions and incorporation ofminerals into the polymers results in increased chemical mechanical and thermalstability without obviously decreasing the properties of the polymers (Kim andLee 2001 Wu et al 2003 Yu Xu et al 2009) Furthermore the hydrogen bondclusters remaining at the surface of the materials after the sol-gel reaction improvedmembrane hydrophilicity and enhanced the stability of composite materials (Cho ampSul 2001 Nagarale et al 2005 Yu Xu et al 2009) A simple method to obtain anorganiceSiO2 hybrid is to mix an organic polymer with a metal alkoxide such astetraethoxysilane (TEOS) followed by a sol-gel process involving hydrolysis andpolycondensation of TEOS (Wu et al 2003 Yu Xu et al 2009) Yu Xu et al(2009) synthesized organiceinorganic PVDFeSiO2 composite hollow fiber ultrafil-tration membranes using a sol-gel and wet-spinning process The microstructuremechanical property thermal stability hydrophilicity permeability and foulingresistance of composite membranes improved significantly by an appropriate choiceof TEOS concentration (3 wt)
The phase inversion process was also used by researchers as a simple and applicabletechnique to prepare polymeresilica-mixed matrix membranes The addition of hydro-philic SiO2 nanoparticles into the casting solution enhanced membrane hydrophilicity(Cui et al 2010 Shen et al 2011) On the other hand the permeability and anti-fouling ability of nanocomposite membranes also improved as the result of increasedhydrophilicity and the decreased adsorption of foulants on the membrane surface
Interfacial polymerization is another valuable method because the characteristics ofthe skin layer as a key factor in determining membrane performance can be optimizedseparately (Jin Shi et al 2012) Jin Yu et al (2012) and Jin Shi et al (2012)prepared a nanofiltration membrane with poly(amidoamine) (PAMAM) dendrimerand TMC containing SiO2 nanoparticles through an interfacial polymerization reactionon PSf ultrafiltration membrane The addition of nano-SiO2 in the skin layer improvedthermal stability and membrane hydrophilicity The results of raw water filtration bypristine PA membrane and PA-SiO2 membrane (1 wt SiO2 nanoparticles) are shownin Figure 15
As shown in the figure the flux of both PA and PA-SiO2 membranes was reducedduring the filtration time however the pristine PA membrane showed a greateramount of flux decline compared with the hydrophilic PA-SiO2 membrane Surfacewater typically contains three potential groups of foulants including microbial (bacte-ria viruses etc) organic and inorganic (humic acid and minerals) compounds Theaddition of SiO2 nanoparticles to the membrane significantly increased the antifoulingability of the PA membrane as the result of the presence of a huge number of hydroxylgroups on the surface of SiO2 nanoparticles This increased the negative charge on themembrane surface weakening the force between the negatively charged micelles andthe surface of the membrane (Jin Shi et al 2012)
24 Advances in Membrane Technologies for Water Treatment
1229 Zirconium oxide nanoparticles
Polymerezirconium dioxide (ZrO2) composite membranes are known to be chemi-cally more stable than titanium dioxide and aluminum oxide membranes and aremore suitable for liquid phase applications under rough conditions (MaximousNakhla Wan et al 2010) although a few studies have investigated the incorporationof ZrO2 into the membrane matrix Bottino Capannelli and Comite (2002) preparedZrO2ePVDF membranes using the phase inversion method Membrane performancedepended on the PVDF solvent (NMP or TEP) or ZrO2 concentration Other work con-ducted by Zheng Zou Nadeeshani Nanayakkara Matsuura and Paul Chen (2011)found that the addition of zirconia nanoparticles in PVDF membrane increased the hy-drophilicity and surface porosity of the membrane and resulted in an increment of fluxin the composite membrane It was proved that the ZrO2PVDF-blend membrane couldbe applied effectively to remove arsenate from an aqueous solution in a wide range ofpHs the optimal pH was 30e80
Furthermore Maximous Nakhla Wan et al (2010) found that ZrO2ePES mem-brane was stronger than pristine membrane and membranes blended with ZrO2 showedless flux decline and fouling resistance compared with unmodified PES membrane Onthe other hand Pang et al (2014) prepared ZrO2ePES hybrid ultrafiltration membranesby combining the ion-exchange process with the immersion precipitation technique toincrease membrane hydrophilicity and decrease membrane fouling The authors reportedthat the adsorption of foulants on the membrane surface decreased considerablycompared with pristine membranes because of an increase in membrane hydrophilicity
123 Nanostructured membranes
Based on the ISO definition membranes with pores in the range of 1e100 nm wouldevidently be considered nanostructured materials Hence ultrafiltration and most
PAPA-SiO2
Timeh
Flux
( L
sdot(m2
sdoth)-1
)
Figure 15 Flux of polyaniline (PA) and PA-SiO2 membranes versus filtration timeReprinted from Jin Shi et al (2012) with permission from Elsevier
Advances in polymeric membranes for water treatment 25
nanofiltration membranes that reject particles about 02e200 kDa corresponding to ananoscale diameter of around 1e100 nm should be classified as nanostructured mem-branes (Bhadra amp Mitra 2013 Muellera et al 2012) Generally membranes aredefined on the basis of the size of the particles that are rejected therefore the termldquonanordquo in ldquonanofiltrationrdquo does not refer to structural elements of the membranesuch as pore size (Bhadra amp Mitra 2013)
Rahimpour Madaeni Shockravi and Ghorbani (2009) added different concentra-tions of PSAm (2 5 10 and 15 wt) into the PES casting solution to prepare nano-structured ultrafiltration membranes Addition of PSAm not only resulted in theformation of an ultrafiltration membrane with nanopores it improved the hydrophilic-ity mechanical strength permeability and protein rejection of PES membranes More-over Mansourpanah and Momeni Habili (2013) employed the interfacialpolymerization technique to prepare a PA skin layer on the PES support using TMCand PIP as reagents Acrylic acid as a hydrophilic monomer and UV irradiation as aphysical procedure were used to modify the obtained thin layers The effect of UV irra-diation times (30 60 and 120 s) and acrylic acid concentration (1 5 and 10 wt)on the performance and morphology of modified TFC membranes was investigated bythe authors This modification technique resulted in the fabrication of a nanoporousmembrane by reducing the membrane pore size from 110 to 30 nm Meanwhile thehydrophilicity permeation flux rejection and antifouling ability of the modified mem-branes improved considerably
124 Bio-inspired membranes
Bio-inspired studies have drawn much attention as a potentially feasible approach toimprove current technologies In this method nature is considered a large database towhich its scientific rules might be applied in engineering applications as efficientsolutions for difficult problems In this way biological materials or natural biopoly-mers are considered for the fabrication of bio-inspired membranes because of theirwell-controlled structures and superior properties and the improvement of anyaspect of biological materials has the possibility of improving the structure and func-tion of bio-inspired membranes Moreover the conditions under which the mem-brane is fabricated are mild an aqueous solution controlled temperature moderatepressure and neutral or near neutral pH As a result of numerous advantages wideareas of application and high efficiency bio-inspired membranes have the chanceto be the next generation of membranes (Morones-Ramırez 2013 RawlingsBramble amp Staniland 2012)
Several bio-inspired platform methods such as bio-adhesion and bio-mimetic adhe-sion have been created (Li Liu et al 2009 Pan et al 2009) In addition a platformcalled bio-mineralization or bio-mimetic mineralization was developed from theformation process of bio-silica (Pan Jia Cheng amp Jiang 2010) Bio-inspired mem-branes have great benefits such as high permeability catalytic reactivity and foulingresistance and hence have the potential to be employed in water treatment processesPan et al (2010) proposed an innovative procedure to fabricate a polymereinorganicskin layer of a composite membrane by combining bio-mimetic mineralization and
26 Advances in Membrane Technologies for Water Treatment
polymer network structure manipulation In this work the silica nanoparticles showeduniform size and homogeneous distribution within the PVA structure The compositemembrane consisting of a PVAesilica skin layer and PSf hollow fiber support layershowed appropriate free volume properties
Inspired by the hierarchical structures in biology Xu et al (2011) proposed theconcept of vertically aligned nanotubes to emulate the biological channels within amembrane matrix As a result of the presence of nanopores inside the membranematrix the permeability selectivity and durability of bio-inspired membraneincreased Furthermore Balme et al (2011) conducted a research project focusingon bio-inspired membranes with biological ion channels These channels wereconfined inside the nanopores of solid-state materials The solvent had an effectiverole on membrane performance in terms of ion permeability and selectivity
Aside from the transport properties and membrane structure the development ofintelligent properties such as self-cleaning self-healing and stimuluseresponse inbio-inspired membranes are other concerns that have drawn the attention of someresearchers (Capadona Shanmuganathan Tyler Rowan amp Weder 2008 Madaeni ampGhaemi 2007 Wang Janout amp Regen 2011)
13 Applications for water treatment
Polymeric membranes are employed extensively for water treatment applications suchas desalination water softening purification of industrial and municipal wastewatersproduction of ultra-pure water and the food chemical and pharmaceutical industriesMembrane processes offer significant advantages such as operational simplicity andflexibility cost-effectiveness reliability low energy consumption good stabilityenvironment compatibility and easy control handling and scale-up under a widespectrum of operating conditions such as temperature pressure and pH Howeverthere are still unresolved problems regarding the employment of polymeric membranesunder more stringent applications Membrane fouling insufficient separation andrejection treatment of concentrates membrane lifetime and resistance to some chem-icals are the most important and well-known problems related to polymericmembranes
Fouling is the most important and well-known problem in membrane separationprocesses The urgent need for pretreatment cleaning limited recovery and short life-time are some negative results of fouling problems Membrane lifetime and chemicalresistance are also related to fouling and consequent cleaning as well as the conditionsunder which the membranes are employed Many researchers have concentrated onproposing efficient solutions and strategies to prolong the lifetime and chemical resis-tance of membranes improve membrane morphology increase hydrophilicitydecrease surface roughness and increase the degree of cross-linking of the polymerictop layer to decrease membrane fouling and increase the chemical resistance andlifetime of the membrane
Another inherent problem for pressure-driven membranes is the concentrate streamConcentrate is usually an unwanted by-product of water treatment using membrane
Advances in polymeric membranes for water treatment 27
technologies and needs to be further treated This treatment may include reuse furthertreatment by removal of contaminants and direct or indirect discharge into the surfacewater or groundwater and landfills
14 Concluding remarks and future trends
One of the most crucial problems facing the world in this century is the provision ofclean water with adequate quality from water resources Given the numerous privilegesof membranes such as relatively high selectivity and permeability low energyconsumption operational simplicity control scale-up and adaptability as well assatisfactory stability under various conditions over the years membranes have beenthe dominant technology in the water treatment industry Hence many studies havebeen conducted and much progress has been achieved in optimizing membrane char-acteristics and performance However some challenges still need to be addressed suchas decreasing membrane fouling increasing the membrane lifetime selectivity andpermeability improving thermal mechanical and chemical resistance and reducingenergy consumption
Some important incentives for the further development of polymeric membranes inwater treatment are fabrication of antifouling and self-cleaning membranes enhance-ment of membrane permselectivity an increase in membrane stability at extreme oper-ational conditions the production of new polymers with special properties for use incomposite membranes synthesis of novel nanomaterials for application in nanocom-posite membranes and the expansion of applications of bio-inspired and environmen-tally responsive membranes
Abbreviations
AA Acrylic acidAAm Acryl amideAPTES AminopropyltriethoxysilaneBPA Bisphenol ABO Bezactiv Orange V-3RCA Citric acidCAc Cellulose acetateCAP Cellulose acetate phthalateCNC Cellulose nanocrystallineCNTs Carbon nanotubesDABA b-Cyclodextrin polyurethaneDMAc DimethylacetamideDW Double-walledHPAA Hyperbranched poly(amidoamine)ICIC 5-Isocyanatoisophthaloyl chlorideIPC Isophthaloyl chloride
28 Advances in Membrane Technologies for Water Treatment
LSMM Hydrophilic surface modifying macromoleculesmm-BTEC 330550-biphenyl tetra acyl chlorideMMT MontmorilloniteMPDA m-PhenylenediamineMWCNT Multi-walled carbon nanotubeMWNTs Multi-walled nanotubesNMP N-methyl-2-pyrrolidoneom-BTEC 220440-biphenyl tetra acyl chlorideOMMT Organically modified montmorillonitesop-BTEC 220550-biphenyl tetra acyl chloridePA PolyanilinePAA Poly acrylic acidPAAm Poly acryl amidePAm PolyamidePAMAM Poly(amidoamine)PAI Poly(amide-imide)PAN PolyacrylonitrilePCA Polycitric acidPDA 13-PhenylenediaminePES PolyethersulfonePI PolyimidePIP PiperazinePMMA Poly(methyl methacrylate)PPD p-PhenylenediaminePPESK Poly(phthalazine ether sulfone ketone)PS PolystyrenePSAm Poly(sulfoxide-amide)PSf PolysulfonePU PolyurethanePVA Poly(vinyl alcohol)PVB Poly(vinyl butyral)PVDF Poly(vinylidenefluoride)PVP PolyvinylpyrrolidoneSEM Scanning electron microscopeSPC Sulfonated polycarbonateSPEK Sulphonated poly(ether ketone)SPES Sulfonated poly(ether sulfone)SPES-NH2 Sulfonated cardo poly(arylene ether sulfone)SPPO Sulfonated poly(26-dimethyl-14-phenylene oxide)SPSf Sulfonated polysulfoneSW Single-walledTEOS TetraethoxysilaneTEP TriethylphosphateTFC Thin-film compositeTFC-FO TFC forward osmosisTMBPA Tetramethyl bisphenol ATMC Trimesoyl chlorideTiO2 Titanium dioxide
Advances in polymeric membranes for water treatment 29
Greek symbol
b-CPU b-Cyclodextrin polyurethane
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Abedini R Mousavi S M amp Aminzadeh R (2011) A novel cellulose acetate (CA) mem-brane using TiO2 nanoparticles Preparation characterization and permeation studyDesalination 277 40e45
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Abu Seman M N Khayet M amp Hilal N (2011) Development of antifouling properties andperformance of nanofiltration membranes modified by interfacial polymerization Desali-nation 273 36e47
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Adams F V Nxumalo E N Krause R W M Hoek E M W amp Mamba B B (2012)Preparation and characterization of polysulfonecyclodextrin polyurethane compositenanofiltration membranes Journal of Membrance Science 405e406 291e299
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Anad~ao P Sato L F Wiebeck H Rolando F amp Diaz V (2010) Montmorillonite as acomponent of polysulfone nanocomposite membranes Applied Clay Science 48 127e132
An J amp Dultz S (2007) Adsorption of tannic acid on chitosanemontmorillonite as a functionof pH and surface charge properties Applied Clay Science 36 256e264
An Q Li F Ji Y amp Chen H (2011) Influence of polyvinyl alcohol on the surfacemorphology separation and anti-fouling performance of the composite polyamide nano-filtration membranes Journal of Membrane Science 367 158e165
Bae T H amp Tak T M (2005) Effect of TiO2 nanoparticles on fouling mitigation of ultra-filtration membranes for activated sludge filtration Journal of Membrane Science 2491e8
Balasubramanian K amp Burghard M (2005) Chemically functionalized carbon nanotubesSmall 1 180e192
Balme S Janot J M Dejardin P Beacuterardo L Henn F Bonhenry D et al (2011) Ionicdiffusion through a bio-inspired membrane Basic Principle of Diffusion Theory Experi-ment and Application 16(31) 1e2
Balta S Sotto A Luis P Benea L Van der Bruggen B amp Kim J (2012) A new outlookon membrane enhancement with nanoparticles The alternative of ZnO Journal ofMembrane Science 389 155e161
30 Advances in Membrane Technologies for Water Treatment
Barud H S Regiani T Marques R F C Lustri W R Messaddeq Y amp Ribeiro S J L(2011) Antimicrobial bacterial cellulose-silver nanoparticles composite membranesJournal of Nanomaterials 2011 1e8
Basri H Ismail A F amp Aziz M (2011) Polyethersulfone (PES)esilver composite UFmembrane Effect of silver loading and PVP molecular weight on membrane morphologyand antibacterial performance Desalination 273 72e80
Basri H Ismail A F Aziz M Nagai K Matsuura T Abdullah M S et al (2010) Silver-filled polyethersulfone membranes for antibacterial applications-effect of PVP and TAPaddition on silver dispersion Desalination 261 264e271
Bebin P Caravanier M amp Galiano H (2006) Nafionclay-SO3H membrane for protonexchange membrane fuel cell application Journal of Membrane Science 278 35e42
Bhadra M amp Mitra S (2013) Nanostructured membranes in analytical chemistry Trends inAnalytical Chemistry 45 248e263
Bleiman N amp Mishael Y G (2010) Selenium removal from drinking water by adsorption tochitosaneclay composites and oxides Batch and columns tests Journal of HazardousMaterials 183 590e595
Bottino A Capannelli G amp Comite A (2002) Preparation and characterization of novelporous PVDFeZrO2 composite membranes Desalination 146 35e40
Cao X Tang M Liu F Nie Y amp Zhao C (2010) Immobilization of silver nanoparticlesonto sulfonated polyethersulfone membranes as antibacterial materials Colloids andSurfaces B 81 555e562
Capadona J R Shanmuganathan K Tyler D J Rowan S J amp Weder C (2008) Stimuli-responsive polymer nanocomposites inspired by the sea cucumber dermis Science 3191370e1374
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Advances in polymeric membranes for water treatment 31
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32 Advances in Membrane Technologies for Water Treatment
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Advances in polymeric membranes for water treatment 33
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34 Advances in Membrane Technologies for Water Treatment
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Advances in polymeric membranes for water treatment 35
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Yang Y Zhang H Wang P Zheng Q amp Li J (2007) The influence of nano-sized TiO2fillers on the morphologies and properties of PSF UF membrane Journal of MembraneScience 288 231e238
Yang Z Chen X H Chen C S Li W H Zhang H Xu L S et al (2007) Noncovalent-wrapped sidewall functionalization of multiwalled carbon nanotubes with polyimidePolymer Composites 28 36e41
Ye X Y Liu Z M Wang Z G Huang X J amp Xu Z K (2009) Preparation and char-acterization of magnetic nanofibrous composite membranes with catalytic activity Mate-rials Letters 63 1810e1813
Yi X S Yu S L Shi W X Sun N Jin L M Wang S et al (2011) The influence ofimportant factors on ultrafiltration of oilwater emulsion using PVDF membrane modifiedby nano-sized TiO2Al2O3 Desalination 281 179e184
Yong Ng L Wahab Mohammad A Peng Leo C amp Hilal N (2010) Polymeric membranesincorporated with metalmetal oxide nanoparticles A comprehensive review Desalination308 15e33
Yu D G Teng M Y Chou W L amp Yang M C (2003) Characterization and inhibitoryeffect of antibacterial PANbased hollow fiber loaded with silver nitrate Journal of Mem-brane Science 225 115e123
40 Advances in Membrane Technologies for Water Treatment
Yu L Y Shen H M amp Xu Z L (2009b) PVDF-TiO2 composite hollow fiber ultrafiltrationmembranes prepared by TiO2 solegel method and blending method Journal of AppliedPolymer Science 113 1763e1772
Yu L Y Xu Z L Shen H M amp Yang H (2009a) Preparation and characterization ofPVDFeSiO2 composite hollow fiber UF membrane by solegel method Journal ofMembrane Science 337 257e265
Yu S Liu M Liu X amp Gao C (2009) Performance enhancement in interfacially synthe-sized thin-film composite polyamide-urethane reverse osmosis membrane for seawaterdesalination Journal of Membrane Science 342 313e320
Zavastin D Cretescu I Bezdadea M Bourceanu M Driagan M Lis G et al (2010)Preparation characterization and applicability of cellulose acetateepolyurethane blendmembrane in separation techniques Colloids and Surfaces A Physicochemical andEngineering Aspects 370(1e3) 120e128
Zhang Y Liu Lu Y Zhao L amp Song L (2013) Investigation of phosphorylatedTiO2eSiO2 particlespolysulfone composite membrane for wastewater treatment Desa-lination 324 118e126
Zhang F Ma X Cao C Li J amp Zhu Y (2014) Poly(vinylidene fluoride)SiO2composite membranes prepared by electrospinning and their excellent properties fornonwoven separators for lithium-ion batteries Journal of Power Sources 251423e431
Zhang M Zhang K S Gusseme B D amp Verstraete W (2012) Biogenic silver nano-particles (bio-Ag0) decrease biofouling of bio-Ag0PES nanocomposite membranesWaterResources 46 2077e2087
Zheng Y M Zou S W Nadeeshani Nanayakkara K G Matsuura T amp Paul Chen J(2011) Adsorptive removal of arsenic from aqueous solution by a PVDFzirconia blend flatsheet membrane Journal of Membrane Science 374 1e11
Zhou Y Zhao H Bai H Zhang L amp Tang H (2012) Papermaking effluent treatment Anew cellulose nanocrystallinepolysulfone composite membrane Procedia EnvironmentalSciences 16 145e151
Zhu X Y Bai R Wee K H Liu C amp Tang S L (2010) Membrane surfaces immobilizedwith ionic or reduced silver and their anti-biofouling performances Journal of MembraneScience 363 278e286
Zodrow K Brunet L Mahendra S Li D Zhang A Li Q et al (2009) Polysulfoneultrafiltration membranes impregnated with silver nanoparticles show improved biofoulingresistance and virus removal Water Resources 43 715e723
Zulfikar M A Mohammad A W amp Hilal N (2006) Preparation and characterization ofnovel porous PMMA-SiO2 hybrid membranes Desalination 192 262e270
Advances in polymeric membranes for water treatment 41
Advances in ceramic membranesfor water treatment 2M Lee Z Wu K LiImperial College London London UK
21 Introduction
211 Water treatment
Water is the most abundant natural resource on our planet but only a small percentageis accessible and fit for use in sustaining human life Many areas of the world have noaccess to safe and clean drinking water and are in urgent need of economical reliableand effective methods of treating local raw water sources In addition the global waterconsumption rate continues to increase because of the rapid rise in population andincreased industrialization and agricultural needs The strain placed on the limitedclean water supply will lead to rises in water price which can significantly increasethe operating costs in many industries where water is used extensively and in largevolumes Furthermore regulations on the quality of drinking ground surface andwastewater qualities are becoming increasingly stringent and thus effective watermanagement is of paramount importance more than ever Many industriesrsquo ideal statewould be to recycle all of their wastewater and reach a lsquozero-dischargersquo conditionSignificant developments have been made over the past half-century in the water treat-ment field in particular in the development of cost-effective low-pressure membraneprocesses such as microfiltration (MF) and ultrafiltration (UF) (Judd 2010 Staff2011 Van Der Bruggen et al 2003)
212 Micro- and ultrafiltration processes in water treatment
The commercialization of cost-effective MFUF membranes may be of the most sig-nificant changes in water and wastewater treatment since the 1960s MF and UF arelow pressure-driven separation processes that are less energy-intensive than traditionaltreatment methods The two processes can separate components by a sieving mecha-nism without the need for chemical or physical pre- or post-treatment Hence they arewidely used in many applications for a range of different purposes such as removingsuspended solids bacteria and viruses and heavy metals achieving oil and water sep-aration and pretreating desalination (Lens et al 2001 Mallada amp Meneacutendez 2008)MFUF membranes are porous with pore sizes ranging from microns to nanometresthey can be easily incorporated into existing processes as a standalone unit or com-bined with other technologies
A hybrid system that has broadened the use of MFUF in wastewater treatment isthe membrane bioreactor (MBR) It combines the biological treatment of wastewater
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400002-2Copyright copy 2015 Elsevier Ltd All rights reserved
with a membrane filtration process and is commonly used to treat industrial and munic-ipal wastewater (Judd 2008 2010 Meng et al 2009) However fouling and replace-ment costs of polymeric membranes still challenge the widening and large-scale useof MBRs Although the capital costs of ceramic membranes are higher their robust-ness higher resistance to microorganisms and ease of cleaning lead to lower main-tenance and replacement requirements cutting down long-term operating costs andoffering a competitive alternative to polymeric systems (Gander Jefferson amp Judd2000 Judd 2008)
213 Ceramic membranes in water treatment
Ceramic membranes may be an alternative material for use in MFUF applications inthe place of polymeric membranes Because of their superior chemical thermal and me-chanical properties they can be backwashed cleaned with harsh cleaning agents andsterilized at high temperatures offering reliable performance over longer periods oftime Ceramic membranes have found commercial success in applications in whichthe operating conditions are harsh such as at high temperatures and in aggressive chem-icals (solvents and highly acidic or caustic solutions) Examples of successfulcommercial-scale plants that use MFUF ceramic membranes include purifying andconcentrating valuable components such as enzymes and (Krstic et al 2007) clarifyingfermentation broths in biotechnology industries (Finley 2005 Sondhi Bhave amp Jung2003) clarification of fruit or sugar cane juices (Daufin et al 2001 Girard FukumotoampSefa Koseoglu 2000 Sondhi et al 2003) and treatment of highly oily wastewaterand degreasing baths (Cheryan amp Rajagopalan 1998 Majewska-Nowak 2010)
In contrast ceramic membranes are much less commonly used in water and waste-water treatment such as for the production of drinking water and treatment of munic-ipal wastewater This has historically been because of their high fabrication costscompared with commercially available polymeric membranes Common configura-tions of ceramic membranes include flat-sheet tubular and monolith elements asshown in Table 21 They are normally thicker than polymeric hollow-fibremembranes leading to lower packing densities They are also intrinsically brittleand therefore more expensive to pack and seal
However as the result of sustainable developments in ceramic membranescheaper more effective higher-performing and more compact ceramic systems canbe formed potentially allowing the widening of their applications Table 21 listssome current major international ceramic MFUF membrane suppliers Tubularmonolithic and flat-sheet ceramic membranes currently dominate over hollow-fibremembranes with great water permeation fluxes
22 Development in ceramic membranes and theirfabrication processes
Amembrane can be defined as a semi-permeable active or passive barrier which whenplaced under a certain driving force separates one or more selected species or
44 Advances in Membrane Technologies for Water Treatment
Table 21 List of commercially available ceramic membranes for MFUF
Company Product Material
Technical specifications
Pore size (mm)
Channeldiameterouterdiameter(mm)
Packingdensity(m2m3)
Purewater flux(L(m2hbar))
Flat sheet ItN NanovationAG
CFM Systems
(Itn-nanovation waterfiltration Itn-nanovation Filtrationrack Itn-nanovationT-series)
Substratea-Al2O3
Selectivelayera-Al2O3ZrO2
02 30e 918 e
MeidenshaCorporation
Ceramic membrane unit(Ceramic membraneunit 2013)
a-Al2O3 01 e 361 1670
LiqTechInternationalInc
Silicon carbide flat sheetmembrane (Liqtech)
SiC 02 e 377 e
Flat disc LiqTechInternationalInc
Silicon carbideFlat disc membrane(Liqtech Dynamic)
SiC 004 01 10 e e 3000 400010000
Tubularmonolith
Tami Industries CeacuteRAM (Inside ceacuteram)ISOFLUX (Isoflux)
e MF 014e14UF MWCO1 kDae150 kDa
e e e
AtechInnovationsGmbH
Ceramic membranes(Atech-innovations)
MF a-Al2O3
UF TiO2
MF 01e12UF up to 005and MWCO5 kDa
25e80100e520
e e
Continued
Table 21 Continued
Company Product Material
Technical specifications
Pore size (mm)
Channeldiameterouterdiameter(mm)
Packingdensity(m2m3)
Purewater flux(L(m2hbar))
LiqTechInternationalInc
SteriMem (LiqTech SiCSteriMem) CoMem(COMEM) CoMemConduit (COMEM
CONDUIT) andAquasolution(AQUASOLUTION)
SiC 004 01 10 30e170250e1460
e e
Pall Corporation Membralox (Pall
MembraloxMembralox)
Substrate Al2O3
Selective layerMF Al2O3 orZrO2
UF TiO2
MF 01e50UF 002e01and MWCO1e5 kDa
30e60e 285 e
Veolia WaterLtd
Ceramem
(TundrasolutionsCeraMem Full-sizemembrane modules2013 Veoliawaterst ampCERAMEM)
MF a-Al2O3SiC TiO2
UF SiC SiO2TiO2
MF 01e05UF 0005e005
20e501420 782 MF 400e1200UF 200e800
Hollowfibre
Hyflux Ltd InoCep (Inocepmembrane InoCep)
a-Al2O3 002e14 3040 157 250e4500
Media andProcessTechnologyInc
MPT hollow-fibremembranes (Mediaand process)
a-Al2O3 0004e02 10e35e 502 e
components of a liquid mixture whilst the rest is rejected (Mulder 1996) Micro- andultrafiltration are low pressure-driven processes in which physical separation ofthe different-sized feed components is achieved via a sieving mechanism across themembrane with a specific pore size
In 1989 the Membrane Technology and Research Inc (MTR) performed a studyaddressing the research priorities in the membrane separation industry (Ruutenhuch1992) For MF and UF the highest priority research topics include fouling-resistantlong-life membranes that are high temperature solvent wide pH and oxidant resistantlow-cost membrane modules and low energy module designs Although the study wasperformed more than 20 years ago many of the issues still remain as importantresearch areas today The development of cheap asymmetric inorganic membraneswith high packing densities can be seen as a possible solution to most of the challengeslisted above Significant progresses have been made in recent years regarding theceramic membranes and their fabrication processes
221 Desirable membrane characteristics and their design
To understand the type of membrane properties desired for pressure-driven membraneprocesses it is important to understand the membrane transport mechanisms Transportmodels can help identify the structural parameters that affect the membrane perfor-mance Different pore geometries exist depending on the fabrication processes andhence different models can be used to describe the transport through these different ge-ometries For example the pores can be depicted as parallel cylinders or a network oftightly packed spheres For pores formed from voids between tightly packed spheresthe CarmaneKozeny relationship can be used to describe membrane flux assumingno interaction exists between the fluids and surface of the membrane (Mulder 1996)
J frac14 ε3
khS2eth1 εTHORN2Dp
Dx(21)
where J is the membrane flux (m3(m2 s)) k is the CarmaneKozeny constant(depending on pore shape and tortuosity) ε is the porosity S is the specific surface area(m2m3) h is the permeate viscosity (N sm2) Dp is the pressure difference (Nm2) andDx is the membrane thickness (m) Membrane selectivity and permeation flux aremajor indicators of membrane performance Eqn (21) shows that structural parameterssuch as high porosity narrow pore size distribution and thin membrane thickness arealways desirable for improved membrane performance The different characteristics ofthe membrane can be designed and tailored using different fabrication techniques andcontrolling the related fabricating parameters during the course of production
222 Material and microstructure
Various membrane materials are available for MFUF processes The choice of themembrane material affects the practicality and sustainability of the filtration operationand hence can be chosen based on the specific application requirements
Advances in ceramic membranes for water treatment 47
Examples of ceramic materials include alumina (g-Al2O3 and a-Al2O3) zirconia(ZrO2) titania (TiO2) glass (SiO2) and silicon carbide (SiC) Ceramic membranesoffer superior temperature and chemical tolerances leading to an extensive range ofapplications across many industries They can be an important and environmentallyfriendly component in helping companies achieve a lsquozero-dischargersquo state Howeverthe main drawback of using ceramic membranes for water treatment is their highcapital cost hence large-scale membrane applications or less challenging water puri-fication processes are still dominated by polymeric membranes As shown in Table21 commercial ceramic membranes for MFUF processes consist mainly ofa-Al2O3 as the substrate and a-Al2O3 TiO2 or ZrO2 as the selective separation layerAccording to experiments in the literature there is a general consensus that chemicalstability (in highly acidic or basic environments) decreases with the following trendTiO2gt ZrO2gt a-Al2O3gt g-Al2O3gt SiO2 (Hofman-Zeurouter 1995 Mallada ampMeneacuten-dez 2008 Van Gestel et al 2003) Generally inorganic membranes can withstandorganic solvents chlorine and other oxidants are less susceptible to microbial attackthan polymeric membranes and can be used at pH values close to 0 and up to 14 (TiO2and a-Al2O3) (Hsieh 1996 Mallada amp Meneacutendez 2008)
Ceramic membranes are generally accepted to have high mechanical strengthBecause of this they can be backwashed at elevated pressures and operated over alonger period of time compared with polymer membranes (Guerra Pellegrino ampDrewes 2012 Sondhi amp Bhave 2001) However a disadvantage of ceramic mem-branes is their brittleness hence they must be handled with care and the choice ofhousing and sealing should be carefully considered The pressure required to driveMF is 01e2 bar and much higher for UF (Van Der Bruggen et al 2003) Themaximum operating pressure specified by a hollow-fibre ceramic membrane manufac-turer is 6 bar (Hyfluxmembranes InoCep) much higher than the 25-bar maximumallowable for polyethersulphone (PESf) membranes (Hyfluxmembranes Kristal)
As mentioned in Section 21 the microstructure across the membrane cross-sectionand surfaces is a key property that dictates the performance and usability of amembrane for a certain applications A membrane can be categorized into symmetricand asymmetric structures A symmetric membrane has a uniform cross-sectionthroughout whereas an asymmetric membrane does not as shown in Figure 21(a)and (b) (or Figure 21(c)) respectively
For MF and UF processes asymmetric membranes consisting of a thin skin layer ontop of a porous sub-layer are desired The top layers of UF membranes are much denserthan MF membranes hence they require higher trans-membrane pressures and gener-ally have lower permeability The asymmetric structures are associated with higherpermeation flux because the effective membrane thickness can be much thinnerthan in symmetric membranes The porous sub-layer acts to provide adequate mechan-ical strength This type of structure combines high selectivity of the skin layer withgood permeation fluxes
An asymmetric membrane can be integral or made of several layers known as acomposite The top and porous layers of an integral asymmetric membrane are normallymade of the same material (Figure 21(c)) and for composite membranes the top skinlayer and the porous sub-layer can be made of different materials (Figure 21(b))
48 Advances in Membrane Technologies for Water Treatment
A composite membranersquos properties and performance can be tailored usingdifferent materials in different layers Some disadvantages of composite membranesinclude the high complexity of the membrane fabrication process which requiresmany steps and the possibility of membrane failure owing to delamination of theseparation layer which occurs during sintering or variations in filtration pressures(wang Jerome Morris amp Kesting 1999) Integral membranes are homogeneous incomposition but can also be highly structurally varied in layers or distinct regionsdepending on their fabrication methods
As shown in Figure 21(c) and (b) the cross-sectional microstructure of the integralmembrane and the composite membrane vary significantly even though both are asym-metric membranes The integral membrane in Figure 21(c) is normally achieved viathe combined phase inversionsintering method (Li 2007) whereas the composite inFigure 21(b) is made by skin deposition on top of a multi-layer substrate supportThe integral membrane consists of two different types of sub-structures sponge-likeand finger-like The sponge-like structure can provide the membrane with the desiredselectivity and the finger-like structures can reduce the overall membrane resistance to
Figure 21 Scanning electron micrograph of the cross-section of (a) a symmetric aluminasubstrate (Reprinted from Kingsbury and Li (2009) with permission from Elsevier) (b) a three-layered alumina membranesupport composite (Reprinted from Hsieh Bhave and Fleming(1988) with permission from Elsevier) and (c) an asymmetric integral alumina membrane(Reprinted from Kingsbury et al (2010) with permission from Elsevier)
Advances in ceramic membranes for water treatment 49
permeate flow On the other hand the composite membrane consists of pores formedby voids between the particle packing only and the membrane selectivity is providedby the finer particles at the top layer and at a lower magnification the entire cross-section will appear to have a sponge-like structure
Integral and composite ceramic membranes can be made via different fabricationmethods the choice of method will largely depend on the membrane configurationand microstructures that are desired
223 Membrane configurations
There is a wide variety of membrane geometries available depending on the shapeformation technique chosen in the fabrication process Ceramic membranes exist intwo main types of element configurations ie flat and cylindrical The choice of mem-brane configuration depends largely on the application such as the required operatingand feed conditions
2231 Flat membranes
Flat ceramic membranes can come in a disc or sheet form The packing density of discmembranes is generally low and hence they are limited to use in small-scale industrialmedical and laboratory applications Commercial ceramic flat-sheet membranes mayhave thicknesses of around 6 mm and can be multi-channelled across their cross-section as shown in Table 21 and Figure 22 Flat-sheet membranes are easy toreplace when packed in modules and can handle high-turbidity feeds Mechanicalcleaning of ceramic flat-sheet membranes is also versatile it can withstand air scrub-bing backwashing and high-pressure water jet cleaning
Figure 22 Product photographof a commercial multi-channelledalumina flat-sheet membrane(CFM Systems)From ItN Nanovation AG(CFM-Flyer) (Reproduced bypermission of ItN NanovationAG)
50 Advances in Membrane Technologies for Water Treatment
2232 Cylindrical membranes
Single-channel tubular membranes generally have diameters of between 10 and25 mm shown in Table 21 and Figure 23 They are more suitable for separating feedsthat have high turbidity and large amounts of suspended solids Because of their largerdiameters and robustness they can be more easily cleaned mechanically and highcross-flow velocities can also be used to control fouling To further improve the pack-ing density of the tubular membranes they can be fabricated as multi-channel tubescalled monoliths with a surface area to volume ratio up to 782 m2m3 as shown inTable 21 In an effort to improve the packing density hexagonal monoliths werealso developed by Pall Corporation (Pall Membralox) (Figure 23(a)) Monolithmembranes offer the same advantages as singular tubular membranes Howeverwhen running in the inside-out configuration the pressure drop of the permeateflow across the monolith could be high as a result of the long and tortuous path ithas to travel to reach the outer surface of the monolith which limits the size of theouter diameter that can be used The patented CeraMem from Veolia Water Ltd (Tun-drasolutions) solves this problem by adding multiple conduits across the monoliththrough which permeate will flow through and then be collected at the end of the mod-ule (Figure 23(c))
Ceramic hollow-fibre membranes have diameters of 20e40 mm (Han et al 2011Kingsbury amp Li 2009 Liu Li amp Hughes 2003 Tan Liu amp Li 2001) (Figure 24)which means that compact modules with highly effective membrane surface areas can
(a) (b)
(c)
Figure 23 Product photograph of (a) Pall Corporation Membralox multi-channelled aluminamembranes (Pall Membralox) (Image courtesy of Pall Corporation) (b) LiqTech Interna-tional Incrsquos SiC SteriMem products (LiqTech SiC SteriMem) (Reproduced by permission ofLiqTech International AS) and (c) Veolia Water Ltd CeraMem element (Tundrasolutions)(Reproduced by permission of Veolia Water Technologies)
Advances in ceramic membranes for water treatment 51
be achieved Furthermore Fraunhofer IGB Ltd stated that ceramic hollow fibres withan outer diameter as low as 05 mm have been fabricated via combined phase inversionand sintering (Ceramic capillary membranes) potentially increasing the packingdensity that can be further achieved In addition they require lower trans-membranepressures to drive permeate flow because of the thinner membrane and can be cleanedeasily using methods such as backwashing and forward flushing
224 Fabrication techniques
Fabrication of composite ceramic membranes is generally a multi-step process It cangenerally be broken down into three different stages preparation of the ceramicpowder paste or suspension shaping of the ceramic powder into the desired geometryand heat treatment which includes calcination and sintering (Li 2007) After thesemain steps additional layer deposition followed by further heat treatment steps cantailor membrane selectivity as well as other membrane properties The choice ofmethod for each step depends on the desired membrane configuration qualitymorphology mechanical and chemical stability and selectivity of the final membranesHowever the fabrication method should also be economical and easy to replicatewithout compromising the quality of the final membrane
2241 Ceramic powder paste and suspension preparation
The ceramic powder preparation stage processes the raw inorganic material intothe shape and size distribution desired for membrane fabrication This can be in the
Figure 24 Product photograph of hollow-fibre alumina membranes from Hyflux Ltd(InoCep) (Reproduced by permission from Hyflux Ltd)
52 Advances in Membrane Technologies for Water Treatment
form of milling or chemical particle preparation Often the ceramic powder is madeinto a paste or suspension so that shape formation is possible During this step addi-tives can also be introduced to the ceramic material The choice of suspension mediumadditives and powder is important and affects the membranersquos microstructure andquality Deflocculants or dispersants often in the form of various fatty acids and esterscan be added to stabilize the ceramic particles in suspension to achieve high particleloadings and continuously high-quality ceramic membranes (Calvert et al 1986 Li2007 Sushumna Gupta amp Ruckenstein 1992 Zeurourcher amp Graule 2005) Magnesiumoxide has been used as a sintering additive to lower the sintering temperature requiredto form alumina hollow-fibre membranes via a combined phase-inversion and sinter-ing method (Choi et al 2006) Other additives such as the polymer binder are used tomaintain the shape of the ceramic membrane precursor and plasticizers can be usedto increase flexibility and ease of handling of the suspensions for tape casting or extru-sion methods (Burggraaf amp Cot 1996)
2242 Shaping
The shaping step is the physical process whereby the ceramic material is transformedfrom a suspension or powder paste form into the membrane geometry Methods thatcan create ceramic membranes and substrates include pressing tape casting slip cast-ing extrusion phase inversion foam techniques and leaching techniques
Pressing is a simple and quick method for producing flat ceramic membranes (Silvaet al 2012 Wang et al 2008a Xin et al 2007) A force is applied over an area ofceramic powder (varying from 10 to 100 MPa (Drioli amp Giorno 2010)) until the parti-cles are consolidated This method produces membranes of a symmetric structure owingto the uniform force applied across the area of ceramic material and are commonly usedto make disc membranes Meanwhile this method is not flexible in fine-tuning micro-structures and there is a limitation on the thickness to diameter ratio of the membranethat can be achieved In addition it is a batch fabrication process and has been usedprimarily to make membranes for use in laboratories rather than commercially
The tape casting method is used to form flat-sheet ceramic membranes (Das ampMaiti 2009 Das et al 1996 Lindqvist amp Lideacuten 1997) A tape cast suspension is firstprepared with the ceramic material a liquid-dispersing medium and organic additivesto provide the suspension with pseudo-plastic behaviour Then the suspension is trans-ferred into a reservoir controlled by a blade that can be height adjusted over a mobilecarrier film Subsequently the suspension film is dried in a controlled environmentTape casting is a more flexible method that can make flat-sheet membranes as wellas coat layers onto membrane supports
Slip casting uses a porous mould to form the shape of the ceramic membrane(Falamaki amp Beyhaghi 2009 Lin amp Tsai 1997) When the ceramic suspension ispoured into the mould the solvent is extracted into the pores of the mould as a resultof the capillary force leaving ceramic particles on the surface of the mould Slip cast-ing is commonly used to form tubular membranes with high surface quality densityand uniformity Important factors that affect the final quality of the membranes createdvia this method include slip density and viscosity
Advances in ceramic membranes for water treatment 53
Extrusion is a method popular for fabricating tubular monolith and hollow-fibrecapillary membranes (Castillon amp Laveniere 1995 Feenstra Terpstra amp Van Eijk1998 Smid et al 1996 Terpstra Bonekamp amp Veringa 1988 Wang et al 1998)It is a process in which an inorganic stiff paste is forced through an orifice of adesigned shape and compacted by applying high pressure The paste must have plasticproperties to form and maintain the shape of the membrane precursors hence addi-tives are added to the ceramic powder The paste is then forced through a die athigh pressure (around 20 MPa) to produce hollow tubes
Combined phase-inversionsintering (Kingsbury amp Li 2009 Liu et al 2003 Tanet al 2001) is a relatively new method for fabricating microstructured ceramic hollow-fibre membranes Solventenon-solvent exchange-induced precipitation of the poly-meric binder and viscous fingering phenomenon occur during membrane fabricationwhich distinguishes this method from conventional ram extrusion and will be furtherintroduced in Section 25
2243 Heat treatment
After the forming step the ceramic membrane precursors are produced They are thendried and treated with heat This heat treatment step consists of three main steps (Li2007) pre-sintering thermolysis and final sintering with the purpose of removingcomponents other than the ceramic material to strengthen the membrane and consol-idate the microstructure and dimensions of the final membrane
Presintering occurs at around 200 C Presintering is used to remove water from themembrane precursors which may be in the form of water chemically bonded to theceramic particle surfaces or as crystallized water within the inorganic phases Thermol-ysis or the calcination step is the process by which all organic components in the mem-brane precursor are removed The sintering stage is where the major changes to theporosity and pore size occur the final shape of the ceramic membrane is consolidatedand mechanical strength is improved In general the higher the sintering temperaturethe higher the mechanical strength but the porosity of the final membrane is lowered
Important parameters that affect thermal stability of ceramic membranes are sinteringactivity and phase transformation of the membrane material and the support (Mallada ampMeneacutendez 2008) During sintering the ceramic material is kept at a certain temperaturelong enough for the material to approach its equilibrium structure The membrane struc-ture can be maintained for temperatures up to 100e150 C below the sintering temper-ature which is normally extremely high (over 1000 C) (Burggraaf amp Cot 1996)
2244 Layer deposition for composite membranes
Additional layer deposition can be used to alter or enhance the ceramic membraneproperties for example by changing the membrane selectivity porosity hydrophilic-ity conductivity permeability biocompatibility etc
To form composite membranes layer deposition methods are required to add layersonto a symmetric substrate Each layer normally consists of ceramic particles ofdifferent sizes to achieve a gradient pore structure across the membrane cross-section Multiple layers are normally required to reach the desired selectivity at the end
54 Advances in Membrane Technologies for Water Treatment
Dip or spin coating is the most commonly used method for layer deposition onto asupport (Babaluo et al 2004 Gu amp Meng 1999 Lindqvist amp Lideacuten 1997) Dipcoating is when a support is dipped into and then withdrawn from a ceramic powdersuspension Upon withdrawal from the suspension the liquid will be sucked into thesupport pores because of capillary forces along with the ceramic particles if they aresmall enough in size Spin coating rotates the substrate while the suspension is depos-ited over the substrate surface
Sol-gel method can be used to fabricate membranes with high selectivity with poresfrom 100 nm down to a few nanometres and is commonly used to form ceramic UFmembranes (Agoudjil Kermadi amp Larbot 2008 Das amp Maiti 2009 Hao et al2004 Kim amp Lin 1998 Larbot et al 1988 Wu amp Cheng 2000) A colloidal (consistsof alkoxide) or polymeric solution is first deposited over a membrane substrate bymeans of dip coating whilst it is converted into a gel form by hydrolysis and conden-sation or polymerization Afterward it is heat treated to form a thin and uniform skinover the membrane
There are many other coating techniques such as chemical vapour depositionneutron sputtering and plasma details about these can be found elsewhere(Burggraaf amp Cot 1996 Hsieh 1996 Li 2007)
225 Combined phase-inversion and sintering fabricationtechnique
As described in the previous section the traditional and common method for pro-ducing hollow-fibre membranes is to use extrusion followed by sintering Howeverthis would give rise to symmetric membranes and additional layer deposition andheat treatment steps are required to produce the required selectivity with a gradientpore size along the membrane cross-section as shown in Figure 21(b) An alter-native and innovative method for producing ceramic hollow-fibre membranes isthe combined phase-inversion and sintering technique (Choi et al 2006 Kings-bury amp Li 2009 Kingsbury Wu amp Li 2010 Koonaphapdeelert amp Li 2007Liu et al 2003 Tan et al 2001 Wang et al 2008b Wang et al 2009 Weiet al 2008) It is a flexible technique that can produce both symmetric ceramicmembranes and a wide range of asymmetric ones in a single step for both flat-sheet and hollow-fibre geometries as shown in Figure 21(a) and (c) Because ofthe presence of large finger-like structures they can reduce mass transfer resistancefor permeation fluxes competitive to the membranes formed via conventionalmethods
A suspension consisting of ceramic particles and polymer binder in a solvent is firstprepared and then through solventenon-solvent exchange induced phase inversion ofthe polymer binder the ceramic particles are immobilized in their desired geometry bycasting (flat membrane) or spinning (hollow-fibre membrane) This method can easilytailor and form membrane precursors with various specific morphologies Thensimilar to all other ceramic fabrication methods the membrane precursors undergo aheat treatment session (Section 243) to remove all organics as well as consolidatethe final membrane structure and mechanical strength
Advances in ceramic membranes for water treatment 55
Preparation of the ceramicesolventepolymer suspension solution is a step that hasa major impact on the properties of the final membrane The particle size and particlesize distribution of the ceramic components can affect the pore size and selectivity ofthe membrane as well as the suspension rheology Porous alumina hollow-fibremembranes developed using this method have selectivity in the MF range when1-mm particles are used (Kingsbury amp Li 2009) and in the UF range when mixturesof 001- and 1-mm particles are used (Tan et al 2001) Yttria-stabilized zirconia(YSZ) porous membranes have pore sizes in the UF range when polydispersed YSZparticles (01 and 002 mm) are used (Wei et al 2008) Other factors such as theceramic to polymer binder ratio can affect the mechanical strength and quality ofthe sintered membrane and a ratio of up to 10 has been suggested (Kingsbury ampLi 2009 Tan et al 2001)
Although the process of spinning ceramic hollow-fibre membranes is similar tospinning polymeric ones the rules and optimization of the polymeric spinning systemcannot be applied directly to the ceramic spinning process because of the significantlydifferent composition of the spinning suspension The presence of a large amount ofceramic particles also changes the mechanisms behind the formation of the differentmembrane sub-structures Two basic sub-structures have been achieved within themembrane cross-section sponge-like denser structures and finger-liker structuresthat can be isolated or micro-channels as shown from Figure 25 The positioningand dimensions of these different sub-structures can be tailored mainly by changingthe parameters during spinning
Improved understanding of spinning parameters such as the air gap bore fluidchoice and bore fluid flow rate on the alumina membrane morphology was achievedby Kingsbury and Li (Kingsbury amp Li 2009 Kingsbury et al 2010) The designingand engineering of the two types of sub-structures led to distinctively differentmembrane morphologies (Figure 25) and generated a string of potential applicationsThese applications include hollow-fibre membrane micro-reactors membrane contac-tors solid-oxide fuel cells gas separation etc (Kanawka et al 2011 Kingsburyet al 2010 Koonaphapdeelert Wu amp Li 2009 Othman et al 2012 Rahman et al2011) In particular the morphology consisting of one sponge-like skin layer on topof a layer of finger-like micro-channels (Figure 25(d)) is expected to dramaticallyreduce the trans-membrane resistance to permeate flow The single thin separation layerprovides the membrane with its selectivity and the finger-like micro-channels canreduce the mass transfer resistance significantly This morphology is expected to excelin aqueous MFUF applications compared with conventional composite asymmetricceramic membranes
Current research has made significant progress in understanding the effects ofdifferent process parameters on membrane morphology and properties but the scientificmechanisms behind the formation of different sub-structures need further researcheffort In recent years the common viscous fingering phenomenon has been appliedto explain formation of the finger-like structures in the ceramicepolymeresolventand non-solvent systems (Kingsbury amp Li 2009 Kingsbury et al 2010 Wang ampLai 2012) Viscous fingering is a hydrodynamic phenomenon in which a less viscousfluid displaces a more viscous fluid forming finger-like patterns Important factors that
56 Advances in Membrane Technologies for Water Treatment
affect the initiation propagation and termination of these finger-like structures includeviscosity difference density difference moving interface velocity and polymer precip-itation rate Finger-shielding coalescing and spitting of the finger-like structures havealso been observed (Wang amp Lai 2012) A clearer understanding of the mechanismsbehind the formation of the unique sub-structures would allow easier and moreadvanced designing of the different morphologies and engineering of the productionprocess
