designofasulﬁdebasednitrogen removalprocessforwastewater ... · elemental sulfur (s0) or...

Design of a sulfide based nitrogenremoval process for wastewater

treatment

Sara Heylen

Promotor: prof. dr. ir. Eveline VolckeTutors: ir. Janis Baeten and ir. Annelies Van den Hove

Master Dissertation submitted to Ghent University in partial fulfilment of the require-ments for the degree of Master of Science in Bioscience Engineering: EnvironmentalTechnology

Academic year 2016–2017

Deze pagina is niet beschikbaar omdat ze persoonsgegevens bevat.Universiteitsbibliotheek Gent, 2021.

This page is not available because it contains personal information.Ghent University, Library, 2021.

Acknowledgements

First and foremost, I would like to express my gratitude to my promotor Eveline Volcke, thegood guidance during meetings throughout the year were always constructive and very helpfulduring the progress of my research. Furthermore, many thanks for both my tutors JanisBaeten and Annelies Van den Hove for their patience to listen to my list of questions and thehelp for solving them.

I would also like to thank my friends and family, who were always there to support me. Specialthanks to Ward, every time after we had a talk when I had a difficult moment, everythinglooked possible again. His willingness to give his time so generously has been very muchappreciated. Also special thanks to Stef and Lieven, who were always willing to help and listen.Furthermore, I also want to thank anyone present during the much enjoyed thesis breaks.

Ghent, June 2017

Sara

Contents

List of abbreviations 1

List of symbols 2

Abstract 5

Samenvatting 7

Introduction 9

1 Literature review 11

1.1 Nitrogen removal processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1.1 Conventional nitrification–denitrification . . . . . . . . . . . . . . . . . . 111.1.2 Nitritation–denitritation . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.1.3 Partial nitritation–anammox . . . . . . . . . . . . . . . . . . . . . . . . 131.1.4 Sulfur based denitrification . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.2 Applications of sulfur based denitrification . . . . . . . . . . . . . . . . . . . . . 151.2.1 Removal of hydrogen sulfide from biogas . . . . . . . . . . . . . . . . . . 151.2.2 Denitrification with elemental sulfur for groundwater treatment . . . . . 161.2.3 Nitrogen removal in domestic wastewater . . . . . . . . . . . . . . . . . 16

1.3 Effect of influent composition on the sulfur based denitrification . . . . . . . . . 181.3.1 Sulfur to nitrogen ratio (S/N ratio) . . . . . . . . . . . . . . . . . . . . . 181.3.2 Alkalinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.3.3 Organic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.4 Effect of reactor design and operation on the sulfur based denitrification . . . . 221.4.1 Anoxic reactor type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221.4.2 Hydraulic residence time . . . . . . . . . . . . . . . . . . . . . . . . . . . 231.4.3 Recirculation flow between anoxic and aerobic reactor . . . . . . . . . . 23

1.5 Conclusions and thesis objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 241.5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241.5.2 Thesis objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2 Methods 27

2.1 Calculation method for the effluent nitrate concentration . . . . . . . . . . . . . 272.1.1 General design of a sulfide based autotrophic denitrification process . . 272.1.2 Mass balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.1.3 COD balance to determine the optimal recirculation ratio . . . . . . . . 31

i

2.1.4 Effect of influent COD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.1.5 System under study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.2 Simulation study set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.3 Minimal volume of the denitrification reactor . . . . . . . . . . . . . . . . . . . 362.4 Applicability of sulfide based nitrogen removal process over nitrate and the

novel process over nitrite in wastewaters with a low sulfate concentration . . . 38

3 Results and discussion 41

3.1 Calculation method for the effluent nitrate concentration . . . . . . . . . . . . . 413.1.1 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.1.2 Effect of influent organic carbon . . . . . . . . . . . . . . . . . . . . . . 44

3.2 Effect of process operation and wastewater characteristics on the effluent nitrateconcentration and optimal recirculation flow . . . . . . . . . . . . . . . . . . . . 453.2.1 Oxygen concentration in the aerobic reactor . . . . . . . . . . . . . . . . 453.2.2 Influent sulfide concentration . . . . . . . . . . . . . . . . . . . . . . . . 463.2.3 Influent total Kjeldahl nitrogen concentration . . . . . . . . . . . . . . . 473.2.4 Interaction between influencing factors . . . . . . . . . . . . . . . . . . . 48

3.3 Minimal volume of the denitrification reactor . . . . . . . . . . . . . . . . . . . 513.4 Applicability of sulfide based nitrogen removal process over nitrate and the

novel process over nitrite in wastewaters with a low sulfate concentration . . . 51

4 General conclusions and perspectives 55

4.1 General conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554.2 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

References 59

A Appendix 65

A.1 Stoichiometric equations of sulfur based nitrogen removal . . . . . . . . . . . . 65

ii

List of abbreviations

Anammox anaerobic ammonium oxidationAOB ammonium oxidizing bacteriaBOD biological oxygen demandCOD chemical oxygen demandFSA free and saline ammoniaHRT hydraulic residence timeNOB nitrite oxidizing bacteriaRBCOD readily biodegradable chemical oxygen demandSANI sulfate reduction, autotrophic denitrification and nitrification integratedSBCOD slowly biodegradable chemical oxygen demandSHARON single reactor high activity ammonia removal over nitriteSOB sulfide oxidizing bacteriaSRB sulfate reducing bacteriaSRT sludge retention timeSRUSB sulfate reduction up–flow sludge bedS/N ratio sulfur to nitrogen ratio (g S.(g N)−1)TKN total Kjeldahl nitrogenTSS total suspended solidsVSS volatile suspended solids

1

List of symbols

Variables

Symbol Name Unit

Concentrations

CH2S,i Influent sulfide concentration g S.m−3

CNH4,e Effluent ammonium concentration g N.m−3

CNH4,i Influent ammonium concentration g N.m−3

Cnob,e Effluent organic biodegradable nitrogen concentration g N.m−3

Cnob,i Influent organic biodegradable nitrogen concentration g N.m−3

Cnou,e Effluent organic unbiodegradable nitrogen concentration g N.m−3

Cnou,i Influent organic unbiodegradable nitrogen concentration g N.m−3

CNO3,a Nitrate concentration in the recirculation flow g N.m−3

CNO3,e Effluent nitrate concentration g N.m−3

CNO3,i Influent nitrate concentration g N.m−3

CO2,a Oxygen concentration in the recirculation flow g O2.m−3

CT KN,e Effluent total Kjeldahl nitrogen concentration g N.m−3

CT KN,i Influent total Kjeldahl nitrogen concentration g N.m−3

Ns Nitrogen uptake in biomass of biofilm g N.m−3

Other variables

a Recirculation ratio (Qa/Qi) -Dp Denitrification potential g N.m−3

Nc Nitrification capacity of the bioreactor g N.m−3

M Total mass of nitrogen g NQa Recirculation flow rate m3.d−1

Qe Effluent flow rate m3.d−1

Qi Influent flow rate m3.d−1

2

Parameters

Symbol Name Unit

Rates

ra,dn,CODMedia surface specific denitrification rate, g N.d−1.m−2with COD as the electron donor

ra,H2S Media surface specific sulfide removal rate g S.d−1.m−2

rv,H2S Volumetric sulfide removal rate g S.d−1.m−3

rdn Denitrification rate g N.d−1

rdn,N.lim Denitrification rate in nitrate limiting conditions g N.d−1

rdn,S.lim Denitrification rate in sulfide limiting conditions g N.d−1

Other parameters

assSpecific media surface of the carrier material, m2.m−3used in the anoxic reactor

fcn COD amount necessary for denitrification of nitrate g COD.(g N)−1

fcv COD content of biomass in the biofilm g COD.(g VSS)−1

ν Sulfide to nitrate stoichiometric conversion ratio g S.(g N)−1

ε Media filling ratio in the anoxic reactor m3.m−3

Van Volume of the anoxic reactor m3

Vmin Minimal volume of the anoxic reactor m3

YADN Yield coefficient of autotrophic denitrifying biomass g VSS.(g N)−1

3

Abstract

This thesis concerns the treatment of saline wastewater with the sulfide based nitrogen removalprocess. The two main reactors in this process are a denitrification (anoxic) reactor followedby a nitrification (aerobic) reactor, both reactors are connected with a nitrate recirculationflow. In the nitrification reactor ammonium is converted to nitrate, then the producednitrate is transported to the denitrification reactor by means of the recirculation flow. In thedenitrification reactor, the nitrate is denitrified to nitrogen gas with sulfide as the electrondonor. Prior to the sulfide based process, an anaerobic process provides the sulfide, necessaryfor the denitrification.

The focus of this master thesis is maximal nitrate removal in the sulfide based denitrificationprocess. Maximal nitrate removal takes place if the recirculation ratio is optimal and if thevolume of the denitrification reactor is big enough. Based on mass balances over the sulfidebased nitrogen removal process, a model was derived to determine these design parameters.With the model, also an estimation of the effluent nitrate concentration for a given recirculationratio can be made. The model complied with the expectations.

Based on simulations, less nitrogen can be removed for wastewater with a low sulfate con-centration compared with saline wastewater. This is due to the limited amount of availablesulfide for denitrification in wastewater with a low sulfide concentration. If organic carbon ispresent in the low sulfate containing wastewater, lower effluent nitrate concentrations can bereached. This is because the additional electron donor enables more denitrification. Presenceof organic carbon did not affect the effluent nitrate concentration in saline wastewaters.

Finally, since this was never examined before, the applicability of the sulfide based processwas assessed for wastewaters with a low sulfate concentration, as not every geographical placeoffers seawater or a sulfate rich water source. The sulfide based process proved to be applicablefor the removal of low ammonium concentrations in part of the wastewaters with a low sulfateconcentration.

To increase this applicability, a novel sulfide based process over nitrite was put forward. Inthis process, ammonium is converted to nitrite in the nitritation reactor. The produced nitriteis then in the denitritation reactor converted to nitrogen gas with sulfide as the electron donor.In this novel process 25% less aeration energy and 40% less sulfide is necessary for the removalof nitrogen, due to the use of nitrite as an electron acceptor instead of nitrate. Because of thelower sulfide requirements, the process over nitrite can remove low ammonium concentrationsin all wastewaters with a low sulfate concentration. Also, medium ammonium concentrationscan be removed for part of the low sulfate containing wastewaters. Further studies shouldinvestigate whether or not this novel process is achievable in practice.

Samenvatting

Deze thesis betreft de behandeling van zout water met het sulfide gebaseerde stikstof ver-wijderings proces. De twee voornaamste reactoren in dit proces zijn de denitrificatie (anoxische)reactor en de nitrificatie (aerobe) reactor, beide reactors zijn verbonden met een nitraatrecirculatie stroom. In de nitrificatie reactor wordt ammonium omgezet naar nitraat, danwordt het geproduceerde nitraat getransporteerd naar de denitrificatie reactor met behulp vande recirculatie stroom. In de denitrificatie reactor wordt nitraat gedenitrificeerd tot stikstofgasmet sulfide als de elektron donor. Voorgaand aan het sulfide gebaseerde proces, voorziet eenanaeroob proces de sulfide, nodig voor de denitrificatie.

De focus van deze master thesis is maximale stikstof verwijdering in de sulfide gebaseerdedenitrificatie. Maximale stikstof verwijdering vindt plaats als de recirculatie stroom optimaalis en als het volume van de denitrificatie reactor groot genoeg is. Gebaseerd op massabalansenover het sulfide gebaseerde stikstof verwijderings proces, werd een model opgesteld om dezeontwerp parameters te bepalen. Met het model kon ook een schatting van de effluent nitraatconcentratie gemaakt worden. Het model voldeed aan de verwachtingen.

Gebaseerd op simulaties, kan minder stikstof verwijderd worden uit sulfaatarm afvalwater invergelijking met uit zout afvalwater. Dit komt door de beperkte hoeveelheid beschikbare sulfidevoor denitrificatie in sulfaatarm afvalwater. Als organisch gebonden koolstof aanwezig is in hetsulfaatarm afvalwater, kunnen lagere effluent nitraat concentraties behaald worden. Dit komtomdat met de extra elektron donor meer nitraat gedenitrificeerd kan worden. De aanwezigheidvan organisch gebonden koolstof heeft geen invloed op de effluent nitraat concentratie bij zoutafvalwater.

Tot slot, aangezien dit niet eerder onderzocht werd, werd de toepasbaarheid van het sulfidegebaseerde proces nagegaan voor sulfaatarme afvalwaters omdat niet overal zeewater of zout,sulfaatrijk water beschikbaar is. Het sulfide gebaseerde proces is enkel toepasbaar voorverwijdering van lage ammonium concentraties in een deel van de sulfaatarme afvalwaters.

Om deze toepasbaarheid te verhogen werd een nieuw sulfide gebaseerd proces over nitrietvoorgesteld. In dit proces wordt ammonium omgezet naar nitriet in de nitritatie reactor. Degeproduceerde nitriet wordt dan in de denitritatie reactor omgezet naar stikstof gas met sulfideals de elektron donor. In dit nieuwe proces is 25% minder beluchtingsenergie nodig en 40%minder sulfide voor de verwijdering van stikstof, dit is te wijten aan het gebruik van nitriet alsde elektron acceptor in plaats van nitraat. Omdat minder sulfide nodig is kan het proces overnitriet lage ammonium concentraties verwijderen in alle sulfaatarme afvalwaters. Ook kon eenmedium ammonium concentratie verwijderd worden in een deel van de sulfaatarme afvalwaters.Verder onderzoek moet uitwijzen of dit proces over nitriet haalbaar is in de praktijk.

Introduction

The safety and accessibility of drinking water is a major concern throughout the world and im-proving access to safe drinking water can result in tangible improvements to health (WHO, n.d.).By decreasing the freshwater use in daily life, more freshwater for drinking water productioncan be provided and hereby the accessibility of drinking water increases. Freshwater couldbe saved by its replacement with seawater for non–drinking purposes in households. In thisway, freshwater used for showering, toilet flushing etc. is saved. A dual water supply allowsthe transportation of the freshwater and seawater to households and next, to the wastew-ater treatment. In Hong–Kong this dual water supply is already successfully applied since1950s (Tang, 2000). Furthermore, to maximize the benefits of the use of seawater, a sulfidebased nitrogen removal process was proposed (Lau et al., 2006). In this way, an additionalreduction in resource use and sludge production can be achieved.

The overall objective of this master thesis is to get a better understanding of the sulfidebased denitrification process. The aim is to optimize design parameters to enable maximumnitrate removal in the treatment of domestic wastewater. First, the sulfur based nitrogenremoval process is explained and advantages, compared to other nitrogen removal processes, arediscussed (chapter 1). Next, the sulfide based denitrification process for domestic wastewatertreatment is explained together with two other applications of the sulfur based process. Finally,the effects of different influent compositions and of design and operation on the sulfur baseddenitrification process were assessed.

Two important design parameters to ensure maximal nitrate removal in the denitrificationprocess are the recirculation flow and the reactor volume. In chapter 2 a calculation methodwas developed to determine the optimal value of these two design parameters. Furthermore,by introducing nitritation and denitritation in the sulfide based process, a novel sulfide basednitrogen removal process over nitrite was put forward to further improve the advantages andapplicability compared with the sulfide based process over nitrate.

In chapter 3, the calculation method to determine the optimal recirculation flow and theminimal required reactor volume was applied to the sulfide based nitrogen removal processfor wastewater treatment. Furthermore, the effect of process operation and wastewatercharacteristics on the minimal effluent nitrate concentration and on the optimal recirculationratio are discussed. Finally, because not all places are favored by the possibility to use seawateror a sulfate rich water source, the applicability of the sulfide based nitrogen removal processover nitrate and the novel process over nitrite were assessed for wastewaters with a low sulfateconcentration. In chapter 4, the general conclusion of the thesis and perspectives for furtherresearch are given.

Chapter 1

Literature review

To optimize a sulfur based nitrogen removal process, background knowledge of nitrogen removalprocesses is required. First, an overview is given on various nitrogen removal techniques insection 1.1. After that, in section 1.2, some example applications of a sulfur based processare given. Next, the effect of the influent composition on the nitrogen removal in a sulfurbased denitrification process is discussed in section 1.3. Furthermore, in section 1.4 the effectof design and operation parameters on nitrogen removal in the sulfur based denitrificationprocess is explained. Finally, a conclusion is formulated and thesis objectives are stated insection 1.5.

1.1 Nitrogen removal processes

In this section, various nitrogen removal processes are discussed. At first, the conventionalnitrification–denitrification process is explained. In this process a high amount of resourcesand energy are used and therefore alternatives for this process have been developed. Of thesealternative processes the nitritation–denitritation process and the partial nitritation–anammoxwill be discussed. Finally, a more recent nitrogen removal process which makes use of reducedsulfur compounds is introduced. Energy and resource consumption relative to the nitrification–denitrification process are given for each alternative nitrogen removal process, in Table 1.1 anoverview is given.