The combined phase-inversion and sintering technique has been successfullyapplied to a wide range of ceramics as well as metals with a range of different pore sizes(Liu et al 2003 Luiten-Olieman et al 2011 2012 Yang Tan amp Ma 2008 Zhanget al 2009b 2012) However a compromise always has to be made between theporosity and mechanical strength when a high sintering temperature is involved Astudy suggested the use of a controlled sintering process by using PESf as not just apolymer binder but also a pore structure lsquostabilizerrsquo (Wu et al 2013) By thermallytreating the membrane precursors the PESf is not completely removed during the firststage of heat treatment Instead some of the PESf is converted to carbon at 1450 C inan environment devoid of oxygen which allows mechanical strengthening of thealumina membranes without losing significant porosity
Figure 25 Scanning electron micrograph of (a) cross-section morphology of a symmetricalumina hollow fibre and (bed) different asymmetric alumina hollow fibres formed by combinedphase-inversion and sintering (Reprinted from Kingsbury amp Li (2009) with permission fromElsevier Reprinted from Kingsbury et al (2010) with permission from Elsevier Reprinted fromTan et al (2011) with permission from Elsevier)
Advances in ceramic membranes for water treatment 57
As a result the combined phase-inversion and sintering method can potentiallycreate the opportunity to use ceramic membranes in large-scale water and wastewatertreatments by offering a hugely reduced fabrication process cost and great flexibilitiesin microstructure design However further improvements are required before thismethod can be fully commercialized for example to improve surface porosity andpure water permeation without sacrificing its mechanical strength and to have a betterfundamental understanding of the microstructure formation mechanisms to enablereliable and reproducible fabrication process on a large scale
23 Development in membrane modules and units
The design of the membrane module is important for achieving the highest packingdensity possible for the particular membrane element whilst offering effective filtra-tion The different membrane elements are packed in a unit called a module and thesemodules are often arranged and connected in a specific way to form a unit offeringgreat efficiency The unit(s) is then integrated with other components such as pumpspipes control systems etc to form a membrane system and part of the water or waste-water treatment plant The following section describes the ceramic MFUF modulesand units available commercially
231 Module and unit designs
Many different module designs are available for the different ceramic membranes andtheir choice depends largely on the application the demand of the process and anyspatial limitations and economical factors The arrangement and packing of themembrane elements can be divided into two main types ie flat membranes andcylindrical membranes
The membrane elements may be contained in pressure vessels or immersed in afluid medium at atmospheric pressure For the contained modules the ceramicmembranes are packed and fitted inside a pressure vessel and trans-membrane pressureis applied at one end of the vessel with the feed The immersed unit allows easierreplacement of the membrane elements or modules and is also more flexible to tailorto specific needs and demands Furthermore it enables air sparging in the unit toimprove permeation and reduce fouling by promoting turbulence Both flat-sheetand cylindrical membranes can be packed in this way and this type of system requiresless energy to operate than contained units
2311 Flat membranes
Ceramic flat membranes can come in the form of flat sheets or flat discs as described inSection 231 and can both be packed into plate-and-frame modules The containedplate-and-frame configuration consists of stacking the flat membranes on top of oneanother with porous spacers in between The size of the spacing depends on the amountand size of suspended solids in the feed The feed enters the module from one end and is
58 Advances in Membrane Technologies for Water Treatment
collected at the membrane support plates Such a module is easy to operate and mem-brane defects can be detected and replaced simply However their packing density islow and so most of the time they are limited to use for small-scale applications
Flat-sheet membranes with multiple channels can be stacked next to each other andform an immersed module as shown in Figure 26 These flat-sheet membranes are con-nected at the two ends to permeate collector(s) and then packed together and supportedby a metal framework that is then immersed into the feed medium Permeate enters fromthe membranersquos outer surface into the channels inside the flat sheets and is thencollected at the end(s) of the membrane These modules can then be further stackedtogether to easily increase productivity The energy consumption of the immersed mod-ule is lower than that of the contained module because of the lower pressureedrivingforce requirements The membranes can also be replaced easily because they are pro-duced and sealed sheet by sheet In addition their ability to handle feeds with a highcontent of suspended solids means that they are commonly used inMBRs to treat waste-water Packing densities of about 918 m2m3 are reached using the CFM Systems
from ItN Nanovation AG by stacking units on top of each other (Table 21) Howeverthis is still significantly lower than the packing densities offered by polymeric flat-sheetmembranes that are spiral wound (700e1000 m2m3) (Crittenden et al 2012)
2312 Cylindrical membranes
Tubular membranes are often packed as a bundle inside a casing which could be a con-tained vessel (Figure 27) or an open vessel The feed solution may be fed though the
(a) (b)
Figure 26 Product photograph of (a) a ceramic flat-sheet module from ItN Nanovation AG(Itn-nanovation) (Reproduced by permission of ItN Nanovation AG) and (b) a ceramic flat sheetsystem from Meidensha Corporation (Ceramic membrane unit 2013) (Reproduced bypermission of Meidensha Corporation)
Advances in ceramic membranes for water treatment 59
centre of the tubular or monolith membranes and permeate is collected outside the tubein the module casing This operating mode requires high operating flow rates and thepressure drop can be large hence energy consumption is highest compared with othermodule designs However because of the relatively large channel diameters of tubularand monolith membranes they are suitable for feed streams with larger particles andcleaning these membranes is also more facile The tubular or monolith membranescan also be packed and supported by a partly open housing support and operated byimmersion into the feed solution similar to the immersed flat-sheet modules offeringsimilar advantages and disadvantages The immersed tubular modules are commonlyused in MBRs for larger-scale applications and also to treat harsh feed streams
Hollow-fibre membranes can also be packed in a similar way in their modules asshown in Figure 24 Ceramic hollow-fibre membranes are generally packed togetherand immobilized at both ends and then placed in a cylindrical housing The fullycontained module is more suited to small-scale applications such as domestic-scaledrinking water treatment Hollow-fibre modules can potentially offer a much higherpacking density and form more compact systems saving space and improving produc-tivity As shown in Table 21 the packing density offered by commercial ceramichollow-fibre modules is not as competitive (502 m2m3 offered by Media and ProcessTechnology Inc) as some of the multi-channel tubular module designs (up to 782 m2m3 offered by Ceramem module from Veolia Water Ltd) This is mainly because ofthe relatively large ceramic hollow-fibre outer diameters from most of the suppliers(3e4 mm from Table 21) Furthermore their intrinsic brittleness makes it more diffi-cult to pack them into bundles and it limits their length Hence cassette-style modulesfor ceramic hollow-fibre membranes are not available in which the hollow fibres aresupported at the top and bottom only and are immersed fully into the feed solutionwith no housing In addition the lack of open vessel designs for hollow-fibre mem-brane elements means that their application in immersed MBR is limited
Figure 27 Product photograph of a containedmodule from Pall Corporation (Pall
Membralox IC) (Image courtesy of PallCorporation)
60 Advances in Membrane Technologies for Water Treatment
232 Housing sealing and operation
Membrane modules consist of the housing sealing materials and membrane The typeof housing used for the membrane varies depending on the membrane and module typeand operating configuration For contained cylindrical membranes the membrane ispacked and enclosed in a vessel shown in Figure 28 For outside-in feed configurationor immersed cylindrical membranes the housing may have openings that exposecertain areas of the membrane element Flat-sheet ceramic membranes are normallysealed at the top and bottom to a permeate collector They are then stacked togetherand supported by a stainless-steel frame into modules such as the Meidensha Corpo-rationrsquos ceramic membrane unit (Ceramic membrane unit 2013) and ItN NanovationAGrsquos CFM System (Itn-nanovation) (Figure 26)
Typical module housing materials include stainless steel titanium and plasticStainless steel is most commonly used because of its good chemical and thermalstability as well as great robustness Although plastic housing is a cheaper option itoften limits the conditions within which the membranes can operate
The type of seal used on the membranes varies depending on the membrane andmodule type They are important because they prevent contaminant leakage into thepermeate stream Mostly O-rings and gaskets are used to cover the perimeter of themembrane ends where both the feed and permeates are exposed The material usedto seal the membranes depends on the required operating conditions of the MFUFwhich will change depending on the application Typical sealing material includesrubber silicon polytetrafluoroethylene and poly(vinyl chloride)
There are many different module designs for flat-sheet and tubular (single andmulti-channel) ceramic membranes However because of the much larger dimensionsof the flat-sheet and tubular elements their packing density is lower than the polymericmembrane modules offered which makes them less competitive for large-scale appli-cations or when space is limited in a treatment plant Hollow-fibre membrane moduleshave the potential to improve the packing density significantly but most of the onesoffered on the market have relatively large outer diameters leading to lower packing
Figure 28 Product photograph of the CeraMem contained module and housing from VeoliaWater LtdReproduced by permission of Veolia Water Technologies (Tundrasolutions)
Advances in ceramic membranes for water treatment 61
densities Commercially available hollow-fibre modules are limited for example thereare limited open-vessel modules and no cassette-style ceramic hollow-fibre modulesfor large-scale immersion applications which is a problem for their application tosmall-scale MFUF applications
24 Ceramic membranes for water treatment
Because of the high chemical and thermal stability and robustness of ceramicmembranes they have been commonly applied in applications for which polymericmembranes cannot be used because of their lower stability For example ceramicscan be used in harsh environments such as at high temperatures and in aggressivechemicals (solvents highly acidic or caustic solutions) and oily water Unlike poly-meric membranes ceramic membranes do not swell in solvent and can be treatedwith strong cleaning agents and sterilized at high temperatures and can withstandhigh pressures for backwashing All of these advantages have made long-termflux stability possible for ceramic membranes to be used on a commercial scale(Mallada ampMeneacutendez 2008) Recently ceramic membranes have garnered increasingattention for applications in milder operating conditions in which polymericmembranes dominate because of the increase in the packing density of the membranemodules and reductions in capital costs
241 Drinking water production
MF and UF membranes are used extensively in producing drinking water and canhandle both small- and large-scale capacities (Staff 2011) Small-scale MFUF appli-cations include portable membrane systems and in large-scale applications MFUFmembranes can be used as stand-alone or hybrid systems in a water treatment processtrain This market has been and is still currently dominated by polymeric membranesbut recently the incorporation of ceramic membranes to produce drinking water hasbeen increasing (Bottino et al 2001 Li et al 2011 Mahesh Kumar Madhu ampRoy 2007 Muhammad et al 2009) Water sources used to produce drinking watercan vary considerably from groundwater lakes and rivers to municipal wastewaterand seawater The final water that is safe to drink must fulfil standards set by the WorldHealth Organisation or by the European Union and be free of suspended solidsmicroorganisms and harmful chemicals
Small-scale ceramic membrane MFUF systems can serve several purposes such asto improve the quality of tap water reduce the volume or recycle household waste-water provide safe drinking water in remote areas with limited resources such as dur-ing camping or hiking and provide safe drinking water in developing nations or duringhumanitarian crises
To produce drinking water from surface water for a large community e in otherwords for municipal use e large systems are required The water purification processwill consist of a process train of different steps Because the design of the process
62 Advances in Membrane Technologies for Water Treatment
depends on the feed quality and the final water quality preparing drinking water fromsurface water is much more expensive because it requires more purification steps Atypical process train for purifying surface water consists of a pre-filtration unit addi-tion of chemicals natural filtration disinfection fine filtration and preservation andstorage Ceramic MFUF membrane units may be used to replace conventional particleremoval units such as coagulation and sedimentation as well as disinfection unit(s)Furthermore the ability to reject microorganisms bacteria and viruses can reducethe amount of chemicals required to be added to drinking water saving considerablespace overall For the production of drinking water from seawater ceramic MFUFunits may be used to provide pre-treatment to reverse osmosis
Studies in literature have shown that ceramic MFUF membranes alone can providepermeate turbidity low enough for drinking standards and are effective in removingmicroorganisms organic matter and disinfection byproduct (DBP) precursors (Bottinoet al 2001 Harman et al 2010 Muhammad et al 2009) Because of the hydrophilicand inorganic characters of the ceramic membranes they are less affected by organicfouling compared with polymeric membranes (Hofs et al 2011) However high-turbidity water (up to 80 NTU) can lead to membrane fouling and significant permeateflux declination which can be recovered via chemical cleaning The membrane poresize also appears to affect the rate of flux decline because Harman et al suggestedthat the smaller 4-nm pore-size membrane fouls mainly by forming a cake layerwhereas pore clogging may dominate in the larger 10-nm pore-size membrane(Harman et al 2010)
To minimize the effect of fouling much research has been carried out to analyse theperformance of hybrid ceramic membrane systems such as the combination of ceramicmembrane MFUF with activated carbon for natural organic matter (NOM) removal(Karnik et al 2005b Konieczny amp Klomfas 2002 Lee Park amp Yoon 2009 Stoquartet al 2012) coagulation for reducing organics and viruses (Fiksdal amp Leiknes 2006Konieczny Bodzek amp Rajca 2006 Lerch et al 2005 Matsushita et al 2005) photo-catalytic reactions (Alem Sarpoolaky amp Keshmiri 2009 Chin et al 2007 Mozia2010) and ozonation to remove NOM (Karnik et al 2005a Kim et al 2008) andsubsequent formation of DBPs (Karnik et al 2005b) The use of the coagulationand MFUF hybrid system is preferred from an economic point of view
242 Municipal wastewatersewage treatment
Treatment and recycling of municipal wastewater is common and a highly importantpart of todayrsquos water management Municipal wastewater consists of household com-mercial and institutional wastewater A typical sewage treatment plant consists of thefollowing steps pre-treatment to remove large and heavy debris followed by primarytreatment used to remove the suspended inorganic and some organic particles and thensecondary treatment for the biological conversion of dissolved and colloidal organicsinto biomass and finally tertiary treatment for the further removal of suspended solidsor nutrients and disinfection
MFUF can be incorporated in the treatment train as stand-alone processes or com-bined with the activated sludge process to form hybrid systems such as the MBR
Advances in ceramic membranes for water treatment 63
This can reduce the steps in secondary and tertiary treatment for example eliminatingthe need for sedimentation and secondary disinfection saving space as well as reducingwaste such as DBPs that need to be treated and disposed of Ceramic membranes can beused to replace the traditional polymeric membranes for sewage treatment because thequick fouling of polymeric membranes limits them to short operating lifetimes Theycan be cleaned under harsher cleaning conditions and therefore offer longer andmore reliable operation as well as less servicing and maintenance reducing long-term operating costs Flat-sheet and cylindrical membrane elements are suitable forbeing combined with MBRs and the modules can be operated in the immersed modeor as a separate external unit The immersed mode consists of the membrane moduleimmersed in the aeration basin and permeate is extracted via suctioning The externalconfiguration consists of pumping the mixed liquor at high pressure through themembrane from one surface Because of the higher trans-membrane pressure requiredfor the external configuration their permeate flux is also generally higher (Marrot et al2004) However the energy and space consumption of the immersed configurationtends to be lower which makes them more suited to large-scale wastewater treatment
Many studies have been carried out to analyse the effectiveness of ceramic MBRsystems to treat municipal wastewater (Waeger Delhaye amp Fuchs 2010 Xinget al 2000 Xu et al 2003 Zhang et al 2009a) Xing et al (2000) looked at usingexternal cross-flow ceramic UF MBRs for urban wastewater reclamation The systemprovided high removal efficiency removing carbon oxygen demand (COD) of anaverage of 97 and the reclaimed water could be reused directly for municipal orindustrial purposes after additional treatment (Xing et al 2000) The lab-scaleimmersed ceramic MBR offered COD removal of more than 92 during the courseof a prolonged sludge retention time (142 days) to treat simulated high-strength waste-water (consisting of glucose and protein) (Sun Zeng amp Tay 2003) Using longersludge retention times can reduce the amount of sludge produced and cut down onthe cost of sludge treatment
Fouling was mainly affected by the membranersquos microstructure surface roughnessand pore sizes in immersed ceramic bioreactor systems (Jin Ong amp Ng 2010) Theceramic membrane with the roughest surface and largest pore size (03 mm) has thehighest fouling potential whereas the membrane with the smoothest surface and small-est pore size (008 mm) fouls the least Air sparging has been found to be an effectivesimple and economical method to improve permeate flux and reduce concentrationpolarization and fouling in external-loop air-lift ceramic MBRs and the immersedceramic MBR (Fan et al 2006 Sun et al 2003 Zhang et al 2009a) AnaerobicMBRs may have lower energy consumption (Liao Kraemer amp Bagley 2006) How-ever inorganic fouling in anaerobic MBRs by struvite has been observed in ceramicmembranes (Kang Yoon amp Lee 2002) and requires periodic alkaline backflushingto recover permeate flux
243 Industrial wastewater treatment
The composition of industrial wastewater varies dramatically across the industries andthe choice of membrane technology will vary according to the contaminants Because
64 Advances in Membrane Technologies for Water Treatment
of increasingly stringent regulations on waste discharge and the increasing cost ofclean water industries are more motivated to treat and recycle wastewater whichcan potentially save significant operating costs
Typical contaminants in industrial wastewater are suspended solids particulatescolloids bacteria and viruses dissolved inorganics such as heavy metals salts nuclearwastes acids or alkalis volatile organics such as aromatics aliphatics alcohols andhalogenated hydrocarbons and non-volatile organics such as phenolics polyaromaticcompounds pesticides insecticides surfactants and dyes Similar to sewage treatmentMBRs are a popular treatment unit in the treatment train Upstream and downstreamtreatment units will vary depending on the desired quality of the final effluent(ultra-pure water process water etc) Ceramic MBRs are commonly used for small-scale treatment of high-strength industrial wastewater because they are robust stableand reliable
244 Produced water
Produced water is the largest waste stream generated from oil and gas operations MFUF is a promising technology for use to treat produced water because it is a low-energyconsumption process and can cut down on the amount of harsh and expensive chem-icals required to treat the wastewater Polymeric membranes have been commonlyused to remove organic contaminants as well as suspended particles before desalina-tion so that the water can be reused However because the feed has a high compositionof organic contaminants oil and grease the polymeric membranes can be degradedand fouled easily requiring regular replacement Ceramic membranes offer higherstability in such conditions and may prolong the lifetime of the membrane and canalso be cleaned more easily
Many studies have been carried out to investigate the performance of ceramicmembranes in oilewater treatment in the laboratory or on a pilot scale (AshaghiEbrahimi amp Czermak 2007 Chen et al 1991 Ebrahimi et al 2010 Li amp Lee2009) Studies showed that using MFUF ceramic membranes gives permeate streamsof very high quality with less than 6 ppm total hydrocarbons and perform better thanpolymeric membranes However an important limitation is permeate flux decline as aresult of fouling by waxes and asphaltenes Backwashing is a useful cleaning methodto prevent permeate flux decline (Abadi et al 2011)
245 Food and beverage industries
MF and UF are important processes in the food and beverage industries They can beused to achieve clarification sterilization concentration and purification of the finalproduct in place of multiple traditional process units For the dairy industry MF canbe used to clarify cheese whey (Skrzypek amp Burger 2010) and to concentrate anddefat milk and sterilize it by rejecting microorganisms (Fernandez Garciacutea AlvarezBlanco amp Riera Rodriacuteguez 2013) In the sugar-refining and beverages industryMF and UF membranes are used to clarify products such as maple syrup wine
Advances in ceramic membranes for water treatment 65
beer juice and vinegar (Ceramic membranes 2000 Fukumoto Delaquis amp Girard1998 Gan et al 1999 Hinkova et al 2002 van der Horst amp Hanemaaijer 1990Li et al 2010) Wastewater generated in these industries can also be recycled withthe help of MFUF membranes to save operating costs Ceramic porous MFUFmembranes have demonstrated success in the food and beverage industries offeringsuperior long-term performance in these applications over polymeric membranes
246 Commercial ceramic applications
Ceramic membranes have been implemented commercially in the industriesmentioned in Sections 41e45 to treat aqueous feeds In terms of producing drinkingwater there are far fewer portable ceramic membrane drinking production systemsthan polymeric ones mainly owing to their high capital costs However Veolia WaterLtd developed a mobile ceramic UF system for providing emergency drinking water(Veoliawaterst Berkefeld) The system consists of a treatment train in the order ofcoagulation adsorption pre-filtration and UF using ceramic monolith elements andultraviolet and chlorination disinfection providing up to 360 m3day permeate flowfit for drinking purposes
For large-scale drinking water production the cost of ceramic membranes can berecuperated by their reliable performance and long operating lifetimes for examplea ceramic MF system by Metawater Co Ltd has been operating for 15 years(Metawater) Metawater Co Ltdrsquos systems typically consist of coagulation dead-end MF using ceramic monolith elements followed by disinfection with chlorineand have the treatment capacity of 1000e100000 m3day for the treatment of clearraw water
Because of the superior stability and operating lifetimes of ceramic MFUFmembranes they can also be used as a pre-treatment step before reverse osmosis fordesalination A drinking water plant in Qassim replaced polymeric UF pre-treatmentmembranes with ceramic flat-sheet ones which reduced the membrane replacementfrequency dramatically and has a treatment capacity of 42000 m3day (Itn-nanovationGround water)
For the treatment of municipal wastewater or sewage the performance of ceramicMBRs is highly competitive compared with polymeric systems Similar to drinkingwater production plants implementation of ceramic membranes is limited mainly bytheir high cost There are examples of commercial ceramic MBR systems being imple-mented in small-scale wastewater treatments such as in a school and an airport byLikuid Nanotek (Likuid-CBR) They use an external cross-flow configuration to treatdomestic wastewater with capacities ranging from 38 to 50 m3day and permeate flux isrecovered solely by periodic chemical cleaning For larger-scale wastewater treatmentthe ceramic immersed MBR is more commonly used for example ItN NanovationAG installed immersed ceramic flat-sheet membrane modules with a capacity to treat150 and 1300 m3day of sewage feed in two different sewage treatment plants(Itn nanovationMBR Itn nanovationMBCR) No membrane servicing or maintenancehas been required since operation began in 2011 for one of the plants and chemicalcleaning is used once a year (oxidant and chelating agent) to maintain permeation flux
66 Advances in Membrane Technologies for Water Treatment
Ceramic membranes have found more frequent use in the treatment of industrialwastewater because they are normally more challenging to treat An example of aceramic MBR includes an external MBR using ceramic tubular membranes fromLikuid Nanotech to treat industrial waste of 3e300 m3day which has been commer-cially used in olive oil wine and narcotics production plants For all of their ceramicMBRs the quality of the final effluent is clean enough to be directly discharged(Likuid-CBR) Another application includes the ceramic MBR installed for treatmentand recycling of up to 1400 m3day of highly pigmented wastewater by Pall Corpora-tion (Membralox pall) Complete retention of insoluble pigments is achieved usingmonolith ceramic membranes running in external configuration and up to 50 ofwastewater can be recycled A larger-scale ceramic MBR has been scheduled forinstallation in a demonstration plant by Meidensha Corporation to treat and recycleup to 4550 m3day of industrial wastewater by combining up-flow anaerobic sludgeblanket technology with the 01-mm alumina flat-sheet MBR system (Singaporecollaborate on ceramic membrane MBR demonstration plant 2012)
The commercial operation of ceramic membranes in produced water treatment ismuch more limited compared with other application areas such as in the food andbeverage industry and industrial wastewater treatment To treat produced water theceramic membranes typically run in cross-flow configuration (Elshorbagy 2013)An example is the full-scale facility in Colorado which uses ceramic membranes asone of the units to treat oily wastewater (Aqwatec) The process train consists ofdissolved air floatation pre-filtration ceramic MF and activated carbon adsorptionand then the water is supplied to a reverse osmosis plant providing municipal drinkingwater In addition Veolia Water Ltd has developed a system consisting of ceramicmonolith membranes to treat produced water which so far has been installed at 60locations (Veoliawaterst CeraMem) Their typical performance includes removalof 987 turbidity and effluent has less than 1 ppm oil and grease (Veoliawaterstna)Furthermore a ceramic (SiC) membrane module will be installed for commercial usein Columbia and Venezuela after the trial period offered consistent reductions of oilconcentration from 20e100 ppm to less than 5 ppm (Success for LiqTech ceramicmembranes in oil amp gas trial 2012)
Ceramic membranes are most commonly used in the food and beverage industry(Fernandez Garciacutea et al 2013) One commercial system for the removal of bacteriaand spores from milk using MF is the Tetra Alcross Bactocatch The raw milk is firstseparated into skim milk and cream and then the former undergoes MF leaving theconcentration of bacteria in permeate to be less than 05 of the original value(Peinemann Nunes amp Giorno 2011) Ceramic membranes were also chosen andinstalled by Atech Innovations GmbH over polymeric membranes in a commercialcitrus fruit juice production plant because of their high performance as well as resis-tance to essential oils from the fruit skins Another example of commercializationincludes the use of tubular ceramic monolith membranes in a high-velocity cross-flowconfiguration in a glucose production plant to minimize gel layer formation which isable to produce 180 m3day of glucose (Bolduan amp Latz 2005)
Ceramic MFUF membranes have been used across a wide range of different ap-plications involving water and wastewater treatment They are most commonly
Advances in ceramic membranes for water treatment 67
used in the food and beverage industry and in the small-scale treatment of industrialwastewater Laboratory pilot and some commercial case studies have shown poten-tial and advantages of using ceramic membranes for large-scale municipal waste-water treatment and the provision of safe drinking water As a result of the manybenefits offered by ceramic membranes their global incorporation into more drink-ing water treatment plants is anticipated Promising developments include theinstallation of ceramic flat-sheet membranes from ItN Nanovation AG in Saudi Ara-bia to pre-treat seawater in 2013 (Itn nanovation receives significant orders fromSaudi Arabia 2013) In addition three pilot plants using ceramic membrane hybridsystems with suspended ion exchange and ozone for drinking water treatment plantswill be installed in the United Kingdom Australia and Singapore by PWN Technol-ogies (Pwntechnologies) The incorporation of ceramic membranes in MBRs alsoclearly offers many advantages including ease of maintenance and long-term reli-ability High capital membrane costs along with energy consumption from contin-uous aeration contribute to a large proportion of the MBRrsquos operating costs andboth deter the growth of ceramic MBRs for large-scale sewage treatment There-fore further research and optimization of the immersed MBR is anticipated towiden the use of ceramic membranes The main overriding limitation to the com-mercial implementation of ceramic membranes for most large-scale water treat-ments is their high capital cost despite their overall longer lifetime and ease ofmaintenance
25 Ceramic membrane cleaning
Over time permeate flux of the ceramic membrane will inevitably decrease which issame as for their polymeric counterparts To recover as much as the initial flux aspossible many different cleaning methods are available The choice of the cleaningmethod(s) depends on the reversibility of the fouling the foulant(s) characteristicsthe membrane material and the membrane element and module design
251 Physical cleaning methods
Physical cleaning methods remove reversible foulants from the membrane surface viamechanical forces Examples of physical cleaning include forward and reverse flush-ing back-pulsingbackwashing use of turbulence promoters air flushing rinsing andsponge ball cleaning
Forward flushing is achieved by using high cross-flow velocities at the feedside to loosen and then remove foulants from the membrane surface The directionof the flush is often reversed for a few seconds to improve the effectiveness of thecleaning which is called reverse flushing Rinsing the membrane can also beused to dislodge and remove fouling layers One study found that rinsing withwater alone for 15 min could recover 90 of the original membrane flux ofmulti-channel tubular membranes fouled by whey proteins (Cabero Riera ampAlvarez 1999)
68 Advances in Membrane Technologies for Water Treatment
Backwashing is when a larger pressure is applied on the permeate side causing thereverse flow of permeate into the feed side and can help flush membrane pores as well(Laitinen et al 2001 Mugnier Howell amp Ruf 2000) For MF tubular ceramicmembranes fouled during treatment of oily wastewater over 95 recovery of the orig-inal flux or prevention of flux decline was achieved by periodic backwashing (Abadiet al 2011) However once permeate flux has dropped to 40e50 of the originalvalue chemical cleaning is required
Air flushingsparging can be used during MFUF to reduce the rate and extent offouling or it can be used periodically to clean the membrane surfaces (Cui Bellaraamp Homewood 1997 Matis et al 2004 Mercier-Bonin Lagane amp Fonade 2000)It can also increase permeate flux when used during filtration because the air bub-bles on the feed side can increase turbulence in the stream but increase energy con-sumption for the entire process Sponge ball cleaning scrubs foulants from themembrane surfaces and is suitable for use in cleaning large-diameter tubular mem-branes fouled by high-turbidity feed streams such as industrial wastewater (Psoch ampSchiewer 2006)
Forward flushing backwashing and air sparging can be combined because back-washing can loosen the fouling cake layer and forward flushing and air spargingcan sweep the foulants away
252 Chemical cleaning methods
Chemical cleaning methods are required to remove physically irreversible fouling bydissolving foulants without damaging the membrane A wide range of chemicals canbe used to clean the membranes their choice depends mainly on the type of foulant aswell as the membrane material For example acids may be used to treat inorganicfouling (Yoon Kang amp Lee 1999) and alkalis are commonly used to remove organicfouling (for membranes fouled by organic contaminants in surface water (Zondervanamp Roffel 2007) and whey protein (Bartlett Bird amp Howell 1995)) Ozone andoxidants have also been used to improve the removal of organic fouling (Changet al 1994) The chemical cleaning cycle normally consists of several steps andfactors such as cleaning temperature chemical concentration pH pressure and flowrates and time all of which can affect the cleaning efficiency (Bartlett et al 1995Bird amp Bartlett 2002) The cleaning agent can be introduced into the membrane chan-nels or the membranes can be immersed into cleaning agents
Often physical cleaning and chemical cleaning methods are combined for betterfouling control and shorter membrane cleaning time Using mechanical rinsing orbackflushing with caustic soda (NaOH) and oxidation (H2O2) leads to fast and effec-tive cleaning in which 87 of permeate flux is recovered within 8 min for ceramicmembrane fouled during beer filtration (Gan et al 1999)
253 Unconventional physical cleaning methods
The use of chemical cleaning methods generates undesired additional waste streams andconventional physical cleaning removes only reversible fouling Unconventional or
Advances in ceramic membranes for water treatment 69
developing techniques targeted for efficient and more environmentally friendly means toremove fouling material include the use of ultrasonic electric or magnetic fields
Ultrasonic fields can be used to aid in cleaning membranes Cavitation caused byultrasound can prevent the membranes from clogging when applied during operationand has the potential to improve permeation flux (Kylleuroonen et al 2006) They work byfirst loosening particles of the cake layer and then transporting them away by acousticstreaming (Lamminen Walker amp Weavers 2004) The same research found that fre-quencies between 70 and 620 kHz could be used without damaging the alumina mem-branes However damage to the membrane has been observed when it is placed withinthe cavitation region of the system (Chen Weavers amp Walker 2006) Furthermorethis may be an expensive method because the cost of energy to implement this maybe high and can potentially be used to remove foulants periodically instead
Electric fields can be used to clean membranes or enhance MFUF processesbecause they can provide an additional driving force to the filtration system Chargedsolids or species on the membrane can be moved as a result of the electric field appliedto the membrane surface thus reducing the effects of concentration polarization aswell (Jagannadh amp Muralidhara 1996) When electric fields are applied to the mem-brane fouling that is irreversible and resistant to physical cleaning methods canbecome physically reversible especially for membranes that are fouled by proteinsMagnetic fields can also potentially be applied to remove or reduce the extent of inor-ganic membrane fouling (Baker Judd amp Parsons 1997) This is because magneticfields change the calcium carbonate scale from its calcite form to the less solubleform aragonite and hence enhance the crystallization of CaCO3 preventing its forma-tion on membrane surfaces (Zaidi et al 2013)
There is a lot of freedom in choosing the cleaning method for ceramic membranesowing to their high thermal chemical and mechanical stabilities Fouling can beremoved effectively via a combination of physical and chemical cleaning methodsbut the use of chemicals to clean introduces new potentially hazardous wastes thatmay then need further treatment Unconventional methods such as using ultrasonicelectric and magnetic fields can potentially be useful in aiding membrane cleaningand reducing the need for chemical cleaning agents However the available literatureregarding the performance and mechanisms of these methods in realistic situations islimited and does not provide adequate evidences to prove their feasibility in improvingmembrane lifetime
26 Prospects and challenges
From a technical viewpoint the outstanding robustness and integrity of ceramic mem-branes have secured their positions in dealing with wastewater systems where at leastan aggressive environment is involved and contribute to sustainable and strong growthtoward wider applications Technical innovations in promoting separation performanceand engineering designs keep driving the development of ceramic membrane products
70 Advances in Membrane Technologies for Water Treatment
toward an application-oriented pattern In particular new membrane fabrication tech-niques such as the combined phase-inversion and sintering methods are expected tosee wider applications in membrane production facilities They offer a much simplerfabrication cycle and can make highly microstructurally varied membrane morphol-ogies that can be tailored according to the application In terms of membrane designthe presence of hollow-fibre membranes is expected to grow in the ceramic membranemarket to offer much greater treatment capacities for larger-scale applications Thepotential of a wider range of ceramic materials other than the commonly used aluminamembranes and membrane substrates is also expected In terms of applications there isa trend toward the increasing use of ceramic membranes for larger-scale wastewatertreatments especially in MBRs because the cost of ceramic membranes is reducedAnticipated cheaper ceramic membrane prices combined with higher packing densitiesof hollow-fibre configuration will increase applications in municipalities and morespace-efficient module designs especially for hollow-fibre membranes are expectedto penetrate the market to meet the higher demands
The widespread use of ceramic MFUF membranes in water treatment is limited bymany problems A key challenge is to reduce the capital cost of ceramic membraneswhich can be met by reducing the steps in the fabrication process and lowering the sin-tering temperature The emergence of cheaper fabrication processes such as the com-bined phase-inversion and sintering method can potentially reduce the fabricationcosts of ceramic membranes However further research and development are requiredto reduce the sintering temperature and maintain a continuously high-quality mem-brane The performance of the porous membranes formed by this method needs tobe improved so that both good mechanical strength and high porosity can be achievedImproved fundamental understanding of the formation mechanisms of ceramic mem-brane microstructures can help improve the reliability and reproducibility of the fabri-cation process which is particularly useful when producing membranes from differentceramic materials and when production is scaled up Furthermore challengesregarding the up-scaling of the combined phase-inversion and sintering processinclude developing a continuous fabrication and sintering process To meet the de-mands for large-scale treatment capacities more space-effective module and unit de-signs would also be required to cut down the costs of housing material as well asimprove productivity Hollow-fibre membrane elements may offer much higher pack-ing densities but improvements in element and module designs will be required suchas creating smaller-diameter hollow fibres and improving their mechanical strengthIn terms of fouling the mechanism varies from case to case depending on the appli-cation Although a combination of physical and chemical cleaning of ceramic mem-branes can be easily achieved this leads to additional waste products that requiretreatment and disposal hence more environmentally friendly approaches to mem-brane cleaning are desired This means that challenges arising in this area include bet-ter understanding of fouling mechanisms in ceramic membranes particularly theinteractions between ceramic membranes and the different foulants to ensure thatthe right choice of cleaning methods is made for reliable long-term operation
Advances in ceramic membranes for water treatment 71
Abbreviations
COD Carbon oxygen demandDBP Disinfectant byproductMBR Membrane bioreactorMF MicrofiltrationNOM Natural organic matterUF Ultrafiltration
Acknowledgements
The authors gratefully acknowledge the research funding provided by EPSRC in the UnitedKingdom (Grant no EPJ0149741)
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Module Specifications Kristal [cited 2014 1001] Retrieved from httpwwwhyfluxmembranescomkristal_modulehtml
Mozia S (2010) Photocatalytic membrane reactors (PMRs) in water and wastewater treatmentA review Separation and Purification Technology 73(2) 71e91 Retrieved from httpdxdoiorg101016jseppur201003021
MPT hollow fiber ceramic membranes [cited 2013 1604] Retrieved from httpwwwmediaandprocesscomproductsproducts01html
Mugnier N Howell J A amp Ruf M (2000) Optimisation of a back-flush sequence for zeolitemicrofiltration Journal of Membrane Science 175(2) 149e161 Retrieved from httpdxdoiorg101016S0376-7388(00) 00412e9
Muhammad N Sinha R Krishnan E R amp Patterson C L (2009) Ceramic filter for smallsystem drinking water treatment Evaluation of membrane pore size and importance ofintegrity monitoring Journal of Environmental Engineering 135(11) 1181e1191
Mulder M (1996) Basic principles of membrane technology The Netherlands SpringerOthman M H D Droushiotis N Wu Z T Kelsall G amp Li K (2012) Dual-layer hollow
fibres with different anode structures for micro-tubular solid oxide fuel cells Journal of PowerSources 205(0) 272e280 Retrieved from httpdxdoiorg101016jjpowsour201201002
Pall Membralox Ceramic Membranes and modules [cited 2014 1001] Retrieved fromhttpwwwpallcompdfsFuels-and-ChemicalsPIMEMBRAENpdf
Membralox IC Pall Membralox IC ceramic membranes and modules [cited 2014 1101]Retrieved from httpwwwpallcompdfsFood-and-BeveragePIMEMBRAICENpdf
Peinemann K V Nunes S P amp Giorno L (2011)Membrane technology Vol 3 Membranesfor food applications Weinheim Germany Wiley
Produced water beneficial use case studies Case studies [cited 2014 1001] Retrieved fromhttpaqwatecmineseduproduced_waterassessbucase
Projects Innovative designs for sustainable advanced water treatment [cited 2014 1001]Retrieved from httpwwwpwntechnologiesnlprojectsindexhtml - six-ceramac-pilot-south-west-water-united-kingdom
Advances in ceramic membranes for water treatment 79
Psoch C amp Schiewer S (2006) Direct filtration of natural and simulated river water with airsparging and sponge ball application for fouling control Desalination 197(1e3)190e204 Retrieved from httpdxdoiorg101016jdesal200511027
Rahman M A Garciacutea-Garciacutea F R Irfan Hatim M D Kingsbury B F K amp Li K(2011) Development of a catalytic hollow fibre membrane micro-reactor for high purityH2 production Journal of Membrane Science 368(1e2) 116e123 Retrieved fromhttpdxdoiorg101016jmemsci201011025
Ruutenhuch R (1992) Membrane separation systems - recent developments and future di-rections Von R W Baker ua Noyes Data Corp Park Ridge 1991 XV 451 S zahlrAbb u Tab geb US-$64 Chemie Ingenieur Technik 64(6) 584 Retrieved from httpdxdoiorg101002cite330640633
Sewage treatment plant ceramicMBR Case studies [cited 2014 1001] Retrieved from httpwwwitn-nanovationcomapplicationscase-studiessewage-treatment-plant-ceramic-mbrhtml
Sewage treatment plant MBCR Case studies [cited 2014 1001] Retrieved from httpwwwitn-nanovationcomapplicationscase-studiessewage-treatment-plant-mbcrhtml
Silva L L O Vasconcelos D C L Nunes E H M Caldeira L Costa V C Musse A Pet al (2012) Processing structural characterization and performance of alumina supportsused in ceramic membranes Ceramics International 38(3) 1943e1949 Retrieved fromhttpdxdoiorg101016jceramint201110025
PUB and Meiden Singapore collaborate on ceramic membrane MBR demonstration plantMembrane Technology 2012(9) (2012) 1e16 Retrieved from httpdxdoiorg101016S0958-2118(12) 70171e4
Skrzypek M amp Burger M (2010) Isoflux ceramic membranes mdash practical experiencesin dairy industry Desalination 250(3) 1095e1100 Retrieved from httpdxdoiorg101016jdesal200909116
Smid J Avci C G Geurounay V Terpstra R A amp Van Eijk J P G M (1996) Preparationand characterization of microporous ceramic hollow fibre membranes Journal ofMembrane Science 112(1) 85e90 Retrieved from httpdxdoiorg1010160376-7388(95)00274-X
Sondhi R amp Bhave R (2001) Role of backpulsing in fouling minimization incrossflow filtration with ceramic membranes Journal of Membrane Science 186(1)41e52 Retrieved from httpdxdoiorg101016S0376-7388(00) 00663e3
Sondhi R Bhave R amp Jung G (2003) Applications and benefits of ceramic membranesMembrane Technology 2003(11) 5e8 Retrieved from httpdxdoiorg101016S0958-2118(03) 11016e6
Staff A (2011) Microfiltration and ultrafiltration membranes for drinking water (M53)Denver Colorado American Water Works Association
Stoquart C Servais P Beacuterubeacute P R amp Barbeau B (2012) Hybrid membrane processes usingactivated carbon treatment for drinking water A review Journal of Membrane Science411e412(0) 1e12 Retrieved from httpdxdoiorg101016jmemsci201204012
Submersible silicon carbide flat sheet membranes Future filtration [cited 2014 1001] Retrievedfrom httpliqtechcomimguserfileSiC Ceramic Flat Sheet Membranepdf
Success for LiqTech ceramic membranes in oil amp gas trial Filtration Industry Analyst2012(11) (2012) 4 Retrieved from httpdxdoiorg101016S1365-6937(12) 70350e7
Sun D Zeng J amp Tay J (2003) A submerged tubular ceramic membrane bioreactor for highstrength wastewater treatment Water Science amp Technology 47(1) 105e111
SURVEY Technical data of atech Al2O3 membranes [cited 2014 1001] Retrieved fromhttpwwwatech-innovationscomfileadmindownloadatech_product_data_en_080206_01pdf
80 Advances in Membrane Technologies for Water Treatment
Sushumna I Gupta R amp Ruckenstein E (1992) Effective dispersants for concentratednonaqueous suspensions Journal of Materials Research-Pittsburgh- 7 2884e2893Retrieved from httpdxdoiorg101557JMR19922884
Tan X Liu N Meng B amp Liu S M (2011) Morphology control of the perovskitehollow fibre membranes for oxygen separation using different bore fluids Journal ofMembrane Science 378(1e2) 308e318 Retrieved from httpdxdoiorg101016jmemsci201105012
Tan X Liu S amp Li K (2001) Preparation and characterization of inorganic hollow fibermembranes Journal of Membrane Science 188(1) 87e95 Retrieved from httpdxdoiorg101016S0376-7388(01) 00369e6
Terpstra R A Bonekamp B C amp Veringa H J (1988) Preparation characterization andsome properties of tubular alpha alumina ceramic membranes for microfiltration and as asupport for ultrafiltration and gas separation membranesDesalination 70(1e3) 395e404Retrieved from httpdxdoiorg1010160011-9164(88) 85069e0
The ROSS System Produced water treatment [cited 2014 1001] Retrieved from httpwwwveoliawaterstnacomnews-resourcesinformation-filesROSSproducedwaterhtm
The Ross System Produced water treatment CeraMem Ceramic Membranes[cited 2014 1001] Retrieved from httpwwwveoliawaterstcomceramemen
Van Der Bruggen B Vandecasteele C Van Gestel T Doyen W amp Leysen R (2003) Areview of pressure-driven membrane processes in wastewater treatment and drinking waterproduction Environmental Progress 22(1) 46e56 Retrieved from httpdxdoiorg101002ep670220116
Van Gestel T Vandecasteele C Buekenhoudt A Dotremont C Luyten J Van derBruggen B et al (2003) Corrosion properties of alumina and titaniaNFmembranes Journalof Membrane Science 214(1) 21e29 Retrieved from httpdxdoiorg101016S0376-7388(02) 00517e3
Wang B amp Lai Z (2012) Finger-like voids induced by viscous fingering during phaseinversion of aluminaPESNMP suspensions Journal of Membrane Science 405e406(0)275e283 Retrieved from httpdxdoiorg101016jmemsci201203020
Waeger F Delhaye T amp Fuchs W (2010) The use of ceramic microfiltration and ultrafil-tration membranes for particle removal from anaerobic digester effluents Separation andPurification Technology 73(2) 271e278 Retrieved from httpdxdoiorg101016jseppur201004013
Wang H T Liu X Q Chen F L Meng G Y amp Soslashrensen O T (1998) Kinetics andmechanism of a sintering process for macroporous alumina ceramics by extrusion Journalof the American Ceramic Society 81(3) 781e784 Retrieved from httpdxdoiorg101111j1151-29161998tb02412x
Wang I-F D Jerome F Morris Richard A amp Kesting Robert (1999) Highly asymmetricultrafiltration membranes US Patent No 5958989 Washington DC US Patent andTrademark Office
Wang T Zhang Y Z Li G F amp Li H (2009) Preparation and characterization of aluminahollow fiber membranes Frontiers of Chemical Engineering in China 3(3) 265e271Retrieved from httpdxdoiorg101007s11705-009-0010-2
Wang Y H Cheng J G Liu X Q Meng G Y amp Ding Y W (2008a) Preparation andsintering of macroporous ceramic membrane support from titania sol-coated aluminapowder Journal of the American Ceramic Society 91(3) 825e830 Retrieved from httpdxdoiorg101111j1551-2916200702207x
Wang Z Yang N T Meng B Tan X Y amp Li K (2008b) Preparation andoxygen permeation properties of highly asymmetric La06Sr04Co02Fe08O3a Perovskite
Advances in ceramic membranes for water treatment 81
hollow-fiber membranes Industrial amp Engineering Chemistry Research 48(1) 510e516Retrieved from httpdxdoiorg101021ie8010462
Water filtration by ceramic flat membranes e CFM ItN water filtration [cited 2014 1101]Retrieved from httpwwwitn-nanovationcomfileadminuser_uploadpdf-downloadsCFM-Flyerpdf
Wei C C Chen O Y Liu Y amp Li K (2008) Ceramic asymmetric hollow fibre mem-branesmdashone step fabrication process Journal of Membrane Science 320(1e2) 191e197Retrieved from httpdxdoiorg101016jmemsci200804003
Wu J C-S amp Cheng L-C (2000) An improved synthesis of ultrafiltration zirconia mem-branes via the solegel route using alkoxide precursor Journal of Membrane Science167(2) 253e261 Retrieved from httpdxdoiorg101016S0376-7388(99)00294-X
Wu Z Faiz R Li T Kingsbury B F K amp Li K (2013) A controlled sintering process formore permeable ceramic hollow fibre membranes Journal of Membrane Science 446(0)286e293 Retrieved from httpdxdoiorg101016jmemsci201305040
Xin X S Lu Z Zhu Q S Huang X Q amp Su W H (2007) Fabrication of dense YSZ electro-lyte membranes by a modified dry-pressing using nanocrystalline powders Journal of Mat-erials Chemistry 17(16) 1627e1630 Retrieved from httpdxdoiorg101039B615548K
Xing C H Tardieu E Qian Y ampWen X H (2000) Ultrafiltration membrane bioreactor forurban wastewater reclamation Journal of Membrane Science 177(1e2) 73e82 Retrievedfrom httpdxdoiorg101016S0376-7388(00)00452-X
Xu N Xing W H Xu N P amp Shi J (2003) Study on ceramic membrane bioreactor withturbulence promoter Separation and Purification Technology 32(1e3) 403e410Retrieved from httpdxdoiorg101016S1383-5866(03)00073-X
Yang N Tan X amp Ma Z (2008) A phase inversionsintering process to fabricate nickelyttria-stabilized zirconia hollow fibers as the anode support for micro-tubular solid oxidefuel cells Journal of Power Sources 183(1) 14e19 Retrieved from httpdxdoiorg101016jjpowsour200805006
Yoon S H Kang I J amp Lee C H (1999) Fouling of inorganic membrane and fluxenhancement in membrane-coupled anaerobic bioreactor Separation Science and Tech-nology 34(5) 709e724 Retrieved from httpdxdoiorg10108001496399908951140
Zaidi N S Sohaili J Muda K amp Sillanpeuroaeuroa M (2013) Magnetic field application and itspotential in water and wastewater treatment systems Separation amp Purification Reviews43(3) 206e240 Retrieved from httpdxdoiorg101080154221192013794148
Zhang F Jing W H Xing W H amp Xu N P (2009a) Experiment and calculation offiltration processes in an external-loop airlift ceramic membrane bioreactor ChemicalEngineering Science 64(12) 2859e2865 Retrieved from httpdxdoiorg101016jces200902046
Zhang J W Fang H Hao L Y Xu X amp Chen C S (2012) Preparation of silicon nitridehollow fibre membrane for desalination Materials Letters 68(0) 457e459 Retrievedfrom httpdxdoiorg101016jmatlet201111041
Zhang X Fang D Lin B Dong Y C Meng G Y amp Liu X Q (2009b) Asymmetric porouscordierite hollow fiber membrane for microfiltration Journal of Alloys and Compounds487(1e2) 631e638 Retrieved from httpdxdoiorg101016jjallcom200908028
Zondervan E amp Roffel B (2007) Evaluation of different cleaning agents used for cleaningultra filtration membranes fouled by surface water Journal of Membrane Science304(1e2) 40e49 Retrieved from httpdxdoiorg101016jmemsci200706041
Zeurourcher S amp Graule T (2005) Influence of dispersant structure on the rheological propertiesof highly-concentrated zirconia dispersions Journal of the European Ceramic Society25(6) 863e873 Retrieved from httpdxdoiorg101016jjeurceramsoc200405002
82 Advances in Membrane Technologies for Water Treatment
Advances in water treatmentby microfiltration ultrafiltrationand nanofiltration
3I Koyuncu12 R Sengur23 T Turken12 S Guclu12 ME Pasaoglu121Istanbul Technical University Civil Engineering Faculty Environmental EngineeringDepartment Istanbul Turkey 2National Research Center on Membrane Technologies(MEM-TEK) Istanbul Turkey 3Istanbul Technical University Nanoscience andNanoengineering Department Istanbul Turkey
31 Introduction
In 2012 access to safe drinking water became the first millennium developmentgoal to be met by the international community (Url-1) The original aim set in2000 was to halve by 2015 the proportion of the population without sustainable ac-cess to safe drinking water and basic sanitation (UN 2010 pp 58e60) This encour-aging achievement should underscore the progress that can be made through thedevelopment of often simple and cheap technologies in low-income economies Atthe same time it can be expected that challenges related to water systems willcontinue to increase requiring further investment and technological innovation tomeet global needs Access to clean water is a simple human need and an importantdriver of social and economic development (UN Water 2006) Predictable andconsistent access to clean drinking water is globally seen as a core function of statesbecause it is crucial to a societyrsquos public health economic vitality and national secu-rity (Elimelech amp Phillip 2011)
311 Global membrane market
The global membrane market for microfiltration (MF) products used in liquidseparations was estimated to be $16 billion in 2013 It was projected to rise at a com-pound annual growth rate (CAGR) of 100 during the next 5 years to reach$26 billion in 2018 (Url-2)
The US market for ultrafiltration (UF) technologies was worth $882 million in2009 and reached $932 million at the end of 2010 By 2015 the market is estimatedto be valued at $12 billion a CAGR of 57 (Url-3)
The global market for nanofiltration (NF) membranes increased from$1728 million in 2012 to 1902 million in 2013 at the end of 2014 and 2019 it is esti-mated to reach $2156 million and 4451 million respectively CAGR will be 156during 2014e2019 (Url-4)
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400003-4Copyright copy 2015 Elsevier Ltd All rights reserved
312 Membrane filtration technology for water treatment
Membrane technology has become a significant separation technology over the past2 decades Continuing advances in regulatory constraints and aesthetic criteria forconsumer water quality have driven the water community to seek new technologiesMembrane technology is one of those new technologies In this technology mem-branes are used as filters in separation processes in various applications They pro-vide effective alternatives to related technologies such as adsorption ionexchangers and sand filters Major uses of this technology are water filtration(including desalination) and purification (including groundwater and wastewater)as well as industries such as food and beverages and biotechnology (Kurt KoseogluDizge Chellam amp Koyuncu 2012 Ozgun et al 2012) It also has medical uses suchas in dialysis
313 Membrane processes
A membrane is defined as a selective barrier between two homogenous phases Inprinciple membranes can carry out most of the separation processes and can com-plement or form an alternative for chemical processes such as distillation extractionfractionation and adsorption Some advantages of membrane filtration are lowenergy consumption the possibility of continuous separation and simpleup-scaling (Baker 2004)
The membrane process is not suitable for every fluid stream because of the contentof feed and product requirements as well as the nature of the membrane process and themembrane designed for that process There are four developed industrial membraneprocesses for water treatment The main difference in these processes is the poresize of the membranes Membrane processes are
1 Microfiltration2 Ultrafiltration3 Nanofiltration4 Reverse osmosis (RO)
RO is beyond the scope of this chapter but the other processes will be covered hereThe schematic separation spectrum given for membrane processes is shown inFigure 31 (Baker 2004)
1Aring 10 Aring 100 AringPore diameter
Reverseosmosis
UltrafiltrationMicrofiltration
Conventionalfiltration
1000 Aring 1 microm 10 microm 100 microm
Figure 31 Schematic spectrum for membrane processes and conventional filtrationReprinted from Baker (2004) Overview of membrane science and technology p 7
84 Advances in Membrane Technologies for Water Treatment
Membrane processes can be used for various applications There are variousguidelines for selecting the proper membrane process For example if particlesgt02 mm are removed the process becomes MF if dissolved contaminants can beprecipitated or coagulated the proper process may be MF or UF In the NF processit is possible to remove both organic and inorganic ions however if monovalentions exist or total dissolved solids removal of 3000 mgL is achieved the RO processis required (AWWA 2007)
General comparisons between MF UF and NF membranes can be found in Table 31
32 Water treatment by MF UF and NF
321 Microfiltration process
The MF and UF processes are commonly used for water treatment systems Af-ter a sand filter or cartridge filter MF can be applied to groundwater and surfacewaters In RO systems generally MF is in the pretreatment section of the treat-ment plant
The pore size of MF typically varies from 01 to 10 mm However the separationmechanism is not simple because particles of sizes are smaller than the pore sizeflow freely through the pore while the larger particles are rejected
Volume flow through MF membranes can be described by Darcyrsquos law where fluxJ through the membrane is directly proportional to the applied pressure (DP)
J frac14 A$DP
where permeability constant A contains structural factors such as the porosity and poresize distribution (Mulder 1996)
MF is used in a wide variety of industrial applications where particles of a sizegt01 mm have to be retained from a suspension The most important applicationsare still based on dead-end filtration using cartridges However for larger-scale appli-cations dead-end filtration will slowly be replaced by cross-flow filtration One of themain industrial applications is the sterilization and clarification of all kinds ofbeverages and pharmaceuticals industries Other applications are summarized below(Eykamp 1995 Mir Micheals Goel amp Kaiser 1992 Mulder 1996)
bull Ultrapure water in the semiconductorbull Drinking water treatmentbull Clarification of fruit juice wine and beerbull Wastewater treatmentbull Pretreatment
322 Ultrafiltration process
For UF pore sizes generally range from 001 to 005 mm (nominally 001 mm) orless UF membranes have the ability to retain larger organic macromolecules
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 85
Table 31 Characteristic features of MF and UF membranes
Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF)
Operation mode Cross-flow and dead-endoperation
Cross-flow and dead-endoperation
Cross-flow operation
Operating pressure 01e3 bar (Transmembrane) 05e10 bar (Transmembrane) 2e40 bar (Transmembrane)
Separating mechanism Separation based on particle size Separation based on basedparticle size
Separation based on differencesin solubility and diffusivity
Molecular separation size Solids gt01 mmSeparation of particles
Colloids 20000e200000 DaSolids gt05 mmSeparation of macromolecules
Dissolved matter200e20000 Dasolids gt0001 mmSeparation of low-MW solutes(salt glucose lactose micro-pollutants)
Membrane types Predominantly symmetricpolymer or ceramic membranes
Asymmetric polymer compositeor ceramic membrane
Asymmetric polymer orcomposite membrane
Module types Spiral-wound hollow-fiber andtube modules plate or cushionmodules
Spiral-wound hollow-fiber andtube modules plate or cushionmodules
Spiral-wound tube and cushionmodules
Osmotic pressure negligible Osmotic pressure negligible Osmotic pressure negligible Osmotic pressure high(1e25 bar)
Thickness of separating layer Symmetric frac14 10e150 mmAsymmetric frac14 1 mm
01e10 mm 01e10 mm
Source Adapted from Pinnekamp and Friedrich (2006) volume 2 2nd edition
86Advances
inMem
braneTechnologies
forWater
Treatm
ent
they have been historically characterized by a molecular weight cutoff (MWCO)rather than by a particular pore size (Membrane Filtration Guidance Manual2005) Both UF and MF membranes can be considered porous membranes in whichrejection is determined mainly by the size and shape of the solutes relative to thepore size in the membrane In fact MF and UF involve similar membrane processeswith the same separation principle However an important difference is that UFmembranes have an asymmetric structure with a much denser top layer (smallerpore size and lower surface porosity) and consequently much higher hydrodynamicresistance (Mulder 1996)
There are two different operation modes for MF and UF systems Figure 32 showsthe cross-flow configuration in which the feed water is pumped tangential to the mem-brane Clean water passes through the membrane while the water that is rejected isrecirculated as concentrate and combined with additional feed water In dead-end ordirect filtration all of the feed water passes through the membrane Therefore recoveryis almost 100 and a small fraction is periodically used for backwash in the system(2e15) (Jacangelo et al 1997)
Permeate
Backwash waste
Recycle
Permeate
Air or liquidbackwash
Air or liquidbackwash
Recirculation pumpPre-screen
Pre-screen
Feed pump
Feed pump
Rawwater
Rawwater
(a)
(b)
Figure 32 (a) Cross-flow configuration MF and (b) dead-end filtration for microfiltration (MF)Reprinted from Jacangelo Trussell and Watson (1997) with permission from Elsevier
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 87
NowadaysmostMFUFplants operate in dead-endfiltrationmode because the energyrequired is lower compared with cross-flow systems as the high cross-flow velocityrequired to prevent fouling results in increased head loss and energy consumption
323 Nanofiltration process
NF membranes have applications in several areas (Koyuncu amp Cakmakci 2010) Themain applications of NF are in water treatment for drinking water production as well aswastewater treatment and also reuse (Debik Kaykioglu Coban amp Koyuncu 2010)NF can be used to treat all kinds of water including groundwater surface water andwastewater or as a pretreatment for desalination (Cakmakci Baspinar BalabanKoyuncu amp Kinaci 2009 Koyuncu Arikan Weisner Mark amp Rice 2008 UyakKoyuncu Oktem Cakmakci amp Toreurooz 2008) The introduction of NF as a pretreat-ment is considered a breakthrough in the desalination process NF membranes haveability to remove turbidity microorganisms and hardness as well as a fraction ofthe dissolved salts This results in a significantly lower operating pressure and providesa much more energy-efficient process compared with RO (Hilal Al-Zoubi DarwishMohammad amp Abu Arabi 2004)
These membranes which fall into a transition region between pure RO membranesand pure UF membranes are called loose RO low-pressure RO or more commonlyNF membranes Typically NF membranes have sodium chloride rejections between20 and 80 andMWCOs for dissolved organic solutes of 200e1000 Da These prop-erties are intermediate between RO membranes with a salt rejection of more than 90andMWCOof less than 50 andUFmembranes with a salt rejection of less than 5 Theneutral NF membrane rejects various salts in proportion to their molecular size so theorder of rejection is simply Na2SO4 gt CaCl2 gt NaCl (Baker 2004)
Many researchers have studies the softening of groundwater using NF Scheap et al(1998) used different NF membranes and their experimental results showed that60e70 ofmultivalent ions can be removed byNFmembranes Sombekke Voorhoeveand Hiemstrap (1997) compared NF membranes with activated carbon adsorptionAccording to the results both processes had goodperformanceHoweverNFmembraneshad some advantages in terms of investment costs (Hilal et al 2004 Schaep et al 1998Sombekke et al 1997)
NF membranes are used for their ion selectivity The retention of a dissolved salt isdetermined by the valence of the anions Therefore most salts with monovalent anions(eg Cl) can pass through the membrane whereas multivalent anions (eg SO2)are retained The separation spectrum for NF membranes is given in Figure 33
Some parameters are crucial for the operation of an NF unit solvent permeabilityflux through the membrane rejection of solutes and yieldrecovery Similar to otherpressure-driven membrane processes the flux J or the permeability (flux per unitof applied pressure) of a membrane is a crucial parameter Most NF membranes arehydrophilic except some used for solvent applications
Rejection in NF is mainly determined by molecular size hydrophobicity andcharge but the effects of the molecular shape and dipole moment for example mighthave an important role as well The porevoid dimensions are statistically distributed
88 Advances in Membrane Technologies for Water Treatment
and can be described by a log-normal distribution (Bowen amp Welfoot 2005) Thisexplains the smooth transition from no rejection to complete rejection in a typicalS-shaped curve when molecular size is varied The MWCO value is often used to indi-cate the lower limit of molecules that are (almost completely) retained similar to UFmembranes For NF membranes with MWCO values between 150 and 1000 (but oftenin the range 150e300) this concept should be used with care Hydrophobic moleculeslarger than the MWCO for example often have a low rejection The pH of the solutionmight change the membranersquos surface charge as well as the charge of the solute so therejection of this solute can be higher or lower than expected
Another parameter is the recovery or yield which is generally used for the design ofan industrial application rather than a membrane characteristic The recovery is theratio of the permeate stream to the feed stream values range from 40 to 90
3231 Applications of NF membranes
NF membranes can reduce the ionic strength of the solution Moreover hardnessorganics and particulate contaminants can be removed by NF membranes Manyresearchers have used NF to achieve those objectives
Some researchers have investigated the softening of groundwater using NFsystems All of the results gave good data Retentions higher than 90 were foundfor multivalent ions whereas monovalent ions were about 60e70 (Hilal et al2004 Scheap et al 1998) NF membranes with pellet softening and granular activatedcarbon for softening water were compared Although all methods had good results NFmembranes had several advantages from the point of health and lower investmentcosts The advantage of NF softening is that a smaller stream of water can be softenedbecause NF membranes remove essentially all hardness cations In municipal watertreatment works it is not necessary to reduce hardness to very low values and there-fore the use of NF membranes allows for side-stream treatment This involves sending
Reverse
osmosis
region
Nano-
filtration
region
100 105
104
103
102
10
80
60
40
20
0001 10 10 10001
Ultrafiltration
region
Water pressure-normalized flux
(m3 m2 middot bar middot day)
Mole
cula
r w
eig
ht cuto
ff
(90
rete
ntion
)
NaC
l re
jection (
)
Figure 33 Separation permeability spectrum of nanofiltration and other processesReprinted from Baker (2004) Membrane transport theory p 83
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 89
only a portion of the flow to be softened to the membranes and permeate is thenblended with the bulk flow stream to obtain a target blended value In contrast precip-itated lime softening cannot reduce hardness to below approximately 50 mgL CaCO3and therefore side-stream treatment is generally not practical
A typical NF membrane system consists of three separate subsystems pretreatmentmembrane processes and posttreatment
The primary application of NF membranes is desalting of saline surface orgroundwater Surface waters often have unsettled chemistry or composition owingto seasonal changes or after dilution with rain NF is a reliable option for surface watertreatment although the focus is on removing organics rather than on softening (Hilalet al 2004 Van der Bruggen amp Vandecasteele 2003)
Disinfection by-products (DBPs) are a significant regulatory concern NFmembranes are increasingly applied to remove DBP precursors such as natural organicmatter (NOM) which can react with various disinfectants used in the water treatmentprocess to form potential carcinogens NOM removal is an important water treatmenttarget for many utilities NF as a standalone process has been shown in many cases toreduce total organic carbon (TOC) to less than 05 mgL (AWWA 2007)
NF is also more effective than RO for lime softening removing naturally occurringcolor and DBP precursors both consist primarily of organic carbon
Semipermeable NF membranes are not porous they have the ability to screenmicroorganisms and particulate matter in the feed water This ability has been verifiedin a number of studies such as one that demonstrated that NF membranes providebetween 4 and 5 log removal of viruses
324 Integrated membrane systems
For acquiring regulation standards for drinking water NF and low pressure ROprocesses are considered however they are too sensitive to fouling and advanced pre-treatment processes like UF and MF are used to increase the productivity of systemsThese integrated systems are called as integrated membrane system (IMS) and theirpurpose is to reduce and control fouling
33 Pretreatment requirements
Pretreatment is typically applied to the feed water before entering the membranesystem to minimize membrane fouling However in some cases it may be used toaddress other water quality concerns or treatment objectives Pretreatment is mostoften used to remove foulants optimize recovery and system productivity and extendmembrane life Pretreatment may also be used to prevent physical damage to themembranes
Different types of pretreatment can be used in conjunction with any givenmembrane type Pilot testing can be used to compare various pretreatment optionsoptimize pretreatment andor demonstrate pretreatment performance (Membrane
90 Advances in Membrane Technologies for Water Treatment
Filtration Guidance Manual 2005) The feed water may contain various concentra-tions of dissolved matter and suspended solids depending on the source Suspendedsolids may include inorganic particles colloids and biological debris such as micro-organisms and algae Dissolved matter may consist of highly soluble matterssuch as chlorides soluble salts and sparingly soluble salts such as carbonate andsulfates
Depending on the raw water quality the pretreatment process may follow stepssuch as
bull Removal of large particles using a coarse strainerbull Water disinfection with chlorinebull Clarification with or without flocculationbull Clarification and hardness reduction using lime treatmentbull Media filtrationbull Reduction of alkalinity by pH adjustmentbull Addition of scale inhibitorbull Reduction of free chlorine using sodium bisulfite or activated carbon filtersbull Water sterilization using ultraviolet (UV) radiationbull Final removal of suspended particles using cartridge filters (Membrane Fouling
Considerations 2001)
331 General pretreatment methods
Membranes can be fouled by organic or inorganic substances Therefore pretreatmentof the feed stream is required to control colloidal organic and biological fouling aswell as scaling For low-pressure membranes a number of pretreatment methods arecurrently used (Abdelrasoul Doan amp Lohi 2013)
UF can sufficiently produce disinfected water directly from surface water fordifferent applications MF can also be used for disinfection although not all virusesare removed Because direct membrane filtration is limited by fouling it leads to acontinuous increase in transmembrane pressure during constant flux filtration(Kennedy et al 2008)
3311 Prefiltration
Prefiltration involving screening or coarse filtration is a common means ofpretreatment for membrane filtration systems that is designed to remove large particlesand debris Prefiltration can be applied to the membrane filtration system as a wholeand to each membrane unit separately The particular pore size associated with theprefiltration process varies depending on the type of membrane filtration system andthe feed water quality Generally hollow-fiber MF and UF systems are designedspecifically to remove suspended solids large particulate matter that can damage orclog the membranes fibers Use of these types of filtration system depend on theinfluent water quality and manufacturer specifications the pore sizemicron rating ofa chosen prefiltration process may range from as small as 100 mm to 3000 mm orhigher Because of this hollow-fiber MFUF membrane systems that operated in an
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 91
inside-out mode are more suitable to fiber clogging and thus may require prefiltration(Membrane Filtration Guidance Manual 2005)
NF and RO are nonporous membranes that cannot be backwashed and are mostcommonly designed in a spiral-wound configuration for municipal water treatment ap-plications NF and RO systems must use finer prefiltration to minimize membraneexposure of particulates of any size At this point spiral-wound membrane modulesare highly sensitive to particulate fouling that can reduce system productivity andreduce membrane life (Membrane Filtration Guidance Manual 2005) Table 32 listsrequired prefiltration systems for NFRO according to the silt density index (SDI) value
In cartridge filtration technology applications prefiltration is not required inhigh-quality source water treatment
Typical prefiltration requirements associated with various types of membranefiltration are presented in Table 33
In some cases one type of membrane filtration may be used as prefiltration foranother This type of treatment scheme is known as an IMS Generally this involvesMFUF use as pretreatment for NFRO applications that require the removal ofparticulates and microorganisms as well as some dissolved contaminants such as hard-ness iron and manganese or DBP precursors The most significant advantages of an
Table 32 Required prefiltration systems for NFRO according to SDIvalue
Feed water SDI value Proposed prefiltration Rating ranges
5 Cartridge filters 5e20 mm
gt5 MFUF 01e001 mm
Source Membrane Filtration Guidance Manual (2005)
Table 33 Typical membrane system prefiltration requirements
Membrane system Prefiltration requirements
Classification Configuration Size (mm) Type(s)
Membrane cartridgefiltrationa
Cartridge 300e3000 Strainersbag filters
MFUF Hollow-fiberinside-out
100e300 Strainersbag filters
Hollow-fiberoutside-in
300e3000 Strainersbag filters
NFRO Spiral-wound 5e20 Cartridge filters
aPrefiltration is not necessarily required for membrane cartridge filtration systemsSource Membrane Filtration Guidance Manual (2005)
92 Advances in Membrane Technologies for Water Treatment
IMS treatment scheme is that MFUF filtrate is of consistently high quality with respectto particulate matter and maintains stable operation by reducing the rate of membranefouling (Membrane Filtration Guidance Manual 2005)
3312 Coagulation and filtration
Coagulation as a pretreatment process can substantially reduce the concentration ofbiodegradable organic matter found in raw water thus it can decrease the potentialfor fouling and enhance membrane rejection However this method can be a problembecause coagulant residuals and increased solids loading can reduce membrane fluxesand increase cleaning requirements Generally coagulation is required for MF pro-cesses in water treatment Some research suggested that flocs need to reach a certaincritical floc size before MF on the contrary pores of MF membranes are irreversiblyfouled by small particles (Judd amp Hillis 2001)
3313 In-line coagulation
In-line coagulation (IC) involves the use of coagulants without the removal ofcoagulated solids After that coagulated water is given to MF or UF Conventionalcoagulation and IC are different from each other in terms of initial membrane fluxdecline IC has better results than conventional coagulation for initial membraneflux (Escobar project report)
The IC (without settling)UF process improves membrane performance and waterquality for surface water treatment Employing coagulation before UF increasedpermeate quality to the extent that dissolved organic matter removal is controlled bythe coagulation step (Guigui Rouch Durand-Bourlier Bonnelye amp Aptel 2002)
Antelmi Cabane Meireles and Aimar (2001) reported that aggregates producedunder sweep floc conditions are more compressible than for charge neutralization con-ditions resulting in compaction when the membrane filtration system is pressurizedThey thought that cake resistance is lower than the resistance resulting from theunsettled flocs and the noncoagulated organics
3314 Coagulationesedimentation process
In coagulation and sedimentation processes a coagulant is applied to form flocs thatare settled out by sedimentation After sedimentation the supernatant is fed to themembranes If coagulation and sedimentation are applied before UF UF membranesare less fouled Experimental studies showed that membrane life can be increased as aresult of coagulationesedimentation processes (Kruithof Nederlof Hoffmanamp Taylor 2004 Minegishi Jang Watanabe Hirata amp Ozawa 2001)
3315 Coagulationeadsorption process
Some research showed that membrane fouling can be minimized by pretreatmentNOM in feed water can be caused trihalomethane (THM) formation in the disinfectionstep Although coagulation and MF are a good flowchart for water treatment THM
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 93
precursors cannot be removed by the MF process When pre-adsorption is used forNOM the removal of THM precursors or NOM is improved (Beacuterube MavinicHall Kenway amp Roett 2002 Carroll King Gray Bolto amp Booker 2000 MaartensSwarta amp Jacobs 1999)
3316 Flocculation and filtration
Colloids can be removed by co-precipitation with Al(III) Fe(III) or Si(IV)hydroxides The negatively charged colloids are surrounded by the metal cationsand thus form a nucleus for their precipitation The same thing happens when limeis used for softening When polymer is added for coagulation the long chains canact as bridges linking colloidal particles and aiding in floc formation
Flocculation and filtration processes are used to achieve three objectiveseliminating the penetration of colloidal particles into the membrane pores increasingthe critical flux and modifying the characteristics of the deposits (Abdelrasoul et al2013 Kennedy et al 2008)
3317 Ion exchange
Some dissolved salts or divalent cations in feed water can cause scaling on themembrane surface To overcome this problem softening of feed water is requiredIon exchange process can be used for this purpose
The softening approach uses cationic resins to replace the calcium ions (or other diva-lent cations) with sodium ions which do not form insoluble salts with carbonate ion Ionexchange is well-suited for incorporation into an RO system Depending on the salinityand pH the RO concentrate may be used as a regenerate solution Cation exchange resinscan be adequately regenerated at lower concentrations than 10 with lower flow ratesand longer regeneration cycles (Wilbert Leitz Abert Boegli amp Linton 1998)
3318 Chemical conditioning
Chemical conditioning is used for various pretreatment purposes such as pHadjustment disinfection biofouling control scale inhibition and coagulation Sometype of chemical conditioning is almost always used with NFRO systems most oftenthe addition of an acid or proprietary scale inhibitor recommended by the membranemanufacturer to prevent the precipitation of sparingly soluble salts such as calciumcarbonate (CaCO3) barium sulfate (BaSO4) strontium sulfate (SrSO4) or silica spe-cies (eg SiO2) (Membrane Filtration Guidance Manual 2005)
A number of different chemicals can be added to MF or UF systems as apretreatment related to treatment objectives However as with NF and ROmembranesapplied pretreatment chemicals must be compatible with the particular membrane ma-terial used An MFUF system may be able to operate efficiently with the in-line addi-tion of lime or coagulants On the other hand presettling in association with thesepretreatment processes can enhance membrane flux and increase system efficiency(Membrane Filtration Guidance Manual 2005)
94 Advances in Membrane Technologies for Water Treatment
332 Pretreatment requirements for membrane bioreactors
The first generation of membrane bioreactors (MBR) in the 1970 and 1980s were builtwith large-diameter tubular membranes and were primarily used for small-scale indus-trial effluents containing little waste Pretreatment to MBR first became an issue whenhollow fibers and plate-immersed membranes were introduced in the 1990s forapplication to municipal wastewater
Today municipal MBR systems capacities are up to 50000 m3d and much largersystems are under construction or in the planning stages Current research goals are toreduce the cost of technology and increase the packing density These improvementsmake it significantly important to install adequate pretreatment to protect the mem-branes at the core of an MBR Possible negative effects of poor pretreatment on themembranes themselves include
bull Buildup of trash hair lint and other fibrous materialsbull Increased risk of sludge accumulationbull Eventually damage to the membrane (Co
ˇ
teacute Brink amp Adnan 2006)
34 Advances in membrane materials for watertreatment by MF UF and NF
Membrane technology is used to improve water quality treat wastewater and reuseprocesses for years The surge of interest in membrane separations increased owingto two developments high-flux defect-free membranes and high-surface-areaeconomical modules Membranes range from porous structures to nonporous onesand can be asymmetric or symmetric They are used for removal from macropollutantsup to ions Because pressure-driven membranes consist of selective barriers pore char-acteristics are important Macrofiltration UF and NF processes are classified by theirpore structure (pore size pore size distribution and porosity) Hydrophilicity surfacecharge roughness etc should also be considered The membrane material has greatsignificance An appropriate membrane material should resist chemical and thermalattack and should be robust thin defect-free and cheap (Baker 2004 Fane Tangamp Wang 2011)
For membrane materials polymers are generally preferred because they areinexpensive and easily formpore structures In additionmetallic ceramic andcompositematerials are used These materials are now thought of as conventional Recently mem-brane materials have improved Some novel membrane materials exist such as nano-structured reactive bioinspired membranes (Ng Mohammad Leo amp Hilal 2010Pendergast amp Hoek 2011)
341 Conventional membrane materials
Conventional membranes include polymeric ceramic and thin-film composite (TFC)membranes in general
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 95
Polymers are by far the most popular membrane material used for membranefabrication A good polymer material should resist thermal and chemical attack andhave good mechanical strength and the ability to form flat-sheet or hollow-fiberconfigurations easily Mostly polymeric membranes are fabricated with the phaseinversion technique Commonly used polymeric membrane materials are cellulose ac-etate (Chou amp Yang 2005 Saljoughi Amirilargani amp Mohammadi 2010) polyviny-lidene fluoride (PVDF) (Han Xu Yu Wei amp Cao 2010 Li Liu Xiao Ma amp Zhao2012) polyacrylonitrile (PAN) polypropylene (PP) polyethersulfone (PES) (ChenLi Han Xu amp Yu 2010 Loh Wang Shi amp Fane 2011 Xu Leurou Zhen Ku amp Yi2008) and polysulfone (PS) (Ohya Shiki amp Kawakami 2009 Rugbani 2009) Poly-meric membranes have varying process uses they can be applied from MF to ROCommercial UFMF membranes can be fabricated from both hydrophilic and hydro-phobic polymers Both polymers have some pros and cons because hydrophilic mem-branes lower the attachment of bacteria owing to adsorption but they are less robustthan hydrophobic polymers Table 34 shows commonly used polymeric materialsand their properties
Cellulose acetate membranes have good permeability and rejection characteristicsare susceptible to hydrolysis have limited pH resistance and low chlorine toleranceand are resistant to fouling On the other hand PES PVDF PS and PAN can be modi-fied as a blend to improve their limited abilities They show good permeability flex-ibility and high strength PP polymer has limited blending capacity and is prone tooxidation (Fane et al 2011)
Ceramic membranes are better than polymeric membranes because of their narrowpore size distribution higher porosity separation characteristics mechanical and chem-ical stability and lower fouling Typical ceramic membranes are fabricated using thesol-gel process and are asymmetric in structure Generally materials used for ceramicmembranes are zirconia silica alumina mullite oxide mixtures and sintered metals(Harman Koseoglu Yigit Beyhan amp Kitis 2010 Hofs Ogier Vries Beerendonkamp Cornelissen 2011 Mohammadi amp Maghsoodloorad 2013 Pendergast amp Hoek2011) They are well-suited for water treatment because they have higher stabilitiesFlux decrease can be easily recovered after fouling because they can resist destructivecleaning conditions so their lifetime is extended However they are expensive inindustry and owing to their inexpensiveness polymeric membranes are preferred
TFC membranes are asymmetric in structure therefore support and selectivelayers can be independently selected for optimization they are generally fabricatedthrough interfacial polymerization or coating and then cross-linking By changingthe properties of cross-linking thin-film structure stability selectivity and perme-ability can be changed Generally used TFC membranes are polyurea polyamideand polyurea-amide (Lau Ismail Misdan amp Kassim 2012 Pendergast amp Hoek2011) The support part of the membrane can be fabricated from PES PS PANetc The film structure is important for TFC membranes By changing monomers sur-factants and additives within the film composition different types of structure can beobtained (Lau et al 2012) TFC membranes are used to obtain high flux and highselectivity in desalination Also these kinds of membranes are used to desalt brackishwater and for water reclamation water softening and removing dissolved organics
96 Advances in Membrane Technologies for Water Treatment
Table 34 Commonly used polymeric materials and their properties
Polymer material Specifications
Cellulose acetate Cellulose acetate and its blends are widelyused for MF and UF membranes Celluloseacetate is susceptible to hydrolysis andmicrobial attacks and has a stable pHbetween 4 and 65
Polysulfone Polysulfone is an amorphous polymer It hasexcellent chemical and thermal stability It ismainly used for UF and MF membranes andas a support layer for NF and ROmembranes Polysulfone membranes have awide range of pore sizes and their modulescan be capillary flat-sheet and hollow-fiber
Polyethersulfone Polyethersulfone membranes have highchemical and thermal stability They aremainly used for UF and MF Flat-sheet orhollow-fiber module configurations exist
Polyacrylonitrile Polyacrylonitrile possesses superior resistanceto hydrolysis and oxidation It is mainlyused to prepare UF membranes and poroussupports of composite membranes
Polyamide Polyamide has good thermal and mechanicalstability It is resistant to organic solventattacks However it is susceptible tochlorine attacks It is used as a thin layer forRO and NF
Polyimide Polyamides exhibit excellent thermal andchemical stability because of their high glasstransition temperature They are used tomake NF membranes
Polycarbonate Polycarbonate is a transparent thermoplasticwith high-performance properties It hasgood mechanical strength and can be used tomake UF and MF membranes by the phaseinversion process
Polyvinylidene fluoride Polyvinylidene fluoride is semicrystalline witha low glass transition temperature It isresistant to chemical and thermal attacks aswell as to most inorganic and organicsolvents It can tolerate a wide range of pH
Source Adapted from Fane et al (2011) and Gupta (2013)
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 97
342 Novel membrane materials
Novel membrane materials can be defined as nanotechnology based membranesThese materials can be zeolite-coated ceramic membranes reactivecatalytic mem-branes nanocomposite membranes which can be either mixed matrix or TFC andbiologically inspired membranes that are aquaporin vertically aligned carbon nano-tube or block copolymer membranes (Pendergast amp Hoek 2011)
Zeolites are naturally occurring alumina silicate minerals that have high uniformityand nanometer-scale crystallinity People prefer zeolite-coated ceramic membranesbecause by using them one can obtain permeability in the range of UF membranesand also selectivity in the range of NF or higher Typical zeolite membranes are fabri-cated by hydrothermal layer-by-layer crystallization etc synthesis methods withamorphous silicate aluminosilicate which is inert and has good thermal and chemicalstability Zeolite crystals are tetrahedral frameworks that consist of cross-linked (SiAl)O4 (Figure 34) (Georgiev Bogdanov Angelova Markovska amp Hristov 2009) Waterpermeability and ion selectivity are primarily affected by the ore and framework den-sity of zeolite particles On the other hand the SiAl ratio is the most important factorfor obtaining chemical stability and hydrophilic properties They can be applied toboth polymers and ceramic membranes (Pendergast amp Hoek 2011) Dahe Teotiaand Bellare (2012) fabricated nanozeolitepolysulfone composite membranes for UFpurposes They found that the addition of nanozeolite increased water permeabilityup to 22 Lm2 h bar and also increased the MWCO value from 9500 to 54000 Da
Other novel membranes are reactive catalytic ones Reactive catalytic membranesactivate semiconductor-based membranes by sunlight or UV to degrade organiccontaminants Semiconductors are characterized by a filled valence band and anempty conduction band When photon energy (hn) exceeds band gap energy an elec-tron leaps from the valence band to the conduction band (Figure 35) This excitedstate of bands can recombine or dissipate input energy as heat and can get trappedin metastable surface states or react with electro-donors and acceptors that areadsorbed on the surface of semiconductors or within the electrical double layer ofcharged particles If there is a surface defect this defect can trap an electron orhole so that the band cannot recombine subsequently a redox reaction mayoccur Valence bands are good oxidants whereas conduction bands are goodreductants For interactions in bulk semiconductors an electron or hole is available
HOxygen
Silicon or Aluminum
Si Al
Ondash
Figure 34 (Left) Chemical structure of zeolite (Right) Building unit of a zeolite structureReprinted from Georgiev Bogdanov Angelova Markovska and Hristov (2009)
98 Advances in Membrane Technologies for Water Treatment
nanomaterials allow highly efficient interactions owing to their surfaces Organicphoto-degradation requires oxidizing and reducing species This degradation couldbe by indirect or direct oxidation With semiconductors contaminants such asalkanes phenols polychlorinated biphenyls alkenes alkanes surfactants and pes-ticides as well as heavy metals such as chromium(VI) can be removed (HoffmannMartin Choi amp Bahnemannt 1995)
Titanium zinc oxide and ferric oxide are the most commonly used semi-conductor-based membranes (Chong Jin Chow amp Saint 2010) The performance of this kind ofmembrane for degrading contaminants and inactivating complex dense cell wall struc-tures is limited Also these types of membranes have a high cost and low packing den-sity (Pendergast amp Hoek 2011) Crock Rogensues Shan and Tarabara (2013)fabricated exfoliated graphite nano-platelets decorated with Au nanoparticles(Figure 36) The resulting membrane was catalytic with UF properties When nano-platelets and the amount of Au were changed different catalytic activities wereobtained Ma Zhang Quan Fan and Zhao (2010) fabricated photocatalytic MF mem-branes with Ag-TiO2hydroxyapatiteAl2O3 to treat surface water According to theirresults humic acid removal synergistically affected filtration and photocatalysis Theantifouling ability of the membrane comes mainly from photocatalytic activityZhu et al (2011) fabricated MF ceramic membranes combined with titaniumoxide and alumina with ozonation for water treatment They obtained a fluxes of79 Lm2 h and 90 Lm2 h with alumina and titanium oxide respectively
Nanocomposite membranes are advantageous because of their low cost easyfabrication and high mechanical stability and because they show both polymericand inorganic material properties Typically used nanoparticles are silver- magne-sium- iron- aluminum- silica- zirconium- and titanium-based as well as carbonnanotubes and graphenes Nanoparticles can be formed by sol-gel processes ion sput-tering spray pyrolysis pulsed laser ablation laser pyrolysis mechanical alloyingmill-ing electrodeposition and so forth (Buonomenna 2013 Goh Ismail amp Ng 2013Mishra amp Ramaprabhu 2011 Ng et al 2010 Pendergast amp Hoek 2011)
Reduction
hv
∆EEnergy
Oxidation
Conductionband
Valenceband
Figure 35 Schematic of photocatalysisReprinted from Pendergast and Hoek (2011) with permission from Royal Society of Chemistry
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 99
Many applications exist for these kinds of membranes in water treatmentapplications Nanoparticles used within these types of membranes enhanced mem-brane selectivity and pore connectivity inhibit macro void formation and increase sur-face to volume ratios Attention is given to nanoparticles because of their ability todisinfect adsorb and degrade organics (Akar Asar Dizge amp Koyuncu 2013 Koseo-glu Kose Altinbas amp Koyuncu 2013) In this manner for example silver nanopar-ticles can be used to decrease fouling and increase antibacterial properties RazmjouResosudarmo Holmes and Li (2012) studied the effects of TiO2 nanoparticles on PEShollow fiber (HF) membranes They used both mechanically modified and chemicallyand mechanically modified TiO2 Higher thermal resistance permeability hydrophi-licity porosity and pore size and lower elasticity and tensile strength were foundfor chemically and mechanically modified TiO2 Han et al (2010) spun hollow-fibermembranes by adding aluminum silica and titanium oxide nanoparticles at thesame dope solution with different concentrations They observed high flux valuesand high tensile strength values Bovine serum albumin (BSA) rejection changeddiscordantly Morphology changed from a macroporous to an asymmetric structureDenser top layers were obtained and the most obvious effect on membranemorphology was on the inner surface Liang Xiao Mo and Huang (2012) fabricateda novel ZnO nanoparticle-blended PVDF membrane for anti-irreversible fouling Theaddition of ZnO nanoparticle increased the hydrophilicity of the membranes and 100water flux recovery was observed Permeability values were doubled compared withpristine membranes In addition they found that adding ZnO increased the mechanicalstability of the membranes Sengur (2013) fabricated PES hollow-fiber membranes
2nd functional nanoparticles
1st ˝carrier mats˝exfoliated graphite nanoplatelets
Hierarchy level
Nanocomposite membraneMembrane casting mixture
PolymerPhase inversion
Hierarchicalnanofiller
Solvent
Porogen
ReactantsProducts
-15 microm -12 nm
-30 nm
Catalyticreaction
Figure 36 Catalytic nanocomposite membrane fabricated with exfoliated graphitenanoplatelets and Au nanoparticlesReprinted from Crock et al (2013) with permission from Elsevier
100 Advances in Membrane Technologies for Water Treatment
with UF properties with both hydroxylated and carboxylated multi-walled carbonnanotube (MWCNT) The effects of different MWCNTs were different Fabricatedcarboxylated MWCNT membranes had antifouling ratios and better water recoveryratios whereas hydroxylated MWCNT membranes had higher permeabilities Turken(2013) fabricated PESAg-nanocomposite UF hollow-fiber membranes According totheir results the membranes showed improved antibacterial properties antifoulingand mechanical stability
Thin-film nanocomposite (TFN) membranes are fabricated through interfacialpolymerization by incorporating nanomaterials into the active layer of TFC mem-branes via doping into casting solutions or by surface modification their advantagesare enhanced separation reduced fouling antimicrobial activity and some other novelproperties For TFN membranes the particles most used are zeolites carbon nano-tubes TiO2 silica and silver (Buonomenna 2013 Kim Hwang El-Din amp Liu2012 Pendergast amp Hoek 2011 Qu Alvarez amp Li 2013) Tiraferri Vecitis andElimelech (2011) used covalently bonded single-walled carbon nanotubes (SWCNT)to a TFC membrane With that method they used a tiny amount of nanomaterials Theresulting membrane had improved antibacterial properties and potentially decreasedfouling In the literature TFN membranes are generally used for RO so there is littleliterature containing NF UF or MF applications
On the other hand biologically inspired membranes offer huge potential andimproved membrane properties However they are far from commercialization becausethere are some difficulties in scaling Aquaporin-based membranes one of the biolog-ically inspired membranes are highly productive and selective but not cost-effectiveAquaporins are protein channels that have the ability to control water flux within mem-branes Their ability to transport to water is higher than any kind of commercial ROmembrane Their water permeability is in the order of glucose glycerol salt andurea (Pendergast amp Hoek 2011 Qu et al 2013 Tang Zhao Wang Heacutelix-Nielsenamp Fane 2013 Zhong Chung Jeyaseelan amp Armugam 2012) Generally they areapplied in desalination and water reuse Table 35 shows studies from the literature
Another kind of biologically inspired membrane is vertically aligned carbonnanotubes (CNTs) (Figure 37) Water transport within CNTs is as high as aquaporinsSelectivity of the vertically aligned CNT membranes is described in Corry (2008)CNTebased membranes also have excellent mechanical properties Aligned CNTsare generally fabricated by chemical vapor deposition (CVD) such as microwaveplasma-enhanced CVD catalytic CVD spray pyrolysis and self-assembly (Pender-gast amp Hoek 2011) Vertically aligned CNT membranes can be used to remove micro-bial contaminants from drinking water They are superior to conventional membranesbecause they can be cleaned by ultrasonication or autoclaving whereas conventionalmembranes should be disposed of at the end of their limited life which is affected byfouling (Pendergast amp Hoek 2011)
The last type is block copolymer membranes These are macromolecules thathave the ability to self-assemble into highly ordered structures when added intothe proper solvent Their densely packed cylindrical pores are ideal for waterseparation In the literature there are many studies using block copolymers tofabricate UF membranes Wandera Himstedt Marroquin Wickramasingh and
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 101
Table 35 Examples of biomimetic membranes designed for water reuse and desalination Performance dataare presented as water permeability (WP) (lm2 h bar) NaCl rejection (RNaCl) () membrane area (A)(cm2) and maximal external pressure applied (Pmax) (bar) when operated in RO CA cellulose acetatePC polycarbonate
Approach WP (lm2 h bar) RNaCl () Area (cm2)Pmax
(bar) Upscaling issues Remarks
Charged lipid mixturevesicles depositionsonto NF membranes
083 nda 35 10 Difficult to producelarge defect-freemembranes
No aquaporinincluded
Vesicle fusionfacilitated byhydraulic pressure onhydrophilic NFmembranes coatedwith positivelycharged lipids
36 02 35 8 126 1 Difficult to producelarge defect-freemembranes
Low RNaCl Onlysuitable for NF
Membranes acrossmultiple micron-scaleapertures asfreestanding lipid orpolymer membranes
nda nda 4 nda Nanofabricationrequired Lowrobustness
WPRNaCl not testedNot suitable forRO
Membranes acrossmultiple micron-scaleapertures andstabilized byhydrogelencapsulation
12e40 nda 35 2 Nanofabricationrequired Highrobustness
Characterization withgramicidinchannels Noaquaporinincluded
102Advances
inMem
braneTechnologies
forWater
Treatm
ent
Aquaporin containingpolymersomes onmethacrylatefunctionalized CAmembranes
342 69 329 91 007 5 Medium robustness Small area High WPbut low RNaClOnly suitable forNF
Detergent-stabilizedHis-tagged aquaporinadded to monolayerswith nickel-chelatinglipids
nda nda nda nda Complex fabricationLow robustness
WPRNaCl not testedMay be notsuitable fordesalination
Proteopolymersomedeposition onto gold-functionalized PCtrack-etchedsubstrates
nda nda 0096 nda Complex fabricationLow robustness
Small areaRelatively highWP in FO No ROdata
Interfacialpolymerizationmethod withembeddedproteoliposomes
4 04 963 13 gt200 14 Simple fabricationHigh robustness
Combined high WPand RNaClSuitable for RO
and not definedSource Adapted from Tang et al (2013) Copyright (2013) with permission from Elsevier
Advances
inwater
treatment
bymicro
filtrationultrafiltrationand
nanofiltration
103
Husson (2012) modified low-MWCO cellulose UF membranes with block copol-ymer nanolayers They used poly(N-isopropylacrylamide)-block-poly([polyethyleneglycol] methacrylate) (PNIPAAm-b-PPEGMA) nanolayers and investigated therole of polymer on the performance of the membrane According to their resultsthe nanolayer structure affected fouling Optimization resulted in high stable fluxThe coating chemistry and structural properties of coating affected antifouling prop-erties Fabricated Ps-b-PMMA films showed unique porous morphology (Figure 38)(Kim Yang Lee amp Kim 2010) Nanopores were observed only at the bottom andtop membrane surfaces The structure had high resistance to all organic solvents andshowed great dimensional stability after filtration tests In another study (Zhao SuChen Peng amp Jiang 2011) the block-like copolymer poly(butyl methacrylate)-b-poly(methacrylic acid)-b-poly(hexafluorobutyl methacrylate) (PBMA-b-PMAA-b-PHFBM) material which has hydrophobic hydrophilic and fluorine-containingsegments was used Copolymer was added into the PES membrane matrix Accord-ing to the results the membrane showed excellent pH-responsive permeability anddesirable fouling release properties To increase the hydrophilicity of PS membranesRoux Jacobs van Reenen Morkel and Meincken (2006) used branchedPEO-block-PS copolymers and incorporated them into the membrane matrix Fluxand rejection data showed that the membrane morphology was changed after itwas modified with copolymer Figure 39 compares conventional and novel-basedmembranes
Figure 37 Carbon nanotube membrane simulationReprinted from Corry (2008) with permission from American Chemical Society
104 Advances in Membrane Technologies for Water Treatment
Figure 38 Scanning electron microscopy image for block copolymer film with mixedorientationReprinted from Kim Yang Lee and Kim (2010) with permission from Elsevier
ZeoliteTFNs
NanoparticleTFNs
NanostructuredceramicmembranesInorganic-organicmembranesBiologicallyinspiredmembranes
Blockcopolymers
Reactivecatalyticsurfaces
Mixedmatrices
Zeoliticfilms
Conventionalmembranes
Potential commercial viability
Pote
ntia
l per
form
ance
enh
ance
men
t
Aquaporins
Alignedcarbon nanotubes
Far
None
Evolutionary
Revolutionary
Near Immediate
Figure 39 Conventional and novel membrane current statusReprinted from Pendergast and Hoek (2011) with permission from Royal Society of Chemistry
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 105
35 Advances in membrane modules and systemconfigurations for water treatmentby MF UF and NF
Industrial membrane plants require a large amount of membrane area to performseparation on a useful scale If membranes are to be used on an industrial scale effi-cient packaging methods are required These packages are called membrane modulesBetween 1960 and the 1970s commercial membrane processes had a significant break-through with the development of low-cost membrane modules The earliest designswhich were tubular plate and frame hollow-fiber and spiral wound were mainly builton basic filtration technology including flat sheet membranes in a type of filter pressBecause of this these systems are called plate-and-frame modules At the same time1- to 3-cm-diameter tubes were developed These designs are still used but becauseof their relatively high cost in most applications two other designs have been largelydisplaced the spiral wound and hollow fiber modules Conventional membranemodules can be seen in Table 36
351 Recent advances in membrane modules
Liu Xu Zhao and Yang (2010) carried out research using a helical membrane moduleunit A terylene filter cloth without modification and with a pore size around 22 nm hadbeen used in the filtration test as a filter membrane Two pieces of membrane sheets(6 18 2 cm total area 0022 m2) were supported on a plastic spacer The fishboneor broom-like structured spacer is shown in the middle in Figure 310(a-1) and (a-3)resembling a deoxyribonucleic acid helix (Figures 310(a-4)) The cover membraneforms an empty sleeve outside the spacer once it is sealed around the edge(Figure 311) It has a tubing outlet at one end of the membrane to conduct thepermeate water The plastic spacer is flexible for helical rotation and its final total rota-tion angle together with the membrane sleeve can be kept at a certain value by fixingthe top and bottom edges The top end of the membrane can be fixed at a final angleto the bottom end while maintaining a certain helical rotation Several short pieces ofhelical membrane or extensions of membrane length can form modules with morehelixes (Figure 310(a-1))
Table 36 Membrane and module forms
Tubular Flat
Conventional membranemodules
Tube module Spiral-wound module
Capillary module Cushion module
Hollow-fiber module Plate module
Disc-tube module
106 Advances in Membrane Technologies for Water Treatment
Figure 310 Helical membrane module support spacer and final helical anglesReprinted from Liu et al (2010) with permission from Elsevier
Cross view of
the module
Mounting
of the
module in
filtration
chamberb
Water
permeation
Spacer
Permeate
Cross flow
Shearing
force
from
bubbles
Membrane
peeled openHold the two pieces
together with
thread
Seal with glue
Figure 311 Cross-view of helical membrane and its mount in a filtration chamberReprinted from Liu et al (2010) with permission from Elsevier
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 107
The long and rectangular new membrane modules are narrower thinner and lighterthan conventional wide flat-sheet membrane modules The helical membrane mayenhance vortex mixing just like a static stirrer in liquid phase Liu et al (2010) showedthat the higher the cross-flow the better the mixing
3511 Rotating disk module systems
Rotating disk module systems including a larger area and capacity module led to theintroduction of multi-disk systems mounted on a single shaft and rotating betweenfixed circular membranes This module is used for biotechnological applications andhas an area of 2 m2 Dyno manufactured by Bokela GmbH (Karlsruhe Germany)has a similar system design This system has a diameter of 137e85 cm and a totalmembrane area from 013 to 12 m2 This system is available with both polymericand ceramic membranes (Figure 312)
Another multi-disk system is produced by Spintek (Huntington CA) with rotatingmembranes of 23 m2 Initially it was available with organic membranes currently it isproduced with mineral membranes (Figure 313)
In Germany multi-shaft systems with overlapping rotating membranes per modulearea are common For example the multi-shaft disk (MSD) system has a diameter of312 cm in several ceramic membrane units All rotating disks turn at the same timethe Hitachi model (Japan) is available at up to a 150- and 100-m2 membrane area atparallel axes at the same time (Figure 314)
3512 Vibrating module systems
Vibrating module systems use an original concept called Vibratory Shear EnhancedProcessing System (VSEP) including a set of circular organic membranes separatedby gaskets and permeate collectors (Figure 315) The most important parameter isthe maximum azimuthal displacement of the membrane rim (Jaffrin 2008)
Figure 312 Dyno rotating disk module (Bokela Germany)Reprinted from Jaffrin (2008) with permission from Elsevier
108 Advances in Membrane Technologies for Water Treatment
Figure 314 Industrial multi-disk shaft module with eight parallel shafts and 31-cm ceramicdisksCourtesy of Westfalia Separator Reprinted from Jaffrin (2008) with permission from Elsevier
Figure 313 Spintek moduleReprinted from Jaffrin (2008) with permission from Elsevier
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 109
36 Applications of water treatment by MF UF and NF
Typically there are four main water sources for drinking and potable water supply sur-face water groundwater seawater and rainwater Different membrane technologiesare applicable to these sources because of their chemical and physical characteristicsFigure 316 shows a schematic describing water sources and possible membrane stepsto safe water For instance surface water can be converted into safe water using an MFUF MF plus UF plus an auxiliary or NF membrane Groundwater has generally highhardness and metal concentrations so MF and UF will not be efficient alone They needan auxiliary system
361 Applications for surface water treatment
Macrofiltration UF and NF membranes can be used in surface water treatmentalone or with an auxiliary system They are becoming popular in surface watertreatment because of their high efficiency Macrofiltration and UF process are effec-tive in removing turbidity particles bacteria and cysts If a high organic content ofwater exists MF and UF may not be used to obtain high water quality On the otherhand the NF process is able to treat many water contaminants such as DBPs andsynthetic organic compounds (Glucina Alvarez amp Laicircneacute 2000) Table 37 listsstudies concerning the use of the MF UF and NF membrane process to treatsurface water
362 Membranes for groundwater treatment
Owing to limited rainfall in some places surface water is scarce In these types of pla-ces groundwater sources become important Although they are variable in nature
Figure 315 Industrial VSEP vibrating modulesReprinted from Jaffrin (2008) with permission from Elsevier
110 Advances in Membrane Technologies for Water Treatment
groundwater sources generally contain calcium magnesium carbonatealkalinity andsilica If these ion concentrations reach high concentrations that exceed mineral solu-bility limits precipitation starts This precipitation forms deposits at the end andscales Scale formation can be problematic To solve this problem membrane technol-ogies are effective (Kinsela Jones Collins amp Waite 2012) Table 38 summarizesstudies applying MF UF and NF to groundwater treatment
363 Nanofiltration for seawater desalination
MWCO values of NF membranes are between 200 and 2000 Da They reject 98 ofdivalent ions and 30e85 of monovalent ions NF membranes have higher perme-ability than RO membranes which gives them an advantage in energy consumptionA flow diagram of dual-step NF is given in Figure 317 There is disagreement
Nanofiltration
Drinking water
MF+UF+auxillary
Surfacewater
MF+UF
Rainharvesting
Seawater
Groundwater
Figure 316 Water sources and applicable membrane steps to achieve potable water
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 111
Table 37 Studies related to microfiltration (MF) ultrafiltration (UF) and nanofiltration (NF) for thetreatment of surface waters
Process used and module type Approach Remarks References
Integrated membrane system wasused (UF thorn RO) spiral-woundand hollow-fiber configuration
Low-salinity water was used toevaluate efficiency of IMS
UF process found to be effective in controllingRO fouling UF reduced SDI
Glucina et al (2000)
MF and UF tubular and ceramicmembranes
Membrane permeability andfouling properties of ceramicand polymeric membraneswere compared
Increase TMP rate owing to fouling inverselyfollows the measured pore size Higherdegree of fouling on the polymericmembranes results from the lower volumearea ratio Aggressive methods are neededto clean non-purgeable organic carbon
Hofs et al (2011)
NF capillary and flat-sheetmembranes
New capillary NF membranewas compared with flat-sheetmembranes to find foulingproperties and pretreatmentrequirements