1.1.1 Conventional nitrification–denitrification

Traditionally, nitrogen is removed from wastewater by subsequent nitrification and denitrifica-tion (Münch et al., 1996). Nitrification is an aerobic process accomplished by autotrophicbacteria (Yang and Zhang, 1995). In the nitrification reactor, the ammonium (NH4

+), presentin the wastewater, is completely oxidized into nitrate (NO3

-) (eq. 1.1). Denitrification is theprocess in which oxidized inorganic nitrogen compounds, such as nitrite (NO2

-) and nitrate, arereduced into nitrogen gas (N2) (Yang and Zhang, 1995; van Rijn et al., 2006). Heterotrophicdenitrifying bacteria derive the required electrons from an organic carbon source (expressed

11

CHAPTER 1. LITERATURE REVIEW

as chemical oxygen demand (COD)). Equation 1.2 shows the denitrification of nitrate withacetate as COD source.

NH4+ + 2 O2 → NO3

- + H2O + 2 H+ (1.1)

5 CH3COO- + 8 NO3- + 3 H+ → 10 HCO3

- + 4 N2(g) + 4 H2O (1.2)

In practice, this nitrogen removal process is often implemented as pre–denitrification (Henze,2008b). In a pre–denitrification process, the influent first goes through the denitrificationreactor (figure 1.1). The denitrification process takes place under anoxic conditions. Next, thedenitrified water is led to the nitrification reactor where air is added to obtain nitrification.Both reactors are connected with a recirculation flow by which nitrate, originating from thenitrification reactor, enters the denitrification reactor (figure 1.1).

Figure 1.1: Schematic representation of the pre–denitrification nitrification process.

Not all nitrate can be removed in organic carbon deficient conditions (Pochana and Keller,1999). If the wastewater has a limited COD content, the addition of an organic chemical,such as methanol, acetate etc., is necessary to enable full denitrification to take place (Yangand Zhang, 1995; van Rijn et al., 2006). These external organic carbon sources increase thecost of the wastewater treatment. On top of that, if too much organic carbon is dosed, moreaeration is necessary to prevent high effluent COD concentrations.

Biological heterotrophic denitrification uses less chemicals, has less energy consumption andreduces the production of waste solids in comparison with chemical nitrate removal techniques,e.g. ion exchange and reverse osmosis (Metcalf and Eddy, 1991). Nevertheless, still a highamount of oxygen is necessary for the aeration of ammonium into nitrate. Furthermore, asmentioned above, organic carbon addition is needed for the heterotrophic denitrification inCOD poor wastewaters.

1.1.2 Nitritation–denitritation

In the nitritation–denitritation process, nitrification and denitrification take place via nitrite.During nitritation, ammonium is converted to nitrite instead of nitrate (Abeling and Seyfried,1992). Nitritation is followed by heterotrophic denitritation of nitrite into nitrogen gas(figure 1.2). An example of a nitritation process is SHARONr (single reactor high activityammonia removal over nitrite) (Hellinga et al., 1998). This process can be easily combinedwith denitritation.

The operating temperature range of the SHARONr process is 30–40°C. At high temperatures,ammonium oxidizing bacteria (AOB) have a higher growth rate in comparison with nitrite

12


Figure 1.2: Nitritation–denitritation process. Oxygen and carbon amounts are expressed relative tothe nitrification–denitrification process (Bowden et al., 2015).

oxidizing bacteria (NOB) (0.9 d−1 for AOB versus 0.4 d−1 for NOB (Comeau, 2008)). Forlow hydraulic residence time (HRT) and at high temperatures, this difference in growth ratecan be exploited, because the NOB are not able to remain in the reactor and thus nitriteoxidation is prevented (Hellinga et al., 1998; Schmidt et al., 2003; Khin and Annachhatre,2004).

Because of the suppression of nitrite oxidation into nitrate, 25% less oxygen is necessaryin comparison with complete nitrification (figure 1.2). Consequently, less energy is usedin comparison with the conventional aerobic nitrification. Furthermore, 40% less organiccarbon is necessary for the denitritation process because nitrite and not nitrate is now reduced(figure 1.2). Overall, less resources and less energy are necessary for the removal of nitrogen incomparison with nitrification–denitrification (section 1.1.1).

1.1.3 Partial nitritation–anammox

Nitritation can also be combined with anaerobic ammonium oxidation (anammox). For this,partial nitritation is required, meaning only part of the influent ammonium is oxidized tonitrite. The partial nitritation step is followed by an anammox step (figure 1.3). In theanammox step, the leftover ammonium and nitrite are combined in anoxic conditions toproduce nitrogen gas (Van de Graaf et al., 1990). During this anammox reaction, nitrite isused as an electron acceptor.

The main limitation of this process is the low growth rate of the anammox bacteria (0.06 d−1

(Strous et al., 1998)). Because of this, a long start-up period is required to obtain sufficientbiomass for complete nitrogen removal (Strous et al., 1998). Together with the producednitrogen gas, a small amount of nitrate is formed. Heterotrophic bacteria can remove theleftover nitrate with a small amount of organic carbon as an electron donor (figure 1.3) (Bowdenet al., 2015).

Because only part of the nitrogen is oxidized to nitrite (partial nitritation), even more oxygencan be saved. Compared to the nitrification reaction, there is an oxygen reduction of 60%(figure 1.3). Furthermore, even less external carbon is necessary (11% of the carbon necessaryduring conventional denitrification) (figure 1.3). It is therefore more energy and resource

13


Figure 1.3: Partial nitritation–anammox process. Oxygen and carbon amounts are expressed relativeto the nitrification–denitrification process (Bowden et al., 2015).

efficient than both nitrification–denitrification and nitritation–denitritation. Furthermore, lesssludge needs to be disposed after treatment because anammox bacteria are autotrophic andhave a lower growth yield compared to heterotrophic denitrifiers (0.12 g VSS.(g N)−1 (Strouset al., 1998)1 versus 1.23 g VSS.(g N)−1 (Marais and Ekama, 1976) as the yield coefficients,respectively).

1.1.4 Sulfur based denitrification

In sulfur based denitrification a reduced sulfur compound, such as hydrogen sulfide (H2S),elemental sulfur (S0) or thiosulfate (S2O3

2−), is used by sulfur oxidizing bacteria (SOB)for autotrophic denitrification (Driscoll and Bisogni, 1978). The sulfur compounds act aselectron donors for the conversion of nitrate or nitrite to nitrogen gas. Furthermore, forbiomass production, ammonia is required as a nitrogen source and carbon dioxide as aninorganic carbon source, the sulfur compounds again acts as the electron donor (Driscoll andBisogni, 1978). For example: when sulfide is used as an electron donor and a yield value of0.57 g VSS.(g NO3_N)−1 is assumed (Claus and Kutzner, 1985b), this results in equation 1.3,appendix A.1 shows the derivation. The nitrification step of the sulfur based process is equalto the nitrification of the conventional process described in section 1.1.1.

2.45 HS- + 3.08 NO3- + 0.2 NH4

+ + HCO3- + 1.43 H+

→ 2.45 SO42- + 1.54 N2 + CH1.8O0.5N0.2 + 1.94 H2O (1.3)

Because sulfur compounds are used as electron donors during denitrification, no externalorganic carbon is necessary. Therefore, less resources are necessary and costs are reduced incomparison with all aforementioned nitrogen removal processes (Claus and Kutzner, 1985a;Oh et al., 2001). Furthermore, the inorganic carbon source CO2 is incorporated into thesludge and not released as CO2 in the atmosphere (Driscoll and Bisogni, 1978). Finally, lowersludge production is achieved in comparison with conventional nitrification–denitrificationbecause of the lower yield (0.57 g VSS.(g NO3_N)−1 (Claus and Kutzner, 1985b) versus1.23 g VSS.(g NO3_N)−1 (Marais and Ekama, 1976)2, respectively). Consequently, less sludgeneeds to be disposed after the wastewater treatment (Campos et al., 2008; Wang et al., 2009).

1To achieve the yield in g VSS.(g N)−1, CH1.8O0.5N0.2 was assumed as the biomass composition.2To achieve the yield in g VSS.(g NO3_N)−1, a conversion factor of 2.86 g COD.(g NO3_N)−1 was used.

14


Table 1.1: Resource and energy input of different nitrogen removal processes. All values are relativeto the conventional nitrification–denitrification process.

Nitrogen removal process Oxygen (%) Organic carbonaddition (%)

Conventional nitrification–denitrification 100 100Nitritation–denitritation 75 60Partial nitritation–anammox 40 11Sulfur based nitrification–denitrification 100 -

1.2 Applications of sulfur based denitrification

Different kinds of applications make use of the sulfur based autotrophic denitrification. In thissection the removal of H2S from biogas, a treatment method for groundwater and nitrogenremoval for domestic wastewater will be discussed.

1.2.1 Removal of hydrogen sulfide from biogas

The wastewater resulting from fish processing industries contains a high concentration oforganic matter and ammonium, both components need to be removed. To take advantage ofthe high amount of COD, an anaerobic treatment step is used to produce biogas (first reactor,figure 1.4). Because the water originates from the sea and thus a high sulfate concentrationis present, there is a concurrent production of hydrogen sulfide by sulfate reducing bacteria(SRB) (Hilton and Archer, 1988; Kleerebezem and Mendez, 2002). Both methane andhydrogen sulfide are produced due to the high amount of COD present in the reactor (Isa et al.,1986). Before the biogas can be used as a fuel for electricity production, the sulfide needs tobe removed because sulfide can cause corrosion to engines (Schweigkofler and Niessner, 2001).

The biogas, consisting of CO2, CH4 and H2S, is captured at the top of the reactor and theremaining wastewater flows towards the nitrification reactor where nitrification takes place(second reactor, figure 1.4). Because the pH was controlled by adding sodium bicarbonate inthe influent, the nitrified wastewater could then be used as an absorption liquid for the sulfidepresent in the biogas (third reactor, figure 1.4) (Kleerebezem and Mendez, 2002). In the finalreactor autotrophic denitrification takes place by SOB with the dissolved sulfide as an electrondonor (eq. 1.3).

An excess of sulfide results in an accumulation of elemental sulfur in the reactor and furthermore,sulfide was detected in the effluent (Kleerebezem and Mendez, 2002). If a limited amountof sulfide is present in the biogas it is suggested to improve the stripping efficiency in thebioreactor or to aim for nitritation into nitrite instead of full nitrification (Kleerebezem andMendez, 2002). In that case, less sulfide is required for complete nitrogen removal.

In the conventional treatment of fish canneries, an anaerobic pre–treatment is followed bysubsequent nitrification and denitrification (Kleerebezem and Mendez, 2002). With thetreatment system pictured in figure 1.4, the sulfide can be removed from the biogas. Anotheradvantage is the elimination of the recirculation flow between the nitrification and denitrification

15


Figure 1.4: Schematic diagram for the treatment of fish cannery effluents by applying autotrophicdenitrification (Fajardo et al., 2014).

reactor. Furthermore, less sludge is produced by SOB compared with heterotrophic denitrifiers(section 1.1.4) (Kleerebezem and Mendez, 2002). However, a disadvantage is the cost of anadditional absorption unit.

1.2.2 Denitrification with elemental sulfur for groundwater treatment

Due to agriculture, groundwater and surface water can be heavily contaminated with nitrate.According to the last European Commission report (EC, 2002), 20% of EU measuring stationshad concentrations of nitrate higher than the maximum allowed concentration of 50 g NO3.m−3.Therefore, remediation of the ground– and surface water is necessary.

Sierra-Alvarez et al. (2007) used a sulfur–limestone autotrophic denitrification process totreat groundwater. The denitrification takes place in a bioreactor packed with a mixture ofgranular elemental sulfur and limestone grit (Sierra-Alvarez et al., 2007). With the use ofthe elemental sulfur particles as the electron donor, the nitrate is converted into nitrogen gas.Limestone acts as an inorganic carbon source for the denitrification. Furthermore, it buffersthe alkalinity decrease, as a consequence of denitrification (section 1.3.2) (Zhang and Lampe,1999; Sierra-Alvarez et al., 2007).

A major advantage compared with a treatment with heterotrophic denitrification is the lowersludge production, which minimizes the handling of the sludge (Claus and Kutzner, 1985a).Furthermore, the elimination of external COD to treat the water lowers the risk of biofoulingand also reduces costs (Zhang and Lampe, 1999).

1.2.3 Nitrogen removal in domestic wastewater

In Hong Kong a dual water supply was installed which saves 750 000 m3.d-1 of fresh water byusing seawater for toilet flushing (WSD, 2006). Nowadays, the used sea– and fresh water aremixed and treated together (Tang, 2000). To maximize the benefit of the use of seawater,

16


the Hong Kong University of Science and Technology developed SANI (sulfate reduction,autotrophic denitrification and nitrification integrated), a new biological organic carbon andnitrogen removal process. This process makes use of the sulfate originating from the seawaterto treat the wastewater.

The first reactor of the SANI process is a sulfate reduction up–flow sludge bed (SRUSB)reactor (figure 1.5). In this anaerobic reactor the saline sewage enters. In the bottom–zone,an active sludge blanked is formed by SRB granules. The granules come in contact with thewaste and COD is removed with sulfate as an oxidizing agent (Hao et al., 2013; van den Brandet al., 2015). Due to the high presence of sulfate and the low HRT applied in the reactor, SRBoutcompete the methanogenic Archaea, and thus no methane is formed (Lau et al., 2006).Finally the sewage is separated from the solid granules in the upper zone of the reactor.

Figure 1.5: Schematic diagram of a SANI pilot plant. Figure adapted from Lu et al. (2011).

The SRUSB is followed by an anoxic reactor (figure 1.5). In this reactor, porous, plasticmedia are present on which the autotrophic SOB form a biofilm (Wang et al., 2009). Theseautotrophic bacteria use the incoming dissolved sulfide, produced in the SRUSB, as an electrondonor for the denitrification (eq. 1.3). Depending on how much COD is removed in the previousreactor, competition between heterotrophic and autotrophic denitrifying bacteria for nitratewill take place (section 1.3.3).

The final reactor is an aerobic reactor (figure 1.5). Similar to the anoxic reactor, porous,plastic media are present. In this third reactor, the nitrification takes place by nitrifyingbacteria (eq. 1.1). Besides the ammonium oxidation, also the remaining sulfide residuals arecompletely oxidized into sulfate before exiting the reactor (Lu et al., 2012b). The producednitrate is recirculated to the anoxic reactor for denitrification (figure 1.5) (Wang et al., 2009).

By using SANI, the primary treatment of the saline sewage can be eliminated. This primarytreatment includes a sedimentation process and chemical treatment to improve the removalefficiency of TSS and BOD (DSD, 2006). Elimination of the primary treatment allows, besidesthe energy savings, also a reduced use of chemicals.

In the SANI process, the major microbial populations are SRB, SOB and nitrifiers (Wang et al.,2009). All three populations have low growth yields and therefore only produce a minimalamount of sludge. The sludge production is only 10% relative to a conventional biologicalnitrogen removal process (Lu et al., 2012b). Furthermore, contrary to the nitrification–

17


denitrification process, no external carbon is necessary for the denitrification because sulfide isused as an electron donor. Because COD is removed through reduction of sulfate, no oxygenis necessary. In this way, according to Lu et al. (2012b), 35% energy is saved.

1.3 Effect of influent composition on the sulfur based denitri-

fication

Influent characteristics of the wastewater have a large impact on the nitrogen removal efficiencyof the process. Therefore, the effect of the most important influent components will be discussedfor the sulfur based denitrification. First the effect of a changing sulfur to nitrogen (S/N)ratio will be discussed. Next, the influence of alkalinity and the influence of organic carbon isexplained.

1.3.1 Sulfur to nitrogen ratio (S/N ratio)

The ratio of reduced sulfur to nitrate in the influent has an influence on the nitrate removal,nitrite accumulation and sulfur end–products.

Nitrate removal

From the chemical reaction equation of sulfide oxidation with nitrate (eq. 1.3) follows thatabout 2.45 moles of sulfide are required to denitrify 3.08 moles of nitrate. Consequently, thestoichiometric optimal sulfur to nitrogen ratio in the influent (S/N ratio) for this reactionwould be 0.80 mol S.(mol N)−1 or equivalently 1.82 g S.(g N)−1, slightly depending on theexact SOB species present in the system (Gu et al., 2004). Reactions with other startingproducts like elemental sulfur or nitrite have other optimal S/N ratios. As long as the S/Nratio is equal or higher then the optimal one, complete denitrification is possible. However,for wastewaters with an S/N ratio lower than the optimal value, the denitrification efficiencycan never be 100%, because the reaction is stoichiometrically limited by a shortage of electrondonors (sulfur compounds). For example, if the S/N ratio is only half of the optimal fornitrate removal via sulfide (0.91 g S.(g N−1)), maximally 50% of the nitrate can be removedvia autotrophic denitrification.

Nitrite accumulation

When nitrate is reduced into nitrogen gas, nitrite acts as an intermediary product. Nitrate willpreferably be used during the oxidation of the reduced sulfur compound, because denitrifyingbacteria prefer nitrate in comparison with nitrite as an electron acceptor (Claus and Kutzner,1985a; An et al., 2010). Furthermore, the reduction rate of nitrite is lower than the nitratereduction rate (Campos et al., 2008; Sahinkaya et al., 2011). Therefore, this compound canaccumulate during the denitrification process.

If the sulfur compound is non–limiting then after nitrate reduction and concomitant nitriteproduction, nitrite is fully reduced in batch experiments. Therefore, neither nitrite nor nitratewill be present in the effluent (Cardoso et al., 2006; Campos et al., 2008; An et al., 2010).On the other hand, in continuous experiments, the presence of nitrite (and nitrate) in theeffluent depends on the HRT (section 1.4.2). At low S/N ratio, denitrification is incomplete,

18


therefore both nitrate and nitrite will be present in the effluent, in both batch and continuousexperiments (Cardoso et al., 2006; Campos et al., 2008; Cai et al., 2008).

The effect of S/N ratio on nitrite ratios is not yet completely understood and in literature,contradictory results are given. Moraes et al. (2012) found a more readily consumption of nitriteinstead of nitrate. Results of this study showed a higher reactivity for nitrite, allowing a betteruse as an electron acceptor. So for a high S/N ratio, no nitrite accumulation was detected.Also, according to a study of Mahmood et al. (2007), nitrite reduction is more effective thannitrate reduction. In both studies nitrate reduction appeared to be the rate–limiting factorinstead of nitrite reduction.

End products of the sulfur based denitrification

In case thiosulfate is used as an electron donor, the oxidized end product is sulfate (figure 1.6).Thiosulfate is not toxic and has a high bioavailability (soluble compound), therefore it is themost readily used electron donor in the sulfur based denitrification. For this reason, thiosulfateis also the end product if elemental sulfur is used as electron donor (Cardoso et al., 2006)(figure 1.6).

Figure 1.6: Oxidation states of sulfur compounds.

When sulfide is used as an electron donor, oxidation can lead to elemental sulfur or sulfateas an end product, depending on the physiological conditions (Krishnakumar and Manilal,1999; Cardoso et al., 2006). Non–partial denitrification is the direct conversion of sulfide intosulfate (eq. 1.3). If elemental sulfur is formed as an intermediate product before completeoxidation into sulfate, the process is referred to as partial denitrification (eq. 1.4 and 1.5).

HS− + 0.4 NO−3 + 1.4 H+ → S0 + 0.2 N2 + 1.2 H2O (1.4)

S0 + 1.2 NO−3 + 0.4 H2O→ SO2−

4 + 0.6 N2 + 0.8 H+ (1.5)

During partial denitrification, the first step is faster than the second one. This is because thereis a limited mass transfer of substrate from the solid phase of elemental sulfur (Cardoso et al.,2006). Therefore, the sulfide has a higher bioavailability to microorganisms and consequentlythe first reaction is faster than the second one (Moraes et al., 2012). Consequently, in sulfidenon–limiting conditions, mainly the first oxidation reaction will occur. Less sulfate will beproduced and elemental sulfur is the main end product for a high S/N ratio (Table 1.2) (Gadekaret al., 2006; An et al., 2010; Moraes et al., 2012).

Non–partial denitrification of sulfide is more likely to occur in sulfide limiting conditions,although also some partial denitrification reaction take place (Moraes et al., 2012). Sulfate isin sulfide limiting conditions the main product in batch reactors. For continuous reactors, theend product is determined by the HRT (Table 1.2).

With the S/N inflow ratio, end product formation can be guided (Table 1.2). The production ofelemental sulfur can be interesting, because the sulfur can then be removed for reuse (Cardoso

19


et al., 2006). On the other hand, sulfate production is easy for discharge in environmentswhere sulfate is present by nature, such as marine environments (Cardoso et al., 2006). If thetreated water is discharged in the sea, sulfate as an end product is desired, meaning a lowS/N ratio is preferred.

Table 1.2: Overview of the end products in case sulfide is used as an electron donor. References:[1] Cardoso et al. (2006), [2] Gadekar et al. (2006), [3] An et al. (2010), [4] Moraes et al.(2012).

S/N ratio Batch or continuous HRT Reaction Referencereactor end product

> optimalBatch - S0 [1,2,3]

Continuous Long S0[2,4]Short S0

< optimalBatch - SO4

2− [1,2,3]

Continuous Long SO42−

[3]Short S0 & SO42−

1.3.2 Alkalinity

Alkalinity is the capacity of a liquid to neutralize an acid, meaning the buffering capacity.Because the alkalinity is primarily a function of carbonate (CO3

2−), bicarbonate (HCO3−) and

the hydroxide content (OH−) in most aquatic environments, it is expressed as the concentrationof these constituents (Standard methods, 1992). In a sulfur rich environment, the sulfurcompound also contributes to the alkalinity because HS– is a weak base and H2S a weak acid.To calculate the total alkalinity, the sulfur compound and the H2CO3 alkalinity, expressed asHCO3

−–equivalents need to be added.

During the autotrophic denitrification a reduced sulfur compound is oxidized into the strongacid sulfate because of which the alkalinity decreases (Batchelor and Lawrence, 1978; Ohet al., 2003). Additionally, HCO3

− is incorporated as a carbon source in the sludge, meaningalkalinity is again consumed (Standard methods, 1992). Because of the alkalinity consumptionin the autotrophic denitrification, the pH can sharply decrease. Without proper intervention,denitrifying bacteria will be inhibited by the low pH. Therefore, biological denitrification willdecrease and nitrate removal efficiency will be lower (Claus and Kutzner, 1985b; Oh et al.,2003).

Alkalinity is produced during heterotrophic denitrification (eq. 1.2). By combining het-erotrophic and autotrophic denitrification, the alkalinity production can be used to preventalkalinity deficiency (Kim et al., 2002; Oh et al., 2003). Heterotrophic denitrification is stimu-lated by the presence of organic carbon in the reactor (section 1.3.3). The more organic carbonis present, the higher the amount of produced alkalinity during heterotrophic denitrification.

20


1.3.3 Organic carbon

Various species of bacteria are present in the reactor for sulfur based denitrification. Acommonly detected sulfoxidizing denitrifying bacteria is Thiobacillus denitrificans (Cardosoet al., 2006). This bacteria is strictly autotrophic (Kuenen, 1979). Beside strictly autotrophicbacteria, some facultative denitrifying bacteria are present. These bacteria can adapt toautotrophic, heterotrophic and mixotrophic3 conditions (Matin, 1978). Heterotrophic denitrifi-cation by facultative and/or heterotrophic bacteria in the reactor is stimulated by presence ofsmall amounts of organic carbon (Oh et al., 2003; Sahinkaya et al., 2011). Consequently, whenorganic carbon is present, denitrification will be partly heterotrophic and partly autotrophic.Organic carbon addition or presence of COD in the reactor will affect the balance betweenboth groups of bacteria.

In a study of Kim et al. (2002), different amounts of methanol were added to a sulfur–packedcolumn. By gradual COD addition, the autotrophic denitrification increases even as theheterotrophic denitrification (figure 1.7, first, second and third bar). This overall increaseis due to the cooperation between heterotrophic and autotrophic denitrifiers. The carbonincreases the heterotrophic denitrification. These bacteria produce alkalinity, which makes surethe pH maintains stable. Furthermore, CO2 is produced in the heterotrophic denitrificationreaction. CO2 can be used by the autotrophic denitrifiers as an inorganic carbon source. Ata certain point, the autotrophic denitrification decreases because of heterotrophic bacteriapredominate the autotrophic ones, the total denitrification efficiency still increases (figure 1.7,fourth and fifth bar).

Figure 1.7: Denitrification efficiency: contribution of autotrophic (SUDN) and heterotrophic (OUDN)denitrification at different methanol doses. W/O: without organics, SUDN: sulfur utilizingdenitrification, OUDN: organic utilizing denitrification, T: total theoretical amountrequired for heterotrophic denitrification. (Kim et al., 2002).

A disadvantage of the addition of organic carbon is the biomass increase. Heterotrophicdenitrification is stimulated by organic carbon addition and therefore, more biomass isproduced. Because sludge reduction is one of the main advantages of a complete autotrophic

3In mixotrophic conditions both autotrophic and heterotrophic denitrification take place (Oh et al., 2001).

21


nitrogen removal process, sludge production by heterotrophic denitrification should be avoidedas much as possible (Wang et al., 2009). Therefore, the presence of the right amount of organiccarbon is essential. Enough carbon should be present in order to achieve sufficient nitratereduction. On the other hand, carbon should be limited because of the sludge productionby the heterotrophic denitrification (Oh et al., 2001). If the influent wastewater containsbiodegradable organics, less additional organic carbon needs to be added (Kim et al., 2002).If too much organics are present in the incoming water, this organic carbon inflow can belimited by removing as much as needed in a previous step, such as the SRUSB in SANI (Wanget al., 2009; Tsang et al., 2009).

1.4 Effect of reactor design and operation on the sulfur based

denitrification

Besides the effect of influent composition, the reactor design and operation is of main impor-tance. Design determines cost of the plant and the efficiency of the process. Process efficiencyincludes energy consumption and removal efficiency of polluting components such as nitrogen.Concerning the sulfur based nitrogen removal process, three important design and operationcharacteristics can be distinguished: the anoxic reactor type, the HRT and the recirculationflow between the anoxic and the aerobic reactor.

1.4.1 Anoxic reactor type

Maintaining sufficient biomass concentrations in the reactor is important for a high denitrifica-tion rate and optimal nitrogen removal. Because the growth yield of SOB is very low (2.6 d−1

for SOB (Claus and Kutzner, 1985b) versus 4 d−1 for heterotrophic denitrifiers (Comeau,2008)) biomass retention is crucial for autotrophic denitrification.

To ensure sufficient biomass in the reactor, a biofilm reactor can be used. In a biofilm,microorganisms form a dense layer attached to a solid surface, also known as the carriermaterial (Henze, 2008a). By upgrading the type of carrier material, an optimal denitrificationrate can be achieved. For example if sulfide is used as an electron donor, the specific surfacearea can be increased by using carrier material with a higher specific surface area per carriervolume (m2.m−3

carrier) or simply by adding more carriers in the reactor (m2.m−3reactor volume) (Wu

et al., 2016). In case elemental sulfur particles are used, decreasing the diameter of thesulfur particles (m2.m−3

carrier) or adding more particles (m2.m−3reactor volume), increases the specific

surface area and consequently denitrification rates will be higher (Koenig and Liu, 2001;Sierra-Alvarez et al., 2007).

Granular sludge is economically interesting, because a granular biofilm can grow without asupport media and therefore no carrier material is necessary. Also granules allow a morecompact reactor design, because of the high number of microorganisms that can be maintainedin a granule (Liu et al., 2002). Furthermore, the extracellular polymeric substance matrix inthe granules is an advantage because of the increased resiliency towards toxins or shocks (Adavet al., 2008). On the other hand, operating conditions should be carefully controlled to allow

22


uniform SOB granules (Yang et al., 2016). For this reason, biofilm reactors are preferredbecause of the more easy use.

1.4.2 Hydraulic residence time

The hydraulic residence time (HRT) is the average time the liquid stays in the reactor. Thismeans that for a high HRT, the wastewater stays longer in the reactor. Consequently for ahigher HRT, less wastewater per day can be treated for an equal reactor volume. For a certainnitrate concentration, a minimal residence time is necessary for complete nitrate removal evenif enough sulfide is present (Claus and Kutzner, 1985a).

The higher the HRT, the more time for the autotrophic denitrification reactions to react andtherefore, the higher the denitrification efficiency (figure 1.8) (Claus and Kutzner, 1985a). Forexample, in case sulfide is used as an electron donor and for high sulfide concentrations, theend product is either elemental sulfur or either sulfate (section 1.3.1). In case of low HRT, thesecond step in partial denitrification will not yet be completed, as a consequence denitrificationwill be incomplete and less sulfate will be produced (Gadekar et al., 2006). In case of theuse of elemental sulfur particles, the limiting factor to reach a high denitrification rate is thelimited dissolution rate of elemental sulfur because of the low solubility in water (Koenig andLiu, 2001). A high HRT is necessary to make sure enough sulfur is dissolved for denitrificationand to allow in this way enough reaction time (Koenig and Liu, 1996).

Figure 1.8: Influence of residence time on the extent of thiosulfate based denitrification using fournitrate concentrations in a synthetic feed medium. � 1.8 g.l-1 NO3

-; 4 3.0 g.l-1 NO3-;

© 4.3 g.l-1 NO3-; 5 6.1 g.l-1 NO3

- (Claus and Kutzner, 1985a).

1.4.3 Recirculation flow between anoxic and aerobic reactor

The recirculation flow (Qa) between the anoxic and the aerobic reactor (figure 1.9) makessure enough nitrate is brought into the anoxic reactor for denitrification. The more nitrate is

23


recirculated, the more can be denitrified into nitrogen gas until a certain maximum amountof nitrate removal is reached. At the optimal recirculation flow maximum removal of nitrateis reached. It is the most important parameter to determine the efficiency of the nitrogenremoval process (Wang et al., 2009; Lu et al., 2009).

Besides the transfer of nitrate, dissolved oxygen is brought into the anoxic reactor by therecirculation flow (Lu et al., 2012a). This oxygen originates from the air supply, necessaryfor the nitrification, in the aerobic reactor (figure 1.9). Because of the presence of DO inthe recirculation flow, part of the sulfide will be oxidized with oxygen and thus less nitrateis denitrified (Lu et al., 2012a; Wu et al., 2016). Therefore the recirculation flow should becarefully determined. A high recirculation flow allows more nitrate inflow for denitrification,but on the other hand, more oxygen will be present which will reduce the denitrificationefficiency (Wang et al., 2009).

1.5 Conclusions and thesis objectives

1.5.1 Conclusions

In Table 1.1 an overview is given of resource and energy input in each discussed nitrogenremoval process. In the sulfur based process, no additional organic carbon is necessary.Although resource use is hereby eliminated, still the same amount of oxygen is necessaryfor the nitrification reaction as for the conventional process. In the (partial) nitritationprocess both aeration energy and addition of organic carbon amounts decreased relative tothe conventional process. The lowest amount concerning aeration is achieved in the partialnitritation–anammox process.

Another advantage of the partial nitritation–anammox and of the sulfur based process is thereduced sludge production during the denitrification reaction (0.12 g VSS.(g N)−1 for anammox,0.57 g VSS.(g N)−1 for SOB versus 1.23 g VSS.(g N)−1 for heterotrophic denitrifiers).

The nitrogen removal in the sulfur based process is influenced by various actors as stated insections 1.3 and 1.4. The most important design criteria is the recirculation flow between theanoxic and the aerobic reactor. Furthermore, the use of a biofilm reactor is preferred becausethe control of the operation conditions for granular sludge is difficult.

1.5.2 Thesis objectives

The main goal of this thesis is to optimize the sulfide based nitrogen removal process. Thenitrogen removal process, which will be further discussed, consists of an anoxic and an aerobicpart for subsequent denitrification and nitrification to take place (figure 1.9).

• Develop a general procedure to find the recirculation flow that maximizes nitrogenremoval based on theoretical calculations and model simulations.

• Develop a general procedure to find the minimal volume that maximizes nitrogen removalbased on theoretical calculations and model simulations.

24


• Improve the sulfide based process by minimization of the necessary aeration energy, byintegration of nitritation and denitritation

• Assess the applicability of the sulfide based nitrogen removal process for wastewaterwith a low sulfate concentration.

• Assess the applicability of the novel sulfide based nitrogen removal process for wastewaterwith a low sulfate concentration.

Figure 1.9: Schematic representation of a subsequent nitrification–denitrification process for a sulfurbased nitrogen removal process. Qi: influent flow rate (m3.d−1), Qa: recirculation flowrate (m3.d−1), Qe: effluent flow rate (m3.d−1).

25


26

Chapter 2

Methods

In this chapter, first a calculation method for the determination of the effluent nitrateconcentration is derived. In the second part, a simulation study is set up to evaluate theeffect of process operation and wastewater characteristics on the effluent nitrate concentrationand on the optimal recirculation ratio. Next, a method to calculate the minimal volumeof the sulfide based denitrification process is derived. In the last section, nitrite instead ofnitrate was used as an electron acceptor to improve the advantages of the sulfide based process.Furthermore, the assesment of the applicability of both the sulfide based process over nitrateand the novel process over nitrite on wastewater with a low sulfate concentration is explained.

2.1 Calculation method for the effluent nitrate concentration

The effluent nitrate concentration of a sulfide based nitrogen removal process was estimatedas a function of a changing recirculation flow to assess the effect of this crucial processparameter and to find the value that maximizes the nitrogen removal. The calculation methodfor the effluent nitrate concentration was based on a calculation method for a conventionalnitrification–denitrification process, developed by Henze (2008b,c). To this end, mass balanceswhere set up over the sulfide based nitrogen removal process for steady state conditions,meaning the behavior of the system did not change over time.