In capillary membranes flux decline can beincreased by forward flushing and airflushing Water permeability of capillarymembranes was 3e15 times higher thanflat-sheet membranes
Van der BruggenHawrijkCornelissen andVandecasteele(2003)
UF-MF Capillary membranes Application of UF polymericmembranes in the disinfectionand treatment of surfacewaters was investigated
Membranes were able to remove ironturbidity bacteria Escherichia coli organiccompounds and mesophilic bacteria
Konieczny (1998)
UF Carbosep M8 tubularinorganic membrane
Different kinds of surface waterswere tested for disinfection byUF membranes Differenthardnesses and organic loadswere tested
UF was a reliable method for disinfectingsurface waters
Di Zio Prisciandaroand Barba (2005)
112Advances
inMem
braneTechnologies
forWater
Treatm
ent
PVDF MF PVDF and PES UF Effects of different magnetic ionexchanges (MIEXs)pretreatment on low-pressuremembrane filtration wereinvestigated by using differentfeed water qualitiesmembrane properties andMIEX doses
Decrease in total and irreversible fouling wasobserved for PVDF MF and PES UF afterMIEX treatment Fouling of PVDF UFmembrane was not decreased after MIEXtreatment which showed the importance ofmembrane properties MIEX treatment waseffective
Huang Cho Schwaband Jacangelo(2012)
UF rotating membrane diskmodule
Irreversible fouling caused byconstituents in surface waterinvestigated
Evolution of irreversible fouling was due topolysaccharide-like organic matterpresumably iron and manganese contributedto some extent
Kimura HaneWatanabe Amyand Ohkuma(2004)
MFUF Irreversible membrane fouling inan MF membrane wasinvestigated Feed watercontaining humic acid ororganic matter was coagulatedwith polyaluminum chloride
Coagulation conditions (dosage) wereeffective for irreversible fouling Acidicconditions enhanced the treated waterquality and increased irreversible fouling
Kimura MaedaYamamura andWatanabe (2008)
NF UF MF spiral wound Efficiency of NF modules for therejection of DBPs from low-turbidity surface waters wasinvestigated
Dissolved organic carbon (DOC)trihalomethane formation potential(THMFP) haloacetic acid formationpotential (HAAFP) and chloral hydrateformation potential (CHFP) rejections werehigher than 86 in NF MF was moderatelyeffective in particle removals UF did notshow significant changes in operationalconditions
Siddiqui Amy Ryanand Odem (2000)
Continued
Advances
inwater
treatment
bymicro
filtrationultrafiltrationand
nanofiltration
113
Table 37 Continued
Process used and module type Approach Remarks References
NF flat-sheet Study aimed to treat surfacewater contaminated withpesticides
NF membranes were able to reduce hardnessCOD and TOC and remove microbialcontent completely
Sarkar et al (2007)
UF hollow-fiber Performance of UF membraneswas investigated by using rawwater from the Bin XianReservoir
Before UF use of coagulation increasedpermeate flux and retarded membrane fluxdecline UF was able to remove turbiditycoliform bacteria iron manganese andaluminum
Xia Nan Liu and Li(2004)
UF hollow-fiber PVDF UF membranes weretested for the treatment ofsurface waters
Permeate quality was stable regardless ofdifferent surface waters TMP should bebelow 1 bar to decrease irreversible fouling
Guo Zhang Fangand Su (2009)
114Advances
inMem
braneTechnologies
forWater
Treatm
ent
concerning whether it is possible to desalinate seawater by using dual-step NF at alower cost than RO In the dual-step NF process seawater is fed to the first NF mem-brane and then the collected permeate is fed to the second NF membrane Finallypotable water is obtained Concentrates are pumped into the NF feed tank optionallyThe energy requirement for seawater desalination using dual-stage NF is proportionalto the salinity of the permeate More energy is needed for lower permeate salinity(AlTaee amp Sharif 2011)
Table 38 Microfiltration (MF) ultrafiltration (UF) andnanofiltration (NF) groundwater applications
Process used andmodule type Approach Remarks References
Electro-ultrafiltration(EUF)
Effectiveness ofEUF system wasdetermined forarsenic removal
Arsenic removalincreased from1 to 14 toover 79
Hsieh WengHuang and Li(2008)
NF piperazinemembrane
Different hardnessand salinityvalues were used
High hardnessretentioncoefficients(70e76)satisfactorypermeate fluxesand high mineralfouling resistancebut low salinityremoval wereobtained
GalanakisFountoulis andGekas (2012)
NF polyamidemembrane
Ozonation slowsand filtrationand NF processeswere combined toremove dissolvedorganic matter(DOM) fromgroundwater
NF was able toremove aromatichumic acid andfulvic acid-likesubstances andDOM
Linlin Xuan andMeng (2011)
NF flat-sheetmembrane module
For fluorideremoval fromcontaminatedgroundwater NFmoduleperformance wasmodeled andsimulated
NF membrane wasable to removefluoride and dropthe level of pH ofgroundwater tothe desired level
Chakrabortty Royand Pal (2013)
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 115
364 Membranes for harvesting rainwater
Harvested rainwater is preferred for irrigation purposes On the other hand there isgrowing interest in harvesting rainwater for drinking and other indoor uses Rainwatercan be filtered by MF UF and NF according to the desired quality MF treatment issuitable for gray water production UF and NF treatment can be used for the drinkingwater supply Home size water treatment units can be used in drinking water produc-tion UF provides a good option for disinfection to remove viruses and bacteriaA number of companies supply UV granulated activated carbon and membranefiltration systems for rainwater purification
365 Applications of membranes for specific contaminantremoval
Arsenic (As) exists in natural water sources in both organic and inorganic forms Long-term exposure to arsenic causes cancer The World Health Organization limits thepresence of As in water to less than 001 mgL Removal of As with membranes is suc-cessful and is popular nowadays There is both pilot and lab-scale research in the liter-ature Nguyen Vigneswaran Ngo Shon and Kandasamy (2009) used both MF andNF NF membranes are more efficient for the removal of As than MF membraneswhich have bigger pores Sato Kang Kamei and Magara (2002) also evaluated Asremoval efficiency using NF and a rapid sand filtration inter-chlorination systemNF membranes with 996 NaCl rejection capacity removed 95 of As(V) and75 of As(III) with no chemical addition under low pressure Floch and Hideg(2004) conducted a study with 200e300 mgL As containing deep well water UsingZenon ZW 1000 hollow-fiber membranes they achieved As concentration less than10 mgL
Pesticides are used to control pests such as weeds and insects They are beneficial inpreventing disease and are used for food production Unfortunately they have anadverse effect on water quality and may have negative effects on human and aquatic
Figure 317 Dual-stage desalination processReprinted from AlTaee and Sharif (2011) with permission from Elsevier
116 Advances in Membrane Technologies for Water Treatment
life Chen Taylor Mulford and Norris (2004) proved that NF membranes can rejectpesticides from 46 up to 100 They found that the rejection rate is directly propor-tional to the molecular weight (MW) of the pesticide When the MW increased therejection rate also increased They also revealed rejection could be increased by adjust-ing the operational flux and recovery Van der Bruggen Schaep Maes Wilms andVandecasteele (1998) researched the removal of pesticides with membrane filtrationsystems that showed good efficiency of membranes in pesticide removal They triedNF membranes and obtained approximately 95 rejection of arsenic with an NF-70membrane
Disinfection is used in the final step of drinking water treatment to deactivate bio-logical agents However NOM in water reacts with disinfectants and causes DBPs toform such as THMs halogenated acetic acids (HAAs) haloacetonitriles (HANs) andhaloketones (HKs) which are detrimental to human health Lee and Lee (2007) usedNF membranes to remove NOMs from surface water and researched the effect ofmembrane pretreatment and membrane material They found that membraneswith an MWCO less than 200 Da were better for controlling NOM in surface waterHydrophobic and positively charged membranes fouled more than hydrophilic andnegatively charged membranes They also revealed that ozone pretreatment wasinefficient for preventing NF fouling but powdered activated carbon (PAC) or UFpretreatment was efficient
Pharmaceuticals have gained attention recently owing to their effects on biologicalactivities Pharmaceutical emissions to the environment should be controlled Radje-novic Petrovic Ventura and Barcelo (2008) studied the removal of pharmaceuticalsin a full-scale NF and RO drinking water treatment plant using groundwater An 85rejection rate was obtained using NF and RO Boleda Galceran and Ventura (2011)studied pharmaceuticals in a full-scale drinking water treatment plant treatment con-sisted of dioxychlorination coagulationflocculation and sand filtration units Aftersand filtration the system was split into two parallel lines conventional (ozonationand carbon) and advanced (UF and RO) treatment A flow diagram of the plant isshown in Figure 318 To remove pharmaceuticals an advanced system was moreeffective than a conventional system
RemineralizationReverseosmosisUVUltrafiltration
CoagulationFloculationSettling
Sandfiltration
Groundwater
Blending Finishedwater
CI2Ozonization GAC
filtration
Riverwater
CIO2
Figure 318 Treatment scheme of drinking water treatment plantReprinted from Boleda et al (2011) with permission from Elsevier
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 117
37 Future trends
Water has been a main problem of mankind for years and will continue to be problem-atic Membrane technology has a significant role in the water industry In the futurenovel membrane materials processes and modules will be used so that we cancome closer to obtaining an optimum membrane structure Membrane technologyhas some shortcomings such as higher energy requirements and fouling Todayrsquos tech-nology related to research on this subject consists of fabricating new membrane mate-rials designing modules calculating hydrodynamics and finding new operationmodes to decrease energy requirements or application setups to treat water or waste-water more effectively Future trends will be the reuse of process waters recoveryof valuable compounds creation of new technologies such as pervaporation and for-ward osmosis monitoring of fouling in real time improvement of new fouling analysismethods and fabrication of tailored novel membranes and developments ofmembranes can be used under extreme conditions
As these aims are realized cost capacity and selectivity should optimized and theirenvironmental impact such as concentrate handling and minimized chemical con-sumption should also be considered
As novel membrane materials are investigated the future lies in block copolymersand biomimetic and patterned membranes such as vertically aligned CNs Thesematerials show great potential because they have greater permeability rates and higherselectivity however commercialization is still a problem If they are commercializedin near future we will obtain lower energy consumption and thus the treatment cost ofpollutants will also decrease The ultimate challenge is extreme conditions such as hightemperatures On this point developments in ceramic membranes are promising(Abetz et al 2006 Url-5 Url-6)
Another problem is housing the membranes Hollow-fiber and spiral wound mod-ule geometry are the main membrane housings as already mentioned in this chapterMembrane module design focuses on mechanical considerations such as pressure butalso on how to limit concentration polarization According to studies on membranesurface there is a valley that causes roughness on membranes surface because ofthat a concentration of rejected species occurs To resolve these problems futureresearch will need to resolve them with fluid mechanical and mass transfer ap-proaches Also designing new membrane modules can decrease the cost and energyrequirements of the membranes For example when the transmembrane pressure islow high fluxes can be obtained Using novel membrane module technologies suchas vibrational modules can decrease the effect of fouling on the membrane surface(Fane et al 2011 Url-7)
Membrane processes are also significant With an improvement in membrane pro-cesses and materials it will become easier to use some new applications for instancepervaporation forward osmosis and new MBR systems Anaerobic MBRs use anaer-obic bacteria to degrade organic substances In this system it is possible to use biogasinstead of air in the submerged reactor Anaerobic MBR systems are better thanconventional systems because of their lower energy consumption Anaerobic MBR
118 Advances in Membrane Technologies for Water Treatment
systems are promising because they can sustain high organic loadings high biomassconcentration can be maintained energy and organic acid are recovered and sludgeproduction is low Another new technology is microbial fuel cells which are a noveltype of MBR Decentralized treatment systems can be used to lower cost and increasesanitation and reuse in wastewater systems (Abetz et al 2006 Fane et al 2011)
In this century great challenges will need to be overcome such as energy waterand a clean environment Membrane technology and its development have an impor-tant role in solving these problems efficiently with new research and development andinnovations in process designs
38 Sources of further information and advice
In the future we will see membranes much more in the water treatment sector Highperformance and low fouling membranes are the main targets in all membrane researchdevelopments and applications Membrane materials module configurations and pro-cesses are becoming more important for improving membrane system performance anddecreasing membrane fouling They will have a critical role in the future performance ofmembranes Interdisciplinary studies should be increased to improve performance basedon materials configurations and processes
39 Conclusion
This chapter summarizes MF UF and NF membranes for water treatment New mem-brane materials have improved membrane technology by enhancing permeability andselectivity As a result of these improvements in membrane performance concentra-tion polarization and fouling phenomena have become important factors in deter-mining membrane performance Membrane modules improve fluid flow so thatfouling and concentration polarization problems decrease In this chapter novel mem-brane materials and module design to increase membrane performance were summa-rized Applications of MF UF and NF membranes were also provided They may beapplied to treat surface water groundwater and seawater desalination and to harvestrainwater Many specific contaminants can be removed efficiently with thesemembrane processes
List of acronyms
AMBR Anaerobic membrane bioreactorCA Cellulose acetateCAGR Compound annual growth rateCHFP Chloral hydrate formation potentialCNT Carbon nanotube
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 119
COD Chemical oxygen demandCVD Chemical vapor depositionDBP Disinfection by-productsED ElectrodialysisEDR Electrodialysis reversalFO Forward osmosisGAC Granulated Activated CarbonHAA Halogenated acetic acidsHAAFP Haloacetic acid formation potentialHAN HaloacetonitrileHK HaloketoneIC In-line coagulationIMS Integrated membrane systemMBR Membrane bioreactorMCF Membrane cartridge filtrationMSD Multi-shaft diskMF MicrofiltrationMIEX Magnetic ion exchangeMWCNT Multi-walled carbon nanotubesMWCO Molecular weight cutoffNF NanofiltrationNOM Natural organic matternZVI Nano zero valent ironPAC Powdered activated carbonPAN PolyacrylonitrilePBMA-B-PMAA-B-PHFBM Poly(butyl methacrylate)-b-poly(methacrylic
acid)-b-poly(hexafluorobutyl methacrylate)PC PolycarbonatePES PolyethersulfonePI PolyimidePNIPAAm-B-PPEGMA poly(N-isopropylacrylamide)-block-poly([polyethylene
glycol] methacrylate)PP PolypropylenePS PolysulfonePS-B-PMMA Polysulfone-b-polymethylmethacrylatePVDF Polyvinylidene fluorideRO Reverse osmosisSDI Silt density indexSWCNT Single-walled carbon nanotubeTDS Total dissolved solidsTFC Thin-film compositeTFN Thin-film nanocompositeTHM TrihalomethanesTHMFP Trihalomethane formation potentialTMP Transmembrane pressureTOC Total organic carbonUF UltrafiltrationUV UltravioletWP Water permeability
120 Advances in Membrane Technologies for Water Treatment
List of symbols
A Membrane areaDa DaltonE Band gap energyhn Photon energyJ Membrane fluxDP Applied pressure
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AlTaee A amp Sharif A O (2011) Alternative design to dual stage NF seawater desalinationusing high rejection brackish water membranes Desalination 273 391e397 httpdxdoiorg101016jdesal201101056
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Carroll T King S Gray S R Bolto B A amp Booker N A (2000) The fouling ofmicrofiltration membranes by NOM after coagulation treatment Water Research 342861e2868 httpdxdoiorg101016S0043-1354(00)00051-8
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Chen S Taylor J Mulford L A amp Norris C D (2004) Influences of molecular weightmolecular size flux and recovery for aromatic pesticide removal by nanofiltration mem-branes Desalination 160 103e111 httpdxdoiorg101016S0011-9164(04)90000-8
Chong M N Jin B Chow C W K amp Saint C (2010) Recent developments in photo-catalytic water treatment technology A reviewWater Research 44 2997e3027 httpdxdoiorg101016jwatres201002039
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Debik E Kaykioglu G Coban A amp Koyuncu I (2010) Reuse of anaerobically andaerobically pre-treated textile wastewater by Uf and Nf membranes Desalination256(1e3) 174e180 httpdxdoiorg101016jdesal201001013
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Eykamp W (1995) Microfiltration and ultrafiltration In R D Noble amp S A Stern (Eds)Membrane separation technology Principles and applications Amsterdam ElsevierISBN 0 444 81633 X
Fane A G Tang C Y amp Wang R (2011) Membrane technology for water Microfiltrationultrafiltration nanofiltration and reverse osmosis Reference Module in Earth Systems andEnvironmental Sciences Treatise on Water Science 4 301e335 Water-Quality Engi-neering httpdxdoiorg101016B978-0-444-53199-500091-9
Floch J ampHidegM (2004) Application of ZW-1000membranes for arsenic removal fromwatersources Desalination 162 75e83 httpdxdoiorg101016S0011-9164(04)00029e3
Galanakis C M Fountoulis G amp Gekas V (2012) Nanofiltration of brackish groundwaterby using a polypiperazine membrane Desalination 286 277e284 httpdxdoiorg101016jdesal201111035
122 Advances in Membrane Technologies for Water Treatment
Georgiev D Bogdanov B Angelova K Markovska I Hristov Y (2009) Synthetic zeolites -structure clasification current trends in zeolite synthesis review International ScienceConference 4e5 June Stara Zagora Bulgaria
Glucina K Alvarez A amp Laicircneacute J M (2000) Assessment of an integrated membrane systemfor surface water treatment Desalination 132 73e82 httpdxdoiorg101016S0011-9164(00)00137e5
Goh P S Ismail A F amp Ng B C (2013) Carbon nanotubes for desalination performanceevaluation and current hurdles Desalination 308 2e14 httpdxdoiorg101016jdesal201207040
Guigui C Rouch J C Durand-Bourlier L Bonnelye V amp Aptel P (2002) Impact ofcoagulation conditions on the in-line coagulationUF process for drinking water produc-tion Desalination 147 95e100 httpdxdoiorg101016S0011-9164(02)00582e9
Guo X Zhang Z Fang L amp Su L (2009) Study on ultrafiltration for surface water by apolyvinylchloride hollow fiber membrane Desalination 238 183e191 httpdxdoiorg101016jdesal200711064
Gupta V K (2013) Water treatment by membrane filtration techniques Environmental Waterhttpdxdoiorg101016B978-0-444-59399-300005-2
Han L F Xu Z L Yu L Y Wei Y M amp Cao Y (2010) Performance of PVDFmul-tinanoparticles composite hollow fibre ultrafiltration membranes Iranian Polymer Journal19(7) 553e565
Harman B I Koseoglu H Yigit N O Beyhan M amp Kitis M (2010) The use of ironoxide-coated ceramic membranes in removing natural organic matter and phenol fromwaters Desalination 261 27e33 httpdxdoiorg101016jdesal201005052
Hilal N Al-Zoubi H Darwish N A Mohammad A W amp Abu Arabi M (2004)A comprehensive review of nanofiltration membranes Treatment pretreatment modellingand atomic force microscopy Desalination 170 281e308 httpdxdoiorg101016jdesal200401007
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Hsieh L C Weng Y Huang C amp Li K (2008) Removal of arsenic from groundwater byelectro-ultrafiltration Desalination 234 402e408 httpdxdoiorg101016jdesal200709110
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Jaffrin M Y (2008) Dynamic shear-enhanced membrane filtration A review of rotating disksrotating membranes and vibrating systems Journal of Membrane Science 324 7e25httpdxdoiorg101016jmemsci200806050
Judd S J amp Hillis P (2001) Optimisation of combined coagulation and microfiltration forwater treatment Water Research 35(12) 2895e2904 httpdxdoiorg101016S0043-1354(00)00586-8
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Kimura K Maeda T Yamamura H amp Watanabe Y (2008) Irreversible membrane foulingin microfiltration membranes filtering coagulated surface water Journal of MembraneScience 320 356e362 httpdxdoiorg101016jmemsci200804018
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Konieczny K (1998) Treatment of natural waters using capillary membranes Polish Journal ofEnvironmental Studies 7(4) 213e220
Koseoglu D Y Kose B Altinbas M amp Koyuncu _I (2013) The production of polysulfone(PS) membrane with silver nanoparticles (AgNP) Physical properties filtration perfor-mances and biofouling resistances of membranes Journal of Membrane Science 428620e628 httpdxdoiorg101016jmemsci201210046
Koyuncu I Arikan O Wiesner M R amp Rice C (2008) Removal of hormones and anti-biotics by nanofiltration membranes Journal of Membrane Science 309 94e101 httpdxdoiorg101016jmemsci200710010
Koyuncu _I amp Cakmakci M (2010) Nanofiltration for water and wastewater treatmentNanotechnology in water treatment applications Horizon Press ISBN 978-1-904455-66-0
Kruithof J C Nederlof M M Hoffman J A M H amp Taylor J S (2004) Integratedmembrane systems Elbert CO AWWA Research Foundation and American Water WorksAssociation
Kurt E Koseoglu D Y Dizge N Chellam S amp Koyuncu _I (2012) Pilot-scaleevaluation of nanofiltration and reverse osmosis for process reuse of segregatedtextile dyewash wastewater Desalination 302 24e32 httpdxdoiorg101016jdesal201205019
Lau W J Ismail A F Misdan N amp Kassim M A (2012) A recent progress in thin filmcomposite membrane A review Desalination 287 190e199 httpdxdoiorg101016jdesal201104004
Lee S amp Lee C (2007) Effect of membrane properties and pretreatment on flux and NOMrejection in surface water nanofiltration Separation and Purification Technology 56 1e8httpdxdoiorg101016jseppur200701007
Li X Liu H Xiao C Ma S amp Zhao X (2012) Effect of take-up speed on polyvinylidenefluoride hollow fiber membrane in a thermally induced phase separation process Journal ofApplied Polymer Science httpdxdoiorg101002app37919
124 Advances in Membrane Technologies for Water Treatment
Liang S Xiao K Mo Y amp Huang X (2012) A novel ZnO nanoparticle blended poly-vinylidene fluoride membrane for anti-irreversible fouling Journal of Membrane Science394e395 184e192 httpdxdoiorg101016jmemsci201112040
Linlin W Xuan Z amp Meng Z (2011) Removal of dissolved organic matter in municipaleffluent with ozonation slow sand filtration and nanofiltration as high quality pre-treatmentoption for artificial groundwater recharge Chemosphere 83 693e699 httpdxdoiorg101016jchemosphere201102022
Liu L Xu X Zhao C amp Yang F (2010) A new helical membrane module for increasingpermeate flux Journal of Membrane Science 360 142e148 httpdxdoiorg101016jmemsci201005014
Loh C H Wang R Shi L amp Fane A G (2011) Fabrication of high performance poly-ethersulfone UF hollow fiber membranes using Pluronic block copolymers as pore formingadditives Journal of Membrane Science 380 111e123 httpdxdoiorg101016jmemsci201106041
Ma N Zhang Y Quan X Fan X amp Zhao H (2010) Performing a microfiltration integratedwith photocatalysis using an Ag-TiO2HAPAl2O3 composite membrane for water treat-ment evaluating effectiveness for humic acid removal and anti-fouling properties WaterResearch 44(20) 6104e6114 httpdxdoiorg101016jwatres201006068
Maartens A Swarta P amp Jacobs E P (1999) Feed-water pretreatment Methods to reducemembrane fouling by natural organic matter Journal of Membrane Science 163 51e62httpdxdoiorg101016S0376-7388(99)00155e6
Membrane Filtration Guidance Manual (November 2005) United States EnvironmentalProtection Agency(EPA) Office of Water(4601) EPA 815-R-06e009
Membrane Fouling Considerations (January 2001) Hydranautics high performance membraneproducts technical paper
Minegishi S Jang N-Y Watanabe Y Hirata S amp Ozawa G (2001) Fouling mechanismof hollow fibre ultrafiltration membrane with pre-treatment by coagulationsedimentationprocess Water Science and Technology Water Supply 1(4) 49e56
Mir L Micheals S L Goel V amp Kaiser R (1992) Crossflow microfiltration Applicationsdesign and cost membrane handbook New York Van Nostrand Reinhold httpdxdoiorg101007978-1-4615-3548-5_35
Mishra A K amp Ramaprabhu S (2011) Functionalized graphene sheets for arsenic removaland desalination of sea water Desalination 282 39e45 httpdxdoiorg101016jdesal201101038
Mohammadi T amp Maghsoodloorad H (2013) Synthesis and characterization of ceramicmembranes (W-type zeolite membranes) International Journal of Applied CeramicTechnology 10(2) 365e375 httpdxdoiorg101111j1744-7402201102749x
Mulder M (1996) Basic principles of membrane technology (2nd ed) Kluwer AcademicPublishers ISBN 978-94-009-1766-8
Ng L Y Mohammad A W Leo C P amp Hilal N (2010) Polymeric membranes incor-porated with metalmetal oxide nanoparticles A comprehensive review Desalinationhttpdxdoiorg101016jdesal201011033
Nguyen V T Vigneswaran S Ngo H H Shon H K amp Kandasamy J (2009) Arsenicremoval by a membrane hybrid filtration system Desalination 236 363e369 httpdxdoiorg101016jdesal200710088
Ohya H Shiki S amp Kawakami H (2009) Fabrication study of polysulfone hollow fibermicrofiltration membranes Optimal dope viscosity for nucleation and growth Journal ofMembrane Science 326 293e302 httpdxdoiorg101016jmemsci200810001
Advances in water treatment by microfiltration ultrafiltration and nanofiltration 125
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Pendergast M T M amp Hoek E M V (2011) A review of water treatment membranenanotechnologies Energy and Environmental Science 4 1946e1971 httpdxdoiorg101039C0EE00541J
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Radjenovic J Petrovic M Ventura F amp Barcelo D (2008) Rejection of pharmaceuticals innanofiltration and reverse osmosis membrane drinking water treatment Water Research42 3601e3610 httpdxdoiorg101016jwatres200805020
Razmjou A Resosudarmo A Holmes R L amp Li H (2012) The effect of modified TiO2
nanoparticles on the polyethersulfone ultrafiltration hollow fiber membranes Desalination287 271e280 httpdxdoiorg101016jdesal201111025
Roux S P Jacobs E P van Reenen A J Morkel C amp Meincken M (2006) Hydro-philisation of polysulphone ultrafiltration membranes by incorporation of branchedPEO-block-PSU copolymers Journal of Membrane Science 276(1e2) 8e15 httpdxdoiorg101016jmemsci200509025
Rugbani A (2009) Investigating the influence of fabrication parameters on the diameter andmechanical properties of polysulfone ultrafiltration hollow-fibre membranes (MScthesis) University of Stellenbosch
Saljoughi E Amirilargani M amp Mohammadi T (2010) Effect of PEG additive andcoagulation bath temperature on the morphology permeability and thermalchemical sta-bility of asymmetric CA membranes Desalination 262 72e78 httpdxdoiorg101016jdesal201005046
Sarkar B Venkateshwarlu N Nageswara Rao R Bhattacharjee C amp Kale V(2007) Potable water production from pesticide contaminated surface watermdashAmembrane based approach Desalination 204 368e373 httpdxdoiorg101016jdesal200602041
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Schaep J Van der Bruggen B Uytterhoeven S Croux R Vandecasteele C Wilms Det al (1998) Removal of hardness from groundwater by nanofiltration Desalination 119295e302 httpdxdoiorg101016S0011-9164(98)00172e6
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Sombekke H D M Voorhoeve D K amp Hiemstrap P (1997) Environmental impactassessment of groundwater treatment with nanofiltration Desalination 113 293e296httpdxdoiorg101016S0011-9164(97)00144-6
126 Advances in Membrane Technologies for Water Treatment
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Advances in water treatment by microfiltration ultrafiltration and nanofiltration 127
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128 Advances in Membrane Technologies for Water Treatment
Water treatment by reverse andforward osmosis 4NK Rastogi1 A Cassano2 A Basile231Department of Food Engineering Central Food Technological Research InstituteCouncil of Scientific and Industrial Research Mysore India 2Institute on MembraneTechnology ITM-CNR University of Calabria Rende (CS) Italy 3Ast-Engineering srlRome Italy
41 Introduction
Water shortages have plagued many communities and humans have long searched fora solution to Earthrsquos limited freshwater supplies Less than 1 of the total water avail-able on the earth is considered freshwater Almost 965 of Earthrsquos water is located inthe seas and oceans 17 is present in icebergs and the remaining percentage is madeup of brackish water (Gleick 1996 Greenlee Lawler Freeman Marrot amp Moulin2009) The population explosion and the expansion of cities have made the productionof potable water undependable and have led to an increase in demand compared withavailability Today the production of potable water has become a worldwide concernfor many communities the projected population growth and demand exceed availableconventional water resources
Over the years purified water standards have become more stringent Watercontaining less than 1000 mgL of salts or total dissolved solids (TDS) is referred asfreshwater above this concentration properties such as taste color corrosion propen-sity and odor are adversely affected (Sandia 2003) Many countries have adoptednational drinking water standards for specific contaminants as well as for TDS butthe standard limits vary from country to country or from region to region within thesame country The World Health Organization and Gulf Drinking Water standardsAustralia recommend a drinking water standard of 1000 mgL TDS (FritzmannLowenberg Wintgens amp Melin 2007) Most desalination facilities are designed toachieve a TDS of 500 mgL or less (Gaid amp Treal 2007 Xu et al 2007) Desalinatedwater used for other purposes such as crop irrigation may have a higher TDS concen-tration Depending on the salinity of the feed water it can be divided into two typessuch as brackish water (often groundwater sources 1000e10000 mgL TDS) andseawater (30000e45000 mgL TDS) (Mickley 2001) The type of feed water decidesthe use of pretreatment method the design of the treatment plant the waste disposalmethod and the recovery of water
The expanding global population increasing water pollution and increasing stan-dard of living have put relentless pressure on water and energy resources Low-costmethods of purifying freshwater and desalting seawater are required to contend withthis destabilizing trend Hence the need for water is increasing rapidly and current
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400004-6Copyright copy 2015 Elsevier Ltd All rights reserved
freshwater resources may not be able to meet all requirements Desalination of sea (orsaline) water has been practiced regularly for over 50 years and is a well-establishedmeans to obtain a water supply in many countries In the 1970s exploration beganinto using membranes for water desalination Proving successful at producing purifiedwater from saltwater membranes became a viable alternative to evaporation-basedtechnologies in the water treatment market Over the years purified water standardshave become more stringent However membranes have risen to the challenge andcontinue to perform efficiently and effectively (Rastogi amp Nayak 2011)
42 Thermal or membrane desalination
Membrane and distillation processes equally share current desalination productioncapacity worldwide However reverse osmosis (RO) will emerge as the leader infuture desalination installations and will be the key to increasing water supplies fordrinking water production RO technology can now produce freshwater (fromseawater) at one-half to one-third the cost of distillation (Miller 2003) Brackish waterRO membranes typically have higher permeate flux and lower salt rejection andrequire lower operating pressures whereas seawater RO membranes have highersalt rejection and lower permeate fluxes and membrane permeability and requirehigher operating pressures to compensate for the higher osmotic pressure of seawaterBrackish water desalination is even less expensive than seawater desalination
The demand for additional freshwater production can be met by adopting waterreuse or desalination (Sauvet-Goichon 2007) Water reuse can be used for irrigationpower plant cooling and groundwater recharge (Focazio et al 2008) Desalination hasbeen identified as a potential source for producing drinking water Desalination is ageneral term for the process of separating salt from saline water for the productionof freshwater Thermal and membrane desalination both the processes are beingused for the recovery of water (Gleick 2006)
Middle East countries initiated the use of seawater thermal desalination and todaytheir share is more than 50 of the worldrsquos desalination capacity (Henthorne 2003Van der Bruggen amp Vandecasteele 2002) Membrane processes have rapidly devel-oped since the 1960s (Drsquosouza amp Mawson 2005 Loeb amp Sourirajan 1963) andnow surpass thermal processes in new plant installations (75 of new production ca-pacity) The advent of such membranes with high retention of low-molecular-weightorganic compounds and good physical and chemical stability has enabled RO to beused more widely on a commercial scale RO membrane technology has been devel-oped for both brackish and seawater applications RO membranes however are able toreject monovalent ions (sodium and chloride) Currently RO membranes are made sothat these can reject more than 99 salts (Brehant Bonnelye amp Perez 2003Reverberi amp Gorenflo 2007)
Membrane and distillation processes equally share current desalination productioncapacity worldwide RO has emerged as the leader in future desalination installationsRO will be the key to increasing water supplies for drinking water productionthroughout the world Although wealthy Middle Eastern countries have been able to
130 Advances in Membrane Technologies for Water Treatment
afford distillation processes RO technology can now produce freshwater (fromseawater) at one-half to one-third the cost of distillation (Miller 2003) Brackish waterdesalination is even less expensive than seawater desalination
Membranes have risen to the challenge of supplying desalinated water and continueto perform efficiently and effectively Another membrane process forward osmosis(FO) can be used to desalinate saline water sources at a notably reduced cost Itinvolves the use of a draw solution of aluminum sulfate or ammonium sulfate whichis used to transport water from saline water These draw solutions on chemical treat-ment or heating result in freshwater (Chanukya Patil amp Rastogi 2013 Frank 1972)This new athermal membrane process has been developed as a possible alternative todesalination It employs a semipermeable dense hydrophilic membrane that separatestwo aqueous solutions (feed and osmotic agent solution) with different osmoticpressures The difference in osmotic pressure acts as a driving force An osmoticpressure-driven process operates on the principle of osmotic transport of water acrossa semipermeable hydrophilic membrane from a dilute feed solution into a concentratedosmotic agent or draw solution (Nayak amp Rastogi 2010) Recently FO concentrationis gaining importance for concentrating liquid foods and natural colors (BabuRastogi amp Raghavarao 2006 Bolin amp Salunke 1971 Garcia-Castello Mccutcheonamp Elimelech 2009 Gusti amp Wrolstad 1996 Loeb amp Bloch 1973 MartinettiChildress amp Cath 2009 Popper et al 1966 Rodriguez-Saona Giusti Durst ampWrolstad 2001) generating electricity (Kravath amp Davis 1975) treating wastewater(Holloway Childress Dennett amp Cath 2007) and desalinating seawater(Mccutcheon Mcginnis amp Elimelech 2006)
43 Difference between osmosis RO and FO
Osmosis is the movement of solvent molecules through a selectively permeable mem-brane into a region of higher solute concentration with the aim of equalizing the soluteconcentrations on both sides The net movement of the solvent from a less concen-trated (hypotonic) to a more concentrated (hypertonic) solution tends to reduce the dif-ference in concentrations When a semipermeable membrane separates two solutionswater always diffuses from the solution with the lower osmotic potential to the onewith the higher osmotic potential This diffusion-driven motion of water through amembrane is termed osmosis It is the tendency of fluids to pass through a membraneso that equal concentrations are achieved on both sides (Figure 41(a)) RO is apressure-driven membrane process that entails forcing fluids through a membrane Itis a separation technique that can be used to concentrate or purify liquids without aphase change The RO process uses hydraulic pressure as the driving force for sepa-ration which serves to counteract the osmotic pressure gradient that would otherwisefavor water flux from the permeate to the feed (Figure 41(b)) FO is an osmotic pro-cess that like RO uses a semipermeable membrane to effect separation of water fromdissolved solutes The driving force for this separation is an osmotic pressure gradientsuch that a draw solution of high concentration (relative to that of the feed solution) isused to induce a net flow of water through the membrane into the draw solution thus
Water treatment by reverse and forward osmosis 131
effectively separating the feed water from its solutes FO uses the osmotic pressure dif-ferential (Dp) across the membrane as the driving force for transport of water throughthe membrane rather than the hydraulic pressure differential (similar to RO) The FOprocess results in the concentration of feed solution and the dilution of draw solution(Figure 41(c)) (Cath Childress amp Elimelech 2006 Rastogi amp Nayak 2011)
44 Fundamentals of water treatment by RO
Osmosis is the basis for RO When a semipermeable membrane separates a dilutesolution from a concentrated solution the solvent (water) crosses from the dilute solu-tion side to the concentrated solution side of the membrane to equalize concentrations onboth sides this process is known as osmosis The flow of solvent can be prevented byapplying an opposing hydrostatic pressure to the concentrated solution The magnitudeof the pressure required to completely impede the flow of water is defined as the osmoticpressure In the case of RO a hydrostatic pressure greater than the osmotic pressure isapplied for water to move from a high-solute to a low-solute concentration(Figure 41(b)) It necessitates high-pressure requirements The positive difference inpressure creates a chemical potential difference (concentration gradient) across the mem-brane that drives the liquid through the membrane against the natural direction ofosmosis (the movement of water molecules from an area of high concentration to anarea of low concentration) while the salts are retained and concentrated on the influentsurface of the membrane Some salt passage through the membrane occurs salt passagefor the same membrane increases with the salt concentration and temperature
The RO process requires high operating pressures ranging from 2300 to 3500 kPafor seawater and 100 to 300 kPa for brackish water to overcome the osmotic pressureof feed water (Perry amp Green 1997 Sagle amp Freeman 2004)
The dominant mechanism governing transport through RO membranes can beexplained as solution-diffusion which involves preferential dissolution of solvent
Feed
Feed
Feed
Feed
Wat
er
Wat
er
Wat
er
Dra
w s
olut
ion
(a) (b) (c)
Force( P)
nabla
Figure 41 Migration of water during (a) osmosis (b) reverse osmosis and (c) forward osmosisThe arrows indicate the direction of mass transfer DP is the hydraulic pressureReprinted from Rastogi (in press) with permission of Taylor amp Francis LLC (httpwwwtandfonlinecom)
132 Advances in Membrane Technologies for Water Treatment
(water) in the membrane and its transport through diffusion Water transport across anRO membrane occurs in three separate steps absorption onto the membrane surfacediffusion through the thickness of the membrane and desorption from the permeatesurface of the membrane Once a water molecule has absorbed onto the membranesurface the water concentration gradient (of the water-membrane system) across themembrane causes the water molecules to diffuse down the concentration gradient tothe permeate side of the membrane The water molecule then desorbs from the mem-brane and becomes part of the bulk permeate (Lonsdale Merten amp Riley 1965 Paul2004)
The equation that gives the water flux through a membrane as a function of pressuredifference during RO can be written in a simple form (Mulder 1996 p 298)
Jw frac14 AwethDp DpTHORN (41)
where Aw is the water permeability coefficient Dp is the transmembrane pressure andDp is the difference in the osmotic pressures of feed and permeate
The permeate flux during the course of RO decreases rapidly owing to reversibleand irreversible fouling of the membrane that significantly affects the process effi-ciency (Kwang-Sup Joo-Heon Dong-Ho Seok-Joong amp Soon-Dong 2004) Thepermeate fluxes are primarily affected by the phenomena of concentration polariza-tion (ie solute buildup) and fouling (eg microbial adhesion gel layer formationand solute adhesion) at the membrane surface Fouling is generally caused by thedeposition of colloidal particles inorganic and organic compounds and microbeson the surface of the membrane (Chien-Hwa Lung-Chen Shaik Khaja Chung-Hsin amp Cheng-Fang 2010) It is considered a group of physical chemical and bio-logical effects leading to irreversible loss of membrane permeability (Chien-Hwaet al 2010 Sablani Goosen AL-Belushi amp Wilf 2001) Concentration polariza-tion is mainly limited to the buildup of retained solute ie accumulation of matteron the surface of the membrane It is considered to be reversible and can becontrolled in a membrane module by means of velocity adjustment pulsation(back flush) ultrasound or an electric field (Vladisavljevic Vukosavljevic ampBukvic 2003)
Considering a situation in which a solution consisting of solvent and solute is sub-jected to a pressure-driven membrane process such as RO the solute is preferentiallyretained on the membrane surface whereas solvent permeates through the membraneAs solvent is removed from the feed the difference in the osmotic pressure (Dp)increases which in turn results in a decrease in flux At the same time a concentrationgradient is developed owing to the rejection of solute with the passage of solventthrough the membrane (Figure 42) The solvent flux through the membrane increaseswith an increase in the applied pressure until the concentration of solute on the mem-brane surface reaches a critical concentration which is referred to as the gel concen-tration (CG) Further increases in pressure do not change the solute concentration atthe membrane surface however they result in a thicker and compacted gel layerAt equilibrium the rate of solvent transport through the membrane (flux) can be calcu-lated on the basis of the convective transport of solutes to the membrane surface
Water treatment by reverse and forward osmosis 133
ethJw$CTHORN by the solvent which is equal to the sum of the diffusive back transport of
soluteD$
dCdx
and permeate flow ethJw$CpTHORN ie
Jw$C frac14D
dCdx
thorn Jw$Cp (42)
where D is the diffusion coefficient for solute transport through solvent C is the soluteconcentration retained on the membrane Cp is the solute concentration in permeate (inthe case of RO the solute is completely retained by the membrane ie Cp frac14 0) anddCdx
is the solute concentration gradient
The integration of Eqn (42) with appropriate boundary conditions (x frac14 0 C frac14 CGx frac14 x C frac14 CB) results in the following equation
Jw frac14 D
d$ln
CG
CB
frac14 k$ln
CG
CB
(43)
where d is the thickness of the boundary layer k is mass transfer coefficient CB and CG
are the bulk and gel layer solute concentrations and CGCB
is the concentration polari-zation modulus Equation (43) shows that at this juncture the flux through themembrane is independent of the transmembrane pressure drop or permeability anddepends only on the solute characteristics (D and CG) and boundary layer thickness(d) Concentration polarization is a reversible fluid dynamic phenomenon If CB
reaches CG solutes will start to precipitate or deposit on the membrane forming a gel
Figure 42 Formation of concentration gradient near the surface of the membrane
134 Advances in Membrane Technologies for Water Treatment
layer which leads to the permanent loss of flux known as fouling (Echavarria TorrasPagan amp Ibarz 2011)
45 Conventional and membrane pretreatmentfor RO feed water
The primary goal of any RO pretreatment system is to lower the fouling propensity ofthe water in the RO membrane system Conventional pretreatment typically consists ofchemical additions including acid coagulant and flocculent that prepare the feedwater for granular media filtration (Isaias 2001 Sauvet-Goichon 2007) Acid treat-ment reduces the pH of the feed water (typical pH range 5e7) which increases thesolubility of calcium carbonate the key potential precipitate in many feed waters(Bonnelye et al 2004) Coagulants are typically small positively charged moleculesthat effectively neutralize like charges (on aqueous particulate and colloidal matter)and allow the suspended solids to group together in flocs (large groups of looselybound suspended particles) Inorganic coagulants are commonly iron or aluminumsalts such as ferric chloride or aluminum sulfate whereas organic coagulants are typi-cally cationic low-molecular-weight (lt500000 Da) polymers (ie dimethyldially-lammonium chloride or polyamines) Granular media filtration includes materialssuch as sand anthracite pumice gravel and garnet (Bonnelye et al 2004) often acombination of materials is used in layers in the filtration bed to reduce the feed watersilt density index (Morenski 1992) Cartridge filtration (filter cartridges are usually1e10 mm) acts as a final polishing step to remove larger particles that passed throughmedia filtration in conventional RO pretreatment (Morenski 1992 Petry et al 2007)
Antiscalants are primarily used as pretreatment typically performed after granularmedia filtration either before or after cartridge filtration (Sauvet-Goichon 2007)Disinfection is achieved by adding a strong oxidant such as ozone chlorine (gaschlorine dioxide or sodium hypochlorite) chloramine or potassium permanganate(Morenski 1992)
A new trend in pretreatment has been the movement toward using larger pore-sizemembranes (microfiltration (MF) ultrafiltration (UF) and nanofiltration (NF)) to useRO as a pretreatment to feed water that avoids the passage of colloids and suspendedparticles contributing to RO membrane fouling (Brehant et al 2003) The mem-branes act as a defined barrier between the RO system and any suspended particlesThey can reduce silt density index as well as turbidity of the feed water (Pearce 2007Vedavyasan 2007) MF membranes are an appropriate choice for the removal oflarger particulate matter whereas NF membranes are used to remove dissolved con-taminants as well as particulate and colloidal material UF membranes represent thebest balance between the removal of contaminants and permeate production of theother three membrane types Typical final permeate fluxes for a UF-RO systemwere reported to be in the range of 15e24 Lm2 h (Kamp Kruithof amp Folmer2000) whereas the permeate flux exiting the UF pretreatment stage was within60e150 Lm2 h (Brehant et al 2003)
Water treatment by reverse and forward osmosis 135
Membrane pretreatment reduces the general aging and destruction of RO mem-branes by feed water components membrane replacement and the frequency of chem-ical (acid or base) cleaning However the RO system can be operated at a higherpermeate flux Membrane pretreatment systems are decreasing in capital cost andare becoming more cost-competitive with conventional systems The risk of membranefouling prevents general operation at high permeate flux especially for feed waters withimpurities such as hydrocarbons (oil) and cellular or extracellular material (frombacteria) (Jian Kitanaka Nishijima Baes amp Okada 1998 Williams amp Edyvean1998) Fouling results in membrane damage and flux decline requires membranereplacement every 5e10 years (Pearce 2007)
46 Fundamentals of water treatment by FO
FO can be considered a technique for the recovery of the water from saline water Itemploys an asymmetric semipermeable dense hydrophilic membrane that separatestwo aqueous solutions (feed and draw solution) with different osmotic pressuresThe osmotic pressure gradient is the sole driving force for the transport of water Con-centration polarization is a significant problem in pressure-driven membrane processessuch as RO It reduces permeate flow as a result of buildup of the retained moleculesleading to enhanced osmotic pressure at the membrane surface The FO phenomenonwith a dense symmetric membrane can result in the occurrence of concentration polar-ization on both sides of the membrane The solute is concentrated and diluted on thefeed and permeate sides leading to concentrative and dilutive external concentrationpolarization respectively (Figure 43(a)) The standard flux equation for forward equa-tion is given by the following equation
Jw frac14 Aethpd pf THORN (44)
where pd and pf are the bulk osmotic pressures of draw and feed solutions respec-tively Jw is the water flux and A is the pure water permeability coefficient
The asymmetric membrane employed in the case of FO consists of a dense activemembrane layer (active layer) and a loosely bound support layer (porous support layer)(Figure 43 Nayak amp Rastogi 2010) The membrane can be placed between the feedand the osmotic agent solutions in two different ways such that they feed toward thesupport layer (normal mode) and toward the active layer (reverse mode) which arereferred as modes I (Figure 43(b)) and II (Figure 43(c)) respectively (GrayMccutcheon amp Elimelech 2006 Nayak amp Rastogi 2010)
461 Modeling of flux when feed toward support layer(internal concentration polarization concentrative)
When the feed (05 M NaCl solution) is placed against a porous support layer and adraw solution is kept on the active layer side (mode I) water is diffused into the poroussupport layer and traversed to the draw solution side through the active layer of mem-brane as a result of the osmotic pressure gradient across the membrane The use of
136 Advances in Membrane Technologies for Water Treatment
sodium chloride solution results in buildup of salt within the porous support layer dueto the diffusion of water to the draw solution side Subsequently it results in a signif-icant internal concentration polarization (concentrative) and negligible external con-centration polarization At the same time permeation of water to the draw solutionside results in external concentration polarization (dilutive) But internal concentrationpolarization may not be highly significant Both the internal (concentrative) andexternal (dilutive) concentration polarization phenomena are responsible for the reduc-tion in the effective osmotic driving force The internal concentration polarization oc-curs within the porous support layer and it cannot be lessened by hydrodynamics suchas turbulence (since it is occurring within the pores of the support layer) leading todrastic reduction of effective osmotic driving force (McCutcheon amp Elimelech2007 Tang amp Ng 2008) The extent of external polarization is much less than the in-ternal concentration polarization during forward osmosis when permeate water fluxesare relatively low (Cath et al 2006 McCutcheon et al 2006)
The concentration profile across asymmetric FO membrane for concentrative inter-nal concentration polarization is illustrated in Figure 43(b) Assuming negligibleexternal concentration polarization at the outer surfaces of the membrane the waterand salt flux (Jw Js) across the membrane is given by
Jw frac14 AethDpeffTHORN frac14 Aethpd pf THORN (45)
Figure 43 Mechanism of forward osmosis indicating water transport (a) with a densesymmetric membrane and with asymmetric membrane (b) feed toward the support layer (mode I)and (c) feed toward active layer (mode II) pd and pf are the bulk osmotic pressures of draw andfeed solutions respectively p
d and pf the osmotic pressures on the membrane surface of draw
and feed solutions respectively p0f and p0
d are the osmotic pressures of the feed and drawsolutions on the inside of the active layer within the porous support for concentrative internalconcentration polarization on the feed side and dilutive internal concentration polarization on thedraw side for modes I and II respectively Dp1 Dp2 and Dp3 are the corresponding effectivedriving forces in (a) (b) and (c) situations respectively ECP and ICP refer to external andinternal concentration polarizations respectivelyReprinted from Rastogi (in press) with permission of Taylor amp Francis LLC (httpwwwtandfonlinecom)
Water treatment by reverse and forward osmosis 137
JS frac14 BethC2 C3THORN (46)
where A and B are the water and solute permeation coefficients respectively pd and pf
are the bulk osmotic pressure of draw and feed solutions respectively at the mem-brane surface and the corresponding solute concentrations are C2 and C3 Because pf
is not known this quantity can be obtained by the following analysis provided byMehta and Loeb (1978) The diffusion of solute is considered to be in the oppositedirection of water flux and hence it was taken as negative The salt flow consists oftwo components acting in opposite directions a diffusive part caused by diffusiondown the salt concentration gradient and a convective part caused by bulk flow ofwater through the membrane The salt flux across the porous support layer can thus bewritten as per the following equation
Js frac14 DεdCdx
JwC (47)
where ε and D are the porosity of the substrate and diffusion coefficient of the salt inthe membrane porous substrate respectively The distance x is measured fromthe membraneesolution interface on the porous support layer Substituting the valueof Js from Eqn (42) to Eqn (43) results in the following equation
BethC2 C3THORN frac14 DεdCdx
JwC
Boundary conditions x frac14 0 C frac14 C4 x frac14 st C frac14 C3
(48)
On rearranging
Z frac14 dCdx
frac14 JwC
Dεthorn BethC2 C3THORN
Dε Boundary conditions
x frac14 0 Z1 frac14 JwC4
Dεthorn BethC2 C3THORN
Dε x frac14 st Z2 frac14 JwC3
Dεthorn BethC2 C3THORN
Dε(49)
where t and s are the thickness and tortuosity of the support layer respectivelySolving Eqn (47) with the boundary conditions above yields Eqn (49)Differentiating Eqn (49) with respect to x results in the following equation
dZdx
frac14 JwDε
dCdx
(410)
Integrating Eqn (49) results in the following equation
Dε
Jw
ZZ2
Z1
1ZdZ frac14
Zst
0
dx orDε
Jwfrac12ln ZZ1
Z2frac14 st (411)
138 Advances in Membrane Technologies for Water Treatment
or lnJwC3 thorn BethC2 C3THORNJwC4 thorn BethC2 C3THORN frac14 st
DεJw frac14 KDJw (412)
where stDε is defined as a membrane (support and active layer) resistivity (KD) (sm or
dm)
orJwC3 thorn BethC2 C3THORNJwC4 thorn BethC2 C3THORN frac14 expethKDJwTHORN (413)
On rearranging Eqn (413)
C3
C2frac14 B
eKDJw 1
thorn JwC4C2eKDJw
BetheKDJw 1THORN thorn Jwor
1 C3
C2frac14 ethC2 C3THORN
C2frac14
1thorn C4
C2eKDJw
1thorn B
JwetheKDJw 1THORN
(414)
pd pf
pdfrac14
1 p
f
pdeKDJw
1thorn B
JwetheKDJw 1THORN
(415)
Substituting the values of (p2 p3) from Eqn (415) to Eqn (41)
Jw frac14 Apd pf
frac14Ap2
1 p
f
pdeKDJw
1thorn B
JwetheKDJw 1THORN
(416)
The rearrangement of Eqn (416) results in the following equation
Jw frac14
1KD
lnethApd thorn B JwTHORN
Bthorn Apf
(417)
where pd and pf are the bulk osmotic pressures at the draw side and osmotic pressures
on membrane surface at the feed side respectivelyFor low water flux pd and p
f can be replaced with bulk osmotic pressures of drawand feed sides (pd and pf) respectively Hence Eqn (417) becomes
Jw frac14
1KD
ln
Apd thorn B Jw
Bthorn Apf
(418)
The value of KD can be inferred from the plot of Jw versus lnApdthornBJwBthornApf
Water treatment by reverse and forward osmosis 139
462 Modeling of flux when feed is toward active layer(internal concentration polarization dilutive)