First, the process layout of the sulfide based process is discussed (section 2.1.1). The massbalances for this process are derived in section 2.1.2. Next, the optimal recirculation flow (Qa)is calculated to obtain maximal nitrogen removal (section 2.1.3). Furthermore, an upgradewas made to the calculation method by also taking into account the COD, present in thewastewater (section 2.1.4). Finally, the values used for the application of the sulfide basedprocess are explained (section 2.1.5).

2.1.1 General design of a sulfide based autotrophic denitrification process

The sulfide based process will be studied in a pre–denitrification configuration, consisting oftwo main reactors: first the denitrification (anoxic) and secondly the nitrification (aerobic)

27

CHAPTER 2. METHODS

reactor (figure 1.9). Both reactors are biofilm reactors. A recirculation flow (Qa) transports theliquid from the aerobic reactor to the anoxic reactor. Due to the production of biomass the TSSin the effluent increases because of which a post–treatment is necessary. In the post–treatmentsedimentation takes place, after adding some flocculants (Wu et al., 2016). Prior to this sulfidebased process, an anaerobic step is required to provide the sulfide, necessary for denitrification(similar as the SRUSB in SANI process, section 1.2.3).

2.1.2 Mass balances

Mass balance of total nitrogen

A nitrogen mass balance over the sulfide based nitrogen removal process is set up over thecomplete reactor system pictured in figure 1.9. The nitrogen enters the denitrification reactorvia the influent flow: part is unbiodegradable and simply leaves the nitrification reactorunchanged (Cnou,e), another part leaves the nitrification reactor as ammonium (CNH4,e), ornitrate (CNO3,e), depending on the nitrification capacity. Furthermore, part of the nitrogenleaves as nitrogen gas due to denitrification (rdn). Finally, some nitrogen which is incorporatedin the biofilm via growth, leaves the nitrification reactor in the form of detached biofilm partsthrough the effluent (Ns). This taken into account and at steady state, equation 2.1 is valid:

dM

dt= (CNO3,i + CNH4,i + Cnou,i + Cnob,i) ∗Qi

− (CNO3,e + CNH4,e + Cnou,e + Cnob,e) ∗Qe − rdn −Ns = 0(2.1)

where:

M total mass of nitrogen (g N),Cnob,e effluent organic biodegradable nitrogen concentration (g N.m−3),Cnou,e effluent organic unbiodegradable nitrogen concentration (g N.m−3),CNO3,e effluent nitrate concentration (g N.m−3),CNH4,e effluent ammonium concentration (g N.m−3),Cnob,i influent organic biodegradable nitrogen concentration (g N.m−3),Cnou,i influent organic unbiodegradable nitrogen concentration (g N.m−3),CNO3,i influent nitrate concentration (g N.m−3),CNH4,i influent ammonium concentration (g N.m−3),Ns nitrogen uptake in biomass of biofilm (g N.m−3),rdn denitrification rate (g N.d−1),Qe effluent flow rate (m3.d−1),Qi influent flow rate (m3.d−1).

The value of interest in the equation is the nitrate concentration in the effluent (CNO3,e).In the influent flow of the denitrification reactor it was assumed that no NO3 was present(CNO3,i = 0 g N.m−3), because the nitrate influent concentration is negligible compared toammonium and organic nitrogen influent (Henze, 2008d). Furthermore, the influent andeffluent organic unbiodegradable nitrogen concentration are equal, because the unbiodegradablematter remains unchanged (Cnou,i = Cnou,e). Also, both influent and effluent flow areequal (Qi = Qe). Finally, there was assumed all biodegradable nitrogen was bio degraded

28

CHAPTER 2. METHODS

(Cnob,e = 0 g N.m−3). Aforementioned assumptions taken into account, only the influentand effluent ammonium concentration (CNH4,i, CNH4,e), the influent biodegradable nitrogenconcentration (Cnob,i), the denitrification rate (rdn) and the nitrogen uptake in the biomass(Ns) are unknown values which need to be calculated to determine the CNO3,e.

Both CNH4,i and Cnob,i can be measured in the influent and are thus available. The CNH4,e

on the other hand, is not measured but can be estimated with a biofilm model by modelingthe complete conversion of the biofilm, e.g. with the use of ASM1 (Henze et al., 1987). Thismodeling lies beyond the scope of this thesis. During calculations, a value for this concentrationwill be assumed for simplicity.

The nitrogen uptake in biomass of the biofilm (Ns) in the process can be neglected due tothe low amount of sludge production and the high sludge retention time (SRT) in the sulfidebased process (Lu et al., 2012a). To validate this, an estimation of Ns was made with formulasfrom Henze (2008b). The formulas are not valid for a biofilm system, because the SRT of theSOB is not the same as the average SRT measured in the system: if SOB are on the inside ofthe biofilm they will leave the system only very slowly, while if they live at the surface a lotwill be detached so the SRT is shorter. Nevertheless, an estimation for Ns could be made.

For the estimation of Ns, necessary concentration values, operation values (volume, SRT andinfluent flow) and characteristics of the SOB (respiration rate and yield) of Lu et al. (2012a)were used. Necessary fractionation values of SOB, such as the nitrogen fraction of biomass,were assumed to be the same as for heterotrophic bacteria, values for these fractionations wereused from Henze (2008b). The estimation of 1.5 g N.m−3 for Ns, compared with an influentnitrogen concentration of 85.3 g N.m−3 (Lu et al., 2012a), confirmed that the contribution ofNs could be neglected (Ns = 0 g N.m−3).

All aforementioned assumptions taken into account, equation 2.1 could be rewritten asequation 2.2. The only unknown, which needs to be determined to solve the equation forCNO3,e, was rdn.

(CNH4,i + Cnob,i − CNO3,e − CNH4,e) ∗Qi − rdn = 0 (2.2)

For the determination of the denitrification rate (rdn) two cases could be distinguished: thecase in which nitrate was limiting and the one in which sulfide was limiting. If nitrate islimiting in the system, enough sulfide will be present for full denitrification. If on the otherhand sulfide is limiting, not all nitrate will be denitrified. In section 2.1.3, more explanation isgiven to determine if nitrate or sulfide limiting conditions are valid. Because the calculationmethod of rdn differs depending on the limiting compound, a distinction was made betweenthe denitrification rate in nitrate (rdn,N.lim) and in sulfide limiting conditions (rdn,S.lim). Forthe determination of both denitrification rates, it was assumed that the anoxic reactor is bigenough to enable full denitrification and thus no kinetic limitation takes place.

Nitrate limiting conditions

In nitrate limiting conditions, all nitrate that enters the anoxic reactor can be denitrified andwould be converted into nitrogen gas. Thus the overall rate of denitrification is equal to therate at which nitrate is pumped into the anoxic reactor as long as sulfide is present in excess:

rdn,N.lim = Qa ∗ CNO3,a (2.3)

where:

29

CHAPTER 2. METHODS

CNO3,a recirculation flow nitrate concentration (g N.m−3),Qa recirculation flow rate (m3.d−1).

Because both flows origins from the nitrification reactor, the effluent nitrate concentrationin the recirculation flow and in the effluent flow are equal to each other (CNO3,a = CNO3,e).After replacing rdn by equation 2.3 in equation 2.2, an expression for the effluent nitrateconcentration was obtained:

CNO3,e = (CNH4,i + Cnob,i − CNH4,e) ∗Qi

Qi +Qa(2.4)

This equation can be simplified by expressing the recirculation flow relative to Qi:

CNO3,e = CNH4,i + Cnob,i − CNH4,e

1 + a(2.5)

where:

a recirculation ratio (Qa/Qi) (-).

Sulfide limiting conditions

In sulfide limiting conditions, all the sulfide from the influent will be oxidized to sulfate(eq. 2.6, first part). During denitrification, part of the sulfide is used for growth, so thisfraction of sulfide should not be considered in the denitrification rate in sulfide limitingconditions (rdn,S.lim) and is therefore subtracted (eq. 2.6, second part). Furthermore, becauseoxygen has a more favorable reduction oxidation potential than nitrate and because growthrates are higher, oxygen is the preferred electron acceptor for SOB (Krekeler and Cypionka,1995; Jing et al., 2010). Therefore, also part of the sulfide will be oxidized with the residualoxygen from the aerobic tank instead of nitrate. Also this part of the sulfide removal does notcontribute to the rdn,S.lim (eq. 2.6, third part). Consequently, the denitrification rate (rdn,S.lim)is given by the total amount of oxidized sulfide minus the part of sulfide used for growthand minus the part reacting with oxygen (eq. 2.6). The paramter values are summarized inTable 2.1.

rdn,S.lim = CH2S,i

ν∗Qi −

CH2S,i

ν∗ YADN ∗ fcv

fcn∗Qi −

Qa ∗ CO2,a

fcn(2.6)

where:

CH2S,i influent sulfide concentration (g S.m−3),CO2,a oxygen concentration in the recirculation flow (g O2.m−3),fcn COD amount necessary for denitrification of nitrate (g COD.(g N)−1),fcv COD content of biomass in the biofilm (g COD.(g VSS)−1),ν sulfide to nitrate stoichiometric conversion ratio (g S.(g N)−1),YADN yield coefficient of autotrophic denitrifying biomass (g VSS.(g N)−1).

After replacing rdn by rdn,S.lim in equation 2.2 and after substitution of the recirculation flowby the recirculation ratio a, the expression for the CNO3,e was obtained:


− CH2S,i

ν∗ (1− YADN ∗ fcv

fcn) + a ∗ CO2,a

fcn

(2.7)

30

CHAPTER 2. METHODS

Table 2.1: Model parameters used in the calculation method.

Symbol Description Value Unitfcn COD amount necessary for denitrification of nitrate 2.86 g COD.(g N)−1

ν sulfide to nitrate stoichiometric conversion ratio 1.43 g S.(g N)−1

fcv COD content of biomass in the biofilm 1.44 g COD.(g VSS)−1

YADN yield coefficient of autotrophic denitrifying biomass 0.57 g VSS.(g N)−1

2.1.3 COD balance to determine the optimal recirculation ratio

To make the calculation of the optimal recirculation ratio (aopt) less complex, the termsnitrification capacity (Nc) and denitrification potential (Dp) were used (Henze, 2008b,c).

The nitrification capacity (Nc) is the amount of nitrate produced by nitrification per liter ofinfluent flow and is expressed in g N.m−3 (Henze, 2008b). The Nc can be found by deriving thenitrogen mass balance over the complete reactor system pictured in figure 1.9 (section 2.1.2).Starting from equation 2.2, the denitrification rate is set equal to zero, because otherwise notall produced nitrate can be taken into account to determine the Nc (rdn = 0 g N.d−1). In thisway the expression for Nc was obtained:

Nc = CNH4,i + Cnob,i − CNH4,e (2.8)

where:

Nc nitrification capacity of the bioreactor (g N.m−3).

The denitrification potential (Dp) is the maximal amount of nitrogen that can be removed bydenitrification, without the presence of oxygen, and is expressed in g N.m−3 (eq. 2.9) (Henze,2008b). A maximal nitrate conversion is reached by using all available electron donors,resulting from sulfide, for denitrification (CH2S,i/ν). Because part of the sulfide is used forgrowth (section 2.1.2), this part was subtracted because this sulfide does not contribute to theconversion of nitrate into nitrogen gas.

Dp = CH2S,i

ν− CH2S,i

ν∗ YADN ∗ fcv

fcn(2.9)

where:

Dp denitrification potential (g N.m−3).

These two terminologies taken into account, the most optimal recirculation ratio (aopt) isdetermined to maximize nitrogen removal. If not enough nitrate is recirculated towards theanoxic reactor, the removal of nitrate will be lower than the maximal removal amount, becausenitrate limiting conditions take place. Furthermore, a too high recirculation ratio results ina sulfide limitation, meaning less nitrate is removed because more sulfide is oxidized withoxygen as the electron acceptor and therefore part of the sulfide is not available anymore fordenitrification.

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CHAPTER 2. METHODS

The COD balance determines the conditions under which an optimal recirculation takes placeand thus, all the sulfide electrons are actually accepted by nitrate and oxygen in the anoxicreactor for a minimal amount of oxygen. The COD balance was derived over the anoxicreactor, units were expressed in g COD.m−3 (electron donors = electron acceptors, eq. 2.10).

fcn ∗Dp = fcn ∗ CNO3,e ∗ aopt + CO2,a ∗ aopt (2.10)

Nitrate is limiting if more electron donors are available than what is necessary for fulldenitrification (eq. 2.11). Because no kinetic limitation takes place, all incoming nitrate wasdenitrified because enough sulfide is present for denitrification. On the other hand, sulfide islimiting if less electron donors are available than electron acceptors (eq. 2.12). Because notenough electron donors are present, no complete denitrification is in this case achieved.

Nitrate limiting: e−H2S > e−

NO3+ e−

O2(2.11)

Sulfide limiting: e−H2S < e−

NO3+ e−

O2(2.12)

To be able to calculate aopt in equation 2.10, only the value for CNO3,e was unknown. Becauseat the aopt all electron donors are used, neither nitrate nor sulfide is limiting. Therefore eitherequation 2.5 or equation 2.7 could be used to substitute the CNO3,e. Because the first equationmentioned is less complicated, equation 2.5 was used to replace CNO3,e.

After replacing CNO3,e by equation 2.5 in equation 2.10 and after including both terms Nc

and Dp, an expression for the aopt was obtained:

a2opt ∗ CO2,a + aopt ∗ (fcn ∗Nc + CO2,a − fcn ∗Dp)− fcn ∗Dp = 0 (2.13)

The obtained quadratic equation was then solved for aopt:

aopt = −B +√B2 − 4 ∗A ∗ C2 ∗A

(2.14)

where:

A = CO2,a,B = fcn*Nc+ CO2,a–fcn*Dp,C = –fcn*Dp.

To find the minimal effluent nitrate concentration possible for a given system, the value ofaopt, calculated with equation 2.14, can be used in equation 2.5.

2.1.4 Effect of influent COD

In the aforementioned calculation method, only sulfide was considered as an electron donor.However, also some organic carbon can be present in the influent of the wastewater treatmentplant. Organic carbon (further referred to as COD) is also an electron donor, meaning moreelectrons will be available for denitrification. Because of this additional electron donor, the

32

CHAPTER 2. METHODS

COD balance was adapted to determine the conditions under which all sulfide and CODelectrons were used by either nitrate or oxygen (eq. 2.15).

e−H2S + e−

COD = e−NO3

+ e−O2

(2.15)

Also in the Dp the extra amount of nitrate removal due to the COD was taken into account(eq. 2.16). In equation 2.16, the ra,dn,COD represents the amount of removed nitrate with theuse of COD as the electron donor per day and per square meter of specific media surface forbiofilm growth. Due to limitations in diffusion, an inactive layer can be present in the biofilm,therefore only the active part is responsible for removal of sulfide (Seker et al., 1995). Dueto this inactive part, the rate is rather influenced by the surface area than by the amountof biomass in the reactor. The ass is the carrier surface per cubic meter filled volume of thereactor, multiplying the ass with ν gives the carrier surface per cubic meter reactor volume.

Dp = CH2S,i


fcn) + ra,dn,COD ∗ ass ∗ ε ∗ Van

Qi(2.16)

where:

ra,dn,COD media surface specific denitrification rate,with COD as the electron donor (g N.d−1.m−2),

ass specific media surface of the carrier material, used in the anoxic reactor (m2.m−3),ε media filling ratio in the anoxic reactor (m3.m−3),Van volume of the anoxic reactor (m3).

Concerning bioavailability, two kinds of COD can be present in the influent: readily biodegrad-able COD (RBCOD) and slowly biodegradable COD (SBCOD). Since an anaerobic step isnecessary to provide the sulfide in this sulfide based system (section 2.1.1), the ratio of theconsumed RBCOD to SBCOD in the outflow of this anaerobic step is not known. Therefore,even if the total incoming COD in the nitrogen removal process is known, no distinction can bemade between RBCOD and SBCOD. Nevertheless, the difference between the denitrificationrate for SBCOD and the rate for RBCOD is important and should be taken into account formore accurate calculations.

2.1.5 System under study

The calculation method was applied for saline wastewater, wastewater with a low sulfateconcentration and saline wastewater containing COD. Concentrations of sulfide and nitrogencompounds in the influent of the anoxic reactor, reactor design and operational parametersand conversion rates can be found in respectively Tables 2.2, 2.3 and 2.4.

Saline wastewater

For the saline wastewater characteristics, data from a pilot plant of the SANI process wasused (Lu et al., 2012a). The effluent TKN concentration of the anaerobic step in SANI(SRUSB reactor, section 1.2.3) was used as the influent TKN concentration of the denitrifica-tion reactor of the nitrogen removal system (figure 1.9, CT KN,i). Furthermore, the effluentTKN concentration of the SANI process was used as the effluent TKN concentration of the

33

CHAPTER 2. METHODS

nitrification reactor (figure 1.9, CT KN,e). With the difference of the influent and effluentTKN concentrations, the effluent nitrate concentration for limiting nitrate conditions can becalculated (eq. 2.17).

CT KN,i − CTKN,e

1 + a= CNH4,i + Cnob,i + Cnou,i − CNH4,e − Cnou,e

1 + a

= CNH4,i + Cnob,i − CNH4,e

1 + a(eq.2.5)= CNO3,e (2.17)

where:

CT KN,e effluent total Kjeldahl nitrogen concentration (g N.m−3),CT KN,i influent total Kjeldahl nitrogen concentration (g N.m−3).

For the calculation of the effluent nitrate concentration in sulfide limiting conditions (eq. 2.7),also the influent sulfide concentration (CH2S,a) and oxygen concentration in the recirculationflow (CO2,a) were necessary. For the CH2S,i, the effluent sulfide concentration of the anaerobicstep in SANI was used (Lu et al., 2012a). No value for the CO2,a was given, therefore theestimation of 3 g O2.m−3 was made, based on oxygen concentrations used in studies of Wanget al. (2009) and Wu et al. (2016).

Wastewater with low sulfate concentrations

The influent TKN concentration for wastewater with a low sulfate concentration was set equalto a medium TKN concentration of 60 g N.m−3 as a typical medium concentration for rawwastewater (Henze, 2008d). Furthermore, a medium sulfide concentration of 30 g S.m−3 wasused for the untreated municipal wastewater (Metcalf and Eddy, 1991). All other values wereassumed to be the same as for saline wastewater.

Effect of influent COD

To illustrate the effect of COD in the influent, the additional removal of nitrate (due tothe extra electron donor) had to be taken into account in the denitrification rate in sulfidelimiting conditions (rdn,S.lim, eq. 2.6). By including the COD in this denitrification rate, anew expression for the effluent nitrate concentration (CNO3,e) could be found:


− CH2S,i


fcn) + a ∗ CO2,a

fcn− ra,dn,COD ∗ ass ∗ ε ∗ Van

Qi

(2.18)

An estimation of the media specific denitrification rate with the use of COD as the electrondonor (ra,dn,COD) was made, based on the article of Welander and Mattiasson (2003). Alow removal rate was chosen because almost no degradable organics enter the anoxic reactorvia the influent (Lu et al., 2012b). The parameter values for the ass and the ε of the anoxicreactor were used from the article of Lu et al. (2012a). Furthermore, the minimal volumedetermined in section 2.3 was used to represent the volume of the anoxic reactor. This volumewas chosen instead of the volume used in the article of Lu et al. (2012a), because in thecalculation method no kinetic limitation was assumed to take place.

34

CHAPTER 2. METHODS

Implementation

With the use of the MATLAB software package (The MathWorks, Inc., 2000), the CNO3,e wasplotted for a range of values for the recirculation ratio in case of saline wastewater, wastewaterwith a low sulfate concentration and saline wastewater in which the COD was included.

2.2 Simulation study set up

The model was submitted to changing concentrations of the oxygen concentration in the aerobicreactor (CO2,a), the influent sulfide concentration (CH2S,i) and the influent total Kjeldahlnitrogen concentration (CT KN,i) to evaluate the effect on the effluent nitrate concentrationand the optimal recirculation ratio. The concentrations were varied between realistic minimaland maximum concentrations, which appear in a municipal wastewater plant (Table 2.2).Figures were plotted with the use of the MATLAB software package (The MathWorks, Inc.,2000). Furthermore, because all three factors are dependent on each other, sensitivity plotswere made to asses the effect on the minimal effluent nitrate concentration and the optimalrecirculation ratio.

Oxygen concentration in the aerobic reactor

The range of oxygen in the aerobic reactor was estimated to be in between 0.3 and 8 g O2.m−3.The lower range value is the minimal amount of oxygen necessary for nitrification (Stenstromand Poduska, 1980). The upper value is based on the maximum dissolved oxygen concentrationin water at a temperature of 25°C (Benson and Krause, 1984). Complete nitrification wasassumed for all oxygen concentrations.

Influent sulfide concentration

The sulfide present in municipal wastewater with a low sulfate concentration lies between20 and 50 g S.m−3 (Metcalf and Eddy, 1991) (all sulfate was assumed to be fully convertedto sulfide in the anaerobic step). However, the value for the pilot plant is higher, becauseseawater instead of freshwater is used as a water source. Therefore, besides the range between20 and 50 g S.m−3, also higher influent sulfide values until 150 g S.m−3 were considered toalso take into account saline wastewater for evaluation of the effect on the effluent nitrateconcentration and optimal recirculation flow.

Influent total Kjeldahl nitrogen concentration

The total Kjeldahl nitrogen concentration present in municipal wastewater ranges between 30and 100 g TKN_N.m−3 (Henze, 2008d). To evaluate the effect of decreasing and increasinginfluent TKN, a plot over this range was made.

Interaction between influencing factors

Because the effect of the changing concentrations are dependent on each other, sensitivityplots were made with changing influent TKN concentrations and changing sulfide for differentoxygen concentrations to evaluate the effect on the minimal effluent nitrate concentration andon the corresponding optimal recirculation flow.

35

CHAPTER 2. METHODS

2.3 Minimal volume of the denitrification reactor

In the calculation method of section 2.1, all sulfide was assumed to be removed in sulfidelimiting conditions and thus, no kinetic limitation was assumed to take place (eq. 2.6). Thisis only true in case the reaction rate is high enough, or in other words, if the anoxic reactorvolume is big enough, assuming a fixed amount of carrier material.

If, due to kinetic limitation, sulfide is not completely removed during denitrification, theleftover sulfide will flow towards the aerobic reactor. In the aerobic reactor, nitrification isinhibited by the presence of sulfide (Bentzen et al., 1995; Æsøy et al., 1998). Furthermore,if not all sulfide is removed, small emissions of hydrogen sulfide can escape from the reactorto the environment. For odour and health reasons, these emissions should be minimized.Finally, if more sulfide is removed, lower effluent nitrate concentrations can be achieved. Forthese reasons, it is interesting to determine the minimal reactor volume at which no kineticlimitation takes place in the anoxic reactor.

To determine the minimal reactor volume, the rate at which sulfide enters the system is setequal to the rate at which sulfide is removed from the system (eq. 2.19). In equation 2.19, themedia specific sulfide removal rate (ra,H2S) expresses the amount of removed sulfide per dayand per square meter of specific media surface for biofilm growth. In the ra,H2S the sulfideremoval through oxidation with nitrate or oxygen is taken into account as well as the removalof sulfide for biomass growth. By multiplying the ra,H2S with the specific surface ass and thefilling ratio ε, the volumetric sulfide removal rate is obtained (rv,H2S) (eq. 2.19).

CH2S,i ∗Qi = ra,H2S ∗ ass ∗ ε ∗ Vmin

= rv,H2S ∗ Vmin (2.19)

where:

ra,H2S media surface specific sulfide removal rate (g S.d−1.m−2),rv,H2S volumetric sulfide removal rate (g S.d−1.m−3),Vmin Minimal volume of the anoxic reactor (m3)

The expression for the minimal volume was then obtained out equation 2.19:

Vmin = CH2S,i ∗Qi

ra,H2S ∗ ass ∗ ε

= CH2S,i ∗Qi

rv,H2S(2.20)

The volumetric sulfide removal rate (rv,H2S) was estimated, because no sulfide removal rate isgiven in the article of Lu et al. (2012a). The estimation of rv,H2S was based on the measuredinfluent and effluent sulfide concentrations of the nitrogen removal process (Lu et al., 2012a).Then, equation 2.20 was used to calculate the minimal reactor volume. Used influent sulfideconcentrations, reactor design and operational parameters and conversion rates can be foundin respectively Table 2.2, 2.3 and Table 2.4.

36

CHAPT

ER2.

MET

HODS

Table 2.2: Wastewater concentration characteristics of the nitrogen removal system.

Symbol Description RangeValues used

Unit Referenceain calculations(Lu et al., 2012a)

CH2S,i influent sulfide concentration 20–150b 124.1 g S.m−3 Metcalf and Eddy (1991)CT KN,e effluent TKN concentration - 23.4 g N.m−3 -CT KN,i influent TKN concentration 30–100 85.3 g N.m−3 Henze (2008d)

CO2,aoxygen concentration in the 0.3–8 3 g O2.m−3 Stenstrom and Poduska (1980) &recirculation flow Benson and Krause (1984)

a References of the range values for municipal wastewater.b The sulfide range for municipal wastewater with a low sulfate concentration is in between 20 and 50 g S.m−3, also values until 150 g S.m−3

were considered to represent the saline wastewater.

Table 2.3: Design and operational parameters of the anoxic reactor. Values from Lu et al. (2012a).

Symbol Description Value Unitass specific media surface of the carrier material, used in the anoxic reactor 115 m2.m−3

ε media filling ratio in the anoxic reactor 1 m3.m−3

Qi influent flow 10 m3.d−1

Table 2.4: Denitrification rates, used in calculations.

Symbol Description Value Unit Reference

ra,dn,CODmedia surface specific denitrification rate, 0.7 g N.m−2.d−1 Welander and Mattiasson (2003)with COD as the electron donor

rv,H2S volumetric sulfide removal rate 252 g S.d−1.m−3 Lu et al. (2012a)

37

CHAPTER 2. METHODS

2.4 Applicability of sulfide based nitrogen removal process

over nitrate and the novel process over nitrite in wastew-

aters with a low sulfate concentration

In the sulfide based nitrogen removal process, external organic carbon addition is not neces-sary and the energy input is equal to the conventional nitrification–denitrification process(Table 1.1). To also minimize this energy input, the idea is put forward to include nitritationand denitritation into the sulfide based nitrogen removal process. In this way in the nitritationreactor, the ammonium is nitritated to nitrite instead of nitrate. Furthermore, the nitriteis then converted to nitrogen gas in the denitritation reactor instead of a denitrification ofnitrate to nitrogen gas. The sulfide based process over nitrate and the novel process overnitrite were compared with each other, based on pure reduction–oxidation stoichiometrics ofthe whole nitrogen removal process.

Furthermore, because seawater contains a lot of sulfate, seawater used for domestic purposesis ideal to treat with the sulfide based process. However, not all places are favored by thepossibility to use seawater or a sulfate rich water source. Therefore, the applicability ofthe sulfide based processes were assessed for wastewaters with a low sulfate concentration.For the availability of sulfide in wastewater with a low sulfate concentration, sulfate valuesfrom a study of Metcalf and Eddy (1991) were used for weak, medium and strong sulfateconcentrations (Table 2.6). To avoid confusion, these will be referred at, as weak, medium andstrong wastewaters with a low sulfate concentration. In the anaerobic step, prior to the sulfidebased process, sulfate is assumed to be fully reduced to sulfide. Furthermore, no strippingis assumed or the stripped sulfide can be absorbed in a separate unit like in Kleerebezemand Mendez (2002). In this way all converted sulfate to sulfide can be used for the nitrogenremoval.

The sulfide based processes were considered to be applicable if the available sulfate in thewastewaters with a low sulfate concentration was higher than the stoichiometrically requiredsulfide for ammonium removal (eq. 2.21). If however, the available sulfate was lower thanwhat is stoichiometrically required for ammonium removal, the sulfide based process could notbe applied due to limited availability of sulfide for the ammonium removal (eq. 2.22).

Process applicable:

H2S in wastewater with a low sulfate concentration ≥ Stoichiometrically required H2S(2.21)

Process not applicable:

H2S in wastewater with a low sulfate concentration < Stoichiometrically required H2S(2.22)

To make the assessment of the applicability more realistic, the production of biomass(CH1.8O0.5N0.2) was also taken into account into the stoichiometric equations. Via theseequations the sulfide to ammonium ratio could then be determined. By multiplying this sulfideto ammonium ratio (g S.(g NH4_N)−1) with the ammonium concentration in the wastewater(g NH4_N.m−3) the stoichiometrically required sulfide amount could be found.

38

CHAPTER 2. METHODS

For example: if nitrate is used as an electron acceptor, additional sulfide and ammoniumshould be taken into account as electron donors for biomass production in the denitrificationand, respectively, nitrification process. Also an additional quantity of ammonium is necessary,as a nitrogen source, for biomass production in both processes. As explained in appendix A.1,the stoichiometric equations including biomass production could then be found with the useof the yield coefficients.

Concerning the denitrification reaction, biomass production was already included in equa-tion 1.3. Via the same methodology, explained in appendix A.1, the nitrification equation withbiomass inclusion could be found (eq. A.6). By combining both equations 1.3 and A.6, thesulfide to ammonium ratio could be calculated:

2.45 mol HS− ∗ 32.07 g S.(mol HS−)−1

(17.97 mol NH4+ ∗ 3.08 mol NO3−

17.77 mol NO3− + 0.2 mol NH4+) ∗ 14.01 g N.(mol NH+

4 )−1= 1.69 g S.(g N)−1

In the same way, the denitritation and nitritation stoichiometric equations including biomassproduction could be determined for the sulfide based process over nitrite. However, becauseno yield coefficients for this nitrogen removal process over nitrite are reported in literature,an estimation of 3/4 times the yield of the process over nitrate was made. 3/4 was chosenbecause nitrite can accept 3/4 times as many electrons as nitrate. After determining bothequations for the denitritation and nitritation, the sulfide to ammonium ratio was calculated(Table 2.5).

Table 2.5: Sulfide to ammonium ratio with and without biomass integration. The ratios are expressedin g S.(g N)−1.

Nitrogen removal process With biomass integrationSulfide based process over nitrate 1.69Sulfide based process over nitrite 1.09

To calculate the stoichiometrically required amount of sulfide, the calculated ratios withbiomass integration were multiplied with the ammonium concentration in the wastewater. Alow, medium and high ammonium contamination of wastewater with a low sulfate concentrationwas considered (Table 2.6). Before the ammonium enters the nitrogen removal system, thewastewater is endured to an anaerobic step to make sure sulfate is converted into sulfide.During this anaerobic step, the ammonium concentration is not affected (Lu et al., 2012a; Wuet al., 2016).

Table 2.6: Assessed low, medium and high ammonium concentrations for weak, medium and strongwastewaters with a low sulfate concentration.

Concentration of Unit ReferenceWeak Medium Strong

sulfate 20 30 50 g S.m−3 Metcalf and Eddy (1991)Low Medium High

ammonium 20 45 75 g N.m−3 Henze (2008d)

39

CHAPTER 2. METHODS

For example, the applicability of the sulfide based process via nitrate is assessed for wastew-ater, contaminated with a low ammonium concentration. 34 g S.m−3 of stoichiometricallyrequired sulfide was calculated by multiplying the ammonium concentration with the sulfideto ammonium ratio (20 g N.m−3*1.69 g S.(g N)−1). Then a comparison was made betweenthis required sulfide amount and the sulfide availability in the wastewater with a low sulfateconcentration. For weak wastewaters with a low sulfate concentration, the sulfide based processis not applicable due to limiting sulfide concentration (20 g S.m−3 < 34 g S.m−3, eq. 2.22).On the other hand for strong wastewaters with a low sulfate concentration, enough sulfide ispresent for the complete removal of ammonium (50 g S.m−3 ≥ 34 g S.m−3, eq. 2.21).

40

Chapter 3

Results and discussion

In the first section, the calculation method for the effluent nitrate concentration will beapplied on the sulfide based nitrogen removal process. Next, the minimal reactor volume ofthis application is determined. Furthermore, the effect of process operation and wastewatercharacteristics on the effluent nitrate is discussed. Also a sensitivity analysis was made toassess the effect on the minimal effluent nitrate concentration and the optimal recirculationratio. Finally, in the last section, the main focus lies on the application of the sulfide basedprocess over nitrate and the novel process over nitrite in wastewaters with a low sulfateconcentration.

3.1 Calculation method for the effluent nitrate concentration

In this section the effluent nitrate concentration is determined for saline and municipalwastewater with a low sulfate concentration by application of the calculation method ona sulfide based nitrogen removal process. Furthermore, the effect of COD inclusion wasevaluated.

3.1.1 Application

Saline wastewater

In figure 3.1, the effluent nitrate concentration is plotted for a sulfide based nitrogen removalprocess of saline wastewater. When the recycle flow (a) increases, the effluent nitrate firstdecreases (figure 3.1, blue curve). This is because at low recycle flows, sulfide is available inexcess and nitrate is limiting (eq. 2.5). When the recycle increases, more nitrate is sent tothe anoxic reactor and consequently, more denitrification takes place, leading to lower effluentnitrate concentrations.

At a certain point, the effluent nitrate reaches a minimum (figure 3.1, blue arrow). Thishappens when all available sulfide is used by the electron acceptors nitrate and oxygen presentin the recycle flow. The recirculation ratio at which this happens is called aopt and is 7 in caseof saline wastewater (figure 3.1, aopt).

41

CHAPTER 3. RESULTS AND DISCUSSION

Figure 3.1: The effluent nitrate concentration in function of a changing recirculation ratio for a sulfidebased nitrogen removal process. For a sulfide concentration of 124.1 g S.m−1.