When the feed (05-M NaCl solution) is placed against the active layer and the drawsolution on the support layer side the water from the feed is diffused into the activelayer which in turn is diffused to the porous support layer and then to the bulk throughthe boundary layer resulting in dilutive internal concentration polarization on the drawsolution side (Figure 43(c)) The external and internal concentration polarizationstoward the feed side are considered negligible The osmotic solute from the drawsolution may penetrate the porous support layer before flux can occur As water fluxcrosses the active layer and enters into the porous support layer it results in dilutionof the draw solution owing to convection The solute diffuses back to the interiorsurface A steady state is quickly reached but the concentration at the interior surfaceof the active layer is far lower than in the bulk draw solution (Gray et al 2006) Thecombined effect of both the diffusion of water through the active layer and thediffusion of the draw solute into the support layer results in dilutive internal concen-tration polarization The extent of external concentration polarization is negligiblecompared with the internal concentration polarization which has a dominant role ina situation when the draw solution in placed against the support layer (Gray et al2006 Tan amp Ng 2008)
Similar to the analysis provided in the above section for concentrative internal con-centration polarization the equation for the dilutive internal concentration polarizationcan be written as
Jw frac14
1KC
ln
Bthorn Apd
Bthorn ApF thorn Jw
(419)
The value of KC can be inferred from the plot Jw versus ln BthornApdBthornApFthornJw
Constants A (0019 matmday) and B (0023 matmday) refer to the water andsolute permeability coefficients of the active layer of the membrane respectively(Gray et al 2006 Loeb Titelman Korngold amp Freiman 1997) and the valueswere determined as per Eqns (45) and (46) respectively based on the data presentedfor the ammonium bicarbonate draw solution by Achilli et al (2012)
463 Quantitative analysis of internal concentrationpolarization
Studies on the effect of draw solution concentration and temperature on transmem-brane flux in the case of modes I and II indicate that transmembrane flux for mode Iwas higher compared with mode II for all values of the draw solution concentrationAt the same time the transmembrane flux in modes I and II increased with an increasein feed temperature from 30 C to 45 C (Figure 44(a) and (b)) The increase in trans-membrane flux with temperature can be explained using the WilkeeChang equation(Treybal 1982) according to which the diffusion coefficient is proportional to theabsolute temperature divided by the viscosity of the solvent The increase in
140 Advances in Membrane Technologies for Water Treatment
temperature reduces the viscosity of solution and increases the diffusion coefficientswhich results in an increase in transmembrane flux (Holloway et al 2007Mccutcheon amp Elimelech 2006)
The severity of concentration polarizations in modes I and II can be inferred fromthe resistance to diffusion within the membrane porous layer (KD and KC values)
(Figure 44(c) and (d)) The values of Jw were plotted against lnApdthornBJw
BthornApf
and
ln BthornApdBthornApFthornJw
as per Eqns (418) and (419) respectively to determine the values of
KD and KC The relevant values are reported in Figure 44(c) and (d) which indicatesthat the predicted values are in good agreement with the experimental values(R2 gt 090) Higher values indicate a higher internal concentration polarization result-ing in lower flux
The low concentration of solute in the feed solution and draw solution results in ahigher effective driving force in mode I compared with mode II (Dpeff1 gt Dpeff2)hence in this situation mode I is most desirable for desalination to have a higher waterflux than mode II In the case of mode I external concentration polarization on both the
25
25
20
2020
15
15
10
10
05
0500
00
252015100500
25
20
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10
05
00
25
20
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10
05
00
25
20
15
10
05
00
00 10 30 40 50
2000 10 30 40 50
(a)
(b) (d)
(c)
Mode IIMode II
Mode I Mode I
Draw solution concentration (M)
Draw solution concentration (M)
Temp (degC) KC (dm)
Temp (degC)
Tran
smem
bran
e flu
xx
10ndash6
(m3 m
2 s)Tr
ansm
embr
ane
flux
x 10
ndash6 (m
3 m2 s)
Tran
smem
bran
e flu
xx
10ndash6
(m3 m
2 s)Tr
ansm
embr
ane
flux
x 10
ndash6 (m
3 m2 s)
KD (dm)30354045
30354045
1692 plusmn 032 1445 plusmn 027
1354 plusmn 0321168 plusmn 0261043 plusmn 012
1478 plusmn 022
1251 plusmn 033 1108 plusmn 018
In((B + Aπd ndashJw)(B + Aπf))
d fIn((B + Aπ )(B + Aπ + Jw))
Figure 44 (a b) Effect of the draw solution concentration at different temperatures on
transmembrane flux (c d) Plot of Jw versus lnBthornApdJw
BthornApF
and Jw versus ln
BthornApdBthornApFthornJw
as per
Eqns (418) and (419) respectively Modes I and II refer to the membrane orientation in whichthe feed was toward the support layer and active layer respectively (A 30 C 35 C- 40 C bull 45 C)Reprinted from Chanukya et al (2013) with permission from Elsevier
Water treatment by reverse and forward osmosis 141
feed and draw solution sides as well as internal concentration polarization on the drawside can be considered insignificant whereas internal concentration polarization(concentrative) toward the feed side will be the only dominant concentration polariza-tion Conversely in the case of mode II internal concentration polarization (dilutive)toward the draw side will be the most dominant concentration polarization which willbe higher because of the higher concentration of draw solution
The orientation of the asymmetric membrane had a significant effect on the perfor-mance of the membrane The feed toward the support layer (mode I) was the bestoption to maximize the flux owing to less concentrative internal concentration polar-ization compared with higher dilutive internal concentration polarization in the othercase when the feed was toward the active layer (mode II) The high agreement betweenthe experimental and predicted values indicated that the proposed model was a good fitand can be used to predict flux at variable conditions
Nayak and Rastogi (2010) and Nayak Valluri and Rastogi (2011) described themechanism of water transport in FO in a situation in which feed solution was a mixtureof low-and high-molecular-weight compounds When the solution of low-molecular-weight compounds was taken as a feed mode I was most desirable However whenthe solution of high-molecular-weight compounds was taken as a feed mode IIgave a higher driving force that in turn led to a higher transmembrane flux Mi andElimelech (2008 2010) also indicated that these high-molecular-weight compoundsmay be deposited within the porous structure of the membrane leading to cake layerformation owing to the lack of shear force as well as hindered back diffusion in theporous structure Fouling of the FO membrane as a result of alginate was reportedto be almost fully reversible after a simple water rinse with no chemical cleaningreagents It was attributed to the less compact formation of the fouling layer owingto the lack of hydraulic pressure Zhao and Zou (2011a b) and Zhao Zou Chuyanget al (2012) demonstrated that internal concentration polarization in the support layerstrongly depended on the physicochemical properties of the solution facing the supportlayer In a situation in which the feed was facing active layer it resulted in a more sta-ble and higher water flux than that of the situation in which support layer was towardthe feed
47 Membranes for FO
Membranes for FO should have high water flux high solute rejection minimumporosity high hydrophilicity reduced fouling and high mechanical strength Thesemembranes are different from standard RO membranes that typically consist of athin active layer (less than 10 mm) and a thick porous support layer Any dense nonpo-rous selectively permeable material commonly used for RO can also be used as amembrane for FO However the presence of a thick support layer results in the occur-rence of concentration polarization within the membrane support structure which mayrequire large osmotic driving forces to sustain adequate water flux (Dova Petrotos ampLazarides 2007ab Gray et al 2006 Mccutcheon Mcginnis amp Elimelech 2005Mcginnis Mccutcheon amp Elimelech 2007)
142 Advances in Membrane Technologies for Water Treatment
A special membrane for FO made of cellulose triacetate was developed by MsHydration Technologies Inc USA (thickness less than 50 mm) (Figure 45(a) and(b)) in which the support layer was embedded in a polyester mesh to provide mechan-ical support (Mccutcheon et al 2005) A scanning electron microscope image of thecross-section of the cellulose acetate polyamide composite and cellulose triacetatemembrane is presented in Figure 46 (Ng Tang amp Wong 2006)
FO membranes can also be made as hydration bags that can be used to recover water(Figure 47) They are double-lined bags The internal bag is made of a FO membraneand is filled with draw solution (eg flavored sucrose) and the external bag is a sealedplastic bag Upon immersing the bag in an aqueous solution water diffuses into the bagbecause of the osmotic pressure difference and slowly dilutes the draw solution Theconcept of a hydration bag was developed for military recreational and emergencyrelief situations in which reliable drinking water was scarce or not available Thehydration bag is one of the few commercial applications of FO (Cath et al 2006)
(a)
(b)
Figure 45 Scanning electron microscope images of cross-sections of cellulosic forwardosmosis membrane (CA) A polyester mesh is embedded within the polymer material formechanical support The membrane thickness is less than 50 mmReprinted from Garcia-Castello et al (2009) and Mccutcheon et al (2005) with permissionfrom Elsevier
Water treatment by reverse and forward osmosis 143
Figure 46 Scanning electron microscope images of the cross-section of (a) cellulose acetate(CA) membrane (b) polyamide composite (AD) membrane (c) forward osmosis (FO)membrane and (d) FO membrane at higher resolution where 1 is the dense selective layer and2 is the support layerReprinted with permission from Ng et al (2006) Copyright 2006 American Chemical Society
Figure 47 Water purification hydration bagReprinted from Cath et al (2006) with permission from Elsevier
144 Advances in Membrane Technologies for Water Treatment
With a view to reducing internal concentration polarization Wang Ong et al(2010) designed a cellulose acetate FO membrane composed of a highly poroussublayer sandwiched between two selective skin layers by using the phase inversiontechnique It prevented salt and other solutes in the draw solution from penetratinginto the membrane porous support It also resulted in higher water flux and lowersalt transport Zhang et al (2010) developed a double dense membrane structure usingphase inversion of cellulose acetate that produced low salt leakage and resulted in lessinternal concentration polarization in the FO process
A novel dual-layer hollow-fiber NF membrane suitable for FO using co-extrusiontechnology was developed consisting of an ultra-thin selective skin (around 10 mm)with a fully open-cell water channels underneath and a micro-porous sponge-like sup-port structure which could achieve high throughput and high salt rejection (YangWang amp Chung 2009ab) Later Wang et al (2010) fabricated a polyethersulfonethin-film composite hollow-fiber membrane for FO using a dry-jet wet spinning pro-cess with an ultra-thin RO-like skin layer (300e600 nm) on either surface of a poroushollow-fiber substrate by interfacial polymerization The active layers presented excel-lent intrinsic separation properties with a hydrophilic rejection layer and good mechan-ical strength A high-flux and high-rejection FO membrane for water reuse andseawater desalination was developed by fabricating polybenzimidazole NF hollow-fiber membranes with a thin wall and desired pore size by nonsolvent induced phaseinversion which was chemically modified by cross-linking with p-xylylene dichloride(Wang Yang Chung amp Rajagopalan 2009) Cross-linking finely tuned the mean poresize and enhanced the salt selectivity Furthermore Su Yang Teo and Chung (2010)developed cellulose acetate NF hollow-fiber membranes suitable for FO processes bysubjecting them to a two-step heat treatment (60 C 60 min and 95 C 20 min) whicheffectively shrank the membrane pores on the membrane surface with a denser outerskin layer (mean pore radius reduced from 063 to 030 nm) The resultant fiber hasa high rejection to NaCl and MgCl2 and low pure water permeability Chou et al(2010) developed a FO hollow-fiber membrane fabricated by the dry-jet wet spinningprocess The outer UF skin was made by increasing the air gap in the spinning processand the inner RO skin layer was made by interfacial polymerization using anm-phenylenediamine aqueous solution and trimesoyl chloride hexane solution Thedeveloped membrane showed excellent intrinsic separation properties with a waterflux of 426 Lm2h Yip Tiraferri Phillip Schiffman and Elimelech (2010) made ahigh-performance thin-film composite membrane consisting of a selective polyamideactive layer formed by interfacial polymerization on top of a polysulfone support layerfabricated by phase separation onto a thin (40 mm) polyester nonwoven fabric
Jia Li Wang Wu and Hu (2010) demonstrated the suitability of carbon nanotubemembranes for seawater desalination using FO The membrane could achieve theoptimal salt rejection property but also the largest water flux which broke the limitof the tradeoff effect between selectivity and permeability existing in traditional liquidseparation membranes The antifouling ability and good mechanical strength renderedit more suitable for FO According to a rough estimation carbon nanotube mem-branes can achieve a higher water flux far in excess of existing commercial FOmembranes
Water treatment by reverse and forward osmosis 145
Qiu Qi and Tang (2011) successfully fabricated FO membranes using layer-by-layer assembly of polyallylamine hydrochloride and polysodium 4-styrene-sulfonate on a porous polyacrylonitrile substrate The chemical cross-linking oflayer-by-layer polyelectrolyte layers was performed with glutaraldehyde The cross-linked and non-cross-linked membranes both had relatively high water permeabilityHowever the cross-linked membranes showed better MgCl2 rejection which clearlydemonstrated the potential of layer-by-layer membranes for high-flux FO applicationsLater Qiu Setiawan Wang Tang and Fane (2012) fabricated high-performance flat-sheet FO membranes using polyamide-imide materials via phase inversion followedby polyelectrolyte polyethyleneimine after treatment to form an NF-like rejection layerwith positive charges Polyamide-imide micro-porous substrate was embedded with awoven fabric the enhanced mechanical strength of the membrane made it possible toreduce the thickness of the substrate to 55 mm and the resultant membranes was able toreach a water flux of 2965 and 192 Lm2h when the active layer was facing draw orfeed solutions respectively
48 Desalination by FO
Research has been reported on the desalination of seawater by FO FO can be used effi-ciently and effectively to desalinate water FO using semipermeable polymeric mem-branes may be a viable alternative to RO as a lower-cost and more environmentallyfriendly desalination technology
Frank (1972) described a method of FO using a solution of aluminum sulfate as anosmotic agent which upon treatment with calcium hydroxide resulted in a precipitateof aluminum hydroxide and calcium sulfate which was removed by standard methodsleaving freshwater Mccutcheon et al (2006) reported FO desalination as a lower-costand more environmentally friendly technology The driving force was provided by adraw solution composed of highly soluble gases ammonia and carbon dioxide Waterfluxes ranging from 36 to 360 Lm2h for a wide range of draw and feed solution con-centrations were reported Ammonium bicarbonate was used as a draw solution toextract water from saline feed water by Mccutcheon Mcginnis and Elimelech(2009) and Chanukya et al (2013) in a FO desalination process High osmotic pres-sures generated by the highly soluble ammonium bicarbonate draw solute yieldedhigh water fluxes leading to high feed water recoveries The ammonium bicarbonatesolution was used as an osmotic solution (Jung et al 2011 Mccutcheon et al2005) that can be decomposed into ammonia and carbon dioxide gases at about60 C The water transferred from the feed side (seawater) to the osmotic solutioncan be obtained by distillation methods (Cath et al 2006 Ng Tang amp Wong2006) and the separated gases can then be recycled to use as a draw solution again(Jung et al 2011) Mcginnis and Elimelech (2007) presented a schematic diagramof the ammonia carbon dioxide FO desalination process to recover freshwater(Figure 48) Similarly Low (2009) studied the performance of FO with respect todifferent ammoniaecarbon dioxide draw solutions and membranes The internalconcentration polarization resulted in lowering of the flux The draw solution used
146 Advances in Membrane Technologies for Water Treatment
for desalination using FO should be highly soluble highly recoverable nontoxicnonreactive with membranes easily separable from water and economically feasible
Kravath and Davis (1975) described a process of seawater desalination by FOacross a cellulose acetate membrane using glucose as the draw solute Upon dilutionsalinity was reduced to a level where ingestion would be possible for short-term con-sumption The performance of flat-sheet cellulose acetate membranes was reported tobe poor in terms of salt rejection Stache (1989) used a semipermeable membrane bagfilled with concentrated fructose solution to desalinate small volumes of seawaterwhile simultaneously creating a nutritious drink Yaeli (1992) used a concentratedglucose solution as a draw solution and water was extracted from seawater by osmosisThe diluted glucose solution was fed to an RO unit in which a low-pressure RO mem-brane separated potable water from the glucose draw solution
An integrated FO process consisting of an RO process to generate potable waterwas proposed FO was used to generate significant hydraulic pressure used to drivingthe RO process in which it could separate salt from seawater to generate potable waterfrom water with a high salt content (Lampi Beaudry amp Herron 2007) BamagaYokochi and Beaudry (2009) designed an integrated desalination unit consisting ofa closed FO and RO process in which RO brine was used as the osmotic draw solutionand thus the chemical energy stored in the RO brine was recovered and used by the FOmembrane process Also Choi et al (2009 2010) investigated systems combining FOand RO for seawater desalination Pilot-scale combined systems with FO resulted inhigher recovery with higher quality and fluxes than conventional RO-based desalina-tion systems Similarly Valladares Yangali Li and Amy (2011) focused on rejecting13 selected micropollutants employing a clean or fouled FO membrane using Red Seawater as a draw solution The resulting effluent was then desalinated at low pressurewith an RO membrane to produce a high-quality permeate Tan and Ng (2010) pro-posed a hybrid FOeNF process for seawater desalination The process achievedgood-quality product water that met the recommended drinking water total dissolvedsolids guideline (500 mgL) Zhao Zou and Mulcahy (2012) designed a hybrid
Salinewater
Membrane
Product water
Brine
DrawsolutionNH3CO2
Soluterecovery system
Figure 48 Schematic diagram of the ammoniaecarbon dioxide forward osmosis desalinationprocessReprinted from Mcginnes and Elimelech (2007) with permission from Elsevier
Water treatment by reverse and forward osmosis 147
FOeNF system designed for brackish water desalination and compared it with a stand-alone RO process Lay et al (2010) examined possible factors for the slower decline offlux in the FO process The transmission of draw solutes from the draw solution intothe feed had significant effect on performance Yangali Li Valladares Li and Amy(2011) indicated that FO coupled with low-pressure RO used for indirect desalinationconsumed only half of the energy used for high-pressure seawater RO desalination andproduced good-quality water extracted from the impaired feed water A cost analysisrevealed that FO is a viable and promising technology
A comparison of vacuum-enhanced direct contact membrane distillation(VEDCMD) and FO for the desalination of brackish water indicated that water recov-ery in FO was higher (90) than with VEDCMD (81) (Martinetti et al 2009) Lingand Chung (2011) developed a potentially sustainable integrated FOeUF system forwater reuse During FO water was transferred from the salt solution to the draw solu-tion (containing super hydrophilic nanoparticles) which was regenerated from thedraw solution using UF
49 Conclusion
Membrane processes for water desalination are becoming increasingly interesting Theyallow improvements in quality enhanced process efficiency and profitability Theexpanding capabilities of membrane processes will certainly continue to benefit the in-dustry in future to obtain the required product quality purity yield and throughputalong with economic viability The use of RO and FO on an industrial scale will beeven more lucrative in the future if membranes with high selectivity improved fluxrobustness and greater chemical and mechanical stability are developed
List of symbols
A or Aw Water permeability coefficientB Solute permeation coefficientsCB Bulk layer solute concentrationCG Gel layer solute concentrationC Solute concentration retained on membraneCp Solute concentration in permeateD Diffusion coefficient for solute transport through solventdCdx
Solute concentration gradient
K Mass transfer coefficientpd and pf Bulk osmotic pressures of draw and feed solutionsJw and Js Water and salt fluxeskd and kf Mass transfer coefficients on draw and feed solution sidesDp Difference in the osmotic pressures of feed and permeateDp Transmembrane pressureD Thickness of the boundary layerε Porosity of the substrate
148 Advances in Membrane Technologies for Water Treatment
x Distance measured from membraneesolution interface on porous support layert Thickness of the support layers Tortuosity of the support layer
Abbreviations
TDS Total dissolved solidsRO Reverse osmosisFO Forward osmosisUF UltrafiltrationMF MicrofiltrationNF Nanofiltration
References
Achilli A Cath T Y amp Childress A E (2010) Selection of inorganic-based draw solutionsfor forward osmosis applications Journal of Membrane Science 364 233e241
Babu B R Rastogi N K amp Raghavarao K S M S (2006) Effect of process parameter ontransmembrane flux during direct osmosis Journal of Membrane Science 280 185e194
Bamaga O A Yokochi A amp Beaudry E G (2009) Application of forward osmosisin pretreatment of seawater for small reverse osmosis desalination units Desalination andWater Treatment 5 183e191
Bolin H R amp Salunke D K (1971) Physicochemical and volatile flavor changes occurringin fruit juices during concentration and foam-mat drying Journal of Food Science 36665e668
Bonnelye V Sanz M A Durand J P Plasse L Gueguen F amp Mazounie P (2004)Reverse osmosis on open intake seawater Pre-treatment strategy Desalination 167191e200
Brehant A Bonnelye V amp Perez M (2003) Assessment of ultrafiltration as a pretreatmentof reverse osmosis membranes for surface seawater desalination Water Science andTechnology Water Supply 3(5e6) 437e445
Cath T Y Childress A E amp Elimelech M (2006) Forward osmosis Principles applica-tions and recent developments Journal of Membrane Science 281 70e87
Chanukya B S Patil S amp Rastogi N K (2013) Influence of concentration polarization onflux behavior in forward osmosis during desalination using ammonium bicarbonateDesalination 312 39e44
Chien-Hwa Y Lung-Chen F Shaik Khaja L Chung-Hsin W amp Cheng-Fang L (2010)Enzymatic treatment for controlling irreversible membrane fouling in cross-flow humicacid-fed ultrafiltration Journal of Hazardous Materials 177 1153e1158
Choi Y J Choi J S Oh H J Lee S Yang D R amp Kim J H (2009) Toward a combinedsystem of forward osmosis and reverse osmosis for seawater desalination Desalination247 239e246
Choi J S Kim H Lee S Hwang T M Oh H Yang D R et al (2010) Theoreticalinvestigation of hybrid desalination system combining reverse osmosis and forwardosmosis Desalination and Water Treatment 15 114e120
Water treatment by reverse and forward osmosis 149
Chou S Shi L Wang R Tang C Y Qiu C amp Fane A G (2010) Characteristics andpotential applications of a novel forward osmosis hollow fiber membrane Desalination261 365e372
Dova M I Petrotos K B amp Lazarides H N (2007a) On the direct osmotic concentration ofliquid foods Part I Impact of process parameters on process performance Journal of FoodEngineering 78 422e430
Dova M I Petrotos K B amp Lazarides H N (2007b) On the direct osmotic concentration ofliquid foods Part II Development of a generalized model Journal of Food Engineering78 431e437
Drsquosouza N amp Mawson A J (2005) Membrane cleaning in the dairy industry A reviewCritical Reviews in Food Science and Nutrition 45(2) 125e134
Echavarria A Torras P C Pagan J amp Ibarz A (2011) Fruit juice processing and membranetechnology application Food Engineering Reviews 3 136e158
Focazio M J Kolpin D W Barnes K K Furlong E T Meyer M T Zaugg S D et al(2008) A national reconnaissance for pharmaceuticals and other organic wastewatercontaminants in the United States e (II) untreated drinking water sources Science of theTotal Environment 402(2e3) 201e216
Frank B S (1972) Desalination of sea water US Patent No 3670897 WI USAFritzmann C Lowenberg J Wintgens T amp Melin T (2007) State of-the-art of reverse
osmosis desalination Desalination 216 1e76Gaid K amp Treal Y (2007) Reverse osmosis desalination The experience of Veacute olia water
Desalination 203 1e14 (original language French)Garcia-Castello E M Mccutcheon J R amp Elimelech M (2009) Performance evaluation of
sucrose concentration using forward osmosis Journal of Membrane Science 338 61e66Gleick P H (1996) In S H Schneider (Ed) Water resources in encyclopedia of climate and
weather (Vol 2 pp 817e823) New York University PressGleick P H (2006) The worldrsquos water 2006e2007 the biennial report on freshwater
resources Chicago IL Island PressGray G T Mccutcheon J R amp Elimelech M (2006) Internal concentration polarization in
forward osmosis Role of membrane orientation Desalination 197 1e8Greenlee L F Lawler D F Freeman B D Marrot B amp Moulin P (2009) Reverse
osmosis desalination Water sources technology and todayrsquos challenges Water Research43 2317e2348
Gusti M M amp Wrolstad R E (1996) Radish anthocyanin extract as a natural red colorantfrom maraschino cherries Journal of Food Science 61 688e694
Henthorne L (2003) Desalination today Southwest Hydrology 12e13Holloway R W Childress A E Dennett K E amp Cath T Y (2007) Forward osmosis for
concentration of anaerobic digester centrate Water Research 41 4005e4014Isaias N P (2001) Experience in reverse osmosis pretreatment Desalination 139 57e64Jia Y X Li H L Wang M Wu L Y amp Hu Y D (2010) Carbon nanotube Possible
candidate for forward osmosis Separation and Purification Technology 75 55e60Jian W Kitanaka A Nishijima W Baes A U amp Okada M (1998) Removal of oil
pollutants in seawater as pretreatment of reverse osmosis desalination process WaterResearch 33(8) 1857e1863
Jung D H Lee J Kim D Y Lee Y G Park M Lee S et al (2011) Simulation offorward osmosis membrane process Effect of membrane orientation and flow direction offeed and draw solutions Desalination 277 83e91
Kamp P C Kruithof J C amp Folmer H C (2000) UFRO treatment plant Heemskerk Fromchallenge to full scale application Desalination 131 27e35
150 Advances in Membrane Technologies for Water Treatment
Kravath R E amp Davis J A (1975) Desalination of seawater by direct osmosis Desalination16 151e155
Kwang-Sup Y Joo-Heon H Dong-Ho B Seok-Joong K amp Soon-Dong K (2004)Effective clarifying process of reconstituted apple juice using membrane filtration withfilter-aid pretreatment Journal of Membrane Science 228 179e186
Lampi K Beaudry E amp Herron J (2007) Forward osmosis pressurized device and processfor generating potable water US Patent No 7303674 B2 Arizona USA
Lay W C L Chong T H Tang C Y Fane A G Zhang J amp Liu Y (2010) Foulingpropensity of forward osmosis Investigation of the slower flux decline phenomenonWaterScience and Technology 61 927e936
Ling M M amp Chung T S (2011) Desalination process using super hydrophilic nanoparticlesvia forward osmosis integrated with ultrafiltration regeneration Desalination 278194e202
Loeb S amp Bloch M R (1973) Counter current flow osmotic processes for the productionof solutions having a high osmotic pressure Desalination 13 207e215
Loeb S amp Sourirajan S (1963) Seawater dimineralization by means of an osmotic membraneAdvances in Chemistry Series 38 117e132
Loeb S Titelman L Korngold E amp Freiman J (1997) Effect of porous support fabric onosmosis through a Loeb-Sourirajan asymmetric membrane Journal of Membrane Science129 243e249
Lonsdale H K Merten U amp Riley R L (1965) Transport properties of cellulose acetateosmotic membranes Journal of Applied Polymer Science 9 1341e1362
Low S C (2009) Preliminary studies of seawater desalination using forward osmosisDesalination and Water Treatment 7 41e46
Martinetti C R Childress A E amp Cath T Y (2009) High recovery of concentrated RObrines using forward osmosis and membrane distillation Journal of Membrane Science331 31e39
Mccutcheon J R amp Elimelech M (2006) Influence of concentrative and dilutive concen-tration polarization on flux behavior in forward osmosis Journal of Membrane Science284 237e247
Mccutcheon J R amp Elimelech M (2007) Modeling water flux in forward osmosis Impli-cations for improved membrane design AIChE Journal 53 1736e1744
Mccutcheon J R Mcginnis R L amp Elimelech M (2005) A novel ammoniaecarbon dioxideforward (direct) osmosis desalination process Desalination 174 1e11
Mccutcheon J R Mcginnis R L amp Elimelech M (2006) Desalination by ammonia-carbondioxide forward osmosis Influence of draw and feed solution concentrations on processperformance Journal of Membrane Science 278 114e123
Mccutcheon J R Mcginnis R L amp Elimelech M (2009) A novel ammonia-carbon dioxideforward (direct) osmosis desalination process Desalination 174 1e11
McGinnis R L amp Elimelech M (2007) Energy requirements of ammoniaecarbon dioxideforward osmosis desalination Desalination 207 370e382
Mcginnis R L Mccutcheon J R amp Elimelech M (2007) A novel ammonia-carbon dioxideosmotic heat engine for power generation Journal of Membrane Science 305 13e19
Mehta G D amp Loeb S (1978) Internal polarization in the porous substructure of asemi-permeable membrane under pressure-retarded osmosis Journal of MembraneScience 4 261
Mickley M C (2001) Membrane concentrate disposal Practices and regulation Report No69 US Department of the Interior Bureau of Reclamation Boulder CO Mickley ampAssociates
Water treatment by reverse and forward osmosis 151
Mi B amp Elimelech M (2008) Chemical and physical aspects of organic fouling of forwardosmosis membranes Journal of Membrane Science 320 292e302
Mi B amp Elimelech M (2010) Organic fouling of forward osmosis membranes Foulingreversibility and cleaning without chemical reagents Journal of Membrane Science 348144e151
Miller J E (2003) Review of water resources and desalination technologies Sandia NationalLaboratories Available from httpwwwprodsandiagovcgi-bintechlibaccess-controlpl2003030800pdf Retrieved on 05012015
Morenski F (1992) Current pretreatment requirements for reverse osmosis membrane appli-cations In Official proceedings of the 53rd international water conference (pp 325e330)
Mulder M (1996) Basic principles of membrane technology The Netherlands KluwerAcademic Publisher
Nayak C A amp Rastogi N K (2010) Forward osmosis for concentration of anthocyanin fromKokum (Garcinia indica Choisy) Separation and Purification Technology 71 144e151
Nayak C A Valluri S S amp Rastogi N K (2011) Effect of high or low molecular weight ofcomponents of feed on transmembrane flux during forward osmosis Journal of FoodEngineering 106 48e52
Ng H Y Tang W amp Wong W S (2006) Performance of forward (direct) osmosis processMembrane structure and transport phenomenon Environmental Science and Technology40 2408e2413
Paul D R (2004) Reformulation of the solution-diffusion theory of reverse osmosis Journalof Membrane Science 241 371e386
Pearce G K (2007) The case for UFMF pretreatment to RO in seawater applicationsDesalination 203 285e295
Perry R H amp Green D W (Eds) (1997) Perryrsquos chemical engineersrsquo handbook New YorkMcGraw Hill
Petry M Sanz M A Langlais C Bonnelye V Durand J P Guevara D et al (2007)The El Coloso (Chile) reverse osmosis plant Desalination 203 141e152
Popper K Camirand W M Nury F amp Stanley W L (1966) Dialyzer concentratesbeverages Food Engineering 38 102e104
Qiu C Qi S amp Tang C Y (2011) Synthesis of high flux forward osmosis membranesby chemically crosslinked layer-by-layer polyelectrolytes Journal of Membrane Science381 74e80
Qiu C Setiawan L Wang R Tang C Y amp Fane A G (2012) High performance flat sheetforward osmosis membrane with an NF-like selective layer on a woven fabric embeddedsubstrate Desalination 287 266e270
Rastogi N K Opportunities and challenges in application of forward osmosis in foodprocessing Critical Reviews in Food Science and Nutrition (in press)
Rastogi N K amp Nayak C A (2011) Membranes for direct osmosis In A Basile ampS P Nunes (Eds) Advanced membrane science and technology for sustainable energyand environmental applications (pp 680e717) Cambridge UK Woodhead Publisher
Reverberi F amp Gorenflo A (2007) Three year operational experience of a spiral-woundSWRO system with a high fouling potential feed water Desalination 203 100e106
Rodriguez-Saona L E Giusti M M Durst R W amp Wrolstad R E (2001) Developmentand process optimization of red radish concentrate extract as potential natural red colourantJournal of Food Processing Preservation 25 165e182
Sablani S Goosen M AL-Belushi R amp Wilf M (2001) Concentration polarization inultrafiltration and reverse osmosis A critical review Desalination 141(3) 269e289
152 Advances in Membrane Technologies for Water Treatment
Sagle A amp Freeman B (2004) Fundamentals of membranes for water treatment InJ A Arroyo (Ed) The future of desalination in Texas (vol II pp 137e153) AustinTX Texas Water Development Board
Sandia (2003) Desalination and water purification Roadmap e a report of the executivecommittee DWPR Program Report 95 US Department of the Interior Bureau ofReclamation and Sandia National Laboratories Available from httpwrrinmsuedutbndrcroadmapreportpdf (accessed 260814)
Sauvet-Goichon B (2007) Ashkelon desalination plante a successful challenge Desalination203 75e81
Stache K (1989) Apparatus for transforming sea water brackish water polluted water or thelike into a nutritious drink by means of osmosis US Patent No 4879030
Su J Yang Q Teo J F amp Chung T S (2010) Cellulose acetate nanofiltration hollow fibermembranes for forward osmosis processes Journal of Membrane Science 355 36e44
Tan C H amp Ng H Y (2008) Modified models to predict flux behavior in forward osmosis inconsideration of external and internal concentration polarizations Journal of MembraneScience 324 209e219
Tan C H amp Ng H Y (2010) A novel hybrid forward osmosisenanofiltration (FO-NF)process for seawater desalination Draw solution selection and system configurationDesalination and Water Treatment 13 356e361
Tang W amp Ng H Y (2008) Concentration of brine by forward osmosis Performance andinfluence of membrane structure Desalination 224 143e153
Treybal R E (1982) Mass transfer operations (3rd ed) New Delhi McGraw-Hill ChemicalEngineering Series
Valladares L R Yangali Q V Li Z amp Amy G (2011) Rejection of micropollutants byclean and fouled forward osmosis membrane Water Research 45 6737e6744
Van der Bruggen B amp Vandecasteele C (2002) Distillation vs membrane filtration Over-view of process evolutions in seawater desalination Desalination 143 207e218
Vedavyasan C V (2007) Pretreatment trends e and overview Desalination 203 296e299Vladisavljevic G T Vukosavljevic P amp Bukvic B (2003) Permeate flux and fouling
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Wang K Y Ong R C amp Chung T S (2010) Double-skinned forward osmosis membranesfor reducing internal concentration polarization within the porous sublayer IndustrialEngineering and Chemistry Research 49 4824e4831
Wang R Shi L Tang C Y Chou S Qiu C amp Fane A G (2010) Characterization ofnovel forward osmosis hollow fiber membranes Journal of Membrane Science 355158e167
Wang K Y Yang Q Chung T S amp Rajagopalan R (2009) Enhanced forward osmosisfrom chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membraneswith a thin wall Chemical Engineering Science 64 1577e1584
Williams C J amp Edyvean R G (1998) An investigation of the biological fouling in thefiltration of seawater Water Science and Technology 38(8e9) 309e316
Xu J Ruan G Chu X Yao Y Su B amp Gao C (2007) A pilot study of UF pretreatmentwithout any chemicals for SWRO desalination in China Desalination 207 216e226
Yaeli J (1992) Method and apparatus for processing liquid solutions of suspensions partic-ularly useful in the desalination of saline water US Patent No 5098575
Yang Q Wang K Y amp Chung T S (2009a) A novel dual-layer forward osmosis membranefor protein enrichment and concentration Separation and Purification Technology 69269e274
Water treatment by reverse and forward osmosis 153
Yang Q Wang K Y amp Chung T S (2009b) Dual-Layer hollow fibers with enhanced fluxas novel forward osmosis membranes for water production Environmental Science andTechnology 43 2800e2805
Yangali Q V Li Z Valladares R Li Q amp Amy G (2011) Indirect desalination of RedSea water with forward osmosis and low pressure reverse osmosis for water reuseDesalination 280 160e166
Yip N Y Tiraferri A Phillip Schiffman J amp Elimelech M (2010) High performancethin-film composite forward osmosis membrane Environmental Science and Technology44 3812e3818
Zhang S Wang K Y Chung T S Chen H Jean Y C amp Amy G (2010) Well-constructed cellulose acetate membranes for forward osmosis Minimized internalconcentration polarization with an ultra-thin selective layer Journal of Membrane Science360 522e535
Zhao S amp Zou L (2011a) Relating solution physicochemical properties to internalconcentration polarization in forward osmosis Journal of Membrane Science 379459e467
Zhao S amp Zou L (2011b) Effects of working temperature on separation performancemembrane scaling and cleaning in forward osmosis desalination Desalination 278157e164
Zhao S Zou L Chuyang T Y amp Mulcahy D (2012) Recent developments in forwardosmosis Opportunities and challenges Journal of Membrane Science 396 1e21
Zhao S Zou L amp Mulcahy D (2012) Brackish water desalination by a hybrid forwardosmosis-nanofiltration system using divalent draw solute Desalination 284 175e181
154 Advances in Membrane Technologies for Water Treatment
Membrane bioreactors for watertreatment 5SA Deowan SI Bouhadjar J HoinkisInstitute of Applied Research (IAF) Karlsruhe University of Applied SciencesKarlsruhe Germany
51 Introduction
Water is another synonym of life According to the World Health Organization themost dangerous threat to the health of mankind emerging within the next few years ispolluted water In underdeveloped countries the shortage of clean freshwater will bethe most important cause of death for children under 5 years The low quantities offreshwater for industrial agricultural and municipal use have to be well preserved byefficient sustainable and cost-effective technologies Water contaminated fromindustry and agriculture with heavy metal ions pesticides organic compoundsendocrine disruptive compounds nutrients (phosphates nitrates and nitrites) hasto be efficiently treated to protect humans from being intoxicated with these com-pounds or with bacteria Furthermore incidental sludge from industrial wastewatertreatment facilities is commonly highly contaminated with toxic compounds(BioNexGen 2013)
Clean water as a basis for health and good living conditions is too far out of reachfor most of the world population Thus neither sustainable consumption nor reinforce-ment of governmental regulations is an effective driver to force the industry to adoptsustainability policies Although polluted water and water shortages demand sustain-able water use and recycling of wastewater the barriers are high for adopt them(BioNexGen 2013)
Water recycling is widely accepted as a sustainable option to respond to the generalincrease in water shortages and the demand for freshwater and for environmentalprotection Water recycling is commonly seen as one of the main options to providea remedy for water shortage caused by the increase in the demand for water anddraughts as well as a response to some economical and environmental drivers Themain options for wastewater recycling are industrial irrigation aquifer rechargeand urban reuse (Pidou 2006)
Membrane bioreactor (MBR) technology is recognized as a promising technologyto provide water with reliable quality for reuse Today MBRs are robust simple tooperate and even more affordable They take up little space need modest technicalsupport and can remove contaminants in one step This makes it practical to providesafely reusable water for nonpotable use
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400005-8Copyright copy 2015 Elsevier Ltd All rights reserved
52 Fundamentals
MBR technology is a combination of the conventional biological sludge process awastewater treatment process characterized by a suspended growth of biomass anda microfiltration (MF) or ultrafiltration (UF) membrane system (Judd 2011) Thebiological unit is responsible for the biodegradation of waste compounds and themembrane module for the physical separation of treated water from the mixed liquorThe pore diametre of the membranes is in the range between 001 and 01 mm so thatparticulates and bacteria can be kept out of permeate and the membrane systemreplaces the traditional gravity sedimentation unit (clarifier) in the biological sludgeprocess Hence MBR offers the advantage of higher product water quality andlow footprint Because of its advantages MBR technology has great potential inwide-ranging applications including municipal and industrial wastewater treatmentand process water recycling By 2006 around 100 municipal full-scale plants(gt500 population equivalent) and around 300 industrial large-scale plants (gt20 m3day) were in operation in Europe (Lesjean amp Huisjeslow 2008) MBR installation ca-pacity grew in 2007 but declined in 2008 and 2009 The market was affected by lowerindustrial spending because of the economic downturn However there was an in-crease in demand for large plants during the past 2e3 years Installations of MBRincreased significantly in the years of 2010 and early 2011 (GIA 2013)
The main industrial applications are in the food and beverage chemical pharma-ceutical and cosmetics and textile industries as well as in laundries The technicalfeasibility of this technology has been demonstrated through a large number of small-and large-scale applications
521 MBR configurations
According to MBR configurations both aerobic and anaerobic MBRs can be dividedinto two classes as side-stream MBR (sMBR) and submerged or immersed MBR(iMBR) (Figure 51)
In side-stream MBRs the membrane module is placed outside the MBR Thesludge from the MBR is pumped into the membrane module which creates cross-flow at the membrane surface and thus permeate is generated The concentratedsludge rejected by the membrane is recycled to the MBR This is a pressure-drivenmembrane filtration process In the early development of stream MBRs both thetransmembrane pressure (TMP) and cross-flow velocity were generated by the recir-culation pump However a few modifications were made to reduce the high-energyconsumption associated with the side-stream configuration Therefore a suctionpump was added to the recirculation pump on the permeate side which increasedoperation flexibility and decreased the cross-flow rate and energy consumption(Shimizu Okuno Uryu Ohtsubo amp Watanabe 1996) The latest side-stream MBRseven introduced air flow into the membrane module which intensified turbulenceon the feed side of the membrane and reduced the fouling and operational costs(Jiang 2007)
156 Advances in Membrane Technologies for Water Treatment
In submerged MBRs the membrane module is directly submerged in the reactor Asuction or vacuum pump is needed to create a TMP difference for permeate produc-tion In this case no circulation pump is needed because cross-flow is created by aera-tion This concept was first developed by Yamamoto Hiasa Mahmood and Matsuo(1989) to reduce energy consumption compared with side-stream MBR configurationIn some cases (eg MF membrane and very low filtration fluxes) the permeate side isplaced in a lower position and gravity itself is the only driving force for the filtration(Ueda amp Hata 1999)
Table 51 compares the side-stream and submerged MBRs The submerged MBRhas a simpler configuration because it needs less equipment The coarse bubbleaeration in the membrane tank is multifunctional In addition to membrane fouling con-trol it supplies oxygen to the biological process (although the efficiency of oxygen useis low) The biggest advantage of submerged over side-stream configuration is energysavings using coarse bubble aeration instead of the high-rate recirculation pump in side-stream MBRs The capillary and hollow-fibre membranes used in many submergedMBRs have high packing density and low cost which make it feasible to use moremembranes However typical tubular membranes used in side-stream MBRs havelow packing density and are more expensive Gander Jefferson and Judd (2000)reviewed four side-stream and four submerged MBR systems and concluded thatside-stream MBRs have higher total energy cost by up to two orders of magnitudemainly owing to the high recycle flow velocity (1e3 ms) and head loss within themembrane module In addition the submerged MBRs experienced fouling and couldbe cleaned more easily than the side-stream MBRs (Gander et al 2000)
However side-stream MBRs have the advantages of more robust physicalstrength and more flexible cross-flow velocity control and hydraulic loading They
Return sludge
InfluentInfluent
EffluentEffluent
AerationAeration
Bioreactor BioreactorExtra sludge
Extra sludge
Side-stream MBR Submerged MBR
Figure 51 Configuration of side-stream and submerged MBRsModified from Jiang (2007)
Membrane bioreactors for water treatment 157
are mostly used in industrial wastewater treatment and small-scale wastewater treat-ment plants where influent flow rate and composition have larger variation andoperational conditions are tough (eg high-temperature conditions) (Jiang 2007)Furthermore both MBR systems can be applied as aerobic (in the presence of anaeration system for the module) and anaerobic (in the absence of an aeration systemfor the module)
522 Membrane material types and morphology
Membrane materials and pore size are important criteria for MBR application Thereare two different types of membrane materials polymeric and ceramic Metallicmembrane filters exist but these have specific applications that do not relate toMBR technology To be made useful the membrane material must then be formed(or configured) to allow water to pass through it In MBRs a dense MF or a looseUF membrane is often applied (average pore size around 05e005 mm)
A number of different polymeric and ceramic materials are used to formmembranes but they are nearly always composed of a thin surface layer that providesthe required permselectivity on top of a more open thicker porous support thatprovides mechanical stability A classical membrane is thus anisotropic in structurewith symmetry only in the plane orthogonal to the membrane surface Polymericmembranes are also usually fabricated to have high surface porosity or percent totalsurface pore cross-sectional area and narrow pore size distribution so as to provideas high throughput and as selective a degree of rejection as possible The membranemust also be mechanically strong (ie have structural integrity) Finally the materialwill normally have some resistance to thermal and chemical attacks (ie extremes of
Table 51 Comparison of side-stream MBRs and submerged MBRs
Parameters Side-stream Submerged
Complexity Complicated Simple
Flexibility Flexible Less flexible
Robustness Robust Less robust
Flux High (40e100 Lm2 h) Low (10e30 Lm2 h)
Fouling reducing methods bull Cross-flowbull Airliftbull Backwashingbull Chemical cleaning
bull Air bubble agitationbull Backwashing (notalways possible)
bull Chemical cleaning
Membrane packing density Low High
Energy consumption withfiltration
High (2e10 kW hm3) Low (02e04 kW hm3)
Source Jiang (2007 p 11)
158 Advances in Membrane Technologies for Water Treatment
temperature pH andor oxidant concentrations that normally arise when the membraneis chemically cleaned) and should ideally offer some resistance fouling (Judd 2006)
In principle any polymer can be used to form a membrane only a limited numberof materials are suitable for the duty of membrane separation the most common ofwhich are
bull Polyvinylidene difluoride (PVDF)bull Polyethylsulphone (PES)bull Polyethylene (PE)bull Polypropylene (PP)bull Polysulphone (PS)
The membrane materials most often used in MBRs are organic polymers eg PEPP and PVDF (Judd 2006) Some are blended with other materials to change their sur-face charge or hydrophobicity (Mulder 1996) Ceramic materials have been shown tobe suitable for MBR technology because of their thermal chemical and mechanicalstability
Through specific manufacturing techniques all of these polymers can be formedinto membrane materials with desirable physical properties and each has reasonablechemical resistance (Judd 2011) Based on the common membrane materials the prin-ciple types of membrane are shown in Figure 52 which describes the typicalmorphology of membranes in MBR technology isotropic and anisotropic micropo-rous membranes are usually applied The two other types of membrane representthe morphology of dense membranes applied in reverse osmosis (RO) and nanofiltra-tion (NF)
Figure 52 Principle types of membranesModified from Baker (2004 p 4)
Membrane bioreactors for water treatment 159
523 Membrane module types
The configuration of the membrane ie its geometry and the way it is mounted andoriented in relation to the flow of water is crucial in determining overall processperformance (Judd 2011) Large membrane areas are normally required whenmembranes have to be applied on an industrial scale The smallest unit into whichthe membranes are packed is called a module (Mulder 1996) Three types ofmembrane modules such as flat sheet (FS) hollow fibre (HF) and multi-tubular(MT) are available that are suited to MBR technologies (Judd 2011) Two populartypes of modules are shown in Figure 53
Flat-sheet capillary and HF membranes are applied in iMBR and typical tubularmembranes are used in sMBR (Gander et al 2000) The geometric structure of amembrane is valuable if it is capable of minimizing fouling during the filtration processand if its module has good specific surface where for lsquospecific surfacersquo meansthe filtering surface per unity of occupied volume Structural simplicity managementflexibility and modularity are also important when deciding whether a membranemodule is valuable An important parameter often taken into account while consid-ering the membrane filtration process is the molecular weight of the compound thatcan be retained by the membrane This is called the molecular weight cutoff it refersto the molecular weight cutoff the solute at which 90 rejection takes place and isexpressed in daltons (Manigas 2008)
53 Aerobic MBR
Aerobic MBR is an MBR configuration associated with aeration systems In thisregard aeration has two functions (1) it supplies oxygen to microorganisms in thebioreactor and (2) it scours the membrane surface to create cross-flow which keeps
(a) (b)
Figure 53 Membrane bioreactor (MBR) module types (a) flat sheet (from Kubota Japan)(b) hollow fibre (from Motian China)
160 Advances in Membrane Technologies for Water Treatment
the membrane surface comparatively clean Normally air is supplied through an airdiffusion system The diffusion rate of oxygen from air to water depends on the airbubble size generated by the diffuser Coarse air bubbles are preferred for betterscouring whereas fine air bubbles are preferred for higher oxygen mass transferinto the water phase
531 Areas of application
Generally aerobic MBR is applied for low- to intermediate-level strength organicload-oriented wastewater such as municipal wastewater (low organic load) industrialwastewater and textile wastewater (organic load of intermediate level) Some aerobicMBR applications for treating wastewater are
bull Municipal wastewaterbull Textile wastewaterbull Agriculture wastewaterbull Wastewater from slaughterhousesbull Fisheriesbull Food processing industry
According to Lesjean et al (2009) and Zheng Zhou Chen Zheng and Zhou(2010) the growth of MBR applications in the European and Chinese marketsis increasing almost exponentially (Figures 54 and 55) Figures 54 and 55 indicatethat the application of MBRs in European countries is more in the industrial sectorswhereas in China it is more in the municipal field Globally there is also a pro-nounced upward trend in plant size as well as in the diversity of technology
0
100
200
300
400
500
600
1990ndash1998
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Num
ber o
f pla
nts
(cum
val
ues)
30 new refsyear
45 new refsyear
65 new refsyearIndustrial (capacity gt20m3d)
Municipal (capacity gt20m3d)
Total municipal in EuropeAbout 2 millions ep (05 population)
Figure 54 Development of industrial andmunicipalMBRmarket inEurope (Lesjean et al 2009)
Membrane bioreactors for water treatment 161
providers - although the largest MBRs are predominantly fitted with GE Zenon Tech-nology (Judd 2011) In 2010 GE Zenon provided more than 40 of the total globalinstalled capacity from MBR treatment (Judd 2011) In 2014 the United States wasthe country with the largest number of large-scale MBR plants (over 50 MLD) world-wide (The MBR Site 2014)
532 Factors affecting membrane performance
MBRs use a membrane filtration process based on pressure gradient The process isaffected by the following factors
bull Intrinsic resistance of the membranebull TMPbull Hydrodynamic regime at the interface between the membrane and the solution to be filteredbull Fouling
Among these factors hydraulic geometry including microbiological aspects ofMBR and fouling of the membranes are the most important
Microbiological species of MBR are influenced by several common parameterssuch as
bull Hydraulic residence time (HRT)bull Mixed liquor suspended solids (MLSS)bull Sludge retention time (SRT)bull Sludge loading (SL) or organic loading rate (OLR)
An overview of these most common influencing parameters is been provided in thefollowing sections
Figure 55 Capacity development of industrial and municipal MBR markets in China (Zhenget al 2010)
162 Advances in Membrane Technologies for Water Treatment
5321 Hydraulic residence time
The HRT indicates the duration in hours that the feed solution remains in the MBRbefore it is processed as permeate by the membrane The formula for HRT is
HRT ethhTHORN frac14 Hydraulic volume ethLTHORNPermeate flow ethL=hTHORN (51)
From Eqn (51) it is evident that HRT depends on the hydraulic volume of thereactor and the permeate flow If HRT is high it is easier for bacterial biocenosis toacclimate to reactor conditions but low HRT affects conditions inversely
HRT is an important operating parameter in aerobic MBR operation (Le ClechChen amp Faine 2006 Meng et al 2009) Lower HRT values result in higher OLRwhich result in a reduction of reactor volumes required to achieve a specified removalperformance On the other hand higher HRTs usually result in better removal perfor-mance Qin Oo Tao and Kekre (2007) reported that for the operation of a submergedMBR for treatment of petrochemical wastewater the use of HRTs in the range13e19 h results in effluents that have acceptable qualities Chang et al (2006) founda negligible effect of HRT in the range of 12e30 h on the removal performanceof MBR used to treat the wastewater of an acrylonitrileebutadieneestyrene unitVisvanathan Thu Jegatheesan and Anotai (2005) reported similar chemical oxygendemand (COD) and pentachlorophenol removal performance at HRTs in the range of12e24 h in an MBR used to treat synthetic wastewater Decreased HRT has also beenreported to result in an increase in the rate of membrane fouling in MBRs although itseffect seems to be mostly indirect rather than direct (Chang et al 2006 Meng ShiYang amp Zhang 2007 Meng et al 2009 Qin et al 2007)
5322 Mixed liquor suspended solids
MLSS are the concentration of suspended solids in mixed liquor usually expressedin grams per litre (Wateronline 2011) Mixed liquor is a mixture of raw or settledwastewater and activated sludge contained in an aeration basin in the activated sludgeprocess MLSS are used to control the wastewater treatment plant in the suspendedgrowth process
According to Lousada-Ferreira et al (2010) MLSS are a critical operational param-eter for aerobic MBR The possibility of using high solid contents results in a reducedfootprint that is considered one of the biggest advantages of MBR technologyHowever the influence of MLSS on fouling is not consistent and sometimes is contra-dictory MLSS concentrations have a direct impact on viscosity (Hasar Kinaci euroUnleurouTogrul amp Ipek 2004 Rosenberger Kruger Witzig Manz Szewyk amp Kraume2002) According to Hasar et al (2004) suspensions with higher viscosity requirehigher cross-flow velocities to create turbulent regimes If the cross-flow is not enoughto scour the solids away from the membrane it is reasonable that the fouling layer willbuild up faster on the membrane surface
Membrane bioreactors for water treatment 163
Therefore the theoretical explanation suggests that increasing concentrations ofsolids result in increasing fouling However some authors report no effect of solidson TMP (Harada Momonoi Yamazaki amp Takizawa 1994) or permeate quality(Rosenberger Kubin amp Kraume 2002) and demonstrate a decrease in TMP for sam-ples with higher MLSS concentrations (Defrance amp Jaffrin 1999 Le Clech Jeffersonamp Judd 2003) One explanation for these apparently contradictory results is that theeffect of MLSS concentration on filtration resistance varies according to the appliedMLSS range in the MBR operation (Lousada-Ferreira et al 2010)
5323 Sludge retention time
SRT indicates how frequently sludge is taken away from the MBR It also means theage of the sludge Laeraa Polliceb Saturnob Giordanob and Sandullia (2009)observed a significant correlation between the SRT and the OLR That study suggestedthat the biomass activity is weakly affected by the age of the sludge Membrane clean-ing requirements also appear to depend slightly on the SRT
According to Ahmed Cho Lim Song and Ahn (2007) the SRT affects the mixedliquor characteristics and induces changes in the physiological state of microorganismsthus variations in membrane fouling substances such as extracellular polymeric sub-stances (EPS) and soluble microbial products (SMP) can occur (Chang amp Lee 1998Masse Sperandio amp Cabassud 2006) However contradictory reports exist in the liter-ature regarding the effects of SRT on membrane biofouling Several studies demon-strated that EPS increases as SRT increases (Chang amp Lee 1998 Cho 2004 Masseet al 2006) others have shown the opposite trend (Lee Kang amp Shin 2003Ng amp Hermanowicz 2005)
5324 Sludge loading or organic loading rate
The SL or ratio of feed to microorganism (FM) is an important design parameter in theMBR process Generally the MBR process can operate at higher FM ratios comparedwith an activated sludge plant (Robinsion 2003) It is defined as kilograms of CODdivided by kilograms of MLSS times days It can also be expressed as kilograms ofCOD divided by cubic metres times days
The OLR has an important role in the treatment of wastewater by MBRs Anincreased OLR decreases the filterability of the MBR Kornboonraksa and Lee (2009)reported that an increase in influent COD biological oxygen demand (BOD) andNH4eN from 1150 to 2050 mgL 683 to 1198 mgL and 154 to 248 mgL respec-tively resulted in a decrease in the removal efficiency of COD BOD and NH4eNTrussell Merlo Hermanowicz and Jenkins (2006) reported increased membranefouling with higher OLR They reported that steady-state membrane fouling ratesincreased 20-fold over a fourfold increase in FM Khoshfetrat Nikakhtari Sadeghifarand Khatibi (2011) found a reduction of COD removal efficiency from 90 to 74when OLR increased from 1 to 25 kg CODm3 day Shen Zhou Mahendran Bagleyand Liss (2010) reported a higher degradation of organics (glucose) of 98 at OLRs
164 Advances in Membrane Technologies for Water Treatment
of 13 g CODL day or less compared with an OLR of 30 g CODL day which had adegradation of about 70
In the biochemical stage of wastewater treatment organic carbon and nutrients areremoved from wastewater by microbes These microbes live and grow enmeshed inEPS that bind them into discrete micro-colonies forming three-dimensional aggre-gated microbial structures called flocs The ability of microorganisms to form flocsis vital for the activated sludge treatment of wastewater The floc structure enablesthe adsorption of soluble substrates but also the adsorption of colloidal matter andmacromolecules found in wastewaters (Liwarska-Bizukojc amp Bizukojc 2005Michael amp Fikret 2002) The diversity of microbial community in activated sludgeis large containing prokaryotes (bacteria) eukaryotes (protozoa nematodes androtifers) and viruses In this complex microsystem bacteria dominate the microbialpopulation and have a critical role in the degradation process (Michael amp Fikret2002)
5325 Fouling
When treating organic solutions (such as macromolecule solutions) the solute concen-tration at the membrane surface can attain a high value and a maximum concentration(the gel concentration) may be reached The formation of a gel layer on the membranesurface is often seen as fouling which causes a decrease in the permeate flux orincrease in TMP during a membrane process The gel concentration depends on thesize shape chemical structure and degree of solvation but is independent of thebulk concentration
Generally membrane fouling can be caused by a variety of constituents amongwhich are bacteria suspended solids and colloids as well as dissolved inorganicand organic compounds (Hilal Kochkodan Al-Khatib amp Levadna 2004)In MBRs fouling can be attributed to pore blocking (deposition within the pores)or to cake layer formation on the membrane surface (Jiang Kennedy van der MeerVanrolleghem amp Schippers 2003) (Figure 56) Colloidal and soluble foulants(SMP) can cause pore blocking and irreversible fouling because of their small size
Soluble microbial products(SMP)Colloids
Extracellular polymericsubstances (EPS)Polysaccarides proteinsnatural organic matter
Floc characteristics Size Structure
Biomass characteristics
Pore blocking Cake formationFigure 56 Biomass characteristics and biofouling
Membrane bioreactors for water treatment 165
533 Case study wastewater reuse by aerobic MBR in acommercial laundry
In addition to municipal wastewater treatment aerobic MBR is widely used in indus-trial wastewater treatment There are a multitude of industrial sectors such as laundrytannery textile wheat starch dairy beverage palm oil and pharmaceutical (Hoinkiset al 2012 Mutamim Noor Hassan Yuniarto amp Olsson 2013)
Several operational factors need to be considered such as HRT MLSS SRT andOLR to achieve high water quality and high water flux (ie to reduce the foulingeffect) The hydraulic performance indicates the productivity of the process in termsof water permeability or flux water quality is usually expressed in terms of COD inpermeate Section 5331 presents a typical case study on the treatment and reuse oflaundry textile wastewater
5331 Background
The process applied in the case study was developed through cooperation betweenthe Karlsruhe University of Applied Sciences and Textile Service KlingelmeyerDarmstadt Germany The wastewater was treated in a two-step process and wasrecycled in the laundryrsquos washing and rinsing process (Figure 57) After coarsescreening using a vibrating sieve to retain suspended particles the wastewater wascollected in a storage tank (WW) Subsequently the wastewater was treated in anMBR as the principal cleaning unit Air was injected into the reactor to scour themembranes and drive the biological treatment The MF permeate was stored in ancollecting tank (TWW) It was free of turbidity and considerably reduced in microbesSome of the MF permeate was treated in a second step by a low-pressure RO unit withspiral-wound modules to retain salts as well as organic residues The MF and RO
TWWTWWWWWW
Rain-Rain-waterwater SurplusSurplus
sludgesludge
SewageSewagetreatmenttreatmentplantplant
Mixed water
MBR ROTWWWWScreen
Sewagetreatmentplant
Surplussludge
Rain-water
MWMWMWMW
Figure 57 Schematic of the water reuse process (WW wastewater TWW treated wastewaterMW mixed wastewater)
166 Advances in Membrane Technologies for Water Treatment
permeates were mixed in a tank (MW) and were used for washing and rinsing Theratio of mixing (RO permeate to MBR permeate) lies between 21 and 11 dependingon the salt level of the MBR permeate Before storage the MBR and RO permeateswere treated with a small amount of chlorine dioxide to prevent the growth of germsBecause rainwater did not meet the water quality criteria for the process it wascollected in the wastewater collecting tank The integrated process generated twokinds of waste that needs to be disposed The surplus sludge was stored in a separatetank and was collected by commercial waste management enterprises The concentratefrom the RO treatment was drained into the municipal treatment plant (Hoinkis et al2012)
5332 Plant components
A commercial-scale MBR plant consists of
bull Vibrating sieve (Sweco) with a mesh size of 200 mmbull An MBRbull An RO unitbull Three storage tanks (wastewater WW MBR permeate TWW mixed MBR plus RO
permeate MW)
The MBR contained a submerged Kubota Type 510 MF plate and frame mem-branes (single plate 08 m2) with 04-mm pore size made of chlorinated polyethyleneThe MBR tank had a total volume of 126 m3 It was separated by a barrier into twocompartments connected by a spill One compartment was designed only for biodeg-radation the other was for biodegradation and membrane filtration This gavemaximum flexibility for adjusting aeration to the needs of the filtration and biodegra-dation process The biomass was circulated between compartments every other dayThe filtration compartment contained two double-deck Kubota stacks (SystemEK300) with 600 plate and frame modules (total membrane area of 480 m2) TheRO unit was fitted with six 8040 spiral-wound modules (Dow LE) The plant wascomposed of three collecting tanks (wastewater MBR drain and MBR plus RO drain)the first with an enamel steel collecting tank of 400 m3 for wastewater and two 200 m3
tanks for the MBR and MBR plus RO drains
5333 Results
The results of the case study are shown in Figures 58 through 510 Average flux wasaround 15 L(m2 h) at a permeability between 300 and 1000 L(m2 h bar) (Figure 58)After 1 year of operation without chemical cleaning the flux was lowered to 12 L(m2 h) at an average permeability of 150e300 L(m2 h bar) as can be seen inFigure 59 Figure 510 shows the COD values in feed and permeate as well as theelimination rate over more than 2 years of operation The feed COD started at600e800 mgL and increased to more than 1000 mgL whereas the COD in permeateremained at values below 100 mgL and the elimination rate was higher than 90The aeration rate in the filtration compartment of the MBR tank was 52 m3minThe biodegradation compartment was intermittently aerated by a fine diffuser at
Membrane bioreactors for water treatment 167
72 m3min The flux rate does not represent full flow capacity the plant was designedfor an eventual flux rate of 12e15 Lm2 h The concentration of MLSS in the reactorincreased from 3 to 10e15 kgm3 The COD sludge loading decreased from 014to 004 kg CODkg MLSS day The average yield factor for biomass growth wascalculated to 013 kg MLSSkg COD degraded The surplus sludge was collected ina storage tank and delivered to a commercial sludge processing unit
Typical water quality parameters of the MBR unit are shown in Table 52 whichdemonstrates that the COD removal efficiency was high (94) and the total N conver-sion rate was also relatively high (72)mdashindications of good permeate quality
54 Anaerobic MBRs
541 Areas of application
Because anaerobic digestion in wastewater treatment combines pollution reductionand energy production it is the most interesting process for industrial effluent
30FL
ux
L(m
2 h)
Per
mea
bilit
y L
(m2
h ba
r)
25
20
15
10
5
0
3000
2500
2000
1500
1000
500
0
0 7 14 21 28Time days
0 7 14TIme days
21 28
Figure 58 Permeate flux and permeability of the MBR treatment plant during a typicaloperation period of 28 days after start-up
168 Advances in Membrane Technologies for Water Treatment
30
25
20
15
10
5
0
400
350
300
250
200
150
100
50
0
0 7 14 21 28
Time days
0 7 14 21 28
Time days
Per
mea
bilit
y L
(m2
h ba
r)FL
ux
L(m
2 h)
Figure 59 Permeate flux and permeability of the MBR treatment plant during a typicaloperation period of 28 days after 1 year of operation
0 0
20
40
60
80
100
200400600800
10001200140016001800
Removal
Feed
Permeate
Timedate
230
620
0623
08
2006
231
020
06
231
220
06
230
220
07
230
420
0723
06
2007
230
820
07
231
020
07
231
220
0723
02
2008
230
420
08
230
620
08
231
020
0823
08
2008
CO
D
mg
L
CO
D re
mov
al
Figure 510 COD values in MBR feed permeate and COD removal efficiency
Membrane bioreactors for water treatment 169
treatment Historically anaerobic processes have been mainly employed for industrialhigh strength wastewater and less for municipal wastewater because (1) it is difficult toretain slow-growth anaerobic microorganisms with short HRT for the treatment oflow-strength wastewater and (2) anaerobic digestion rarely meets discharge standards(Lin et al 2013)
Similar to aerobic MBRs (see Section 53) AnMBRs combine biodegradation withthe MF and UF membrane process which provides solideliquid separation The mostsalient features of AnMBRs compared with aerobic MBRs are no need of oxygensupply biogas (methane) production and lower sludge yield which significantlylowers the operating costs
Compared with conventional anaerobic digestion AnMBRrsquos most important aspectis its ability to offer complete biomass retention owing to membrane filtration Thisprovides sufficient SRT for the methanogens Conventional anaerobic biodegradationshows relatively poor settling properties of the biomass and hence results in the loss ofbiomass into the effluent (Lin et al 2013) In conventional systems only the strategyof biofilm or granule formation in modern high-rate anaerobic reactors offers highbiomass retention but at the cost of a long start-up period and complex process con-ditions (Lin et al 2013) Many industrial wastewater characterized by hightemperature high fat oil and grease content toxicity high salinity or drastic changesin OLR eg have a negative impact on the sludge granulation process and eventuallycould lead to degranulation (Dereli Ersahin et al 2012) Table 53 compares the mainfeatures of conventional anaerobic treatment (CAT) and AnMBR treatment in whichAnMBR treatment shows better effluent quality than conventional anaerobic treat-ment but also total pretreatment and complete biomass retention Because of its ben-efits such as complete biomass retention enhanced effluent water quality andpotential net energy production AnMBR is becoming increasingly interesting formunicipal wastewater treatment (Lew Tarre Beliavski Dosoretz amp Green 2009)
Similar to aerobic MBRs two configurations are used side-stream and submerged(see Section 521) In a side-stream configuration the membrane filtration module isplaced outside the reactor and after filtration the biomass is recirculated into the
Table 52 Typical water quality parameters of the MBR unit
Parameter UnitFeeda
(wastewater)Permeatea
(microfiltrate) Removal rate ()
BOD5 mgL 390 lt9 gt97
COD mgL 1120 67 94
NeNO3 mgL 77 21 73
NeNH4thorn mgL 15 01 93
Total N mgL 160 45 72
PePO43 mgL 14 13 7
aTypically average values based on several measurements
170 Advances in Membrane Technologies for Water Treatment
reactor This offers high fluxes but at the cost of frequent cleaning and high energyconsumption around 10 kW hm3 (Le Clech et al 2006)
In addition high cross-flow velocity has a negative impact on biomass activities(Choo amp Lee 1996 Ghyoot amp Verstraete 1997) In submerged MBR the membranesare placed inside the reactor Because air cannot be used to provide cross-flow on themembranes as in aerobic MBR maintaining high permeability is more difficult inAnMBRs In many cases gas sparging with generated biogas is used to scour themembranes (Smith Stadler Love Skerlos amp Raskin 2012) Most laboratory researchor small-scale pilot trials have been conducted with side-stream AnMBRs Howeveron a large or full scale submerged systems are preferred for their lower energyconsumption
Most research work was done at laboratory and on a small pilot scale only a fewstudies have been conducted with large-scale AnMBR (Dereli Ersahin et al 2012)Although laboratory-scale studies provide invaluable information on treatabilityand membrane fouling mechanisms with scientific insight hydraulics and shear forcesacting on a small membrane module will significantly differ from a full-scale mem-brane module (Dereli Ersahin et al 2012) Table 54 shows treatment performanceon a large pilot and full scale for various industrial wastewaters All of these studiesused submerged membrane configurations
Table 53 Comparison of conventional anaerobic treatment (CAT)and anaerobic membrane bioreactor (AnMBR)
Feature CAT AnMBR
Organic removal efficiency thorn thornEffluent quality o thornOrganic loading rate thorn thornSludge production Footprint thorno Biomass retention Complete
Nutrient requirement Alkalinity requirement thorn For certain industrial
streamsthorno
Energy requirement Temperature sensitivity o o
Start-up time 2e4 months lt2 weeks
Bioenergy recovery Yes Yes
Mode of treatment Primarily pretreatment Total or pretreatment
thornthorn Excellent thorn high o moderate lowpoorModified from Lin et al (2013)
Membrane bioreactors for water treatment 171
Table 54 Treatment performance of various industrial wastewaters
Reactorvolumem3
temperatureCOLRkgCODm3 day HRTdays TSSgL
CODremoval
MembranetypePore sizeFluxLm2 hTMPbar Reference
Food processing 870033
12 29 23 994 Flat sheet04 mm25e42003
Christian et al(2011)
Potato processing 3335
2e12 35e14 40 99 Flat sheet04 mm083e5003e004
Singh Burkeand Grant(2010)
Stillage from tequilaproduction
1337
48 124 e 95 Flat sheetee
Grant ChristianVite andJuarez (2010)
Snack factory 07635
51 e 79e104 97 Hollow fibre04 mm65e8e
Diez Ramosand Cabezas(2012)
Ethanol thin stillage 1237
45e7 16 24 98 Flat sheet008 mm43 1101e02
Dereli Urbanet al (2012)
Modified from Dereli Urban et al (2012)
172Advances
inMem
braneTechnologies
forWater
Treatm
ent
542 Factors affecting performance
Regarding aerobic MBRs membrane fouling presents one of the main bottlenecks forAnMBR application (Dereli Ersahin et al 2012) Because of the fouling of organicand inorganic compounds permeate flux through the membrane can be significantlyreduced Compared with aerobic MBRs AnMBRs show lower permeate fluxes as aresult of less flocculation of the sludge and hence increased concentrations of fineparticles and colloidal solids at the membrane surface Cake layer formation was identi-fied as themost important foulingmechanism inAnMBRs (Dereli Ersahin et al 2012)
5421 Temperature
Regarding process temperature anaerobic digestion basically falls into one ofthe following categories (Rajeshwari Balakrishnan Kansal Lata amp Kishore 2000)psychrophilic (0e20 C) mesophilic (20e42 C) or thermophilic (ca 42e75 C) InAnMBRs a higher temperature can increase membrane flux owing to reducedviscosity and higher membrane permeability This is advantageous for loweringenergy requirements With lower sludge viscosity lower shear rates will be requiredto obtain the same shear stress (Jeison amp van Lier 2006) Because of higher membranepermeability the same water flux can be achieved at a lower TMP (Smith et al 2012)Higher temperatures in thermophilic operations proved to be advantageous only in short-term experiments in the long-term theywere disadvantageous (JeisonampvanLier 2007)
Jeison and van Lier (2007) showed that permeate water flux in a thermophilic oper-ation was two to three times lower compared with water flux in a mesophilic operationHence it was concluded that in the long-term adverse temperature effects on the prop-erties and composition of the sludge were more important than the beneficial influenceof membrane filtration These findings are in line with the results of Lin et al (2009)who investigated the influence of sludge properties on membrane fouling in sub-merged AnMBRs treating kraft evaporation condensate The AnMBRs were operatedunder similar hydrodynamic conditions and MLSS concentrations under both meso-philic and thermophilic conditions In the long-term the permeate water flux of themesophilic system was significantly higher than that for the thermophilic system(74 L(m2 h) versus 18 L(m2 h) respectively) (Lin et al 2009) This was attributedto the increased formation of small particles under thermophilic conditions whichresulted in a more compact and less porous cake layer on the membrane in the caseof thermophilic operation In addition Lin et al (2009) found that the concentrationof bound EPS and protein in the cake layer was higher under thermophilic conditionswhich might be because of the increased decay rate of bacteria at elevated tempera-tures Furthermore the permeate water quality of the mesophilic system was consid-erably better than that of the thermophilic reactor
5422 Organic loading rate
In theory a high OLR and short HRT can be employed in AnMBRs However OLRshould always be evaluated together with SRT and sludge activity (Dereli Ersahinet al 2012) A variety of experimental studies carried out up to very high OLR still
Membrane bioreactors for water treatment 173
showed high COD removal efficiency Treatment of simulated wastewater from thepetrochemical industry demonstrated high COD removal efficiency at OLR up to25 kg CODm3day (van Zyl Wenzel Ekama amp Riedel 2008) Similar high treat-ment efficiency has been shown for slaughterhouse and palm oil wastewater aswell at ORL up to 133 and 11 kg CODm3day respectively (Abdurraham Rosli ampAzhari 2011 Saddoud amp Sayadi 2007)
5423 Hydraulic residence time and SRT
The performance of AnMBR and membrane fouling is affected by HRT and SRTGenerally speaking a low HRT is preferred because it reduces tank volume and hencethe overall footprint of the system A high SRT is required to achieve better treatmentperformance especially at lower temperatures (OFlaherty Collins amp Mahony 2006)However increasing the SRT causes higher SMP and EPS production and results in ahigher propensity for membrane fouling Moreover increasing the SRT under constantHRT increases the biomass concentration resulting in lower permeate flux (Smith et al2012) A variety of publications are concerned with the study of treatment performanceon HRT An insignificant decrease in COD removal efficiency (ca 5) was observedby Hu and Stuckey when they studied the treatment of simulated domestic wastewaterat mesophilic temperatures (35 C) lowering HRT from 48 h down to 3 h (Hu ampStuckey 2006) Even at an HRT of 3 h COD removal efficiency was higher than90 Several other studies similarly concluded that HRT had little effect on AnMBRpermeate quality (Smith et al 2012) Those studies suggested that adequate AnMBRperformance may be obtained at relatively short HRTs even at lower temperaturesbut that a lower limit on HRT may exist primarily owing to concerns regarding mem-brane fouling (Smith et al 2012) For AnMBR SRT is an easily controllable opera-tional parameter because membrane separation allows complete retention of biomassHuang et al studied low-strength domestic wastewater and observed better treatmentperformance at a longer SRT but at the cost of increased membrane fouling as a resultof higher biomass concentrations and SMP production (Huang Ong amp Ng 2011)Despite this many publications observed that EPS is a major contributor to directfouling However the role of EPS quantity and characteristics in membrane foulingas a function of SRT is not well understood in AnMBRs (Smith et al 2012)
5424 Membrane properties
Fouling in AnMBRs is very much affected by membrane properties such as materialspore size and surface characteristics Although hydrophobic membrane materials aremore durable under harsh chemical and thermal conditions hydrophilic membranesare typically favoured because they are less prone to fouling (Dereli Urban et al2012) Membrane materials used for AnMBRs can be classified into three major cat-egories ceramic metallic and polymeric In early experimental studies ceramic wasthe most widely used material for AnMBRs (Lin et al 2013) In AnMBRs metallicmembranes have also been applied because of their better hydraulic performance bet-ter fouling recovery and better tolerance to oxidation and high temperature (Kim ampJung 2007 Zhang et al 2005) However because of economic constraints in
174 Advances in Membrane Technologies for Water Treatment
commercial applications cheaper polymeric materials have gained more interestTypically materials such as PVDF PES PE and PP are applied with pore sizes inthe MF or UF range (003e1 mm) and in hollow fibre flat sheet or tubular configu-rations In general preferred polymeric membrane materials for MBRs are PVDFand PES which account for around 75 of total products on the market including9 of the 11 commercially most important products (Santos amp Judd 2010) Becauseof the high fouling propensity modification of the hydrophobic membrane surfaceby hydrophilic coating is a main goal in reducing fouling propensity Choo et al(2000) modified the surface of an organic membrane by grafting with2-hydroxyethyl methacrylate (HEMA) which resulted in 35 flux increase A135 flux increase was reported by Sainbayar Kim Jung Lee and Lee (2001)who modified the surface of a PP membrane material by 70 grafting withHEMA Kochan Wintgens Melin and Wong (2009) studied surface coating viathe adsorption of surfactants in which different UF flat-sheet membranes were coatedwith branched poly(ethyleneimine) poly(diallyldimethyl ammonium chloride) andpoly(allylamine chloride) and filtered with sludge supernatant The experimentsshowed lower fouling but also low physical tolerance and chemical stability underMBR conditions To overcome this drawback Asatekin et al (2006) developed aself-assembling technique by coating commercial PVDF UF membranes with theamphophilic graft copolymer PVDFegraft-polyoxyethylene methacrylate
543 Case study treatment of high-strength industrialwastewater using a full-scale AnMBR
Most AnMBR research has been carried out at the laboratory or small pilot scale onlya few studies have been concerned with large- or full-scale applications Most full-scale AnMBR installations were established in Japan Kanai reported about 14large-scale AnMBR implementations in the food and beverage industries in Japan(Kanai Ferre Wakahara Yamamoto amp Moro 2010)
A typical case study on the treatment of high-strength industrial wastewater using afull-scale AnMBR is presented here
The study is the first AnMBR installation in North America and the largest AnMBRinstallation in the world It was built at Kenrsquos Foods in Massachusetts (ChristianGrant McCarthy Wilson amp Mills 2011)
Due to lack of space positive economics and the ability to provide additional ca-pacity for flow and organic load the system was converted from Kenrsquos Foodsrsquo existinganaerobic process to AnMBR in July 2008 It provides waste treatment but also a largegain in energy production from salad dressing and barbeque sauce wastewater
The process uses Kubota flat-plate membrane cartridges (with nominal pore size of04 mm) that are submerged directly into the anaerobic biomass and completely blockall suspended solids from escaping to the effluent Figure 511 shows a general processflow diagram of the AnMBR process
The design of this AnMBR system has an effluent flow rate of 475 m3day with39000 mgL COD 18000 mgL BOD and 12000 mgL total suspended solids(TSS) under mesophilic conditions ensured by an average operating temperature of
Membrane bioreactors for water treatment 175
33 C for all four anaerobic membrane tanks a pH of 69 01 for anaerobic sludgeacclimatization an operating volumetric flux rate in the range of 006e010 m3m2
day and a TMP fluctuating between 0005 and 01 bar Once the TMP reaches01 bar membrane cleaning is required (three of four membrane compartments werecleaned once during the first 20 months of operation) Table 55 presents the rawwastewater characteristics and average effluent quality after 20 months of runningthe pilot test
In Table 55 this AnMBR process consistently provides an effluent with undetect-able TSS concentrations and COD and BOD concentrations of 209 39 and16 5 mgL corresponding to COD and BOD removals of 994 and 999 respec-tively By dint of biogas scouring across the membrane surface a very low rate ofmembrane fouling is an important result in this study because membrane foulingunder anaerobic conditions is a considerable issue for the AnMBR process
Anaerobic MBR
Biogas
Feed
Methanefermentation
tank
Separation tankwith submerged
membrane
Wastewatertreatmentequipment
Powergenerating
facility
Sludgedehydrationequipment
Pretreatmentequipment
Optional
Figure 511 General process flow diagram of the AnMBR processModified from Christian et al (2011)
Table 55 Raw wastewater and AnMBR effluent characteristics(July 2008 to April 2010)
Parameter Raw wastewater AnMBR effluent Removals
Flow rate m3day 300 300 e
BOD mgL 18000 20 999
COD mgL 33500 210 994
TSS mgL 10900 lt1 100
pH e 705 e
176 Advances in Membrane Technologies for Water Treatment
Upgrading the existing anaerobic treatment system to an AnMBR demonstratedthe simplicity and suitability of the process for a wastewater treatment system that pre-viously experienced difficulty with solids management and sludge settling ability(Christian et al 2011)
This installation at Kenrsquos Foods can be considered an ideal technology forthe development of renewable energy from waste especially when containing ahigh suspended solids concentration
55 Forward osmosis MBRs
The combination of FO and activate sludge treatment is a relatively new process FO isa natural process driven by the osmotic pressure difference that retains solutes but al-lows water to permeate through a semipermeable membrane (Cath Childress ampElimlech 2006) FO is attractive because of the high rejection efficiency of its mem-branes hence the drawback of conventional MF and UF MBRs can be overcomewhich are limited in the retention of low-molecular-weight compounds (LMWC)To increase efficiency in LMWC removal conventional MBRs need to be combinedwith downstream RONF units which results in high capital cost and energy consump-tion (Alturki et al 2010)
In FO-MBRs treated water is drained without a pump by osmotic pressure into adraw solution with higher solute concentration than the feed solution this results indilution of the draw solution Therefore the draw solution needs to be concentratedby either mechanical (RO) or thermal processes (distillation) In doing so the treatedclean water can be drained off the system Figure 512 shows a schematic of theprocess FO-MBR offers the possibility of high cleaning efficiency in wastewatertreatment and reuse Achilli Cath Marchand and Childress (2009) reported morethan 99 of organic carbon and more than 98 of NH4eN removal combining thebiological and FO processes using flat-sheet cellulose triacetate FO membranes(Achilli et al 2009) With this process the draw solution is reconcentrated by RO
Draw solution
Reconcentration
Treated water
MBR
Feed
Figure 512 Schematic of FO-MBR system
Membrane bioreactors for water treatment 177
Most research work on FO-MBRs was conducted on the laboratory scale There areonly a few examples of larger pilot-scale studies Quin et al (2009) studied optimi-zation of an FO-MBR on the pilot scale using NaCl and MgSO4 salts in draw solu-tions The NaCl performed at much higher efficiency than MgSO4 as an osmoticagent owing to a greater solute diffusion coefficient
Chen et al (2014) combined a submerged AnMBR with an FO membrane to treatsynthetic low-strength wastewater The FO-AnMBR showed greater than 96 carbonremoval and 100 of total phosphorous and 62 of NH4eN removal This was betterremoval efficiency than for the conventional AnMBR The most salient feature ofFO-AnMBR is its ability to provide biogas that can be used as an energy source todrive the combined process
56 Conclusion and perspectives
Over the past 2 decades the development of MBRs has made great progress In partic-ular the application of aerobic MBRs showed rapid growth in many municipal andindustrial large-scale implementations This is mainly because of their potential forrecycling and reusing grey water and industrial effluents Major improvements havebeen achieved for aerobic MBRs (Brepols 2011)
bull Improved module hydraulics and optimized process control helped reduce energy consump-tion for air scouring
bull Advances in membrane production led to improvement in materials stability and resiliencewhereas market prices have come down
bull In terms of investment cost todayrsquos MBR plants are not necessarily more expensive thancomparable conventional activated sludge plants
MBR technology is expected to grow in the near future because of the increasingscarcity of water worldwide which makes wastewater reclamation necessary Thiswill be further aggravated by climate change Based on the estimated global MBRmarket of US$8382 million in 2011 the MBR market is projected to grow at anaverage rate of 224 reaching a total market size of US$344 billion in 2018(WaterWorld 2012) The Chinese market is expected to grow at an even higher rateof 289 with the key drivers of high economic growth rate increased confidencein technology and public awareness as well as a strong need for wastewater reclama-tion (Sartorius Walz amp Orzanna 2013)
Besides China the MBR market in Middle Eastern and North African countries isexpected to witness a high growth trajectory in the next 5e10 years because of a largenumber of projects commissioned in medium and large range plants with capacitiesranging between 5000 and 50000 m3day (Frost amp Sullivan 2012)
Future research and development should focus on further lowering energy demands(operating costs) and membrane costs (capital costs) as well as FO-MBRs because oftheir advantages regarding the high rejection of low-molecular-weight contaminantsand their lower fouling propensity
178 Advances in Membrane Technologies for Water Treatment
List of abbreviations
AnMBR Anaerobic MBRBOD Biological oxygen demandCAT Conventional anaerobic treatmentCOD Chemical oxygen demandEPS Extracellular polymeric substancesFS Flat sheetFM Feed to microorganismsHF Hollow fibreHRARs High-rate anaerobic reactorsHRT Hydraulic residence timeiMBR Immersed MBRLMWC Low molecular-weight compoundsMBR Membrane BioreactorMF MicrofiltrationMT Multi-tubularMLD Million liter per dayMLSS Mixed liquor suspended solidsMW Mixed wastewaterOLR Organic loading ratePCP PentachlorophenolPE Polyethylenepe Population equivalentPP PolypropylenePES PolyethylsulphonePS PolysulphonePVDF Poly vinylidene diflurideRO Reverse osmosisSL Sludge loadingSMP Soluble microbial productssMBR Side-stream MBRSRT Sludge retention timeTMP Transmembrane pressureTSS Total suspended solidsTWW Treated wastewaterUF UltrafiltrationWW Wastewater
References
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Achilli A Cath T Y Marchand E A amp Childress A E (2009) The forward osmosis mem-brane bioreactor A low fouling alternative to MBR processes Desalination 239 10e21
Membrane bioreactors for water treatment 179
Ahmed Z Cho J Lim B-R Song K-G amp Ahn K-H (2007) Effects of sludge retentiontime on membrane fouling and microbial community structure in a membrane bioreactorJournal of Membrane Science 287 211e218
Alturki A A Tadkaew N McDonald J A Khan S J Price W E et al (2010) CombiningMBR and NFRO membranes filtration fort the removal of trace organics in indirect potablewater reuse applications Journal of Membrane Science 365 206e215
Asatekin A Menniti A Kang S Elimelech M Morgenroth E amp Mayes A M (2006)Antifouling nanofiltration membranes for membrane bioreactors from self-assembling graftcopolymers Journal of Membrane Science 285 81e89
Baker R W (2004) Membrane technology and applications (2nd ed) New York WileyBioNexGen (2013) Development of the next generation membrane bioreactor system www
bionexgeneu Accessed 60913Brepols C (2011) Operating large scale membrane bioreactors for municipal wastewater
treatment London UK IWA PublishingCath T Y Childress A E amp Elimlech M (2006) Forward osmosis Principles applications
and recent developments Journal of Membrane Sciences 281 70e87Chang I-S amp Lee C-H (1998) Membrane filtration characteristics in membrane coupled
activated sludge systemmdashthe effect of physiological states of activated sludge on mem-brane fouling Desalination 120 221e233
Chang J S Chang CY Chen A C Erdei L ampVigneswaran S (2006) Long term operationof submerged membrane bioreactor for the treatment of high strength acrylonitrile-butadiene-styrene (ABS) wastewater Effect of hydraulic retention time Desalination 19145e51
Chen L Gu Y Cao C Zhang J Ng J W amp Tang C (2014) Performance of a submergedanaerobic membrane bioreactor with forward osmosis membrane for low-strength waste-water treatement Water Research 50 114e123
Cho J (2004) Model development for a submerged membrane bioreactor with the effect ofextracellular polymeric substances Seoul Seoul National University
Choo K H Kang I J Joon S H Park H Kim J H Adia S et al (2000) Approaches tomembrane fouling control in anaerobic membrane bioreactors Water Science Technology41 363e371
Choo K H amp Lee C H (1996) Membrane fouling mechanisms in the membrane coupledanaerobic bioreactor Water Research 30 1771e1789
Christian S Grant S McCarthy P Wilson D ampMills D (2011) The first two years of full-scale anaerobic membrane bioreactor (AnMBR) operation treating high-strength industrialwastewater Proceedings of IWA 12th world congress on anaerobic digestion GuadalajaraMexico
Defrance L amp Jaffrin M Y (1999) Reversibility of fouling formed in activated sludgefiltration Journal of Membrane Science 157 73e84
Dereli R K Ersahin M E Ozgun H Ozturk I Jeison D van der Zee F et al (2012)Potentials of anaerobic membrane bioreactors to overcome treatment limitations induced byindustrial wastewaters Bioresource Technology 122 160e170
Dereli R K Urban D R Heffernan B Jordan J A Ewing J Rosenberger G T et al(2012) Performance evaluation of a pilot scale anaerobic membrane bioreactor (AnMBR)treating ethanol thin stillage Environmental Technology 33 1511e1516
Diez V Ramos C amp Cabezas J L (2012) Treating wastewater with high oil and greasecontent using an anaerobic membrane bioreactor (AnMBR) filtration and cleaning assaysWater Science and Technology 65(10) 1847e1853
180 Advances in Membrane Technologies for Water Treatment
Frost amp Sullivan (2012) The Middle East and North Africa membrane bioreactor market to growat a CAGR of 1777 per cent by 2015 httpwwwfrostcomprodservletpress-releasepagdocidfrac14251074704 Accessed 07012015 at 1132h
Gander M Jefferson B amp Judd S (2000) Aerobic MBRs for domestic wastewater treatment Areview with cost considerations Separation and Purification Technology 18(2) 119e130
Ghyoot W R amp Verstraete W H (1997) Coupling membrane filtration to anaerobic primarysludge digestion Environmental Technology 18 569e580
GIA Global Industrial Analysts Inc (2013) Global membrane bioreactors market to reachUS$888 million by 2017 httpwwwprwebcomreleasesmembrane_bioreactorsMBRprweb8973361htm Accessed 221113
Grant S Christian S Vite E amp Juarez V (2010) Anaerobic membrane bioreactor(AnMBR) pilot-scale treatment of stillage from tequila production Proceedings of IWA12th world congress on anaerobic digestion Guadalajara Mexico
Harada H Momonoi K Yamazaki S amp Takizawa S (1994) Application of anaerobic UFmembrane reactor for treatment of a wastewater containing high strength particulate or-ganics Water Science and Technology 30(12) 307e319
Hasar H Kinaci C euroUnleurou A Togrul H amp Ipek U (2004) Rheological properties ofactivated sludge in a sMBR Biochemical Engineering Journal 20 1e6
Hilal N Kochkodan V Al-Khatib L amp Levadna T (2004) Surface modified polymericmembranes to reduce (bio) fouling A microbiological study using E coli Desalination167 293e300
Hoinkis J Deowan S A Panten V Figoli A Huang R R amp Drioli E (2012) Membranebioreactor (MBR) technology e a promising approach for industrial water reuse ProcediaEngineering 33 234e241
Hu A Y amp Stuckey D C (2006) Treatment of dilute wastewaters using a novel sub-merged anaerobic membrane bioreactor Journal of Environmental Engineering 132190e198
Huang Z Ong S L amp Ng H Y (2011) Submerged anaerobic membrane bioreactor for low-strength wastewater treatment Effect of HRT and SRT on treatment performance andmembrane fouling Water Research 45 705e713
Jeison D amp van Lier J B (2006) Cake layer formation in anaerobic submerged membranebioreactors (AnSMBR) for wastewater treatment Journal of Membrane Science 284227e236
Jeison D amp van Lier J B (2007) Thermophilic treatment of acidified and partially acidifiedwastewater using anaerobic submerged MBR Factors governing long-term operationalflux Water Research 41 3868e3879
Jiang T (2007) Characterization and modelling of soluble microbial products in membranebioreactors (PhD thesis) (pp 241e248) Ghent University Belgium
Jiang T Kennedy M D van der Meer W G J Vanrolleghem P A amp Schippers J C(2003) The role of blocking and cake filtration in MBR fouling Desalination 157(1e3)335e343
Judd S (2006) The MBR book Principles and applications of membrane bioreactors for waterand wastewater treatment Cranfield University UK Elsevier ISBN 978-1-85617-481-7
Judd S (2011) The MBR book Principles and applications of membrane bioreactors for waterand wastewater treatment Cranfield University UK Elsevier
Kanai M Ferre V Wakahara S Yamamoto T amp Moro M (2010) A novel combinationof methane fermentation and MBR-Kubota submerged anaerobic membrane bioreactorprocess Desalination 250 960e967
Membrane bioreactors for water treatment 181
Khoshfetrat A B Nikakhtari H Sadeghifar M amp Khatibi M S (2011) Influence of organicloading and aeration rates on performance of a lab-scale upflow aerated submerged fixed-film bioreactor Process Safety and Environmental Protection 89 193e197
Kim J O amp Jung J T (2007) Performance of membrane-coupled organic acid fermenter fortthe resource recovery from municipal sewage sludge Water Science and Technology 55245e252
Kochan J Wintgens T Melin T amp Wong J (2009) Characterisation and filtrationperformance of coating-modified polymeric membranes used in membrane bioreactorsChemical Papers 63 152e157
Kornboonraksa T amp Lee S H (2009) Factors affecting the performance of membranebioreactor for piggery wastewater treatment Bioresource Technology 100 2926e2932
Laeraa G Polliceb A Saturnob D Giordanob C amp Sandullia R (2009) Influence ofsludge retention time on biomass characteristics and cleaning requirements in a membranebioreactor for municipal wastewater treatment Desalination 236 104e110
Le Clech P Chen V amp Faine T A G (2006) Fouling in membrane bioreactors usedin wastewater treatment Journal of Membrane Science 284 17e53
Le Clech P Jefferson B amp Judd S J (2003) Impact of aeration solids concentration andmembrane characteristics on the hydraulic performance of a membrane bioreactor Journalof Membrane Science 218 117e129
Lee W Kang S amp Shin H (2003) Sludge characteristics and their contribution to micro-filtration in submerged membrane bioreactors Journal of Membrane Science 216217e227
Lesjean B Ferre V Vonghia E amp Moeslang H (2009) Market and design considerationsof the 37 larger MBR plants in Europe Desalination and Water Treatment 6 227
Lesjean B amp Huisjeslow E H (2008) Survey of the European MBR market Trends andperspectives Desalination 231 71e81
Lew B Tarre S Beliavski M Dosoretz C amp Green M (2009) Anaerobic membranebioreactor (AnMBR) for domestic wastewater treatment Desalination 243 251e257
Lin H Peng W Zhang M Chen J Hong H amp Zhang Y (2013) A review on anaerobicmembrane bioreactors Applications membrane fouling and future perspectives Desali-nation 314 169e188
Lin H J Xie K Mahedran B Bagley D M Leung K T Liss S N et al (2009) Sludgeproperties and their effects in submerged anaerobic membrane bioreactors (SAnMBR)Water Research 43 3827e3837
Liwarska-Bizukojc E ampBizukojcM (2005) Digital image analysis to estimate the influence ofsodium dodecyl sulphate on activated sludge flocs Process Biochemistry 40 2067e2072
Lousada-Ferreira M Geilvoet S Moreau A Atasoy E Krzeminski P vanNieuwenhuijzen A et al (2010) MLSS concentration Still a poorly understoodparameter in MBR filterability Desalination 250 618e622
Manigas L (2008) Use of membrane bioreactor for the bioremediation of groundwaterpolluted by chlorinated compounds (PhD thesis) University of Cagliari Italy
Masse A Sperandio M amp Cabassud C (2006) Comparison of sludge characteristics andperformance of a submerged membrane bioreactor and an activated sludge process at highsolids retention time Water Research 40 2405e2415
The MBR Site (2014) Immersed anaerobic MBRs are they viable wwwthembrsitecomAccessed 150314
Meng F Chae S R Drews A Kraume M Shin H S amp Yang F (2009) Recent advancesin membrane bioreactors (MBRs) Membrane fouling and membrane material WaterResearch 43 1489e1512
182 Advances in Membrane Technologies for Water Treatment
Meng F Shi B Yang F amp Zhang H (2007) Effect of hydraulic retention time on membranefouling and biomass characteristics in submerged membrane bioreactors Bioprocess andBiosystems Engineering 30 359e367
Meng F amp Yang F (2007) Fouling mechanisms of deflocculated sludge normal sludge andbulking sludge in membrane bioreactor Journal of Membrane Science 305 48e56
Michael L S amp Fikret K (2002) Bioprocess engineering Basic concepts (2nd ed) UpperSaddle River NJ Prentice Hall
Mulder M (1996) Basic principles of membrane technology (2nd ed) Boston KluwerAcademic ISBN 0-7923-4248
Mutamim N S A Noor Z Z Hassan M A A Yuniarto A amp Olsson G (2013)Membrane bioreactor Applications and limitations in treating high strength industrialwastewater Chemical Engineering Journal 225 109e119
Ng H Y amp Hermanowicz S W (2005) Membrane bioreactor operation at short solidsretention times Performance and biomass characteristics Water Research 39 981e992
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Pidou M (2006)Hybrid membrane processes for water reuse (PhD thesis) School of AppliedScience Department of Sustainable Systems Centre for Water Science CranfieldUniversity UK
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Qin J J Oo M H Tao G amp Kekre K A (2007) Feasibility study on petrochemicalwastewater treatment and reuse using submerged MBR Journal of Membrane Science293 161e166
Rajeshwari K V Balakrishnan M Kansal A Lata K amp Kishore V V N (2000) State-of-the-art of anaerobic digestion technology for industrial wastewater treatment Renewableand Sustainable Energy Reviews 4 135e156
Robinsion T (2003) The MBR for industrial effluent Wakefield UK CIWEM amp Aqua EnviroConference
Rosenberger S Kruger U Witzig R Manz W Szewyk U amp Kraume M (2002) Per-formance of a bioreactor with submerged membranes for aerobic treatment of municipalwaste water Water Research 36 413e420
Rosenberger S Kubin K amp Kraume M (2002) Rheology of activated sludge in membranebioreactors Engineering in Life Sciences 2 269e275
Saddoud A amp Sayadi S (2007) Application of acidogenic fixed bed reactor prior to anaerobicmembrane bioreactor for sustainable slaughterhouse wastewater treatment Journal ofHazardous Materials 149 700e706
Sainbayar A Kim J S Jung W J Lee Y S amp Lee C H (2001) Application of surfacemodified polypropylene membranes to an anaerobic membrane bioreactor EnvironmentalTechnology 22(9) 1035e1042
Santos A amp Judd S (2010) The commercial status of membrane bioreactors for municipalwastewater Separation Science and Technology 45 850e857
Sartorius C Walz R amp Orzanna R (2013) Lead market for membrane bioreactor (MBR)technologyeChinarsquos second-mover strategy for the development and exploitation of its leadmarket potential Karlsruhe Fraunhofer Institute for Systems and Innovation Research ISIhttpkooperationenzewdefileadminuser_uploadRedaktionLead_MarketsWerkstattberichteWB_11_MBR_Sartorius_et_alpdf Accessed 010314
Membrane bioreactors for water treatment 183
Shen L Zhou Y Mahendran B Bagley D M amp Liss S N (2010) Membrane fouling in afermentative hydrogen producing membrane bioreactor at different organic loading ratesJournal of Membrane Science 360(1) 226e233
Shimizu Y Okuno Y Uryu K Ohtsubo S ampWatanabe A (1996) Filtration characteristicsof hollow fiber microfiltration membranes used in MBR for domestic wastewater treatmentWater Research 30(10) 2385e2392
Singh K Burke D amp Grant S (2010) Anaerobic flat sheet membrane bioreactor treating foodprocessing wastewater Pilot-scale performance Proceedings of IWA 12th world congresson anaerobic digestion Guadalajara Mexico
Smith A L Stadler L B Love N G Skerlos S J amp Raskin L (2012) Perspectiveson anaerobic membrane bioreactor treatment of domestic wastewater A critical reviewBioresource Technology 122 149e159
Trussell R S Merlo R P Hermanowicz S W amp Jenkins D (2006) The effect of organicloading on process performance and membrane fouling in a submerged membrane biore-actor treating municipal wastewater Water Research 40 2675e2683
Ueda T amp Hata K (1999) Domestic wastewater treatment by a submerged membranebioreactor with gravitational filtration Water Research 33(12) 2888e2892
Visvanathan C Thu L N Jegatheesan V amp Anotai J (2005) Biodegradation of penta-chlorophenol in a membrane bioreactor Desalination 183 455e464
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Yamamoto K Hiasa M Mahmood T amp Matsuo T (1989) Direct solid-liquid separationusing hollow fiber membrane in an activated-sludge aeration tank Water Science andTechnology 21 43e54
Zhang S QuY Liu Y Yang F ZhangX FurukawaK et al (2005) Experimental study ofdomestic sewage treatment with a metal membrane bioreactor Desalination 177 83e93
Zheng X Zhou Y Chen S Zheng H amp Zhou C (2010) Survey of MBR market Trendsand perspectives in China Desalination 250 609e612
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184 Advances in Membrane Technologies for Water Treatment
Advances in electrodialysis forwater treatment 6B Van der BruggenKU Leuven Leuven Belgium
61 Introduction
Electrodialysis is a membrane process driven by a difference in electrical potentialover a membrane stack in which charged compounds are removed from a feed solu-tion It is a process with a relatively long history with several papers published in the1950s on applications such as the demineralization of sugar solutions (Anderson ampWylam 1956) desalination (Dewhalley 1958) and protein separation (Laurence1956) which are still among applications of interest more than half a century laterWhat is remarkable is that although the direction has not substantially changed the in-terest in electrodialysis has boomed during the past decade The number of publica-tions on electrodialysis has doubled in 10 years and has increased by a factor of 10in 30 years Moorersquos law seems to be valid for electrodialysis as well This is partlythe result of the global tendency to publish more and more papers not necessarilyof higher or even the same quality Nevertheless it is interesting to observe thechanges in research interest over the years The most conservative conclusion fromthe numbers is that electrodialysis has remained a process of particular interest in termsof its potential for separation as well as in terms of applications over 6 decades It wasalready considered a mature process in 1972 (Solt 1972) and it is not surprising thatthe research topics have only partially shifted since the early days Scaling problemsfor example were reported in one of the earliest publications on electrodialysis(Cooke 1958) and are still on the research agenda today Examples include the devel-opment of pretreatment methods such as pre-softening with a pellet reactor (TranJullok Meesschaert Pinoy amp Van der Bruggen 2013) and the use of electrochemicalprocesses to remove scaling from membranes (Zaslavschi Shemer Hasson amp Semiat2013) The separation of proteins was already understood in earlier days to have po-tential (Laurence 1956) although the breakthrough in this area came much latermany applications today are in the dairy industry for whey protein concentrationmilk protein standardization etc (Daufin et al 2001) Applications and methodshowever are much more advanced today Electrodialysis with ultrafiltration mem-branes for example allows for advanced separation of peptides by exploiting the com-bined effect of charge and size separation by fine-tuning the pH and cutoff of themembrane (Firdaous et al 2009 Roblet Doyen Amiot amp Bazinet 2013) Electrodi-alysis in that context would touch the performance of a nanofiltration membranealthough the potential for specific separations is different (Langevin Roblet MoresoliRamassamy amp Bazinet 2012) Coupling nanofiltration and electrodialysis (with
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400006-XCopyright copy 2015 Elsevier Ltd All rights reserved
ultrafiltration membranes) in a single process line would optimize the overall separa-tion performance and allow the production of more specific peptide fractions The prin-ciple of electrodialysis can be further extended in view of the improved separation ofpeptides In the electrophoretic membrane contactor a porous membrane is used toestablish contact across two flowing liquids between which an electrically drivenmass transfer takes place (Galier amp Roux-de Balmann 2011)
In terms of applications comparison with the early days yields more surprisesUrine separation is currently a topic of considerable interest Historically the use ofelectrodialysis for salt separation was described in 1960 in a publication that appearedin Nature (Wood 1960) At that moment it did not attract much interest (it was citedan impressive three times) but today urine separation has been revived Scientific in-terest has shifted and the separation needed today is more challenging than only theremoval of salts the focus is rather on recycling of nutrients while micropollutantsshould be kept out of the eventual product (Pronk Biebow amp Boller 2006) Electro-dialysis is probably a critical process to achieve this although none of the processessuggested for urine separation including electrodialysis have so far advanced beyondthe laboratory stage a combination of solutions may be needed to meet the require-ments (Maurer Pronk amp Larsen 2006) A combination of electrodialysisprecipitation and evaporation has perspectives of application in developing countriesthis can be linked to local business development which is an important prerequisitefor the implementation of new technologies in developing countries (Pronk ampKone 2009)
Despite the continuous presence of electrodialysis on the separations scene it is byno means a static process Apart from the logical shift in attention for various applica-tions the main advances in electrodialysis are related to the breakthrough of electro-dialysis with bipolar membranes (which allows for integrated reactions in situcombination of fermentation and electrodialysis and in general the conversion ofsalts to their corresponding acids and base in a wide range of applications) and theuse of non-classical membrane materials and configurations for advanced separationsThese are described in more detail in this chapter Other advances are the integration ofelectrodialysis with traditional unit operations and other membrane separations in thechemical biochemical food and pharmaceutical industries (Xu amp Huang 2008) thedevelopment of reverse electrodialysis to capture renewable energy from mixing saltand freshwater (Vermaas et al 2013) and ionic liquideassisted electrodialysis forconcentrating acidifying and removing organic salts from aqueous solution(Lopez amp Hestekin 2013) or for the selective recovery of lithium from seawater(Hoshino 2013)
In addition even after over half a century the fundamental understanding ofall phenomena related to electrodialysis is still a point of attention For examplemechanisms for overlimiting current and concentration polarization are not yet well un-derstood and only recently was a first-principles model involving the NernstePlanckePoisson equations fully coupled with the NaviereStokes equations proposed(Urtenov et al 2013) At the same time the leaky membrane model was proposed as asimplified model based on the NernstePlanck equation (Dydek amp Bazant 2013) Otherefforts aim to translate the process fundamentals to a simulation model (Tanaka 2013)
186 Advances in Membrane Technologies for Water Treatment
which surprisingly has not yet converged for electrodialysis To understand the flow inan electrodialysis stack computational fluid dynamics can be used (Tamburini La Bar-bera Cipollina Ciofalo amp Micale 2012)
This chapter describes the fundamentals of electrodialysis in brief with the aim ofunderstanding the process principles (for a more in-depth analysis of the fundamentalsthe reader is referred to the publications mentioned above) followed by a review of theadvances of electrodialysis in water treatment as stated previously
62 Fundamentals of electrodialysis for water treatment
Two kinds of membranes are used in electrodialysis anion exchange and cationexchange These two membrane types are alternated in a membrane stack so that arepeating unit is obtained consisting of a compartment with an anion exchangemembraneon the left side and a cation exchange membrane on the right followed by anothercompartment with an anion exchange membrane on the right side and a cation exchangemembrane on the left
Over this membrane stack a difference in electrical potential is applied using elec-trodes at both ends of the stack The feed solution is pumped through the stack Cationsmigrate in the direction of the negatively charged cathode and are allowed to migratethrough the first cation exchange membrane The next membrane is an anion exchangemembrane so that the cation cannot migrate further Anions migrate in the direction ofthe positively charged anode and can migrate through the first anion exchange mem-brane The next membrane is a cation exchange membrane so that the anion cannotmigrate further (Figure 61) The membranes are in principle not permeable for wateralthough osmosis and electro-osmosis are known to occur in an electrodialysis stack
Cathode(ndash)
Anode(+)
+
ndash ndash ndash ndash ndash
+ + + + +
Electrode rinse
ConcentrateDiluate
Figure 61 Ion migration in a salt solution through the membrane stack
Advances in electrodialysis for water treatment 187
In this way positive as well as negative ions are removed from the feed solutionwhereas the neighboring compartments are concentrated