When the recirculation increases even more, the effluent nitrate increases again (figure 3.1,blue curve). This is because more oxygen from the aerobic reactor is recycled at higherrecirculation flows and oxygen is preferred above nitrate as an electron acceptor by SOB.A higher recirculation flow consequently leads to more sulfide consumption by oxygen andless by nitrate. As a results, an increase in the recirculation flow rate above aopt leads toless denitrification, implying higher nitrate concentrations in the effluent. Therefore, a linearincreasing trend of the accumulation of nitrate can be seen (eq. 2.7).

Depending on the required effluent nitrate concentration, a corresponding recirculation ratiocan be determined for the design of the recirculation flow of the plant. For example, if the TNlimit is 25 g N.m−3 and the nitrification reactor reduces the TKN to 10 g N.m−3, then therecan maximally be 15 g N.m−3 of nitrate in the effluent. Both corresponding recirculation ratios3 and 14 for a nitrate value of 15 g N.m−3 can then be determined on the graph (figure 3.1,red dots).

It is economically more interesting to choose the recirculation ratio in the first part of the graph(for a ≤ aopt), because for high recirculation ratios, pumping costs and pipe construction costsare higher. Therefore, for economical reasons the recirculation ratio is limited at a practicalmaximum of 5 (Henze, 2008b). At this maximum practical recirculation ratio, the effluentnitrate concentration is close to the minimal nitrate concentration (10 g N.m−3 compared to8 g N.m−3 at aopt), so it is practically possible to go close to the maximal nitrate removal.

In the article of Lu et al. (2012a), a recirculation ratio of 2.5 is applied and an effluent nitrateconcentration of 16.8 g N.m−3 is achieved. Out of figure 3.1, the corresponding effluent nitrateconcentration for a recirculation ratio of 2.5 is equal to 17.7 g N.m−3 (figure 3.1, blue dot)and thus very close to the effluent value from the article. Important to note is that the usedconcentrations are subjected to high measurement errors (up to 10–20%).

42


Wastewater with low sulfate concentrations

For wastewater with a low sulfate concentration the effluent nitrate concentration is plottedin figure 3.2. The effluent nitrate concentration first decreases for an increasing recycleflow (a) (figure 3.2, blue curve). This first decreasing part is different compared to thedecreasing part of the saline wastewater, because both wastewater have different influentnitrogen concentrations (saline wastewater: 85 g N.m−3 versus wastewater with a low sulfateconcentration: 60 g N.m−3).

Figure 3.2: The effluent nitrate concentration in function of a changing recirculation ratio for a sulfidebased nitrogen removal process. For a medium sulfide concentration of 30 g S.m−1.

Similar as for saline wastewater, a minimal effluent nitrate concentration is reached, thisminimal concentration is higher than the minimal concentration for the saline wastewater(figures 3.1 and 3.2, blue arrows). This is because less sulfide is available in wastewater with alow sulfate concentration and therefore, the small amount of available sulfide is faster oxidizedby the recirculated nitrate and oxygen. Consequently, the corresponding recirculation ratio atwhich all the available sulfide is used, is lower for wastewater with a low sulfate concentrationthan for saline wastewaters. In case of wastewater with a low sulfate concentration this optimalrecirculation ratio is 1 (figure 3.2, aopt).

For higher recirculation ratios, the effluent nitrate concentration increases linearly (figure 3.2).The increase in the effluent nitrate is equal to the increase for saline wastewater. This isbecause the oxygen concentrations in the aerobic reactors of both wastewaters are the same(eq. 2.7).

Since the minimal effluent nitrate is very high for wastewater with a low sulfate concentration(figure 3.2, blue arrow), the required effluent nitrate concentration is often not reached andconsequently the sulfide based process is not sufficient to remove nitrogen out of the wastewater.For example, if again a TN limit of 25 g N.m−3 is assumed and a reduction of the TKNto 10 g N.m−3, then the effluent nitrate concentration can maximally be 15 g N.m−3, butthis concentration can not be reached for wastewater with a low sulfate concentration sincethe minimal effluent nitrate concentration is only 22 g N.m−3 (figure 3.2, blue arrow). In

43


section 3.4, an improvement is made to the sulfide based process to increase its applicabilityfor wastewaters with a low sulfate concentration.

3.1.2 Effect of influent organic carbon

In figure 3.3, the COD is included in the determination of the effluent nitrate concentrationfor saline wastewater. For increasing recycle flows (a), the effluent nitrate concentrationdecreases (figure 3.3, blue curve). In the decrease of the blue curve, the same numericalvalues can be observed as in figure 3.1 up to the aopt of 7. The only difference is that infigure 3.3 the nitrate remains limiting since there are much more electron donors, resultingfrom COD. Therefore, the effluent nitrate concentration keeps decreasing and thus, also lowereffluent nitrate concentrations can be reached compared with the minimal effluent nitrateconcentration for the saline wastewater.

Figure 3.3: The effluent nitrate concentration in function of a changing recirculation ratio for asulfide based nitrogen removal process. Both sulfide and COD were included as theelectron donors (for a sulfide concentration of 124.1 g S.m−3).

The minimal effluent nitrate concentration is lower than for saline wastewater (figures 3.1 and 3.3,blue arrows). This minimal effluent nitrate concentration is reached at an optimal recircu-lation ratio of 39 (figure 3.3, aopt). The optimal recirculation ratio is higher than for salinewastewater because more electron donors are present and thus it takes longer before they arecompletely used by the recirculated nitrate and oxygen.

Similar as in case of the saline wastewater, a corresponding recirculation ratio of 3 can bedetermined with a required effluent nitrate concentration of 15 g N.m−3 (figure 3.3, first reddot). Furthermore, the second corresponding recirculation ratio is 52 (figure 3.3, second reddot).

If the practical recirculation ratio of 5 is considered, the corresponding effluent nitrateconcentration is approximately 10 g N.m−3. This concentration is way higher than the minimal

44


effluent nitrate concentration in case COD is present in the saline wastewater (figure 3.3, bluearrow). So it is practically impossible to go close to the minimal effluent nitrate concentration(at aopt). Due to the practical limitation of the recirculation flow, the same effluent nitrateconcentration as for saline wastewater can be reached. Therefore, presence or addition of CODis only advantageous for wastewaters in which sulfide is limiting to lower the minimal effluentnitrate concentration.

Important to note is that the estimated ra,dn,COD, used in the calculations, only takes RBCODinto account. Because not all incoming COD will be RBCOD, the real aopt will most likely belower, because for SBCOD lower removal rates are valid. Furthermore, Lu et al. (2012b) saysthere are almost no biodegradable organics in the influent of the anoxic reactor. Althougha low biodegradability was chosen, figure 3.3 is mainly gives an image of the trends of thegraph, but is no realistic representation of reality.

3.2 Effect of process operation and wastewater characteristics

on the effluent nitrate concentration and optimal recircu-

lation flow

For the given range of oxygen concentration in the reactor, influent sulfide concentrationand TKN concentration, the individual effect on the effluent nitrate concentration and onthe optimal recirculation ratio is studied in sections 3.2.1, 3.2.2 and 3.2.3, respectively.Furthermore, in section 3.2.4 the inter dependency of these influencing factors is assessed.

3.2.1 Oxygen concentration in the aerobic reactor

In figure 3.4, the effluent nitrate concentration is shown for a changing oxygen concentration. Inthe absence of a recycle flow (a), effluent nitrate concentrations for all oxygen concentrationsstart at the same value. Furthermore, for increasing recycle ratios, the effluent nitrateconcentration decreases for each oxygen concentration with the same trend. When therecirculation flow is low, sulfide is present in excess, so all nitrate that is recycled will bedenitrified, irrespective of the oxygen concentration (eq. 2.5).

For lower oxygen concentrations, lower minimal effluent nitrate concentrations can be reached.For example at 8 g O2.m−3, the minimal effluent nitrate concentration is 12 g N.m−3 (figure 3.4,purple curve), while at 0.3 g O2.m−3 this concentration is 3 g N.m−3 (figure 3.4, green curve).If the oxygen concentration in the recycle is high, the sulfide will become limiting earlier thanwhen the oxygen concentration is low. This is because less oxygen is recirculated togetherwith nitrate to the anoxic reactor and therefore, more sulfide can react with nitrate fordenitrification instead of reaction with oxygen. Consequently, for a lower oxygen concentrationand the same influent sulfide concentration, more nitrate can be denitrified and the optimalrecirculation ratio (aopt) will be higher (figure 3.4).

Recirculation ratios above the aopt result in a linear increase in the nitrate effluent concentration.The higher the oxygen concentration, the steeper the linear increase is (figure 3.4). For highoxygen concentrations, more oxygen is recirculated to the anoxic reactor. Because oxygen is

45


Figure 3.4: The effluent nitrate concentration in function of a changing recirculation ratio. Plottedfor the oxygen concentrations 0.3, 2, 3, 5 and 8 g O2.m−3. Influent TKN concentration= 85.3 g N.m−3 and influent sulfide concentration = 124.1 g S.m−3.

the preferred electron acceptor, less nitrate will consequently be denitrified and therefore, theeffluent nitrate concentration increases (eq. 2.7).

3.2.2 Influent sulfide concentration

In figure 3.5, the effluent nitrate concentration is plotted for a changing influent sulfideconcentration. Similar as for a change in oxygen concentration, the start value of the effluentnitrate concentration is for all sulfide concentrations the same (figure 3.5). Furthermore, alsothe first decreasing trend in the effluent nitrate concentration is identical, because all nitratethat is recycled will be denitrified, irrespective of the sulfide concentration (eq. 2.5).

For high influent sulfide concentrations, lower minimal effluent nitrate concentrations can beachieved. For example, for saline wastewater at 150 g S.m−3 the minimal effluent nitrateconcentration is 4 g N.m−3 (figure 3.5, purple curve), while for wastewater with a low sulfateconcentration of 20 g S.m−3, the minimal effluent nitrate concentration is only 51 g N.m−3

(figure 3.5, green curve). For high sulfide concentrations, more sulfide is available and thusmore nitrate can be denitrified, resulting in lower effluent nitrate concentrations (eq. 2.7). Thesulfide will become limiting earlier if only low amounts are available, therefore the optimalrecirculation ratio at which minimal effluent is achieved are lower for low sulfide concentrations(figure 3.5, green versus purple curve).

After the recirculation exceeds the aopt, there is a parallel increase in the effluent nitrateconcentration for all influent sulfide concentrations (figure 3.5). More oxygen is providedby the increasing recirculation flow and since oxygen is the preferred electron acceptor, lessnitrate can be denitrified. This accumulation of nitrate is independent of the influent sulfide

46


Figure 3.5: The effluent nitrate concentration in function of a changing recirculation ratio. Plottedfor the influent sulfide concentrations 20, 35, 50, 90 and 150 g S.m−3. Influent TKNconcentration = 85.3 g N.m−3 and oxygen concentration = 3 g O2.m−3.

concentration.

3.2.3 Influent total Kjeldahl nitrogen concentration

For a ranging influent TKN concentration, the effluent nitrate concentration is shown infigure 3.6. The higher the influent TKN concentration, the more nitrate can be produced bynitrification. For this reason, if no recirculation takes place, the starting value of the effluentnitrate concentration is higher for higher TKN influent values (figure 3.6, e.g. purple versusgreen curve).

From the moment recirculation occurs, denitrification takes place and nitrate is thus removedfrom the system. For high TKN concentrations a given recirculation ratio implies a largeramount of nitrogen transferred to the anoxic reactor. Consequently, for high influent TKNconcentrations more nitrate is denitrified at a low recirculation ratio than for low influentTKN concentrations, therefore the decrease in effluent nitrate concentration is steeper for highTKN concentrations (figure 3.6, e.g. purple versus green curve).

Because the amount of sulfide is limited and fixed, the minimal effluent nitrate concentrationsis higher for higher influent TKN concentrations (figure 3.6, purple versus green curve).Furthermore, a higher TKN concentration will lead to sulfide limitation at lower recycle flowsso aopt is smaller. For example, the optimal recirculation flow is equal to 3 at an influentTKN concentration of 100 g N.m−3 (figure 3.6, purple curve) and 21 for an influent TKNconcentration of 65 g N.m−3 (figure 3.6, black curve).

Finally, with the same slope as for changes in the influent sulfide concentration, the effluent

47


Figure 3.6: The effluent nitrate concentration in function of a changing recirculation ratio. Plottedfor the incoming total Kjeldahl concentrations 30, 50, 65, 85.3 and 100 g TKN_N.m−3.Influent sulfide concentration = 124.1 g S.m−3 and oxygen concentration = 3 g O2.m−3.

nitrate concentration increases parallel for all influent TKN concentrations (figure 3.6). Theincrease of nitrate is independent of the influent TKN concentration.

3.2.4 Interaction between influencing factors

The individual effects of the changing oxygen, sulfide and TKN concentrations were dealtwith in the previous sections. However, the effect of the influent TKN concentration on theminimal effluent nitrate concentration and on the optimal recirculation ratio is also dependentof the sulfide and oxygen concentration. In figures 3.7 and 3.8 this dependency and sensitivityis explained.

Minimal effluent nitrate concentration

Figure 3.7a shows the minimal effluent nitrate concentration for different values of influentTKN concentrations and sulfide concentrations for an oxygen concentration of 0.3 g O2.m−3.In figure 3.7b, the same plot is made but for an oxygen concentration of 3 g O2.m−3.

Even though, for an increasing oxygen concentration, an increasing trend in the minimaleffluent nitrate concentration can be noticed, only minimal changes are visible over the twofigures (figures 3.7a and 3.7b, e.g. trend of green curves are almost identical). Thus, a changingoxygen concentration does not have a big impact on the minimal effluent nitrate concentration.

The minimal effluent nitrate concentration is very sensitive for changes in the influent TKNconcentration for a low sulfide concentration. For example for a sulfide concentration of20 g S.m−3, the minimal effluent nitrate concentration ranges from 0 to about 70 g N.m−3

(figure 3.7b, red curve). For high sulfide concentrations on the other hand, the changes in

48


(a) For CO2,a = 0.3 g O2.m−3. (b) For CO2,a = 3 g O2.m−3.

Figure 3.7: Sensitivity plots for a range of TKN and sulfide concentrations: effect on the minimalCNO3,e.

the influent TKN concentration have less impact. To illustrate, the minimal effluent nitrateconcentration only ranges between 0 and 20 g N.m−3 for a sulfide concentration of 150 g S.m−3

(figure 3.7b, black curve). At a low sulfide concentration, only a limited amount of nitrate canbe denitrified. Because at low sulfide concentrations only a limited amount of nitrate can beremoved, the minimal effluent nitrate is highly sensitive to a change in TKN concentration. Onthe other hand, for a high sulfide concentration a lot of nitrate can be removed and the minimaleffluent nitrate concentration is thus less dependent on the influent TKN concentration.

Finally, for high influent TKN concentrations, the minimal effluent nitrate concentration is verysensitive for a changing sulfide concentration. For example, for an influent TKN concentrationof 100 g N.m−3 the minimal nitrate concentration is equal to 67 g N.m−3 at a low sulfideconcentration (figure 3.7b, red curve). However, when for the same TKN concentration of100 g N.m−3 the sulfide concentration is high, the minimal nitrate concentration is only9 g N.m−3 (figure 3.7b, black curve). The more sulfide is available for high TKN concentration,the more nitrate can be removed and therefore the minimal effluent nitrate concentration is alot lower than for low sulfide concentrations. Because of the high sensitivity for high TKNconcentrations, addition of an electron donor is very effective to lower the minimal effluentnitrate concentration.

Optimal recirculation ratio

To reach aforementioned minimal effluent nitrate concentrations, recirculation at the optimalratio is required. Therefore, the effect of the changing concentrations on the recirculation ratiowas evaluated as well (figure 3.8). Because a change in the oxygen concentration barely affectedthe minimal effluent nitrate concentration, the effect of a changing oxygen concentration onthe optimal recirculation rate was not considered.

The optimal recirculation flow (aopt) is very sensitive for a change in the influent TKNconcentration for high influent sulfide concentrations (figure 3.8, e.g. black curve). Onthe other hand, the aopt is less sensitive to a change in TKN concentration for low sulfideconcentrations (figure 3.8, e.g. red curve). For low sulfide concentrations, not much sulfide isavailable for denitrification and thus the optimal recirculation at which the minimal effluent

49


Figure 3.8: Sensitivity plots for a range of TKN and sulfide concentrations: effect on the aopt. Foran oxygen concentration of 3 g O2.m−3.

nitrate concentration is reached is low over the changing TKN concentration.