The flow through the feed compartments is usually denoted as the diluate streamthe flow with increasing concentration is denoted as the concentrate stream
The combination of a series of cells (consisting of an anion exchange and a cationexchange membrane and a diluate compartment and a concentrate compartment) isnecessary to decrease the time needed to purify a given stream (in a batch configurationwith recycling) or decrease the size of the membranes in the system (in a flow-throughsystem)
Because of the flow of ions through the stack the electrical current is transferredThe extent to which the electrical current is effectively used for migration of ionsthat should be removed determines the efficiency of electrodialysis
Apart from the diluate and concentrate flow an electrode rinse solution is generallyapplied The electrodes are positioned in separate compartments to protect the elec-trode material At the cathode hydrogen gas and hydroxyl ions may be formed owingto the dissociation of water (after application of a voltage) The hydroxyl ions maypossibly damage the electrode when the electrode material is not carefully selectedAn acid is usually added to the rinse liquid of the cathode to prevent damage
At the anode the electrode material is at risk because of the formation of metal ox-ides at the electrode surface (corrosion) which can then dissolve in an acid environ-ment Possible materials with sufficient resistance are graphite stainless steel nickelalloys or a Platinum coating Reactions with a metal M are
Mthorn xOH5MethOHTHORNx thorn xe
2Mthorn 2xOH5M2Ox thorn xH2Othorn 2xe
Because of the consumption of OH hydrogen ions are left which make the solu-tion more acidic This dissolves metal ions
MethOHTHORNx thorn xHthorn5Mxthorn thorn xH2O
M2Ox thorn xHthorn52Mxthorn thorn xH2O
H2 Cl2 and O2 may be formed as a result of oxidation reactions at the anode Thesecause negative effects Therefore it is important to make sure that negative ions (suchas Cl) do not have access to the electrode compartments For the cathode this is notproblematic because it repels anions but the anode needs sufficient protection Thiscan be achieved by using two cation exchange membranes The electrode rinse solu-tion usually contains SO2
4 which would not cause problems at the anodeMembranes are often exposed to aggressive compounds Furthermore because of
the electrical resistance the temperature may increase Many feed flows (eg surfacewater) contain suspended solids These may damage the membrane as the result of fric-tion and scouring The chemical thermal and mechanical resistance of the membranesis therefore an important parameter to consider
188 Advances in Membrane Technologies for Water Treatment
Between the membranes a spacer is positioned that is usually made of a polymer(polyethylene polyvinylchloride (PVC) or specialty polymers) The spacers providemechanical support to the membranes They are made in a maze configuration thatensures the membranes are not pressed onto each other They also provide goodflow dynamics by promoting turbulence
When the membranes are brought into the solution a chemical potential differencearises between the solution and the inner structure of the membrane This brings ionsof the same sign as the fixed ions into the membrane These are called co-ions For thesame reason counter-ions migrate from the membrane to the bulk of the solutionThen a dominant charge emerges in the membrane (co-ions and fixed ions have ahigher concentration and the same charge) giving rise to an electrical potential thatopposes the chemical potential This electrical potential (the Donnan potential) limitsthe co-ion concentration in the membrane and the removal of counter-ions from themembrane
When the electrical potential difference is applied over the electrodes a current willarise in which the counter-ions and co-ions take part Mainly counter-ions are trans-ported because their concentration is higher
This also demonstrates that there cannot be an ideally selective membrane becausetransport of ions always starts with the transport of co-ions The co-ion concentrationin the membrane is never zero at the startup This selectivity is described by the perm-selectivity and therefore also by the transport number
The selectivity of ion exchange membranes for anions or cations is related to thecharge which is in turn due to the presence of specific functional groups Anion ex-change membranes contain positively charged groups (typically quaternary ammo-nium salts) that attract anions Cations on the other hand are repelled Cationexchange membranes contain negatively charged groups (sulfonic acid or carboxylicacid groups) that attract cations Anions on the other hand are repelled
The selectivity of the membranes is determined by the transport number andpermselectivity A transport number of 1 for a cation exchange membrane meansthat it permeates all cations and retains all anions and vice versa for an anionexchange membrane In practice membranes with a transport number of 09 areconsidered good
The transport number ti of a component c is defined as
tc frac14 jci
(61)
where jc is the ion flow density and i is the current density
The permselectivity Pthorn is Pthorn frac14 tthorn tthorn1 tthorn
(62)
where tthorn is the value in the middle of the membrane The permselectivity is con-centration dependent when the concentration in the electrolyte solution is high moreco-ions end up in the membrane
Advances in electrodialysis for water treatment 189
The electrical resistance of a membrane largely determines the current losses occur-ring in the stack The lower this resistance is the less the temperature in the stack willincrease and therefore the fewer losses will occur Thus the electrical resistance of themembranes should be kept as low as possible to limit losses because the resistance ofthe flows through the stacks cannot be changed The order of magnitude of the elec-trical resistance of a membrane is 2e10 ohmcm2
When the electrical potential of the process is increased the current density in-creases as well More ions are transported This may lead to a total depletion ofions in the boundary layer at the diluate side of the membrane Then the current densitycannot increase further because no ions are left to transport the electrical current Thisis called the limiting current density At this point the efficiency of the process hasbecome low The electrical energy is consumed to split water into its ions whichcan transport the electrical charge and maintain the potential difference and currentdensity The electrical resistance has enormously increased It is important not toexceed this limiting current density The limiting current density can be determinedexperimentally using the method of Cowan and Brown which involves measuringelectrical resistance R as a function of the reciprocal current I 1 (Figure 62) Theintersection of the two lines tangential to the legs of the curves determines the limitingcurrent (from which the limiting current density can be calculated)
Another phenomenon that may occur is back-diffusion This occurs when thedriving force of the concentration difference between the diluate and the concentratecompartment has increased so much that ions diffuse from the concentrate compart-ment back to the diluate compartment against the electrical flow This should be taken
1I (Andash1)
R (VA)
Limiting current
Figure 62 Determination of the limiting current and the limiting current density inelectrodialysis
190 Advances in Membrane Technologies for Water Treatment
into account when applying concentrated solutions to an electrodialysis unit Electro-dialysis allows one to obtain a concentration factor up to ca 200e250
At the diluate side the ion concentration in the boundary layer (close to the mem-brane) is lower than in the bulk of the flow At the concentrate side the ion concentra-tion in the boundary layer (close to the membrane) is higher than in the bulk of theflow This is the result of diffusion Both concentrations can be calculated as
cm frac14 cb tm tbl
$i$d
z$F$Dethfor the diluate sideTHORN (63)
cm frac14 cb thorntm tbl
$i$d
z$F$Dethfor the concentrate sideTHORN (64)
The limiting current density in the boundary layer can be calculated from (eg atthe diluate side)
ilim frac14 z$D$F$ethcb cmTHORNd$tm tbl
(65)
where cb is the bulk concentration of the solute at a distance d from the membrane cmis the concentration of the solute near the membrane F is the Faradayrsquos constant(96487 Cmol) D is the diffusion coefficient (ms) z is the valence of the ion d isthe thickness of the boundary layer (m) tm is the transport number of the ion in themembrane and tbl is the transport number of the ion in the boundary layer
The limiting current density is reached when cm goes to zero The above equationfor the current density for cm frac14 0 (limiting current density) is
ilim frac14 z$D$F$cbd$tm tbl
It has been experimentally demonstrated that the limiting current density of a solu-tion increases linearly with temperature
ilim frac14 Athorn 4$T
where A is the limiting current density (ilim) at 0 C T is the temperature and 4 is thetemperature coefficient for the limiting current density
A and 4 are constants for a given concentration and a given species The concen-tration dependency for a given compound is linear as well
The temperature in the stack increases during electrodialysis so that during the pro-cess the limiting current density increases The flow velocity also increases themaximal current
Membrane fouling however will decrease the limiting current density All of thisshould be taken into account to ensure that the limiting current density is not exceeded
Advances in electrodialysis for water treatment 191
The limiting current density of the cation exchange and anion exchange membranesfor a solution with a single salt is not necessarily the same It is only the same when thecation and the anion have the same transport number If not the transport number ofthe cation is usually smaller than that of the anion and in that case the limiting currentdensity will first be attained at the cation-selective membrane In the ReI1 curve twoseparate minima are then visible for the two types of membranes
For solutions containing several solutes or a mixture of ions each compound willhave its own limiting current density
63 Advances in membrane materials for electrodialysisfor water treatment
Electrodialysis membranes are dense membranes that can be either heterogeneous orhomogeneous Heterogeneous membranes are made of an ion exchange resin whichis milled to a small grain size and then mixed with a solution of binding polymers(PVC rubber styrenedivinylbenzene copolymer etc) The ratio of resin grains andbinding polymer determines the electrical and mechanical properties of the membraneMore binding polymer improves the mechanical strength but also increases the elec-trical resistance Heterogeneous membranes have a limited mechanical strength and ahigh electrical resistance in general
Homogeneous membranes are synthesized by adding ionizable functional groups toa polymeric film The polymers that are used have to
bull induce a uniform charge distributionbull have good mechanical stability and low electrical resistancebull be cross-linked to reduce swelling when applied in water
The more charges are introduced the better is the conductivity but this leads toswelling because the material becomes more hydrophilic Cross-linking anchors thepolymeric chains in the polymer by chemically linking the chains to one anotherthis reduces swelling
Homogeneous membranes are thinner than heterogeneous membranes which re-duces their resistance They are mechanically stronger because they are directlymade as a film (unlike heterogeneous membranes)
The charge density of typical membranes is 1e2 mEqg (dry polymer)The membrane material contains functional groups that enable ion transport The
charged groups remaining on the membrane surface are the fixed ions the ions thatare dissociated from their functional group are the counter-ions Both are hydratedwhen an aqueous feed is applied Anion exchange membranes often contain quater-nary ammonium groups as fixed ions cation exchange membranes contain sulfonylor carboxyl groups Anion exchange membranes can be recognized by their trans-parent white color Cation exchange membranes vary in color from amber-brown towhite depending on the type
Regular ion exchange membranes are not selective However some anion exchangemembranes have been developed with selectivity for monovalent ions (such as the
192 Advances in Membrane Technologies for Water Treatment
ACS series of anion exchange membranes of Eurodia Industrie SA (WissousFrance)) This allows for the selective removal of eg nitrates compared with (diva-lent) sulfates This is important to manage the total dissolved solids (TDS) of the prod-uct stream which should not be too low for most applications including potable waterwhereas nitrates should be removed to a larger extent A typical outcome is a reductionof the nitrate concentration by 84 with a reduction in TDS by 49 The advantage ofthis is that remineralization after nitrate removal is no longer necessary
Whereas the number of ion-exchange membranes was limited until a decade agoresearch efforts have resulted in a much wider range of polymeric membranes (eventhough these are generally not commercially available) Synthesis of anion exchangemembranes includes poly(vinyltrimethoxysilane-co-2-(dimethylamine)ethylmethacry-late) copolymer (Chakrabarty Prakash amp Shahi 2013) membranes composed of4-vinylbenzyl chloride styrene and ethylmethacrylate (Koo Kwak amp Hwang2012) cross-linked polystyrene (Sachdeva Ram Singh amp Kumar 2008) acryloni-trilebutadienestyrene with activated carbon and silver fillers (Zendehnam RobarmiliHosseini Arabzadegan amp Madaeni 2013) and aliphatic-hydrocarbonebased anionexchange membranes prepared from glycidyl methacrylate and divinylbenzene(Tanaka Nagase amp Higa 2011) to name just a few Such membranes may be appli-cable in other electromembrane applications such as fuel cells (Merle Wessling ampNijmeijer 2011)
Cation exchange membranes include polystyrene and polyaniline blends (AmadoGondran Ferreira Rodrigues amp Ferreira 2004) heterogeneous polyvinylchloridestyreneebutadieneerubber blends (Hosseini Madaeni Heidari amp Moghadassi2011) and monovalent selective membranes made of blends of sulfonated poly(ethersulfone) with sulfonated poly(ether ether ketone) (Gohil Nagarale Binsu amp Shahi2006) Furthermore ceramic cation exchange membranes synthesized by impregna-tion of microporous ceramic supports with zirconium phosphate have been developedfor aggressive wastewater applications (Marti-Calatayud Garcia-Gabaldon Perez-Herranz Sales amp Mestre 2013) and phosphotungstic acidebased membranes ongraphite support intended for use as electrolyte-filled separators for batteries andsuper-capacitors (Seepana Pandey amp Shukla 2012)
This shows the large potential of various materials as anion exchange or cationexchange membranes although commercialization today is still limited
A bipolar membrane consists of a combination of a cation exchange and an anionexchange membrane (Figure 63) Between the two membranes a water film is presentfrom which water is dissociated the bipolar membrane is a source of anions (OH) aswell as cations (Hthorn) although they end up in different compartments on both sides ofthe bipolar membrane
Development and application of bipolar membranes have emerged especially in thepast decade Research on synthesis has resulted in eg sulfonated polystyrene ethylenebutylene polystyrene-poly(vinyl alcohol)equaternized polystyrene ethylene butylenepolystyrene bipolar membranes (Venugopal amp Dharmalingam 2013) poly(vinyl alco-hol)esodium alginate chitosan with TiO2 and ZnO (Chen Hu Chen Chen amp Zheng2011) and poly(vinyl alcohol)epoly(anetholesulfonic acid sodium salt) (cation selec-tive) and poly(vinyl alcohol)epoly(diallyldiethylammonium bromide) (anion selective)
Advances in electrodialysis for water treatment 193
bipolar membranes (Espinoza-Gomez Flores-Lopez Rogel-Hernandez amp Martinez2009) Several more examples can be found in the literature
64 Advances in membrane modules and systemconfigurations for electrodialysis for watertreatment
The classical stacked module is still the standard configuration Advances in moduledesign are primarily related to the use of spacers in the stack Spacers increase the elec-trical resistance which translates into higher power consumption Optimization of thefluid dynamics within the channels is still a problem as demonstrated by computa-tional fluid dynamics simulations to predict the fluid flow field inside a single channelinside an electrodialysis stack (Tamburini et al 2012) An appropriate choice of netspacer material can influence the slipno-slip condition of the flow on the spacer wiresthus significantly affecting the channel fluid dynamics in terms of pressure drops(Gurreri Tamburini Cipollina amp Micale 2012) A conducting spacer instead of thetraditional non-conducting spacer may help overcome electrical resistance (ZhangWang amp Goa 2012) whereas fluid dynamics and electrical resistance may beimproved simultaneously when using ion exchange membranes in which the spaceris formed directly on the membrane surface (Balster Stamatialis amp Wessling2010) so that it is no longer a separate entity
Further innovations are related to the stack configuration The use of bipolar mem-branes as described above yields a four-compartment membrane stack which is anextension of the classical three-compartment approach Such configuration is alsoapplied in selectrodialysis a process using a four-compartment stack without bipolar
Cathode
OHndash
H+
H2O
Bipolar membrane
Anode
Figure 63 Representation of a bipolar membrane for electrodialysis
194 Advances in Membrane Technologies for Water Treatment
membranes but with a combination of selective and nonselective membranes toachieve fractionation of ions so that for example phosphates can be selectivelyremoved from a feed stream (Zhang Paepen Pinoy Meesschaert amp Van der Bruggen2012) This is shown in Figure 64
A feed solution containing cations monovalent anions and divalent anions ispumped through the first compartment which is sealed by a cation exchangemembrane (CM) and an anion exchange-selectivemembrane (AM) The cationsmigrateunder the influence of the applied electrical potential difference toward the cathodepassing the CM The anions monovalent as well as bivalent migrate toward the anodeand thus pass theAMThus the solution of themiddle compartment the product will beenriched with anions At the same time monovalent ions available in this productcompartment migrate toward the anode passing the AMV This compartment iswhere the selection of the monovalent versus bivalent ions occurs and is denoted asthe selector This compartment is enriched in bivalent anions monovalent ions areremoved The third compartment (brine compartment) is enriched by the migratingmonovalent anions This process was shown to be feasible for concentrating phosphateions selectively from wastewater (Zhang et al 2013)
Other innovations relate to the flow through electrodialysis stacks A co-current hy-draulic flow mode of the salt solutions through the compartments of the stack is oftenapplied which requires a series of separation modules This can be improved bychanging the classical co-current hydraulic flow mode into a mixed flow mode inwhich the internal hydraulic flow of diluate and concentrate is co-current in the stackcompartments but is externally counter-current over the total series of stacks (Braunset al 2012)
Integrated processes may further extend the range of applications for water treat-ment As an example the separation of Co2thorn and Ni2thorn is discussed two cations
AM AMV CM
ProductFeed Brine
Clndash
ERS
SO42ndash
Na+
Na+
CM
Clndash
SO42ndash
Na+Na+Clndash
Clndash
Figure 64 Selectrodialysis stack for ion fractionation ERS electrode rinse solutionCM cation exchange membrane AM anion exchange membrane (nonselective) AMV anionexchange membrane (selective for monovalent ions)
Advances in electrodialysis for water treatment 195
with the same charge and approximately the same size (Xu 2001) Separation can beachieved by two electrodialysis stacks in series the first one of which is a conventionalelectrodialysis unit followed by an electrodialysis unit with bipolar membranes(Figure 65)
A wastewater stream containing a mixture of Co2thorn and Ni2thorn is supplied to compart-ment 1 The positive ions in this case Co2thorn and Ni2thorn migrate through the cationexchange membrane to compartment 2 In this compartment a complexing agent ispresent denoted as HR This agent selectively forms a complex with one of the twocations according to the reactions
Co2thorn thorn 2R Kc
CoR2
Ni2thorn thorn 2R Kc
NiR2
For the separation considered here Kc is much larger for Ni2thorn than for Co2thorn so thatonly the nickel complex is formed The cobalt ions are not complexed and can there-fore migrate through the two consecutive cation exchange membranes to compartment4 There they combine with an anion to the corresponding metal salt This concentratestream contains only the cobalt salt and no nickel
Nickel occurs now as a neutral complex in the solution and therefore cannot migrateto another compartment
++++++ +
++++++ +
++++++ +
++++++ +
ndashndashndashndashndashndashndashndash
ndashndashndashndashndashndashndashndash
ndashndashndashndashndashndashndashndash
ndashndashndashndashndashndashndashndash
ndashndashndashndashndashndashndashndash
ndashndashndashndashndashndashndashndash
++++++ +
Ni2+
Co2+
Clndash
Co2+
Ni2+
Co2+Co2+
Clndash
CoCl2
ClndashH+
Ni2+ Ni2+
NiCl2
OHndash
1 2 3 4
5 6 7
Ni2+
Co2+
Clndash
8
HR
Figure 65 Separation of ions with the same charge and similar size using electrodialysis andelectrodialysis with bipolar membranes (EDBM)
196 Advances in Membrane Technologies for Water Treatment
The solution in compartment 2 is fed to compartment 7 of the EDBM system In thiscompartment there is an excess of Hthorn ions so that the pH is sufficiently low for thefollowing reaction to occur
NiR2 thorn 2Hthorn Kc
Ni2thorn thorn 2HR
At low pH the Ni2thorn ions are exchanged for protons and are again released The pro-tons come from the bipolar membrane The free nickel ions can further migratethrough the cation exchange membrane to compartment 8 where they can be separatedas a metal salt
The complexing agents from compartment 5 are recycled to compartment 2together with the hydroxyl ions that are released in compartment 6 to increase thepH again to a neutral value These hydroxyl ions are also generated by the bipolarmembrane
The net result is that the Co2thorn ions accumulate in compartment 4 and the Ni2thorn ionsare collected in compartment 8
The concentration difference between Ni2thorn and Co2thorn ions in the correspondingcompartments can reach up to 120
65 Applications of electrodialysis for water treatment
The dominant application of electrodialysis is still in desalination This is a well-established application that can be readily applied Emerging applications are thereforein (1) the use of electrodialysis for alternative feed waters (2) ion fractionation and (3)the use of bipolar membranes in a wide range of production or conversion processes
A typical example of alternative feed water is concentrated brine from an reverseosmosis (RO) plant in which electrodialysis is used as a technology to prevent damageto marine ecosystems (Jiang Wang Zhang amp Xu 2014) Similarly the concentrateresulting from RO of treated wastewater is applied to reduce the volume of salty waterdischarge (Zhang Ghyselbrecht Meesschaert Pinoy amp Van der Bruggen 2011)Electrodialysis was found to be a viable technology but scaling may be a problembecause of the enhanced concentration of carbonates after wastewater treatmentIndustrial wastewater applications have been considered as well (Silva Poiesz ampvan der Heijden 2013) electrodialysis is feasible in terms of the separation of mono-valent and divalent ions with energy requirements ranging from 6 to 11 kW hm3 offeed stream depending on the water reclamation rate
Fractionation using electrodialysis has main applications for amino acids Aminoacids can be transported selectively across a double membrane system composed of acation exchange and an anion exchange membrane via pH adjustment of the sourcephase solution (Tsukahara Nanzai amp Igawa 2013) Electrodialysis with commerciallyavailable ion exchange membranes was applied to isolate the basic amino acids L-lysine(Lys) and L-arginine (Arg) (Readi Girones Wiratha amp Nijmeijer 2013) This has evenmore potential when electrodialysis is applied with ultrafiltration membranes This wasdemonstrated in several applications such as the separation of a pepsinepancreatin soy
Advances in electrodialysis for water treatment 197
protein isolate hydrolysate (Roblet et al 2013) and to recover and concentrate the activeantibacterial fraction of a snow crab by-products hydrolysate which is the result of apeptide with a molecular weight of about 800 Da (Doyen Saucier Beaulieu Pouliotamp Bazinet 2012) The attraction of using ultrafiltration membranes for electrodialysisapplications is in the possibility of fine-tuning the separation on the basis of either chargeseparation or size exclusion which allows for specific separation of charged organiccompounds typically related to peptides
However selective removal of other ions is also interesting One example is the useof monovalent permselective membranes for selective removal of arsenic and mono-valent ions from brackish water RO concentrate (Xu Capito amp Cath 2013) Mono-valent permselective anion exchange membranes have high selectivity in removingmonovalent anions over di- and multivalent anions In general transport of monova-lent and divalent ions is different because of steric effects and different charge inter-actions for example a removal rate of more than 70 for monovalent ions mightcorrespond to a removal rate below 50 for divalent ions (Silva et al 2013) Thiscan be exploited and optimized for eg the removal of nutrients from wastewater
The use of bipolar membranes has been considered in many studies The moststraightforward one is when concentrates from seawater desalination are used asfeed This yields a mixture of sulfuric acid and hydrochloric acid on the one sideand sodium hydroxide on the other A concentration of 1 molL can be obtained inrealistic conditions with a good prospect for long-term operation (Shen Huanget al 2013) Similar observations were made for industrial saline water mainlycomposed of NaCl and KCl which yielded an acid and a base stream with a concen-tration of around 2 molL (Ghyselbrecht et al 2013) Bipolar membranes were used inanother application on a pilot scale to recover glyphosate and the production of baseacid with high concentration in view of the zero discharge of wastewater (Yang GaoFan Fu amp Gao 2014) Electrodialysis with bipolar membranes may be coupled withdiffusion dialysis for two-step recovery with electrodialysis as the second step torecover the remaining concentrations of acid (Zhuang et al 2013) More applicationsinclude the production of morpholine (Jiang Wang amp Xu 2013) the separation oflithium and cobalt in view of the recycling of waste lithiumeion batteries in a similarapproach as described above for nickel and cobalt (Iizuka Yamashita NagasawaYamasaki amp Yanagisawa 2013) the production of tetrapropyl ammonium hydroxideon a pilot scale with a perspective for full-scale industrial application (Shen YuHuang amp Van der Bruggen 2013) and the production of L-ascorbyl-2-monophosphate a stable substitute of vitamin C (Song et al 2012) Many moreapplications have been reported which indicates the potential of electrodialysis withbipolar membranes as a versatile process A complete overview however is beyondthe scope of this chapter
66 Future trends
Electrodialysis has been of interest for the past 5 decades although for a limited rangeof applications such as the desalination of brackish water This will not substantially
198 Advances in Membrane Technologies for Water Treatment
change in years to come although an increase in these applications is not expectedbecause of competition with more cost-effective processes (at higher salt concentra-tions) such as RO and membrane distillation Other applications related to selectiveremoval of one or more ions however may become attractive when membrane mate-rials are developed with better selectivity (eg for nitrate specific peptides or phos-phate) or when alternative stack configurations are applied in which a fractionationeffect can be obtained
Since the turn of the century bipolar membranes have been increasingly studiedand applied This opens up a new range of applications related to combined reactionand purification owing to the acidebase splitting effect Numerous applications canbe based on this feature and many more are on the agenda to be further developedin coming years This will in turn stimulate the availability of bipolar membraneswith a further positive effect on the application market
One question remains for electrodialysis which is related to the sustainability of thesystem Although the process is claimed to be a green technology this may be arguedfrom an energetic point of view particularly when taking the exergetic value of therequired electrical energy into account Therefore more attention to this aspect is crit-ical Combined systems based on renewable energy may overcome this issue whichthen will allow for example remote applications of electrodialysis
Sources of further information and advice
Tanaka Y (2013) Ion exchange membrane electrodialysis Fundamentals desalination sep-aration (Water resource planning development and management series) New YorkNova
Bernardes A Siqueira Rodrigues M A amp Zoppas Ferreira J (Eds) (2014) Electrodialysisand water Reuse Berlin Heidelberg Springer
Strathmann H (2011) Introduction to membrane science and technology WeinheimGermany Wiley
Sata T (2004) Ion exchange membranes Preparation characterization modification andapplication London Royal Society of Chemistry
Strathmann H (2004) Ion-exchange membrane separation processes New York Elsevier
References
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Anderson A M amp Wylam C B (1956) Ion exchange electrodialysis for demineralisation ofsugar solutions Chemistry and Industry 12 191e192
Balster J Stamatialis D F ampWessling M (2010) Membrane with integrated spacer Journalof Membrane Science 360(1e2) 185e189
Brauns E Bossaer J Toye S Mijnendonckx K Pinoy L amp Van der Bruggen B (2012) Astudy of electrodialysis operating with mixed flow mode Separation and PurificationTechnology 98 356e365
Advances in electrodialysis for water treatment 199
Chakrabarty T Prakash S amp Shahi V K (2013) End group cross-linked 2-(dimethylamino)ethylmethacrylate based anion exchange membrane for electrodialysis Journal of Mem-brane Science 428 86e94
Chen R Y Hu Y Y Chen Z Chen X amp Zheng X (2011) Preparation and char-acterization of bipolar membranes modified using photocatalyst nano-TiO2 and nano-ZnO Journal of Applied Polymer Science 122(2) 1245e1250
Cooke B A (1958) Scaling problems in electrodialysis using permselective membranesChemistry and Industry 19 555e556
Daufin G Escudier J P Carrere H Berot S Fillaudeau L amp Decloux M (2001) Recentand emerging applications of membrane processes in the food and dairy industry Food andBioproducts Processing 79(C2) 89e102
Dewhalley C H (1958) Desalting of water by electrodialysis Chemistry and Industry 18e13
Doyen A Saucier L Beaulieu L Pouliot Y amp Bazinet L (2012) Electroseparation of anantibacterial peptide fraction from snow crab by-products hydrolysate by electrodialysiswith ultrafiltration membranes Food Chemistry 132(3) 1177e1184
Dydek E V amp Bazant M Z (2013) Nonlinear dynamics of ion concentration polarization inporous media The leaky membrane model AIChE Journal 59(9) 3539e3555
Espinoza-Gomez H Flores-Lopez L Z Rogel-Hernandez E amp Martinez M (2009)Preparation and characterization of PVAPASA-PVAPDDAB bipolar membrane Journalof the Brazilian Chemical Society 20(7) 1294e1301
Firdaous L Dhulster P Amiot J Gaudreau A Lecouturier D Kapel R et al (2009)Concentration and selective separation of bioactive peptides from an alfalfa white proteinhydrolysate by electrodialysis with ultrafiltration membranes Journal of Membrane Science329(1e2) 60e67
Galier S amp Roux-de Balmann H (2011) The electrophoretic membrane contactor A mass-transfer-based methodology applied to the separation of whey proteins Separation andPurification Technology 77(2) 237e244
Ghyselbrecht K Huygebaert M Van der Bruggen B Ballet R Meesschaert B amp Pinoy L(2013) Desalination of an industrial saline water with conventional and bipolar membraneelectrodialysis Desalination 318 9e18
Gohil G S Nagarale R K Binsu V V amp Shahi V K (2006) Preparation and charac-terization of monovalent cation selective sulfonated poly(ether ether ketone) and poly(ethersulfone) composite membranes Journal of Colloid and Interface Science 298(2)845e853
Gurreri L Tamburini A Cipollina A amp Micale G (2012) CFD analysis of the fluid flowbehavior in a reverse electrodialysis stack Desalination and Water Treatment 48(1e3)390e403
Hoshino T (2013) Preliminary studies of lithium recovery technology from seawater byelectrodialysis using ionic liquid membrane Desalination 317 11e16
Hosseini S M Madaeni S S Heidari A R amp Moghadassi A R (2011) Preparation andcharacterization of polyvinyl chloridestyrene butadiene rubber blend heterogeneous cationexchange membrane modified by potassium perchlorate Desalination 279(1e3)306e314
Iizuka A Yamashita Y Nagasawa H Yamasaki A amp Yanagisawa Y (2013) Separationof lithium and cobalt from waste lithium-ion batteries via bipolar membrane electrodialysiscoupled with chelation Separation and Purification Technology 113 33e41
Jiang C X Wang Y M amp Xu T W (2013) An excellent method to produce morpholineby bipolar membrane electrodialysis Separation and Purification Technology 115 100e106
200 Advances in Membrane Technologies for Water Treatment
Jiang C X Wang Y M Zhang Z H amp Xu T W (2014) Electrodialysis of concentratedbrine from RO plant to produce coarse salt and freshwater Journal of Membrane Science450 323e330
Koo J S Kwak N S amp Hwang T S (2012) Synthesis and properties of an anion-exchangemembrane based on vinylbenzyl chloride-styrene-ethyl methacrylate copolymers Journalof Membrane Science 423 293e301
Langevin M E Roblet C Moresoli C Ramassamy C amp Bazinet L (2012) Comparativeapplication of pressure- and electrically-driven membrane processes for isolation ofbioactive peptides from soy protein hydrolysate Journal of Membrane Science 40315e24
Laurence D J R (1956) Separation of proteins by electrodialysis through filter-paper mem-branes Biochemical Journal 62(3) 36
Lopez A M amp Hestekin J A (2013) Separation of organic acids from water using ionicliquid assisted electrodialysis Separation and Purification Technology 116 162e169
Marti-Calatayud M C Garcia-Gabaldon M Perez-Herranz V Sales S amp Mestre S(2013) Synthesis and electrochemical behavior of ceramic cation-exchange membranesbased on zirconium phosphate Ceramics International 39(4) 4045e4054
Maurer M Pronk W amp Larsen T A (2006) Treatment processes for source-separated urineWater Research 40(17) 3151e3166
Merle G Wessling M amp Nijmeijer K (2011) Anion exchange membranes for alkaline fuelcells A review Journal of Membrane Science 277(1e2) 1e35
Pronk W Biebow M amp Boller M (2006) Electrodialysis for recovering salts from a urine so-lution containing micropollutants Environmental Science amp Technology 40(7) 2414e2420
Pronk W amp Kone D (2009) Options for urine treatment in developing countries Desali-nation 248(1e3) 360e368
Readi O M K Girones M Wiratha W amp Nijmeijer K (2013) On the isolation of singlebasic amino acids with electrodialysis for the production of biobased chemicals Industrialand Engineering Chemistry Research 52(3) 1069e1078
Roblet C Doyen A Amiot J amp Bazinet L (2013) Impact of pH on ultrafiltration membraneselectivity during electrodialysis with ultrafiltration membrane (EDUF) purification of soypeptides from a complex matrix Journal of Membrane Science 435 207e217
Sachdeva S Ram R P Singh J K amp Kumar A (2008) Synthesis of anion exchangepolystyrene membranes for the electrolysis of sodium chloride AIChE Journal 54(4)940e949
Seepana M M Pandey J amp Shukla A (2012) Synthesis and characterization of PWA basedinorganic ion-exchange membrane Separation and Purification Technology 98 193e198
Shen J N Huang J Liu L F Ye W Y Lin J Y amp Van der Bruggen B (2013) The useof BMED for glyphosate recovery from glyphosate neutralization liquor in view of zerodischarge Journal of Hazardous Materials 260 660e667
Shen J Yu C Huang J amp Van der Bruggen B (2013) Preparation of highly purified tet-rapropyl ammonium hydroxide using continuous bipolar membrane electrodialysisChemical Engineering Journal 220 311e319
Silva V Poiesz E amp van der Heijden P (2013) Industrial wastewater desalination using elec-trodialysis Evaluation and plant design Journal of Applied Electrochemistry 43(11)1057e1067
Solt G (1972) Electrodialysis comes of age Effluent and Water Treatment Journal 12(6) 293Song S Tao Y F Shen H Q Chen B Q Qin P Y amp Tan T W (2012) Use of bipolar
membrane electrodialysis (BME) in purification of L-ascorbyl-2-monophosphate Separationand Purification Technology 98 158e164
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Tamburini A La Barbera G Cipollina A Ciofalo M amp Micale G (2012) CFD simulationof channels for direct and reverse electrodialysis Desalination and Water Treatment48(1e3) 370e389
Tanaka Y (2013) Development of a computer simulation program of batch ion-exchangemembrane electrodialysis for saline water desalination Desalination 320 118e133
Tanaka N Nagase M amp Higa M (2011) Preparation of aliphatic-hydrocarbon-based anion-exchange membranes and their anti-organic-fouling properties Journal of MembraneScience 384(1e2) 27e36
Tran A T K Jullok N Meesschaert B Pinoy L amp Van der Bruggen B (2013) Pelletreactor pre-treatment A feasible method to reduce scaling in bipolar membrane electro-dialysis Journal of Colloid and Interface Science 401 107e115
Tsukahara S Nanzai B amp Igawa M (2013) Selective transport of amino acids across adouble membrane system composed of a cation- and an anion-exchange membraneJournal of Membrane Science 448 300e307
Urtenov M K Uzdenova A M Kovalenko A V Nikonenko V V Pismenskaya N DVasilrsquoeva V I et al (2013) Basic mathematical model of overlimiting transfer enhancedby electroconvection in flow-through electrodialysis membrane cells Journal of MembraneScience 447 190e202
Venugopal K amp Dharmalingam S (2013) Fundamental studies on a new series ofSPSEBS-PVA-QPSEBS bipolar membrane Membrane preparation and characterizationJournal of Applied Polymer Science 127(6) 4983e4990
Vermaas D A Veerman J Yip N Y Elimelech M Saakes M amp Nijmeijer K (2013)High efficiency in energy generation from salinity gradients with reverse electrodialysisACS Sustainable Chemistry and Engineering 1(10) 1295e1302
Wood T (1960) Desalting of urine by electrodialysis Nature 186(4725) 634e635Xu T W (2001) Development of bipolar membrane based processes Desalination 140(3)
247e258Xu P Capito M amp Cath T Y (2013) Selective removal of arsenic and monovalent ions from
brackish water reverse osmosis concentrate Journal of Hazardous Materials 260885e891
Xu T W amp Huang C (2008) Electrodialysis-based separation technologies A critical reviewAIChE Journal 54(12) 3147e3159
Yang Y Gao X L Fan A Y Fu L L amp Gao C J (2014) An innovative beneficial reuseof seawater concentrate using bipolar membrane electrodialysis Journal of MembraneScience 449 119e126
Zaslavschi I Shemer H Hasson D amp Semiat R (2013) Electrochemical CaCO3 scaleremoval with a bipolar membrane system Journal of Membrane Science 445 88e95
Zendehnam A Robarmili N Hosseini S M Arabzadegan M amp Madaeni S S (2013)Fabrication of novel (acrylonitrile butadiene styreneactivated carbonsilver nanoparticles)heterogeneous anion exchange membrane Physico-chemical and antibacterial character-istics Journal of the Taiwan Institute of Chemical Engineers 44(4) 670e677
Zhang Y Desmidt E Van Looveren A Pinoy L Meesschaert B amp Van der Bruggen B(2013) Phosphate separation and recovery from wastewater by novel electrodialysisEnvironmental Science amp Technology 47(11) 5888e5895
Zhang Y Ghyselbrecht K Meesschaert B Pinoy L amp Van der Bruggen B (2011)Electrodialysis on RO concentrate to improve water recovery in wastewater reclamationJournal of Membrane Science 378(1e2) 101e110
202 Advances in Membrane Technologies for Water Treatment
Zhang Y Paepen S Pinoy L Meesschaert B amp Van der Bruggen B (2012) Selec-trodialysis Fractionation of divalent ions from monovalent ions in a novel electrodialysisstack Separation and Purification Technology 88 191e201
Zhang K Wang M amp Gao C J (2012) Ion conductive spacers for the energy-saving pro-duction of the tartaric acid in bipolar membrane electrodialysis Journal of MembraneScience 387 48e53
Zhuang J X Chen Q Wang S Zhang W M Song W G Wan L J et al (2013) Zerodischarge process for foil industry waste acid reclamation Coupling of diffusion dialysisand electrodialysis with bipolar membranes Journal of Membrane Science 432 90e96
Advances in electrodialysis for water treatment 203
Photocatalytic membranereactors for water treatment 7R Molinari1 P Argurio1 L Palmisano21Universita della Calabria Rende (CS) Italy 2Universita di Palermo Palermo Italy
71 Introduction
Heterogeneous photocatalysis is an advanced oxidation process (AOP) based on theuse of a semiconductor (the photocatalyst) It has been extensively studied for aboutfour decades when Fujishima and Honda (1972) discovered the photocatalytic split-ting of water on TiO2 electrodes It has been object of a large amount of studies relatedto environment recovery by the total degradation to innocuous substances of organicand inorganic pollutants (Hoffmann Martin Choi amp Bahnemann 1995) and for syn-thesis (Molinari Caruso amp Poerio 2009 Palmisano et al 2010) It is a discipline thatincludes a large variety of reactions (Molinari Argurio amp Lavorato 2013) partial ortotal oxidations dehydrogenation hydrogenation (eg hydrogen transfer) metal depo-sition water detoxification gaseous pollutant removal bactericidal action etc
The main advantages of photocatalysis making it a very promising lsquogreenrsquo processconsist in the mild operating conditions and the possibility to abate refractoryvery toxic and nonbiodegradable molecules (Molinari Caruso amp Palmisano 2010Palmisano Augugliaro Pagliaro amp Palmisano 2007) Besides photocatalysis (1)avoids the use of environmentally and unhealthy dangerous heavy metal catalystsby using safer photocatalysts (mainly TiO2) (2) uses mild oxidants (O2 in some casesfrom the air) (3) needs few auxiliary additives (4) does not produce harmful chemi-cals (5) offers a good alternative to the energy-intensive conventional treatmentmethods and (6) can be combined with other physical and chemical technologies(eg membrane separations)
Because the photocatalyst is generally used as a powder suspended in a liquidmedium the catalyst-recovering step from the reaction environment is a pivotal stagein view of large-scale applications Photocatalytic membrane reactors (PMRs) repre-sent a promising approach to overcome this limitation A PMR can be defined as a de-vice existing in various configurations that combines a photocatalyst and a membraneto produce chemical transformations A PMR improves the potentialities of classicalphotoreactors (PRs) and those of membrane processes with a synergy of both technol-ogies thus minimizing environmental and economic impacts (Molinari CarusoArgurio amp Poerio 2008 Molinari et al 2009) The membrane permits continuousoperation in systems in which the recovery of the photocatalyst (immobilized or in sus-pension) the reaction and the product separation simultaneously occurs resulting in adevice that is competitive with other separation technologies in terms of material
Advances in Membrane Technologies for Water Treatment httpdxdoiorg101016B978-1-78242-121-400007-1Copyright copy 2015 Elsevier Ltd All rights reserved
recovery energy costs reduction of the environmental impact and selective removalof the components Higher energy efficiency modularity and easy scale-up are someother advantages of PMRs compared to conventional PRs
Water treatment by removing both organic and inorganic pollutants represents themain use of the photocatalytic techniques Water pollution caused by hazardousorganic chemicals used in industry and agriculture is a serious problem Environ-mental laws are severe and they will become more and more restrictive in the nextyears Furthermore various directives suggest the use of green chemistry conceptsand clean technologies inside the manufacturing processes to protect the environmentTraditional chemical and biological treatments (eg adsorption on active carbonchemical oxidation and aerobic biological treatments) to clean-up water often failto remove most organic pollutants because of the high resistance of these compoundsresulting in high concentrations discharged in treated effluents On the basis of this inthe last years the attention of the international scientific community has been focusedon the development of alternative methods In this context photocatalytic processesrepresent a promising alternative green process (Huo et al 2013 Mozia amp Morawski2006 2012 Mozia Tomaszewska amp Morawski 2005 2007) because as a conse-quence of the highly unselective reactions involved in the photocatalytic processesa wide range of organic pollutants can be totally degraded (ie mineralized) into smalland harmless species (carbon to carbon dioxide hydrogen to water nitrogen to nitrateetc) without using chemicals avoiding sludge production and its disposal
In the pertinent literature different studies in which photocatalysis is coupled withpressure-driven membrane techniques such as nanofiltration (NF) and ultrafiltration(UF) for the degradation of organic pollutants are reported (Molinari Borgese DrioliPalmisano amp Schiavello 2002 Molinari Palmisano Drioli amp Schiavello 2002Patsios Sarasidis amp Karabelas 2013 Zhang et al 2013) Membrane fouling is oneof the main drawbacks of these systems On the basis of this some authors proposedto couple photocatalysis with (1) membrane distillation (Huo et al 2013 Mozia ampMorawski 2006 Mozia amp Morawski 2012 Mozia Tomaszewska amp Morawski2007) obtaining an almost complete total organic carbon (TOC) removal butrequiring energy to activate the evaporation phenomenon (2) dialysis (AzragueAimar Benoit-Marquieacute amp Maurette 2007) to mineralize organic compounds con-tained in artificial turbid water operating at ambient temperature and (3) pervaporation(Camera-Roda amp Santarelli 2007 Camera-Roda et al 2011) improving the detoxi-fication efficiency of wastewater containing organic pollutants at low concentrationlevels
In this chapter the basic principles of photocatalysis and PMRs are reportedwith the advantages related to their coupling thanks to the synergistic effectsThe importance in making a correct choice of membrane (type and material) is evi-denced discussing the choices made in literature and the criteria used for selectionMoreover a classification of the PMRs based both on their configuration (pressurizedand depressurizedsubmerged) and on the particular membrane separation with photo-catalysis (pressure-driven membrane separation membrane distillation dialysis andpervaporation) is also presented providing evidence that the opportune choice ofPMR configuration and membrane separation is a key step to limit membrane fouling
206 Advances in Membrane Technologies for Water Treatment
Finally some applications of PMRs for water treatment in particular for pharmaceu-ticals removal are reported with evidence of the potentialities the drawbacks and thefuture trends
72 Fundamentals of PMRs for water treatment
721 Basic principles of photocatalysis and typesof photocatalysts and membranes
The main difference of photocatalysis compared to conventional catalysis is the pho-tonic activation mode of the catalyst which replaces the thermal activation (Herrmann2005) The electronic structure of a semiconductor is characterized by a valence band(VB) and a conduction band (CB) which are separated by an energy band gap (Eg)When a semiconductor particle is excited by irradiation with photons of energy (hn)equal or higher than its energy band gap (Eg) valence electrons (e) are promotedfrom VB to CB thus leaving a positive hole (hthorn) in the VB For semiconductorTiO2 this step is expressed as
TiO2 thorn hvTiO2e thorn hthorn
(71)
In the absence of suitable electron andor hole scavengers the photo-generated elec-trons and holes can recombine in bulk or on the surface of the semiconductor within ashort time releasing energy in the form of heat or photons Electrons and holes thatmigrate to the surface of the semiconductor without recombination can reduce andoxidize respectively the substrates adsorbed on the semiconductor A schematizationof both the photonic activation of the photocatalyst and the photocatalytic oxidationand reduction reactions that take place onto a photo-activated semiconductor particleis reported in Figure 71
The process in Figure 71 can be divided in four steps
1 Absorption of light followed by the separation of the electronehole couple2 Adsorption of the reagents3 Redox (reduction and oxidation) reaction4 Desorption of the products
The redox reactions involving the species adsorbed onto the semiconductor surfaceoccur only if the potentials of the redox couple to which the substrates belong (E0
1 E02)
are compatible with both the VB and CB potentials (EVB ECB) In particular the redoxpotential of the species to be oxidized E0
2 should be less positive than EVB so that thephoto-produced holes hthorn can oxidize the reduced form of this species (red2 ox2 seeFigure 71) At the same time the redox potential of the species to be reduced E0
1should be more positive than ECB so that the photo-produced electrons ee can reducethe oxidized form of this species (ox1 red1 see Figure 71) Only if both E0
2 lt EVB
and E01 lt ECB conditions are met both reduction and oxidation are thermodynami-
cally favoured and the overall photocatalytic process takes place
Photocatalytic membrane reactors for water treatment 207
The redox reactions that take place onto the semiconductor surface represent thebasic mechanisms of photocatalytic waterair purification and photocatalytic hydrogenproductionsubstrate hydrogenation respectively In the VB photo-generated holescan oxidize surface hydroxyl groups into hydroxyl radical in the aqueous reactingenvironment
OH thorn hthornOH (72)
As widely accepted hydroxyl radicals are the primary oxidants that attack thesubstrates to be degraded by reducing them to their elemental form
In the CB photo-promoted electrons can reduce the molecular oxygen dissolved inthe aqueous phase to superoxide radical metal ions to their lower oxidation states orHthorn ions to H
O2 thorn eO2 (73)
Menthorn thorn eMeethn1THORNthorn (74)
Hthorn thorn eH (75)
The so-generated hydrogen radicals can follow two main fates combination forH2 production or hydrogenation of adsorbed substrate
Thanks to (1) the favourable energetics of its band structure (2) its relatively highquantum yield (3) its stability under irradiation and (4) its low cost and availabilityTiO2 represents the archetypical photocatalyst a virtual synonym for photocatalysis
hν
Eg
endash
h+
ox1Red1
ox2Red2
CB
VB
Light
Figure 71 Schematization of the photocatalytic process that occurs on a photo-activatedsemiconductor particle ox1 red1 ox2 and red2 represent respectively the oxidized and thereduced species of two different redox couples indicated as 1 and 2
208 Advances in Membrane Technologies for Water Treatment
Nevertheless this material does not present photo-response under visible light illumi-nation because of its wide band gap taking advantage of only less than 6 of the solarenergy Thus its potential as a green technology cannot be entirely fulfilled Conse-quently in the last years a great number of new photocatalysts have been synthesizedand tested as possible alternatives to TiO2 (Di Paola Garciacutea-Lopez Marcigrave ampPalmisano 2012 Hernandez-Alonso Fresno Suarez amp Coronado 2009) particularlyin view of the solar application The most important requirements that such a materialshould possess are a suitable band gap chemical and physical stability nontoxic na-ture good availability and low cost The most common materials used are oxidesor sulfides which redox potentials for the VBs and the CBs range are betweenthorn40 and 15 V versus a normal hydrogen electrode (NHE) Their photocatalyticproperties depend mainly on the position of the energetic level and on the band gapTable 71 summarizes the most common semiconductors used as photocatalystsreporting their band gap and wavelength of the radiation needed for the activation
The photoactivity of a semiconductor depends also on the mobility and lifetimeof the photo-produced electronhole couples on the light absorbance coefficientand on the method used to prepare the photocatalytic powder which affects manyphysicochemical properties of the semiconductor such as surface area distributionof particle size and crystallinity
Despite the photocatalytic process offering interesting advantages with respect toother catalytic process (eg milder operating conditions use of safer catalyst etc)its use in the industry is still limited for three different reasons (Ni Leung Leungamp Sumathy 2007) (1) recombination of photo-generated electronhole pairs that dissi-pate their energy as heat (2) fast backward reaction and (3) difficulty to use visiblelight which limits the effectiveness of the use of solar energy
Table 71 Band gaps and wavelengths of the most commonsemiconductors used as photocatalysts
Photocatalyst Band gap (eV) Wavelength (nm)
SnO2 38 318
TiO2 anatase 32 387
TiO2 rutile 30 380
WO3 28 443
Fe2O3 22 560
ZnO 32 387e390
ZnS 37 335e336
CdS 25 496e497
CdSe 17 729e730
GaAs 14 886e887
Photocatalytic membrane reactors for water treatment 209
To solve these problems continuous efforts have been made to enhance the photo-catalytic activity and promote the visible light response
A simple approach consists in the addition of electron donors or electron scaven-gers able to react irreversibly with the photo-generated VB holes or CB electronsrespectively Practically electron scavengers or electron donors can be reduced byCB electrons or oxidized by VB holes respectively the remaining strong oxidizingVB holes or strong reducing CB electrons can oxidizereduce the substrate Operatingin this way the electronhole recombination can be suppressed or reduced and higherquantum efficiency can be achieved
Another way to suppress or to reduce the recombination of photo-generatedelectronhole pairs consists in doping the semiconductor with a noble metal like PtAu Pd Rh Ni Cu and Ag By using this method photo-promoted electrons canbe transferred from CB of the semiconductor to metal particles deposited on its surfacewhile photo-generated VB holes remain on the photocatalyst Thus the possibility ofelectronehole recombination is greatly reduced resulting in an enhancement of photo-catalytic efficiency (Ara~na et al 2008 Chen et al 2008 Colmenares AramendiacuteaMarinas Marinas amp Urbano 2006 Rosu Suciu Lazar amp Bratu 2013 Wu et al2008 Yang et al 2013) A combined effect of metal ion doping consists in the reduc-tion of the band gap energy of the photocatalyst thus shifting the radiation absorptiontowards higher wavelengths permitting the use of visible light (Ge 2008 Ling Sunamp Zhou 2008) The same effect also can be obtained by doping the catalyst with an-ions such as N F C S and B (Collazzo Foletto Jahn amp Villetti 2012 Di Valentin ampPacchioni 2013)
Other ways to overcome previous limitations consist in dye sensitization compositesemiconductors metal-ion implantation etc
PMRs are hybrid systems in which photocatalysis is coupled with a membrane sep-aration In general a membrane can be defined as a barrier that separates two phasescontrolling the mass transport between them
The most important classification of membranes is based on their morphologicalproperties In particular a membrane can be porous allowing the permeation throughits pores and then the transport mechanisms (sieving mechanism andor Knudsendiffusion) depend on the pore size or dense in which the mass transport takes placeby the so-called solutiondiffusion mechanism ie dissolution of the permeationspecies in the membrane phase and diffusion across the membrane thickness
Another morphological classification of membrane distinguishes symmetric andasymmetric membranes In a symmetric membrane structural and transport proper-ties are the same throughout its section and the thickness of the entire membrane de-termines the flow In asymmetric membranes consisting of a dense thin layersupported by a porous layer that acts as mechanical support of the fragile skin layerstructural and transport properties change along the membrane thickness In this waythe high selectivity of a dense membrane is combined with the high permeation rateof a thin layer membrane Asymmetric membranes are mainly used in membrane sep-aration processes that use hydrostatic pressure as the driving force (the so-calledpressure-driven processes) reverse osmosis (RO) NF UF and microfiltration(MF) (Mulder 1996)
210 Advances in Membrane Technologies for Water Treatment
In a PMR system the appropriate choice of the membrane (material and type) andmembrane module configuration is mainly determined by the type of photocatalyticreaction Indeed the membrane can assume many roles The main purpose in couplinga membrane process with a photocatalytic reaction is the necessity to recover and reusethe catalyst So it is important to choose a membrane (type and material) with com-plete catalyst rejection thus maintaining it in the reaction environment Besideswhen the process is used for the degradation of organic pollutants the membranemust be able to reject the substrates and their intermediate products which otherwisepass in the permeate For example when the photocatalytic process is used as asynthetic pathway the main objective can be the selective separation of the producteg minimizing its successive oxidation which leads to undesirable byproducts
In this context the rejection (R) is a useful parameter that expresses the ability ofthe membrane to maintain the substrate and its intermediates in the reactiveenvironment
R frac14 Cf Cp
Cf frac14 1
CpCf
(76)
in which Cf and Cp are the solute concentrations in the feed and permeate respectivelyThe retention of the substrates and degradation byproducts is important also to con-
trol the residence time of substrates in the photocatalytic system This parameterstrongly influences the efficiency of photodegradation Longer retention times usuallyobtained by reducing the permeate flux results in higher pollutant degradation dueto the greater contact time between the molecules to be degraded and the catalystHowever since the PMR must be able to offer a high water permeate flux it is impor-tant to find a good compromise among the permeate flux and the residence time toachieve a system for application purpose
722 Membrane materials development and designfor photocatalysis
Because the main problem encountered when the photocatalytic process is coupled to apressure-driven process such as MF and UF (which requires porous membranes) is thedecrease of permeate flux caused by concentration polarization and fouling the choiceof membrane material is strongly determined by the need to limit these phenomena andby the chemical and thermal stability of the materials
Polymeric membranes are generally used in photocatalysis although inorganicones which generally present higher chemical and thermal stability are attractivefor PMR applications But they have the disadvantage of a significantly higher costcompared to polymeric ones (Chin Chiang amp Fane 2006 Moritz Benfer Arki ampTomandl 2001) In view of this economic factor suitable polymeric membranesshould be sought for this application
Molinari et al (2000) reported an investigation of the stability under ultraviolet(UV) irradiation of some eligible commercial membranes composed by variousmaterials Tests of photo-resistance under UV light were carried out by irradiating
Photocatalytic membrane reactors for water treatment 211
the membranes immersed in distilled water Samples were periodically withdrawn andanalysed by TOC optical microscopy (OM) and scanning electron microscopy (SEM)to verify any damage to the membrane surface Membrane stability was also deter-mined by measuring the changes in pure water permeation flux (WPF) before and afterUV irradiation Table 72 shows the main characteristic of the 11 commercialmembranes used in the UV irradiation tests
Membranes made of polyacrylonitrile (PAN) fluoride thorn polypropylene (PP) andpolysulphone thorn PP seemed to be stable to UV light over a 24-h period of irradiationPAN membrane was chosen to carry out some photo-reactivity experiments because itshowed the highest permeate flux which was constant in the time TiO2 was immobi-lized on the flat sheet membrane by ultrafiltrating TiO2 suspensions in water (600 mL)at 005 01 02 and 03 gL the amounts of immobilized TiO2 were respectively076 204 408 and 612 mgcm2 of membrane The best uniform coverage was givenby loading 408 mgcm2 although the best results for the photocatalytic reaction wereobtained in a suspended TiO2 system 4-nitrophenol (4-NP) was used as model mole-cule and 80 degradation was obtained in around 5 h by using 05 gL of suspendedTiO2 whereas degradation around 51 was obtained with the same irradiation time inthe system with deposited TiO2 regardless of the amount of catalyst
Some years later Chin et al (2006) reported a study focused on the selection ofpolymeric membranes for PMR applications providing a protocol similar to that
Table 72 Types of commercial membranes used in the UV irradiationtest
Type of membrane Pore size Polymer Supplier
MPPS 0000 u002 15 kDa Polysulphone Separem
MPPS 0000 u006 40 kDa Polysulphone Separem
MPPS 0000 u20 25 kDa Polyamide Separem
MPPS 0000 u25 20 kDa Polyamide Separem
P-12-10 e PEEK Homemade
FS 50 PP 50 kDa Fluoride thorn PP Dow
PES 40 kDa Polyethersulphone TechSep
PVDF 01 mm Polyvinyldenefluoride
TechSep
GR 51 PP 50 kDa Polysulphone thorn PP TechSep
PAN 40 kDa Polyacrylonitrile TechSep
CA 600 PP 20 kDa Cellulose acetate-PP
Dow
Note PP polypropyleneSource Molinari et al (2000)
212 Advances in Membrane Technologies for Water Treatment
one proposed byMolinari et al (2000) for testing the stability of polymeric membranesbefore using them with photocatalytic reactions Membrane stability was characterizedby (1) changes in WPF before and after UV irradiation (2) release of TOC in the50 mL of Milli-Q water in which the membrane were immersed before and afterthe predefined time of UV exposure and (3) SEM to observe the surface morphologyof the membranes before and after the exposure to UV light The overall change in sur-face hydrophobicity or hydrophilicity of the membranes was determined by contactangle measurements (CAM) Ten types of polymeric membranes (Table 73) were sub-jected to the initial UV screening test Polytetrafluoroethylene (PTFE) hydrophobicpolyvinylidene fluoride (PVDFphobic) and PAN membranes were stable after30 days of UV illumination However in a study of the oxidative stability of mem-branes (under UV hydrogen peroxide [H2O2] and combined UVH2O2 conditions)it was found that the stability of PAN membrane declined considerably when it wasexposed to 10 days of 200 mM H2O2UV conditions Then the author concludedthat on the basis of their UV and oxidative screening tests PTFE and hydrophobicPVDF are the better choice for photocatalytic applications
Different papers reported in the literature suggest the use of PVDF for PMRs Fu JiWang Jin and An (2006) successfully used a submerged membrane photocatalyticreactor (SMPR) in which the MF module was made of PVDF hollow fibre membranes(pore size frac14 02 mm filtration area frac14 02 m2) for the degradation of fulvic acid bynanostructured TiO2 Chin Lim Chiang and Fane (2007) studied a low-pressurePVDF submerged hollow fibre membrane module to retain the TiO2 particles in thephotodegradation of bisphenol A (BPA) used as a model pollutant
Table 73 Types of commercial membranes used in the UV irradiationtest
Type of membrane Pore size Suppliers
Polyvinyldene fluoride(PVDF)
022 mm Millipore
PVDF-MP 022 mm Millipore
PVDF-Pall 01 mm Pall filtration
Polycarbonate (PC) 01 mm Millipore
Polysulphone (PS) 600 kDa Osmonics
Polytetrafluoroethylene(PTFE)
02 mm Cole Parmer
Polypropylene (PP) 01 mm Osmonics
Polyacrylonitrile (PAN) 40 kDa Cleanseas
Polyethersulphone (PES) 50 kDa Millipore
Cellulose acetate (CA) 02 mm MFS
Source Chin et al (2006)
Photocatalytic membrane reactors for water treatment 213
Despite all the properties that make PVDF one of the most extensively applied UFmembrane materials (eg antioxidation good thermal and hydrolytic stabilities aswell as good mechanical and membrane-forming properties) the hydrophobic natureof PVDF often results in severe membrane fouling and permeability decline sinceorganic contaminants (hydrophobic) deposition on this materials results favoureddue to chemical affinity This has become a conspicuous drawback for their applicationin water treatment (Lang Xu Yang amp Tong 2007) Many methods have been testedto improve the membrane performances Among these ones blending with inorganicmaterials especially nanoparticles has attracted great attention owing to the relativelyeasy preparation by the method of phase inversion which is carried out under mildconditions (Shon et al 2010 Wei et al 2011) On the basis of this DamodarYou and Chou (2009) reported a study on the self-cleaning antibacterial and photo-catalytic properties of TiO2-entrapped PVDF membranes The modified PVDF mem-branes were prepared by using the phase inversion method adding different amountsof TiO2 particles (0e4 wt) into the casting solution The TiO2ePVDF membraneswere tested for their antibacterial property by using Escherichia coliform photoactiveproperty using reactive black 5 (RB5) dye and self-cleaning (antifouling) propertiesusing 1 Bovine Serum Albumin (BSA) solution Obtained results evidenced thatthe hydrophilicity and pore size of composite PVDFTiO2 membranes were variedby addition of different amounts of TiO2 This also improves the permeability of modi-fied PVDFTiO2 membrane The PVDFTiO2 membrane showed better bactericidalability as compared to neat PVDF membrane under UV light The rate of RB5 removalwas higher as compared to neat PVDF membrane Among the tested membranes theones with 2e4 PVDFTiO2 showed better antifoulingself-cleaning ability ascompared to neat PVDF membrane
On the same topic Wei et al (2011) proposed a new PVDFeTiO2 nanowire hybridUF membrane prepared via phase inversion by dispersing TiO2 nanowires in PVDFcasting solutions Obtained results showed that the microstructure mechanical prop-erty thermal stability hydrophilicity permeation and antifouling performance ofhybrid membranes were improved significantly by addition of hydrophilic inorganicTiO2 nanowires
In a recent work You Semblante Lu Damodar andWei (2012) evaluated the anti-fouling and photocatalytic properties of a membrane in which polyacrylic acid (PAA)was plasma-grafted on commercial PVDF membrane to introduce functional groupson the membrane surface that can support the TiO2 nanoparticles Obtained resultsshowed that the membrane hydrophilicity was tremendously enhanced by the self-assembly of TiO2 following a direct proportionality to TiO2 loading However greaterhydrophilicity did not necessarily implicate better antifouling properties since exces-sive nanoparticles plugged membrane pores The membrane with 05 TiO2 loadingmaintained the highest pure water flux and the best antifouling property Beyond thisconcentration a significant permeate flux decline was observed The TiO2-modifiedmembranes were able to remove 30e42 of 50 mgL aqueous RB5 dye and thedegradation reaction was highly dependent on TiO2 amount The overall resultsevidenced that the fabricated membranes are good candidates for use in membranereactors with high hydrophilicity fouling mitigation and photocatalytic capability
214 Advances in Membrane Technologies for Water Treatment
Chin et al (2006) demonstrated that PTFE (a highly crystalline polymer thatexhibits excellent thermal stability and high chemical resistance) membranes arealso good candidates for PMR applications On the basis of this Damodar and You(2010) proposed a PMR assembled by integrating a novel flat plate PTFE membranemodule that was placed at the centre of the tank and surrounded by two UV lamps (seeFigure 72)
The performance of the assembled PMR was evaluated in the mineralization ofRB5 and the efficiency of catalyst recycling and reuse was studied using batchand long-term continuous experiments The obtained data showed that during contin-uous long-term operations the membrane module was able to maintain the catalysteffectively within the reactor and the catalyst was also reused without much loss indye degradation efficiency Indeed the catalyst that gradually deposited on mem-brane surface during continuous operation was effectively recycled by using simpleand quick cleaning method after which the original behaviour of the membrane andRB5 degradation performance were restored The performance of the reactor wasstable with nearly 75e82 TOC removal and 85e90 chemical oxygen demand(COD) removal and showed similar trend for about five to six days after eachmembrane-cleaning step
Raja et al (2007) studied the discolouration of orange II on synthesized PTFECo3O4 films under visible light irradiation An innovative way to prepare PTFECo3O4 supported catalyst by a suitable thermal treatment of the PTFE film in contactwith the Co3O4 colloid was presented The so prepared catalyst allowed the accelerateddiscolouration of orange II
Another hydrophobic polymer largely used in PMR is PP PP hollow fibre mem-branes were used by Erdim Soyer Tasiyici and Koyuncu (2009) to study the effectof natural organic matter (NOM) fouling Synthetic and natural raw waters wereused in their photocatalytic experiments The experimental data showed that the in-crease in NOM concentration increased the pressure while raw water experimentsshowed a higher pressure increase and lower removal efficiencies compared to thatof synthetic water
Magnetic stirrer
Feed
Permeate
Air inlet
UV lampsMembrane module
Air diffuser
Figure 72 Schematization of the assembled photocatalytic membrane reactor (PMR)Adapted from Damodar and You (2010)
Photocatalytic membrane reactors for water treatment 215
Because of their hydrophobic nature PP membranes can be used in MD (Mozia ampMorawski 2006 Mozia Morawski Toyoda amp Tsumura 2010)
Many works are reported in literature on the coupling of the photocatalytic processwith NF membranes which have the advantage of retaining low molecular weightmolecules better in comparison to low-pressure MF and UF modules
Different NF membranes were tested in the photodegradation of different pharma-ceuticals by Molinari Pirillo Loddo and Palmisano (2006) Table 74 reports the maincharacteristic of these NF membranes
Obtained results evidenced that membrane retention depended on both the pH of theaqueous solution to be treated and on the chemical characteristics of the particularpharmaceutical compound to be degraded NF PES 10 at alkaline pHs NTR 7410at neutral and alkaline pHs and N 30 F at acidic pHs were the membranes that showed
Table 74 Main characteristics of commercial membranes tested
Type of membrane Material Characteristics Manufacturer
NTR 7410 Sulfonatedpolysulfone
Rejection 10 with02 NaCl at49 bar 25 Cand pH 65
Nitto DenkoTokyo
PAN GKSS HV3T Polyacrylonitrile Cut-off frac14 30 kDawater flux4231 Lhm2 at2 bar and8462 Lhm2 at4 bar
GKSS Germany
N 30 F Modifiedpolysulfone
Rejection 25e35with 02 NaCland 85e95with 05 ofNa2SO4 waterflux 40e70 Lhm2 at 40 bar and20 C
Hoechst CelgardGermany
NF PES 10 Polyethersulfone Rejection 10e20with 05 NaCland 40e70with 05 ofNa2SO4 waterflux 200e400 Lhm2 at 40 barand 20 C
Hoechst CelgardGermany
Source Molinari et al (2006)
216 Advances in Membrane Technologies for Water Treatment
the best rejection percentages for furosemide at pressures of 4e8 bar NTR 7410 wasthe best membrane for ranitidine rejection over the whole pH range although its rejec-tion at 8 bar was lower than that found for NF PES 10
In another work (Molinari et al 2002) NF PES 10 and NTR 7410 membraneswere tested for the photocatalytic removal of 4-NP and benzoic acid from aqueoussolutions TiO2 P25 Degussa was used as the catalyst both suspended and depositedon the polymeric membranes Obtained results showed that the treatment of 4-NPsolution in acidic medium allowed the total removal of the substrate by using a contin-uous system NTR 7410 was also an efficient NF membrane for separating two dyesCongo red and direct blue from aqueous solutions (Molinari Pirillo Falco Loddoamp Palmisano 2004)
73 Advances in membrane modules and systemconfigurations for PMRs for water treatment
Despite the important advantages of the photocatalytic processes with respect to thetraditional ones their application at industrial level is limited by different drawbacksrelated to the involved reactions and reactor configuration
With regard to the configuration of the reactor and in particular by considering thephotocatalyst two operative configurations can be identified the first one in whichthe photocatalyst is suspended in the aqueous reacting phase and another one withthe photocatalyst immobilized on a support
Slurry photocatalytic reactors in which the catalyst is suspended in the aqueousreacting phase have been widely used In these reactors the nondegraded moleculesor their byproducts are freely transported in the final stream Photocatalyst recoveryand recycle is another problem The coupling with pressure-driven membrane separa-tions in PMRs can solve these problems
731 Pressurized membrane PRs
Different types of PMRs were built with the purpose of obtaining an easy separation ofthe catalyst from the reaction environment and an efficient removal of pollutants fromaqueous media The most studied configurations were pressurized systems in whichpressure-driven membrane processes such as MF UF and NF were combined tothe photocatalytic process In these systems the catalyst used both in the suspendedconfiguration and immobilized on the membrane is confined in the pressurized sideof the permeation cell
In 2002 Molinari et al (2002) reported some experimental results obtained byusing two different pressurized PMR configurations for the degradation of 4-NPIn particular the configurations studied were (1) a first one in which the irradiationof the catalyst was performed in the permeation cell containing the membrane withthree sub-cases (a) catalyst in suspension (b) catalyst deposited on the membrane(c) catalyst entrapped in the membrane and (2) a second one in which the irradiation
Photocatalytic membrane reactors for water treatment 217
of the suspended catalyst was performed in the recirculation tank The results showedthat the second configuration appeared to be the most interesting for industrial appli-cations in terms of irradiation efficiency and membrane permeability For example inreactor optimization high irradiation efficiency high membrane permeate flow rateand selectivity can be obtained by sizing separately the lsquophotocatalytic systemrsquo andthe lsquomembrane systemrsquo and taking advantage of all the best research results for eachsystem
In a successive work the same group Molinari et al (2004) studied the photodegra-dation process of two commercial azo dyes ie Congo red (C32H22N6Na2O6S2) andpatent blue (C27H31N2NaO6S2) in a membrane PR by using TiO2 Degussa P25 asthe photocatalyst and an NF membrane Two different PR configurations a cylindricalone with an external lamp and an annular one with an immersed lamp in the axial po-sition (Figure 73) were studied and the influence of some operational parameterssuch as the pressure in the membrane cell and the initial concentration of the substrateswas determined A comparison between suspended and entrapped TiO2 was also done
The rate of pollutant photodegradation was strongly affected by the UV irradiationmode In particular the rate of photodegradation with the immersed lamp resulted 50times higher than that of suspended one (0274 versus 000548 mgmin) although thepower of the last was four times greater Congo red was photodegraded with higherrate under the same experimental conditions probably due to its higher adsorptiononto the catalyst surface The reactor containing the suspended photocatalyst wassignificantly more efficient than the reactor containing the catalyst entrapped intothe membrane as also found by other authors (Mascolo et al 2007) The observed
(5)
(2)
(6)
(3)
(1)(1)
(2)
(3)
(6)
(5)
(4)(4)
Figure 73 Scheme of the batch system with external lamp and with immersed lamp used in theexperimental runs without membrane with the suspended catalyst (1) oxygen cylinder (2)cylindrical reactor with cooling jacket (3) thermostatic baths with cooling water (4) powersupplies (5) medium pressure Hg lamp (6) magnetic stirrer (Reprinted from Molinari et al(2004) with permission from Elsevier)
218 Advances in Membrane Technologies for Water Treatment
reduction of photoactivity was probably due to the lower available active surface area ofthe catalyst compared to the suspended immobilized system The results evidenced thatit was possible to treat successfully highly concentrated solutions (500 mgL) of bothdyes by means of a continuous process obtaining good values of permeate fluxes(30e70 Lm2h) this could be interesting for industrial applications
732 Depressurized (submerged) membrane PRs
The main problem observed by coupling photocatalysis with a pressure-driven mem-brane process is the decrease of permeate flux caused by membrane fouling due tocatalyst deposition on the membrane Submerged membrane systems represent oneof the different approaches proposed to overcome this limitation In this configurationthe catalyst is suspended in an open-air reaction environment the membrane isimmersed in the batch and the permeate is sucked by means a vacuum pump
Fu et al (2006) reported the degradation of FA by using synthesized nanostructuredTiO2silica gel catalyst particles in a submerged membrane photocatalytic reactor(SMPR) schematized in Figure 74 It was found that the photocatalyst at 05 gLand airflow at 006 m3h were the optimal conditions for the removal of FA The resultsshowed a reduction of fouling phenomena when nanostructured TiO2 was used as thephotocatalyst
An additional way to prevent photocatalyst deposition and then reduce membranefouling consists in controlling the hydrodynamic conditions near the membrane sur-face In this context gas sparging at the bottom of the membrane represents a prom-ising strategy On this basis Chin et al (2007) studied the removal of BPA in waterby using low-pressure submerged module and they observed that the aeration allow-ing a mechanical agitation reduced the fouling of the membrane and maintained theTiO2 well suspended in the solution In addition in this study another strategy toreduce membrane fouling was proposed an intermittent membrane filtration whichpermitted to maintain high flux at low aeration rate Practically when suction across
(1)
(2)
Feed
(3)
(4)
Submerged membrane module
Permeate
Aeration
Cooling water
(5)
Figure 74 Schematic diagram of the submerged membrane photocatalytic reactor (SMPR)system (1) feed tank (2) feed pump (3) thermostated jacket protoreactor (4) UV lamp(5) suction pumpReprinted from Fu et al (2006) with permission form Elsevier
Photocatalytic membrane reactors for water treatment 219
the membrane was stopped the aeration could shear the membrane surface facilitatingthe detachment of catalyst particles The results summarized in Figure 75 showedthat TOC photomineralization by operating with intermittent operation was similarto that ones obtained with continuous operation ie 94 BPA reduction at250 min Moreover it was found that with the intermittent mode the transmembranepressure (TMP) of the system decreased by five times compared to the continuousoperation Because TMP is proportional to the filtration energy cost intermittentoperation would reduce the operating cost of the continuous SMPR
The advantages of this approachwere also studied byHuangMeng Liang andQian(2007) They demonstrated that sedimentation of the suspended semiconductor can becontrolled by applying fine-bubble aeration and intermittent membrane filtration
Choi (2006) used the same intermittent approach studying the performance of apilot-scale SMPR in the degradation of 4-Chlorophenol (4-CP) The results evidenceda complete degradation of the pollutant in 2 h Besides in continuous runs no mem-brane fouling was observed when the intermittent operation was used
Therefore the SMPR with fine-bubble aeration and intermittent membrane filtra-tion can be potentially applied in the photocatalytic oxidation process during drinkingwater treatment
733 Coupling of photocatalysis and membrane distillation
Coupling of photocatalysis and MD could avoid fouling problems related to the use ofpressure-driven membrane separations MD is a separation process based on the prin-ciple of vapoureliquid equilibrium The nonvolatile components (eg ions macromol-ecules etc) are retained on the feed side whereas the volatile components passthrough a porous hydrophobic membrane and then they condense in a cold distillate(usually distilled water)
0
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Continuous membranefiltration
Intermittent membranefiltration
Perc
enta
ge (
)
Change in TMPEfficiency of TOC degradationEfficiency of production of water
Figure 75 Comparison of the transmembrane pressure (TMP) change total organic carbon(TOC) degradation and production of water between continuous and intermittent permeationoperationReprinted from Chin et al (2007) with permission from Elsevier
220 Advances in Membrane Technologies for Water Treatment
The possibility on the way to use a PMR obtained by coupling photocatalysis anddirect contact MD (DCMD) for degradation of azo dyes (acid red 18 acid yellow 36and direct green 99) in aqueous solution was investigated by Mozia et al (2007) Theinitial concentration of a dye was equal to 30 mgL and the catalyst (TiO2 Aeroxide
P25 Degussa Germany) loadings amounted to 01 03 and 05 gL The resultsshowed that the MD process was very effective in separation of photocatalyst particlesIndeed after 5 h of the experiment the turbidity of distillate was similar to thatmeasured for ultrapure water regardless of the TiO2 amount in the reacting phaseThe highest effectiveness of photodecomposition was obtained in the case of acidred 18 and the most difficult compound to be degraded was acid yellow 36 havingthe lowest molecular weight among all the dyes used (Figure 76)
A complete rejection of the dye and other nonvolatile compounds (organic mol-ecules and inorganic ions) was achieved Some volatile compounds crossed themembrane as indicated by the measurements of TOC concentration into the distil-late However the amount of these substances remained in the range 04e10 mgL so that it can be concluded that the product (distillate) was practically pure waterA permeate flux of 034 m3m2d was obtained similarly to those obtained during theprocess in which ultrapure water was used regardless of the TiO2 amount This resultevidenced that MD permitted to avoid the significant fouling observed whenpressure-driven membrane processes were coupled with photocatalysis althoughthe higher energetic consumption of MD represents an important disadvantage
Thus the hybrid process coupling the photocatalysis and DCMD is a promisingmethod for the removal of organic compounds such as azo dyes from water becausethe MD membrane is a very effective barrier for the catalysts particles and the nonvol-atile compounds present in the feed solution
0
25
50
75
100
Acid red 18 Acid yellow 36 Direct green 99
Deg
rada
tion
()
01 gL03 gL05 gL
Figure 76 Photodegradation of Acid red 18 Acid yellow 36 and Direct green 99 in the PMRobtained by coupling photocatalysis and direct contact membrane distillation (DCMD) usingdifferent TiO2 concentration in the reacting phase (evaluated on the basis of dyes concentrationin the feed phase after 5 h of illumination)Data from Mozia et al (2007)
Photocatalytic membrane reactors for water treatment 221
734 Coupling of photocatalysis and dialysis
The results summarized in the previous section show that only the combination ofphotocatalysis and MD avoids fouling but it needs energy to reach evaporationphenomena
On this basis Azrague et al (2007) proposed the combination of dialysis and photo-catalysis to mineralize organic compounds (24-dihydroxybenzoic acid [24-DHBA]was used as a model pollutant) contained in artificial turbid waters obtained by usinga natural clay named bentonite In this PMR configuration the dialysis membraneused as a membrane contactor permits to separate the polluted turbid water fromthe PR compartment (Figure 77)
Operating in this way the membrane acts as a barrier for the photocatalyst particlesand it allows to extract the organic compounds from the turbid water thanks to theconcentration difference between the turbid polluted compartment and the one inwhich the photocatalytic reaction takes place The absence of TMP avoids the forma-tion of fouling even in case of highly turbid water Besides the membrane permits (1)to maintain the TiO2 in the PR compartment avoiding a final filtration stage and (2) tokeep the bentonite away from the PR thus avoiding the loss of efficient irradiationsdue to the scattering by bentonite particles
All these advantages combined with the complete removal of a high 24-DHBAconcentration demonstrate the effectiveness of the proposed PMR combiningphotocatalysis and dialysis These results should lead to the design of a PMR workingin a continuous mode
735 Coupling of photocatalysis and pervaporation
Recently it has been demonstrated that the integration of photocatalysis with PV is apromising method to improve the efficiency of the detoxification of water streams con-taining recalcitrant organic pollutants at a low concentration (Camera-Roda amp Santar-elli 2007) as the integration of the two processes generates a synergistic effect In PVseparation is not based only on the relative volatility of the components in the mixture
(1)
(2)
(3)
(4)
(5) (6)
(7)
(8)
Figure 77 Schematization of the experimental set-up of the integrated photocatalysiseMDprocess (1) magnetic stirrer (2) feed (artificial turbid water) tank (3) and (4) circulation pumps(5) hollow fibre module or flat sheet membrane (6) thermostated photoreactor (7) pressureregulation valve (8) oxygen cylinderAdapted from Azrague et al (2007)
222 Advances in Membrane Technologies for Water Treatment
but also depends on the relative affinity of the components with the membrane Thusthe choice of membrane material is important to obtain a selective separation of themolecules
The experimental apparatus used for the integrated photocatalysis-PV process isschematically represented in Figure 78 The liquid is continuously recycled by a mem-brane pump to an annular photocatalytic reactor (APR) and kept at a constant temper-ature after passing through a small tank immersed in a thermostatic bath Oxygen iscontinuously supplied inside the tank to maintain a constant concentration of8 mg L1 in the system At the exit of the APR the water passes through one ortwo plane PV modules (membrane surface area frac14 160 cm2) through which the compo-nents permeate selectively thanks to the vacuum (6 mbar) that is kept downstream
The results (Figure 79) showed that the rate of disappearance of a model pollutant(4-chlorophenol [4-CP]) is highly improved by using the integrated system as the
(1)
(2)
(3)
(4)Permeate
Retentate
Figure 78 Schematization of the experimental set-up utilized for the integrated photocatalysis-PV process (1) termostated feed tank (2) membrane pump (3) annular photocatalytic reactor(APR) (4) plane pervaporation moduleAdapted from Camera-Roda and Santarelli (2007)
0
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50
60
0 5 10 15 20 25 30 35Time (h)
4-C
P re
mov
ed (
)
IntegrationPC+PV no integrationOnly photocatalysisOnly pervaporation
Figure 79 Percentage of 4-chlorophenol (4-CP) removed versus time in the integratedphotocatalysis-PV processAdapted from Camera-Roda and Santarelli (2007)
Photocatalytic membrane reactors for water treatment 223
membrane was efficient in continuously eliminating some intermediate productsthat could slow down the rate of the photocatalytic reaction and concurrently thephotocatalysis transforms the weakly permeable 4-CP into organic compounds thatPV can remove at a high rate
Process intensification by synergistic effect depends on the optimization of the ratiobetween the characteristic rates of the photocatalytic and PV processes
Recently the use of a highly selective membrane allowed to recover somechemical products with high added value as vanillin starting from trans-ferulic acid(Camera-Roda et al 2013)
74 Applications of PMRs for water treatment
In the last years the interest in the presence of pharmaceutical active compounds(PhACs) and their metabolites in waterways has increased significantly (JonesVoulvoulis amp Lester 2002 Kasprzyk-Hordern Dinsdale amp Guwy 2008) Largeamount of PhACs in terms of thousands of tons are annually used for therapeuticpurposes or in animal farming in each European country and may be excreted bothas unmetabolized and as active metabolites thus reaching the aquatic environmentBesides improper disposal of industrial waste may also contribute to their occurrencein aquatic environment The wastewater is generally treated in sewage treatment plants(STP) that often fail to remove the majority of PhACs resulting in high concentrationsdischarged in treated effluents (Braga Smythe Schafer amp Feitz 2005 Carballa et al2004 Castiglione et al 2006 Colpin et al 2002 Comeau Surette Brun amp Losier2008) so PhACs are detectable in the aquatic environment with concentration levelsup to the mgL These amounts are much lower than maximum concentrations reportedfor typical industrial contaminants but their toxicological chronic effects on aquaticenvironment due to the continuous exposure to mixtures of pharmaceuticals areunknown
On this basis the demand for developing efficient systems alternative to the tradi-tional purification methods to remove PhACs from water has assumed a great researchinterest PMRs could represent a useful solution to this problem
The photodegradation of different pharmaceuticals (furosemide ranitidine (hydro-chloride) ofloxacine phenazone naproxen carbamazepine and clofibric acid)contained in aqueous medium at various pHs by using a batch PR and a PMR workingin recirculation regime was studied by Molinari et al (2006) The experimental testswere performed by using polycrystalline TiO2 as the photocatalyst determining theperformances of different commercial membranes mainly of the NF type (Table 74)in the PMR To analyse the results of the photodegradation experiments in the PMRthe main factors involved in the overall performance of the PR were separately studiedby carrying out the following tests (1) adsorption tests on the TiO2 particles (2) degra-dation tests in the absence of the membrane (3) membrane rejection by changing theoperating pressures and (4) degradation tests in the PMR The results evidenced thatthe adsorption of the substrates onto the catalyst is affected by the pH owing to theinfluence of this operating parameter on the hydrophilichydrophobic character of
224 Advances in Membrane Technologies for Water Treatment
the photocatalyst All the seven considered molecules were successfully photode-graded in the batch reactor (without the membrane) with pseudo-first order kinetics
Furosemide and ranitidine were selected to carry out the study of rejection and pho-todegradation in the PMR During the photocatalytic tests in the PMR the permeateflux through the NTR 7410 membrane had an average value of 45 Lhm2 at bothacidic and alkaline pHs Rejection values were in the range 10e60 for furosemideand 5e30 for ranitidine in the dark (without photoreaction) but a net decreasedown to zero was observed in the contemporary presence of light photocatalystand oxygen Then the degradation results in the PMR (Figure 710) showed that rejec-tion measured in the presence of photocatalyst and oxygen both in the dark and underirradiation conditions was almost zero so that the membrane in this case was benefi-cial only because it allowed the confinement of the photocatalyst
In a successive work the same group Molinari et al (2008) reported the degradationof two pharmaceuticals (Gemfibrozil [GEM] and Tamoxifen [TAM]) selected astarget molecule using suspended TiO2 as catalyst in two configurations of PMRs(pressurized and depressurized) operated in both batch and continuous mode Thelatter mode consisted in the continuous withdrawn of the permeate from the reactorand its replacing by an equal volume of polluted water until reaching a steady state(approximately 190 min) in terms of flux and rejection values of intermediates Theobjective of these tests was to simulate the continuous photodegradation processthat could be applied at industrial level The experimental plant consisted of an annularPR with an immersed UV lamp connected to the permeation cell in which a pressurizedflat sheet membrane or a submerged membrane module was located The effects of pa-rameters such as pH of the aqueous TiO2 suspensions pump flow rate and membraneclean-up on the efficiency of the PMR were studied Closed and continuous
0
4
8
12
16
20
0 50 100 150 200Time (min)
Con
cent
ratio
n (m
gL)
Figure 710 Substrate concentrations versus time for runs carried out by using the hybridsystem with the NTR 7410 membrane at initial pH 3 (TiO2 frac14 1 gL O2 frac14 22 ppm immersedlamp 125 W) Furosemide (A) retentate (gt) permeate Ranitidine (-) retentate ()permeateReprinted from Molinari et al (2006) with permission from Elsevier
Photocatalytic membrane reactors for water treatment 225
procedures were used to investigate the behaviour of the pressurized PMRs The dataobtained for the closed membrane system showed complete photodegradation of GEMand TAM with an abatement of 99 after 20 min and mineralization higher than 90after approximately 120 min
Regard to the pressurized system the membrane used (commercial flat sheet NFmembrane NTR 7410 Nitto Denko Tokyo) allowed to maintain the drug and the cata-lyst in the reaction environment but it was not able to reject significantly the degrada-tion intermediates which were also found in the permeate A good operating stabilitywas observed by operating in continuous mode reaching a steady state for approxi-mately 120 min (Figure 711) with a complete abatement of the drug and values ofmineralization (60) and permeate flux (386 Lhm2) that remained constant untilthe end of a run A TOC rejection of about 62 at steady state confirmed the needto identify a membrane with higher rejection to the intermediate products Catalystdeposition on the membrane surface and fouling caused a flux decline during the pho-tocatalytic process
The depressurized system in which the submerged membrane module was locatedseparately from the PR and the oxygen was bubbled on the membrane surface couldbe of interest both to separate the photocatalytic zone from the separation zone and toreduce membrane fouling thanks to oxygen sparging The UF membrane (PES com-mercial capillary membrane 005e01 mm average pore size) used in the submergedsystem showed retention of only the catalyst in the reaction environment whileGEM and its oxidation products moved in the permeate However the submergedPMR permitted to obtain a steady-state flux of 651 Lhm2 higher than those obtainedoperating with the pressurized module resulting it interesting for application purposes
Retentate
Permeate
0 50 100 150 200 250 3000
Time (min)
TOC
(mg
L)
2
4
6
8
10
Figure 711 Total organic carbon (TOC) values versus the time in the degradation ofGemfibrozil (GEM) in the continuous submerged membrane photoreactor (continuous feed ofGEM at concentration of 10 mgL Reprinted from Molinari et al (2008) with permission fromElsevier)
226 Advances in Membrane Technologies for Water Treatment
The overall results reported by Molinari et al (2006) and Molinari et al (2008)confirmed that PMRs can be of interest in the removal of organic pollutants from waterbecause they allowed the recovery and reuse of the catalyst permitted to achieve acontinuous process and if a suitable membrane is found it is possible to retain thepollutant and its degradation products in the reaction environment
With regard to the possibility of using solar radiation to activate the photocatalystin 2005 Augugliaro et al studied lincomycin (a common antibiotic) degradation inaqueous suspensions of polycrystalline TiO2 Degussa P25 irradiated by sunlight Ahybrid system consisting of a solar PR coupled with a membrane module was testedwith the aim to improve system sustainability and performance The results of prelim-inary tests without the membrane separation step showed that lincomycin was success-fully oxidized in the pilot plant solar PR The presence of the membrane whichconfined both photocatalyst and pollutants in the reaction environment allowed oper-ation in continuous mode The data collected by using the hybrid system both in totalrecycle (batch) and in continuous mode (Figure 712) indicated that the presence of themembrane allows the reduction of both the substrate and intermediates down to lowconcentration levels
Benotti Stanford Wert and Snyder (2009) reported a study on the use of a PMRpilot system using UVTiO2 photocatalysis in the removal of 32 PhACs from waterThe results showed that concentrations of all compounds decreased during the treat-ment In particular removal efficiency higher than 70 was obtained for 29 of thetargeted compounds while a removal efficiency lower than 50 was achieved foronly three compounds During the treatment of all the considered PhACs no oestro-genically active transformation products were formed Thus the PMR developed inthis work is a useful technology for water treatment as determined by pharmaceuticaland endocrine-disrupting compound destruction as well as the removal of oestrogenicactivity
In 2012 Mozia and Morawski on the basis of their previous encouraging resultsobtained in dye removal from water compared the efficiency of photodegradationof ibuprofen (IBU) sodium salt present in tap water in a PMR using DCMD under a
90(a) (b)8070605040302010
00 1 2 3 0
E [einstein]E [einstein]
C [μ
M]
C [μ
M]
0
5
10
15
20
25
1 2 3 4
Figure 712 Lincomycin concentrations versus the cumulative photon energy E for runscarried out in (a) total recirculation regimens and (b) in continuous regimen by using the hybridsystem at different initial lincomycin concentrations Full symbol retentate open symbolpermeateReprinted from Augugliaro et al (2005) with permission from Elsevier
Photocatalytic membrane reactors for water treatment 227
batch and continuous operation The results revealed that the influence of the operationmode on the effectiveness of IBU decomposition and mineralization and on the prod-uct (distillate) quality was not significant The permeate did not contain IBU regard-less of the process conditions Despite that the authors recommended the operation ofthe PMR in the continuous mode due to its higher potential for a large-scaleapplication
Membrane scaling during a long-term process was also studied The results sum-marized in Figure 713 showed a decline of permeate flux during the long-term exper-iments In particular after 54 h of the PMR continuous operation the flux was lowerby approximately 7 compared to pure water flux (272 Lm2d versus 294 Lm2d)whereas after 188 h an 86 flux decline was detected The deposit layer formed ona membrane surface had a composite structure on a thin porous TiO2 layer the calciteand aragonite crystals were present The application of cleaning with HCl solutionallowed to dissolve the CaCO3 scale deposit and to recover the flux However about70 h after washing with HCl solution the flux started decreasing with a similar rate asbefore cleaning
In a successive work the same authors (Mozia Darowna Przepiorski amp Moraw-ski 2013) reported the removal of a nonsteroidal anti-inflammatory drug (diclofenacsodium salt [DCF]) from water in two hybrid systems coupling photolysis or photoca-talysis with DCMD The results are summarized in Figure 714 in terms of TOCamounts in distillate (permeate) at the end of various experiments in comparisonwith the initial and final amounts of organic carbon in feed solutions A completeDCF decomposition in the feed was obtained in both processes In the case of the pho-tolysiseDCMD hybrid system the effectiveness of TOC mineralization after 5 h ofirradiation ranged from 273 to 487 depending on the DCF initial concentrationThe addition of TiO2 allowed the improvement of the efficiency of TOC removal Af-ter 5 h of the hybrid photocatalysiseDCMD process the mineralization efficiency was
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300Time (h)
Perm
eate
fulx
(Lm
2 d)
HCl cleaning
Figure 713 Changes of permeate flux during long-term operation of photocatalytic membranereactor (PMR) using direct contact membrane distillation (DCMD) in continuous modeAdapted from Mozia and Morawski (2012)
228 Advances in Membrane Technologies for Water Treatment
in the range of 825e100 the highest for the lowest DCF concentration Applicationof DCMD in the hybrid systems ensured high efficiency of separation of DCF as wellas the products of its degradation Indeed regardless of the process applied distillatewas high purity water DCF was not detected TOC concentration did not exceed08 mgdm3 and conductivity was lower than 16 mScm
The results obtained by Mozia and Morawski (2012) and Mozia et al (2013)showed that because the MD efficiently separated not only TiO2 particles but alsoorganic contaminants present in feed solution the PMR using DCMD represents apromising method for treatment of waters containing pharmaceuticals Low permeate(distillate) flux assured relatively long residence time of contaminants which resultedin high effectiveness of their photodegradation
The overall results reported in this section confirm that PMRs can be of interest inthe removal of organic pollutants (eg pharmaceuticals) from water because the mem-brane (1) separates TiO2 particles efficiently thus permitting the recovery and reuse ofthe catalyst (2) permits the realization of a continuous process and (3) allows theretention of organic pollutants and their degradation products thus avoiding theirrelease in the permeate and controlling the residence time of contaminants in the reac-tion environment Then PMRs can be considered a useful technology for water treat-ment as determined by PhACs destruction and the removal of oestrogenic activity Animportant issue to take into consideration in developing photocatalysisemembranehybrid processes is membrane fouling mainly due to photocatalyst deposition onthe membrane feed side In this context the approach proposed by Mozia et al inwhich MD is coupled with photocatalysis seems to be promising because MD allowsto obtain the points (1) (2) and (3) previously reported and also permits the avoidanceof the significant fouling observed when a pressure-driven membrane process is usedalthough the higher energetic consumption of MD could represent a limitation of thishybrid process
642 640
00
627
377
05
640
110
0700
100
200
300
400
500
600
700
Feed 0 Feed 5 h Permeate 5 h
TOC
(mg)
DCMD
Photolysis - DCMDPhotocatalysis - DCMD
Figure 714 Comparison of TOC amounts in distillate (permeate) after 5 h of direct contactmembrane distillation (DCMD) photolysiseDCMD and photocatalysiseDCMD processeswith the initial and final total organic carbon (TOC) amounts in feed solutionsData from Mozia et al (2013)
Photocatalytic membrane reactors for water treatment 229
75 Advantages and limitations of PMRs in watertreatment
Based on the results summarized in the previous sections the use of PMRs can be pro-posed as an attractive green process because it is effective in abating harmful andrecalcitrant substances present in aqueous effluents
The main advantages related to the use of the PMRs to degrade pollutants are
1 Possibility to be applied to a wide range of compounds in aqueous gaseous and solid phase2 Short reaction times and mild experimental conditions usually ambient temperature and pressure3 Generally only oxygen from air without any additional additive is necessary4 Effectiveness with low concentration of pollutant(s)5 Possibility to destroy a variety of hazardous molecules with the formation of innocuous prod-
ucts solving the disposal pollutant problem associated to the conventional wastewatertreatment methods
6 Possibility to convert toxic metal ions in their nontoxic forms which can be recovered andreused
7 Synergistic effects by coupling the potentialities of classical PRs and those of membraneprocesses ie catalyst recovery and reuse (immobilized or in suspension) rejection of thesubstrate andor separation of degradation products thus allowing to perform the reactionof interest and the separation of the desired product(s) simultaneously in continuous mode
8 Possibility to control the residence time of substrates in the photocatalytic system thus verifyingthe contact time between the molecules to be degraded and the catalyst and then the photocata-lytic degradatione this feature is very important when the process is used for synthetic purposes
9 Possibility to use sunlight
Despite these important advantages the application of the photocatalytic process at in-dustrial level is limited by different drawbacks mainly related to the membrane fouling
As reported in previous sections the choice of the membrane is a key step forachieving a successful application of PMRs For example Molinari et al (2006) andMollinari et al (2008) demonstrated the potentialities of PMR applications in both pres-surized and submerged configurations in water detoxification because they allowed therecovery and reuse of the catalyst and permitted the achievement of a continuous pro-cess while the membrane rejection towards the pollutants was not very satisfactoryThese results show the necessity to identify a membrane selective to intermediate prod-ucts to retain the pollutant and its degradation products in the reaction environment
76 Conclusion
All the results reported in this chapter on coupling the photocatalytic process withmembrane separations show numerous advantages derived from a synergistic effectof these two techniques In particular it has been shown that PMRs can be considereda useful technology for the removal of organic pollutants (eg pharmaceuticals) fromwater thanks to PhAC destruction and the removal of oestrogenic activity
An important limitation related to photocatalysisemembrane coupling is related tothe decrease of permeate flux caused by concentration polarization and membrane
230 Advances in Membrane Technologies for Water Treatment
fouling mainly due to photocatalyst deposition on membrane surface This drawbackcan be limited by appropriately choosing membrane material which must also ensureadequate chemical and thermal stability and type and PMRs configuration
The ideal membrane process to be coupled with photocatalysis should possess thesefeatures (1) complete retention of the catalyst and controlled fouling and (2) goodrejection of the substrates and of degradation byproducts to avoid the release ofdangerous substances in the treated water and to assure a residence time in the photo-catalytic system sufficient for obtaining complete pollutant degradation and minerali-zation Longer residence times are usually obtained by reducing the permeate flux so itis important to find a good compromise among the permeate flux and the residencetime to achieve a system for application purpose In other words the process intensi-fication by synergistic effect depends on the optimization of the ratio between the char-acteristic rates of the photocatalytic and membrane separation processes
Many methods have been tested to improve the membrane performances Amongthese blending with inorganic materials (eg immobilization of TiO2) to increasethe hydrophilic character of the membrane surface exposed to the reacting environ-ment of TiO2 is a promising approach that produces photocatalytic membranes withantifoulingself-cleaning ability The so-prepared membranes are good candidatesfor use in membrane reactors for wastewater treatment and reuse processes
Considering PMR configuration the use of SMPRs with fine-bubble aeration andintermittent membrane filtration can be potentially applied in the photocatalytic oxida-tion process during drinking water treatment because this approach significantly re-duces membrane fouling
The hybrid process obtained by coupling photocatalysis with MD seems very prom-ising for the removal of organic pollutants from waters because it possesses the idealfeaturespreviously reported In particular this couplingavoids fouling but it needs energyto obtain evaporation On the basis of this the combination of dialysis and pervaporationwith photocatalysis has been also reported showing that this coupling also possesses thefeatures of the ideal membrane process to be coupled with photocatalysis
77 Future trends
The use of submerged membrane systems operated with fine-bubble aeration in theintermittent membrane filtration mode and the coupling of photocatalysis with MDdialysis and pervaporation represent some promising approaches for overcomingthe limitation related to membrane fouling In particular the coupling with DCMDseems to be more promising asMD (1) separates TiO2 particles efficiently (2) permitsthe retention of organic pollutants and their degradation byproducts (the MDdistillate is practically high purity water) (3) permits the realization of a continuousprocess and (4) allows the avoidance of membrane fouling although the higherenergetic consumption of MD could represent a limitation of this hybrid process
On this basis it is foreseen that hybrid system photocatalysisemembranes will havea significant impact in designing processes for the abatement of organic pollutantscontained in waters
Photocatalytic membrane reactors for water treatment 231
78 Sources of further information
An exhaustive reference book on the fundamentals and applications of heterogeneousphotocatalysis that was recently published is Clean by light irradiation Practicalapplications of supported TiO2 by Augugliaro V Loddo V Pagliaro M PalmisanoG amp Palmisano L (2010) Cambridge (UK) RCS Publishing
Fundamentals of photocatalysis and some interesting information about membranePR regarding membrane materials and some operational issues can be found inChapter 6 - Photocatalytic membrane reactors Fundamentals membrane materialsand operational issues by Mozia S Morawski AW Molinari R Palmisano Lamp Loddo V (2013) In A Basile (Ed) Handbook of membrane reactors - volume2 Reactor types and industrial applications (pp 236e295) Cambridge (UK) Wood-head Publishing Limited ISBN 978-0-85709-415-5
Other interesting information about configuration and performance of PMRs and anexample of this integrated technology in water treatment and chemical production canbe found in Chapter 21 - Photocatalytic membrane reactors Configurations perfor-mance and applications in water treatment and chemical production by MolinariR Palmisano L Loddo V Mozia S amp Morawski AW (2013) In A Basile(Ed) Handbook of membrane reactors - volume 2 Reactor types and industrialapplications (pp 808e845) Cambridge (UK) Woodhead Publishing LimitedISBN 978-0-85709-415-5
List of symbols
Cf Solute concentration in the feedCp Solute concentration in the permeateeL Photogenerated electronEg Energy band gaph Plankrsquos constanthD Photogenerated holen Frequency of light radiation
List of acronyms
E01 Redox potential of the species to be reduced
E02 Redox potential of the species to be oxidized
24-DHBA 24-dihydroxybenzoic acid4-CP 4-Chlorophenol4-NP 4-NitrophenolAOP Advanced oxidation processAPR Annular photocatalytic reactorBPA Bisphenol ACAM Contact angle measurementCB Conduction band
232 Advances in Membrane Technologies for Water Treatment
COD Chemical oxygen demandDCF DiclofenacDCMD Direct contact membrane distillationECB Redox potential of the conduction bandEVB Redox potential of the valence bandFA Fulvic acidGEM GemfibrozilIBU IbuprofenMD Membrane distillationMF MicrofiltrationNF NanofiltrationNHE Normal hydrogen electrodeNOM Natural organic matterOM Optical microscopyPAA Polyacrylic acidPAN PolyacrylonitrilePES PolyethersulfonePhAC Pharmaceutical active compoundPMR Photocatalytic membrane reactorPP PolypropylenePR PhotoreactorPTFE PolytetrafluoroethylenePV PervaporationPVDF Polyvinylidene fluorideR Membrane rejectionRB5 Reactive black 5 dyeRO Reverse osmosisSEM Scanning electron microscopySMPR Submerged membrane photocatalytic reactorSTP Sewage treatment plantsTAM TamoxifenTOC Total organic carbonUF UltrafiltrationUV Ultraviolet irradiationVB Valence bandWPF Water permeation flux
References
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Augugliaro V Garciacutea-Lopez E Loddo V Malato-Rodriacuteguez S Maldonado I Marcigrave Get al (2005) Degradation of lincomycin in aqueous medium Coupling of solar photo-catalysis and membrane separation Solar Energy 79 402e408 Retrieved from httpdxdoiorg101016jsolener200502020
Photocatalytic membrane reactors for water treatment 233
Azrague K Aimar P Benoit-Marquieacute F amp Maurette M T (2007) A new combination of amembrane and a photocatalytic reactor for the depollution of turbid water AppliedCatalysis B-Environmental 72 197e204 Retrieved from httpdxdoiorg101016japcatb200610007
Benotti M J Stanford B D Wert E C amp Snyder S A (2009) Evaluation of a photo-catalytic reactor membrane pilot system for the removal of pharmaceuticals and endocrinedisrupting compounds from waterWater Research 43 1513e1522 Retrieved from httpdxdoiorg101016jwatres200812049
Braga O Smythe G A Schafer A amp Feitz A J (2005) Fate of steroid estrogens inAustralian inland and coastal wastewater treatment plants Environmental Science ampTechnology 39 3351e3358 Retrieved from httpdxdoiorg101021es0501767
Camera-Roda G Augugliaro V Cardillo A Loddo V Palmisano G amp Palmisano L (2013)A pervaporation photocatalytic reactor for the green synthesis of vanillin Chemical Engi-neering Journal 224 136e143 Retrieved from httpdxdoiorg101016jcej201210037
Camera-Roda G amp Santarelli F (2007) Intensification of water detoxification by integratingphotocatalysis and pervaporation Journal of Solar Energy Engineering 129 68e73Retrieved from httpdxdoiorg10111512391204
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234 Advances in Membrane Technologies for Water Treatment
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236 Advances in Membrane Technologies for Water Treatment