Relatedness between the minimal effluent and the optimal recirculation ratio

Important to notice is that most optimal recirculation ratios are not achievable in practicebecause they exceed the practical limit of 5 (figure 3.8, above dotted line). Consequently,also the corresponding minimal effluent nitrate concentrations are not realistic. However,due to the steep decreasing slope for the effluent nitrate concentration for low recirculationratios (figures 3.4, 3.5 and 3.6, slope of curves before the aopt), already very low effluentnitrate concentrations can be reached at the practical recirculation ratio of 5. Therefore, therecirculation ratio of 5 can easily be applied as an upper boundary recirculation ratio whilestill low effluent nitrate concentrations can be obtained. For example, for an influent TKNconcentration of 50 g N.m−3 and a sulfide concentration of 124 g S.m−3, an effluent nitrateconcentration of 4 g N.m−3 can be obtained for a recirculation ratio of 5 (figure 3.6, redcurve).

On the other hand, if the optimal recirculation ration lies below the practical maximum of5 (figure 3.8, below dotted line), the minimal effluent nitrate concentrations are achievablein practice. These minimal nitrate concentrations are however very high (figure 3.7, e.g. redcurve and blue curve for TKN > 50 g N.m−3). Therefore it is important to regulate thisoptimal recirculation well, to enable maximal nitrogen removal. For example, for influentTKN concentrations higher than 65 g N.m−3, only minimal effluent nitrate concentrationsabove 20 g N.m−3 are achievable for wastewaters with a low sulfate concentration (figure 3.7b,blue and red curve). If the applied recirculation ratio is lower or exceeds the optimal one,even higher effluent nitrate concentrations than the minimal concentration will be achieved.Therefore, regulation of this optimal recirculation ratio is very important to remove as muchnitrogen as possible.

50


3.3 Minimal volume of the denitrification reactor

The calculated minimal volume is 4.9 m−3 (eq. 2.20). This volume is higher than the usedvolume of 3.9 m−3 for the anoxic reactor in the article of Lu et al. (2012a). Due to kineticlimitation, sulfide can thus not be fully removed. Important to note is that the calculatedminimal volume was subjected to the measurement error of the used sulfide concentrations.

If kinetic limitation takes place in the article of Lu et al. (2012a), less sulfide will be usedand consequently, less nitrate will be denitrified meaning more nitrate will be present in theeffluent. Because in the simulations no kinetic limitation is taken into account, predictedeffluent nitrate concentrations of the calculation method (figure 3.1), are expected to be slightlylower than the values measured in the article. Due to high measurement errors, this could notbe confirmed.

The main goal of the sulfide based nitrogen removal process is to reach the effluent standardfor the effluent nitrate concentration. Although the effluent nitrate concentration is lowerfor the removal of all sulfide, increasing the reactor volume to prevent kinetic limitation canbe very expensive and thus not always economical compared to the gain in nitrate removal.Therefore, it can be presumed that for the applied reactor volume in a wastewater treatmentinstallation, kinetic limitation occurs. The calculation method for the minimal reactor volumewill thus give an overestimation of the real applied reactor volume. More research is necessaryto determine the minimal volume in case kinetic limitation takes place.

3.4 Applicability of sulfide based nitrogen removal process

over nitrate and the novel process over nitrite in wastew-

aters with a low sulfate concentration

In equations 3.1 & 3.2 the reduction–oxidation stoichiometric reactions are shown for thesulfide based process over nitrate (nitrification–denitrification) and respectively the novelsulfide based process over nitrite (nitritation–denitritation).

8 NH+4 + 16 O2 + 5 HS− → 13 H+ + 4 N2 + 5 SO2−

4 + 12 H2O (3.1)

8 NH+4 + 12 O2 + 3 HS− → 11 H+ + 4 N2 + 3 SO2−

4 + 12 H2O (3.2)

Figure 3.9 shows a comparison between both processes. Percentages in the figure representthe reduction–oxidation stoichiometrics, relative to the process over nitrate (eq. 3.1 & 3.2).

The use of nitrite instead of nitrate as an electron acceptor, allows a reduction in oxygen inputwith 25% (figure 3.9). For the same nitrogen removal, aeration energy is thus more efficientlyused for the process over nitrite. Another advantage is the lower amount of necessary sulfidefor the nitrogen removal. 40% less sulfide is necessary as electron donor to reduce nitrite tonitrogen gas instead of nitrate. This reduction in the required sulfide allows a wider range ofwastewaters to be treated.

Assessments were made for the applicability of the sulfide based nitrogen removal process overnitrate and nitrite for low, medium and high ammonium contaminated wastewaters. Results

51


Figure 3.9: Schematic representation of the sulfide based nitrogen removal process over nitrate(NO3

−, right) and over nitrite (NO2−, left). Percentages are expressed relative to the

amount of oxygen/ sulfide used in the nitrogen removal process over nitrate.

can be found in figure 3.10. The green zone represents the range in which sulfate can bepresent in municipal wastewater with a low sulfate concentration. The green zone representsthe range in which sulfate is present in wastewaters for weak, medium and strong sulfateconcentrations (table 2.6).

Figure 3.10: Stoichiometrically required sulfide for wastewater treatment with the sulfide basedprocess over nitrate and nitrate, for the removal of a low, medium and high ammoniumconcentration in municipal wastewater (20, 45 and 75 g N.m−3).

With nitrate as an electron acceptor, only 34 g S.m−3 sulfide is necessary for the removal of

52


a low amount of ammonium (figure 3.10, first purple dot). Consequently, enough sulfide isavailable in medium and strong wastewaters with a low sulfate concent for the treatment oflow ammonium concentrations with the sulfide based process over nitrate. For medium andhigh ammonium concentrations, the sulfide based process over nitrate cannot be used becausenot enough sulfide is present for complete nitrogen removal (figure 3.10, second and thirdpurple dot).

Low ammonium concentrations in weak, medium and strong wastewaters with a low sulfateconcentration can be treated with the sulfide based process over nitrite (figure 3.10, first greendot). Furthermore, also medium ammonium concentrations can be treated with the processover nitrate for stong sulfate wastewaters e.g. in coastal areas with some salt water intrusioninto the sewer system (figure 3.10, second green dot). For high ammonium concentrations notenough sulfide is present in the wastewater for complete nitrogen removal.

The use of nitrite instead of nitrate as an electron acceptor allows sulfide based denitrificationfor a wider range of wastewaters, because of the lower amount of required sulfide for theremoval of nitrogen. Whether it is achievable in practice still needs further research.

If not enough sulfide is present for full ammonium removal, another electron donor or a post–treatment will be necessary for further nitrogen removal. An example of another electron donoris COD, the presence of COD can partly compensate the weak autotrophic denitrification. Inthis way overall denitrification efficiency in sulfide limiting conditions can be increased (Wanget al., 2009; An et al., 2010) (section 1.3.3).

If the stoichiometrically required sulfide is higher than the available amount in the wastewaterwith a low sulfate concentration, not all sulfide is used to achieve complete nitrogen removal.In the sulfide based autotrophic denitrification nitrogen removal process, leftover sulfide isoxidized in the aerobic reactor or stripped from the reactor. In case of oxidation, problemssuch as odor, toxicity and corrosion are prevented (Buisman et al., 1991).

53


54

Chapter 4

General conclusions and

perspectives

4.1 General conclusion

The sulfide based nitrogen removal process consists out of two main reactors, a denitrification(anoxic) reactor and following a nitrification (aerobic) reactor, both reactors are connectedwith a nitrate recirculation flow. In the denitrification reactor the nitrate, resulting from thenitrification reactor, is denitrified with the use of sulfide as an electron donor. Prior to thesulfide based process, an anaerobic process provides the sulfide necessary for the denitrification.

Maximal nitrate removal is achieved by optimization of the recirculation flow between bothreactors and by using a sufficiently big volume for the anoxic reactor. In this thesis a calculationmethod was developed to determine this optimal recirculation ratio and the minimal volume ofthe denitrification reactor. Furthermore, the model allowed estimations of the effluent nitrateconcentration for a given recirculation ratio. The model complied with the expectations, forfurther application upgrades are suggested in section 4.2.

For the sulfide based treatment of wastewater with a low sulfate concentration, more nitratewas present in the effluent as for the treatment of saline wastewater. This was due to thelimited amount of available sulfide for denitrification in the wastewater with a low sulfateconcentration. If COD is present in the low sulfate containing wastewater, lower effluentnitrate concentrations can be reached due to this additional electron donor. Presence of CODdid not affect the effluent nitrate concentration in saline wastewaters.

For different wastewater characteristics, the minimal effluent nitrate concentrations weredetermined at the optimal recirculation ratio. The lowest minimal effluent concentrationswere achieved for wastewaters with a high sulfide concentration and/or low influent TKNconcentrations. For these wastewaters also the highest optimal recirculation ratios werefound, these high recirculation ratios are however not economically possible because theyexceed the practical limit of 5. Consequently, also the corresponding minimal effluent nitrateconcentrations can not be reached. Nevertheless, due to the steep decreasing slope for theeffluent nitrate concentration at low recirculation ratios, already very low effluent concentrationscan be reached for a recirculation ratio of 5. Therefore, if the optimal recirculation ratio

55

CHAPTER 4. GENERAL CONCLUSIONS AND PERSPECTIVES

exceeds the practical limit of 5, this practical limit can be used as the upper boundary valuewhile still low effluent nitrate concentrations can be obtained.

For wastewaters with a high TKN concentrations and a low sulfide concentration, only alimited amount of nitrate can be removed and consequently the minimal effluent nitrateconcentration is high. Since optimal recirculation ratios for these wastewaters are below thepractical limit of 5, optimal regulation is possible to make sure the applied recirculation ratiois not below or above the optimal ratio. In this way, although only high minimal effluentnitrate concentrations can be achieved, a maximal nitrogen removal can be aimed at.

The applicability of the sulfide based process was assessed for wastewaters with a low sulfateconcentration, as not every geographical place offers seawater or a sulfate rich water source.With the sulfide based process over nitrate, low ammonium concentrations can be removed inpart of the wastewaters with a low sulfate concentration. The process is not applicable formedium or high ammonium concentrations.

Furthermore, a novel sulfide based process over nitrite was put forward. In the process overnitrite denitritation and nitritation take place because nitrite instead of nitrate is used asan electron acceptor. This process needs 25% less aeration energy and 40% less sulfide fornitrogen removal in comparison with the process over nitrate. With the process over nitrate,low ammonium concentrations can be removed in all low sulfate containing wastewaters. Also,medium ammonium concentrations can be treated for part of the wastewaters with a lowsulfate concentration. The novel process over nitrite thus allows the treatment of a widerrange of wastewaters. Further studies should investigate whether or not this process overnitrite is achievable in practice.

4.2 Perspectives

For the application of the derived calculation method, a value was estimated for the effluentammonium concentration of the process. For the design of a process, this effluent value isunknown, because the wastewater is not yet subjected to treatment. Therefore further researchcan focus on how to determine this value, to be able to use the developed model for designprocedures.

In the model an estimation is made of the effluent nitrate concentration and the correspondingrecirculation ratio. To improve this estimation kinetic limitation should be included inthe model. Also the inclusion of COD is important to reach more accurate predictions.Furthermore, the effect of a changing oxygen concentration on the nitrification reaction wasin this thesis not considered. Changing oxygen concentrations could induce an effect onthe nitrification of ammonium because of which less or more nitrate could enter the reactor.This decrease or increase in nitrate production will have an impact on the effluent nitrateconcentration and is therefore also important to take into account.

Because there was presumed the calculated minimal volume was an overestimation of theapplied volume in real installations, kinetic limitation should be included in the calculationmethod. Kinetic limitation can be taken into account by estimation of the sulfide removalrate in case kinetic limitation takes place. For this, more research is necessary.

56


For the assessment of the applicability of the sulfide based process over nitrate and nitriteon wastewater with a low sulfate concentration, a 100% conversion of sulfate to sulfide wasassumed. However, during sulfate conversion into sulfide, stripping can occur because of whichpart of the sulfide is no more available in the wastewater. Besides the stripping of sulfide,some sulfate could be left in the water and thus not available for denitrification. Due toaforementioned reasons, less sulfide is available for denitrification. In further studies, this isimportant to take into account for application of the sulfide based process.

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58

Bibliography

Abeling, U. and Seyfried, C. F. (1992). Anaerobic-aerobic treatment of high-strength am-monium wastewater - nitrogen removal via nitrite. Water Science and Technology,26(5-6):1007–1015.

Adav, S. S., Lee, D. J., Show, K. Y., and Tay, J. H. (2008). Aerobic granular sludge: Recentadvances. Biotechnology Advances, 26(5):411–423.

Æsøy, A., Ødegaard, H., and Bentzen, G. (1998). The effect of sulphide and organic matter onthe nitrification activity in a biofilm process. Water Science and Technology, 37(1):115–122.

An, S., Tang, K., and Nemati, M. (2010). Simultaneous biodesulphurization and denitrificationusing an oil reservoir microbial culture: effects of sulphide loading rate and sulphide tonitrate loading ratio. Water Research, 44(5):1531–1541.

Batchelor, B. and Lawrence, A. W. (1978). Autotrophic denitrifucation using elemental sulfur.Journal of the Water Pollution Control Federation, 50(8):1986–2001.

Benson, B. B. and Krause, D. (1984). The concentration and isotopic fractionation of oxygendissolved in freshwater and seawater in equilibrium with the atmosphere1. Limnology and

Oceanography, 29(3):620–632.

Bentzen, G., Smit, A., Bennett, D., Webster, N., Reinholt, F., Sletholt, E., and Hobsont, J.(1995). Controlled dosing of nitrate for prevention of h2s in a sewer network and the effectson the subsequent treatment processes. Water Science and Technology, 31(7):293–302.

Bowden, G., Tsuchihashi, R., and Stensel, H. D. (2015). Technologies for Sidestream NitrogenRemoval. Technical report.

Buisman, C. J. N., Jspeert, P. I., Hof, A., Janssen, A. J. H., Hagen, R. T., and Lettinga, G.(1991). Kinetic parameters of a mixed culture oxidizing sulfide and sulfur with oxygen.Biotechnology and Bioengineering.

Cai, J., Zheng, P., and Mahmood, Q. (2008). Effect of sulfide to nitrate ratios on thesimultaneous anaerobic sulfide and nitrate removal. Bioresource Technology, 99(13):5520–5527.

Campos, J., Carvalho, S., Portela, R., Mosquera-Corral, A., and Méndez, R. (2008). Kinetics ofdenitrification using sulphur compounds: effects of S/N ratio, endogenous and exogenouscompounds. Bioresource Technology, 99(5):1293–1299.

59

BIBLIOGRAPHY

Cardoso, R. B., Sierra-Alvarez, R., Rowlette, P., Flores, E. R., Gomez, J., and Field, J. A.(2006). Sulfide oxidation under chemolithoautotrophic denitrifying conditions. Biotech-nology and Bioengineering, 95(6):1148–1157.

Claus, G. and Kutzner, H. (1985a). Autotrophic denitrification by Thiobacillus denitrificansin a packed bed reactor. Applied Microbiology and Biotechnology, 22(4):289–296.

Claus, G. and Kutzner, H. (1985b). Physiology and kinetics of autotrophic denitrification byThiobacillus denitrificans. Applied Microbiology and Biotechnology, 22(4):283–288.

Comeau, Y. (2008). Microbial metabolism. In Henze, M., van Loosdrecht, M., Ekama, G.,and Brdjanovic, D., editors, Biological wastewater treatment: principles, modelling and

design, chapter 2, pages 9–32. IWA Publishing.

Drainage Services Department (WSD) (2016). Seawater for flushing. http:

//www.dsd.gov.hk/EN/Sewerage/Sewage_Treatment_Facilities/Type_of_Sewage_

Treatment_Facilities/index.html, Accessed: 2016-08-03.

Driscoll, C. T. and Bisogni, J. J. (1978). The use of sulfur and sulfide in packed bed reactorsfor autotrophic denitrification. Journal (Water Pollution Control Federation), pages569–577.

European Commission (EC) (2002). Report from the commission: Implementation of councildirective 91/676/eec concerning the protection of waters against pollution caused bynitrates from agricultural sources – synthesis from year 2000 member states reports.

Fajardo, C., Mora, M., Fernández, I., Mosquera-Corral, A., Campos, J. L., and Méndez, R.(2014). Cross effect of temperature, pH and free ammonia on autotrophic denitrificationprocess with sulphide as electron donor. Chemosphere, 97:10–15.

Gadekar, S., Nemati, M., and Hill, G. A. (2006). Batch and continuous biooxidation ofsulphide by Thiomicrospira sp. CVO: Reaction kinetics and stoichiometry. Water Research,40(12):2436–2446.

Gu, J.-D., Qiu, W., Koenig, A., and Fan, Y. (2004). Removal of high NO3- concentrations in

saline water through autotrophic denitrification by the bacterium Thiobacillus denitrificansstrain mp. Water Science and Technology, 49(5-6):105–112.

Hao, T., Wei, L., Lu, H., Chui, H., Mackey, H. R., van Loosdrecht, M. C. M., and Chen, G.(2013). Characterization of sulfate-reducing granular sludge in the SANI® process. Water

Research, 47(19):7042–7052.

Hellinga, C., Schellen, A., Mulder, J., van Loosdrecht, M. C. M., and Heijnen, J. (1998). Thesharon process: An innovative method for nitrogen removal from ammonium-rich wastewater. Water Science and Technology, 37(9):135–142.

Henze, M. (2008a). Modelling biofilms. In Mogens Henze, Mark C. M. van Loosdrecht, G.A. E. D. B., editor, Biological wastewater treatment: principles, modelling and design,chapter 17, pages 457–492. IWA Publishing.

Henze, M. (2008b). Nitrogen removal. In Ekama, G. A. and Wentzel, M. C., editors, Biologicalwastewater treatment: principles, modelling and design, chapter 5, pages 87–138. IWAPublishing.

60

http://www.dsd.gov.hk/EN/Sewerage/Sewage_Treatment_Facilities/Type_of_Sewage_Treatment_Facilities/index.html



BIBLIOGRAPHY

Henze, M. (2008c). Organic material removal. In Henze, M., van Loosdrecht, M. C. M.,Ekama, G. A., and Brdjanovic, D., editors, Biological wastewater treatment: principles,

modelling and design, chapter 4, pages 53–86. IWA Publishing.

Henze, M. (2008d). Wastewater characterization. In Henze, M. and Comeau, Y., editors,Biological wastewater treatment: principles, modelling and design, chapter 3, pages 33–52.IWA Publishing.

Henze, M., Grady Jr, C., Gujer, W., Marais, G., and Matsuo, T. (1987). Activated sludgemodel no. 1: Iawprc scientific and technical report no. 1. IAWPRC, London.

Hilton, M. G. and Archer, D. B. (1988). Anaerobic digestion of a sulfate-rich molasseswastewater: Inhibition of hydrogen sulfide production. Biotechnology and Bioengineering,31(8):885–888.

Isa, Z., Grusenmeyer, S., Verstraete, W., and Grusenmeyer, S. (1986). Sulfate ReductionRelative to Methane Production in High-Rate Anaerobic Digestion : Technical AspectsSulfate Reduction Relative to Methane Production in High-Rate Anaerobic Digestion :Microbiological Aspects. 51(3):580–587.

Jing, C., Ping, Z., and Mahmood, Q. (2010). Influence of various nitrogenous electron acceptorson the anaerobic sulfide oxidation. Bioresource Technology, 101(9):2931–2937.

Khin, T. and Annachhatre, A. P. (2004). Novel microbial nitrogen removal processes. Biotech-nology Advances, 22(7):519–532.

Kim, I., Oh, S., Bum, M., Lee, J., and Lee, S. (2002). Monitoring the denitrification ofwastewater containing high concentrations of nitrate with methanol in a sulfur-packedreactor. Applied Microbiology and Biotechnology, 59(1):91–96.

Kleerebezem, R. and Mendez, R. (2002). Autotrophic denitrification for combined hydrogensulfide removal from biogas and post-denitrification. Water Science and Technology,45(10):349–356.

Koenig, A. and Liu, L. (1996). Autotrophic denitrification of landfill leachate using elementalsulphur. Water Science and Technology, 34(5-6):469–476.

Koenig, A. and Liu, L. H. (2001). Kinetic model of autotrophic denitrification in sulphurpacked-bed reactors. Water Research, 35(8):1969–1978.

Krekeler, D. and Cypionka, H. (1995). The preferred electron acceptor of Desulfovibriodesulfiricans CSN. FEMS Microbiology Ecology, 17:271–278.

Krishnakumar, B. and Manilal, V. B. (1999). Bacterial oxidation of sulphide under denitrifyingconditions. Biotechnology Letters, 21(5):437–440.

Kuenen, J. G. (1979). Growth yields and “maintenance energy requirement" in Thiobacillusspecies under energy limitation. Archives of Microbiology, 122(2):183–188.

Lau, G. N., Sharma, K. R., Chen, G. H., and van Loosdrecht, M. C. M. (2006). Integration ofsulphate reduction, autotrophic denitrification and nitrification to achieve low-cost excesssludge minimisation for Hong Kong sewage. Water Science and Technology, 53(3):227–235.

61

BIBLIOGRAPHY

Liu, Y., Xu, H. L., Show, K. Y., and Tay, J. H. (2002). Anaerobic granulation technology forwastewater treatment. World Journal of Microbiology and Biotechnology, 18(2):99–113.

Lu, H., Ekama, G. A., Wu, D., Feng, J., van Loosdrecht, M. C. M., and Chen, G. H. (2012a).SANI process realizes sustainable saline sewage treatment: steady state model-basedevaluation of the pilot-scale trial of the process. Water Research, 46(2):475–490.

Lu, H., Wang, J., Li, S., Chen, G. H., van Loosdrecht, M. C. M., and Ekama, G. A. (2009).Steady-state model-based evaluation of sulfate reduction, autotrophic denitrification andnitrification integrated (SANI) process. Water Research, 43(14):3613–3621.

Lu, H., Wu, D., Jiang, F., Ekama, G. A., van Loosdrecht, M. C. M., and Chen, G. H.(2012b). The demonstration of a novel sulfur cycle-based wastewater treatment process:sulfate reduction, autotrophic denitrification, and nitrification integrated (SANI) biologicalnitrogen removal process. Biotechnology and Bioengineering, 109(11):2778–2789.

Lu, H., Wu, D., Tang, D. T. W., Chen, G. H., van Loosdrecht, M. C. M., and Ekama, G.(2011). Pilot scale evaluation of SANI® process for sludge minimization and greenhouse gasreduction in saline sewage treatment. Water Science and Technology, 63(10):2149–2154.

Mahmood, Q., Zheng, P., Cai, J., Wu, D., Hu, B., Islam, E., and Rashid Azim, M. (2007).Comparison of anoxic sulfide biooxidation using nitrate/nitrite as electron acceptor.Environmental progress, 26(2):169–177.

Marais, G. and Ekama, G. (1976). The activated sludge process part I-steady state behaviour.Water south africa, 2(4):164–200.

Matin, A. (1978). Organic nutrition of chemolithotrophic bacteria. Annual Reviews in

Microbiology, 32(1):433–468.

Metcalf and Eddy (1991). Wastewater engineering: treatment, disposal, and reuse. McGraw-

Hill, New York.

Moraes, B., Souza, T., and Foresti, E. (2012). Effect of sulfide concentration on autotrophicdenitrification from nitrate and nitrite in vertical fixed-bed reactors. Process Biochemistry,47(9):1395–1401.

Münch, E. V., Lant, P., and Keller, J. (1996). Simultaneous nitrification and denitrification inbench-scale sequencing batch reactors. Water Research.

Oh, S., Yoo, Y., Young, J., and Kim, I. (2001). Effect of organics on sulfur-utilizing autotrophicdenitrification under mixotrophic conditions. Journal of Biotechnology, 92(1):1–8.

Oh, S. E., Bum, M. S., Yoo, Y. B., Zubair, A., and Kim, I. S. (2003). Nitrate removal bysimultaneous sulfur utilizing autotrophic and heterotrophic denitrification under differentorganics and alkalinity conditions: batch experiments. Water Science and Technology,47(1):237–244.

Pochana, K. and Keller, J. (1999). Study of factors affecting simultaneous nitrification anddenitrification (SND). Water Science and Technology, 39(6):61–68.

Sahinkaya, E., Dursun, N., Kilic, A., Demirel, S., Uyanik, S., and Cinar, O. (2011). Simulta-neous heterotrophic and sulfur-oxidizing autotrophic denitrification process for drinkingwater treatment: control of sulfate production. Water Research, 45(20):6661–6667.

62

BIBLIOGRAPHY

Schmidt, I., Sliekers, O., Schmid, M., Bock, E., Fuerst, J., Kuenen, J., Jetten, M. S., andStrous, M. (2003). New concepts of microbial treatment processes for the nitrogenremoval in wastewater. FEMS Microbiology Reviews, 27(4):481–492.

Schweigkofler, M. and Niessner, R. (2001). Removal of siloxanes in biogases. Journal of

Hazardous Materials, 83(3):183–196.

Seker, S., Beyenal, H., and Tanyolaç, A. (1995). The effects of biofilm thickness on biofilmdensity and substrate consumption rate in a differential fluidizied bed biofilm reactor(DFBBR). Journal of Biotechnology, 41(1):39–47.

Sierra-Alvarez, R., Beristain-Cardoso, R., Salazar, M., Gómez, J., Razo-Flores, E., and Field,J. A. (2007). Chemolithotrophic denitrification with elemental sulfur for groundwatertreatment. Water Research, 41(6):1253–1262.

Standard methods (1992). Standard methods for the examination of water and wastewater.American Water Works Association. chapter 29, http://www.mwa.co.th/download/

file_upload/SMWW_1000-3000.pdf, Accessed: 2016-12-28.

Stenstrom, M. K. and Poduska, R. A. (1980). The effect of dissolved oxygen concentration onnitrification. Water Research, 14(6):643–649.

Strous, M., Heijnen, J. J., Kuenen, J. G., and Jetten, M. S. M. (1998). The sequencing batchreactor as a powerful tool for the study of slowly growing anaerobic ammonium-oxidizingmicroorganisms. Applied microbiology and biotechnology, 50(5):589–596.

Tang, S. (2000). Dual water supply in Hong Kong. WEDC Conference, pages 364–366.

The MathWorks, Inc. (2000). Matlab. Natick, MA, http://nl.mathworks.com/?s_tid=gn_

logo.

Tsang, W. L., Wang, J., Lu, H., Li, S., Chen, G. H., and Van Loosdrecht, M. C. M. (2009). Anovel sludge minimized biological nitrogen removal process for saline sewage treatment.Water Science and Technology, 59(10):1893–1899.

Van de Graaf, A., Mulder, A., Slijkhuis, H., Robertson, L., and Kuenen, J. (1990). AnoxicAmmonium Oxidation. In Proceedings of the fifth european congress on biotechnology. 1,pages 388–391.

van den Brand, T. P. H., Roest, K., Chen, G. H., Brdjanovic, D., and van Loosdrecht,M. C. M. (2015). Potential for beneficial application of sulfate reducing bacteria insulfate containing domestic wastewater treatment. World Journal of Microbiology and

Biotechnology, 31(11):1675–1681.

van Rijn, J., Tal, Y., and Schreier, H. J. (2006). Denitrification in recirculating systems:theory and applications. Aquacultural Engineering, 34(3):364–376.

Wang, J., Lu, H., Chen, G. H., Lau, G. N., Tsang, W. L., and van Loosdrecht, M. C. M.(2009). A novel sulfate reduction, autotrophic denitrification, nitrification integrated(SANI) process for saline wastewater treatment. Water Research, 43(9):2363–2372.

Water Supplies Department (WSD) (2016). Type of sewage treatment facilities.http://www.wsd.gov.hk/en/water_resources/water_treatment_and_distribution_

process/seawater_for_flushing/index.html, Accessed: 2017-04-15.

63

http://www.mwa.co.th/download/file_upload/SMWW_1000-3000.pdf

http://www.mwa.co.th/download/file_upload/SMWW_1000-3000.pdf

http://nl.mathworks.com/?s_tid=gn_logo

http://nl.mathworks.com/?s_tid=gn_logo

http://www.wsd.gov.hk/en/water_resources/water_treatment_and_distribution_process/seawater_for_flushing/index.html

http://www.wsd.gov.hk/en/water_resources/water_treatment_and_distribution_process/seawater_for_flushing/index.html

BIBLIOGRAPHY

Welander, U. and Mattiasson, B. (2003). Denitrification at low temperatures using a suspendedcarrier biofilm process. Water Research, 37(10):2394–2398.

World Health Organization (WHO) (n.d.). Drink water. http://www.who.int/topics/

drinking_water/en/, Accessed: 2016-08-03.

Wu, D., Ekama, G. A., Chui, H.-K., Wang, B., Cui, Y.-X., Hao, T.-W., van Loosdrecht, M. C.,and Chen, G.-H. (2016). Large-scale demonstration of the sulfate reduction autotrophicdenitrification nitrification integrated (SANI®) process in saline sewage treatment. Water

Research, 100:496–507.

Yang, P. and Zhang, Z. (1995). Nitrification and denitrification in the wastewater treatmentsystem. In Ishizuka, K., Hisajima, S., and Macer, D. R., editors, Traditional technologyfor environmental conservation and sustainable development in the asian-pacific region,chapter 4, pages 145–158. Master’s Program in Environmental Science and Master’sProgram in Biosystem Studies, University of Tsukuba.

Yang, W., Lu, H., Khanal, S. K., Zhao, Q., Meng, L., and Chen, G.-H. (2016). Granulation ofsulfur-oxidizing bacteria for autotrophic denitrification. Water Research, 104:507–519.

Zhang, T. C. and Lampe, D. G. (1999). Sulfur:limestone autotrophic denitrification pro-cesses for treatment of nitrate-contaminated water: batch experiments. Water Research,33(3):599–608.

64

http://www.who.int/topics/drinking_water/en/

http://www.who.int/topics/drinking_water/en/

Appendix A

Appendix

A.1 Stoichiometric equations of sulfur based nitrogen removal

As an example the stoichiometric equations in case sulfide is used as an electron donor will beexplained. In a similar way equations for thiosulfate or elemental sulfur as an electron donorcan be found. Furthermore, nitrate is assumed to be the electron acceptor.

Denitrification

The autotrophic denitrification reaction consists of two main processes: first the actualreduction of nitrate into nitrogen gas (eq. A.1) and secondly the inclusion of anorganic carbonin biomass (CH1.8O0.5N0.2) (eq. A.2). Equations of both individual processes can be deductedby balancing the reduction–oxidation reactions. Sulfide is assumed to be the electron donorfor biomass production.

0.625 HS− + NO−3 + 0.375 H+ → 0.625 SO2−

4 + 0.5 N2 + 0.5 H2O (A.1)

0.2 NH+4 + HCO−

3 + 0.275 H+ + 0.525 HS− → CH1.8O0.5N0.2 + 0.4 H2O + 0.525 SO2−4 (A.2)

The link between both equations is given by the yield coefficient (YDN ). Most commonly forsulfur based denitrification, the yield coefficient is expressed in g VSS.(g NO3_N)−1. For eachgram reduced nitrate, a certain amount of biomass (VSS) is produced. Because aforementionedequations are expressed in mol, also the yield coefficient should be converted to mol VSS.(molNO3)−1. Per mol of nitrate reduced in the denitrification, yield moles of biomass are produced(eq. A.1 + YDN* (eq. A.2)). Or in other words, per mol of biomass produced, mol times nitratedivided by the yield is denitrified ((eq. A.1)/YDN + eq. A.2). This last way of reasoning isused to construct the overall equation (eq. A.3), because in this way the biomass in the overallequation is equal to one mol.

(0.625YDN

+ 0.525) HS- + 1YDN

NO3- + (0.375

YDN+ 0.275) H+ + 0.2 NH+

4 + HCO−3

→ (0.625YDN

+ 0.525) SO42- + 0.5

YDNN2 + CH1.8O0.5N0.2 + ( 0.5

YDN+ 0.4) H2O (A.3)

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APPENDIX A. APPENDIX

Suppose YDN is equal to 0.57 g VSS.(g NO3_N)−1 (Claus and Kutzner, 1985b), converted tomol units this becomes 0.324 mol VSS.(mol NO3)−1 (calculation below). After implementingthe yield coefficient into equation A.3, equation 1.3 is obtained.

0.57 g VSS.(g NO3_N)−1 ∗ 14.01 g NO3_N.mol NO3−1

24.6 g VSS.mol−1 = 0.324 mol VSS.(mol NO3)−1

Nitrification

The derivation of the nitrification reaction is very similar to the one of the denitrification. Thetwo main processes of the nitrification reaction are the oxidation of ammonium into nitrate(eq. 1.1) and the inclusion of anorganic carbon in biomass (eq. A.4). Both equations can bededucted by balancing the reduction–oxidation reactions. Ammonium is assumed to be theelectron donor for biomass production.

0.73 NH+4 + CO2 + 0.07 H2O→ CH1.8O0.5N0.2 + 0.53 NO−

3 + 1.25 H+ (A.4)

The yield coefficient (YN ) is most commonly expressed in g VSS.(g FSA_N)−1. Afterconversion of the yield coefficient into mol units, both equations 1.1 & A.4 can be combinedin a similar way as for denitrification ((eq. 1.1)/YN+eq. A.4). In this way, equation A.5 isachieved.

( 1YN

+ 0.725) NH+4 + CO2 + 2

YNO2

→ CH1.8O0.5N0.2 + ( 1YN

+ 0.525) NO−3 + ( 2

YN+ 1.25) H+ + ( 1

YN− 0.075) H2O (A.5)

Suppose YN is equal to 0.10 g VSS.(g FSA_N)−1 (Claus and Kutzner, 1985b), convertedto mol units this becomes 0.058 mol VSS.(mol N)−1 (calculation similar to YDN ). Afterimplementing the yield into equation A.3, equation A.6 is obtained.

17.97 NH+4 + CO2 + 34.48 O2

→ CH1.8O0.5N0.2 + 17.77 NO−3 + 35.73 H+ + 17.32 H2O (A.6)

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