overview of treatment technology in organic waste removal

7/26/2019 Overview of Treatment Technology in Organic Waste Removal

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Introduction

Organic matters refer to degradable carbon based compounds. In water and wastewater treatment, organicmatter is one of the major pollutants. Organic compounds, such as PCBs, pesticides and herbicides, polycylic

aromatic hydrocarbons (PAHs), aliphatic and heterocyclic compounds are often prevalent in wastewater and

can compromise the safety of people as well as the environment.

The amount of organic compounds in wastewater is generally evaluated by the chemical oxygendemand (COD) test and the biological oxygen demand (BOD) test. The basis for the COD test is that nearly

all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidicconditions. The COD value is always measured using acidic potassium permanganate and potassium

dichromate and could reflect the pollution degree of reducing matter in water including ammonia and reducing

sulfide. Thus, COD value often overestimate organic pollutants in wastewater with high quantity of organic

matter. BOD value is the amount of dissolved oxygen needed by aerobic biological organisms in a body of

water to break down organic material present in a given water sample at certain temperature over a specific

time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of

sample during 5 days of incubation at 20 C. This is not a precise quantitative test, although it is widely used

as an indication of the organic quality of water. Both COD and BOD test could quickly reflect the organicpollution in the wastewater although they cannot reflect the types of organic matter and composition of the

water and therefore cannot reflect the total amount of the same total organic carbon pollution caused bydifferent consequences.

Due to the prevalence of organic matter in water and wastewater sources, many different technologies

have been developed throughout the years dedicated to the removal of organic pollutants. Some of these

technologies was invented as early as the beginning of 19th century. These technologies are listed under past

technologies in this review report. Although developed in the past, these technologies are still used in certain

parts of the world. Since then, newer technologies have been developed and are slowly replacing past

technologies. These technologies are listed as present technologies. There is also a section for future

technologies which reviews advanced removal techniques are still being researched and developed.

Review and Evaluation

Past Technologies

1.

Constructed Wetlands

Natural wetlands are land areas saturated with water and certain characteristic vegetation of aquatic plants and

various microorganisms. Constructed wetlands are specifically engineered systems that contains a built,

controlled and sustained wetland environment designed to treat wastewater by utilising the natural processes

carried out by the vegetation and microorganisms present in the wetland ecosystem (Vymazal, 2014).

Although the construction of wetlands requires large plots of land, energy consumption as well as operationalrequirements are relatively lower than compared to other wastewater treatment technologies (Wu et al., 2014).

1.1Types and Classifications

According to Wu et al (2014), constructed wetlands (CWs) can be largely classified into three categories

namely traditional CWs, hybrid CWs and enhanced CWs. The diagram below illustrates the types and

classifications of constructed wetlands. As a limiting scope, enhanced CWs will not be discussed in this report.


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Figure 1.1: The Classification of CWs Used in Wastewater Treatment (Wu et al, 2014).

1.1.1 Traditional CWs

As seen in figure 1.1, traditional CWs can be grouped into free water surface CWs (FWS CWs) and subsurfaceCWs which are further divided into horizontal flow CWs (HF CWs) and vertical flow CWs (VF CWs). Free

water surface constructed wetlands, as the name suggests, consists of a relatively shallow depth of water with

a free surface and its vegetation partially or totally submerged in the water. The vegetation are supported by alayer of substrate containing soil situated at the bottom of the wetland (Wu et al, 2014). According to Vymazal

(2014), the existence of a free water surface allows for the diffusion of oxygen gas into the water forming adecreasing oxygen concentration gradient down the water depth. This give rise to the major presence of

aerobic microorganisms near the free surface as well as a small population of anaerobic microorganisms near

the bottom of the wetland where little to no oxygen molecules are present.

Wastewater enters the wetland horizontally along and through the water channel. Sufficiently large

and heavy organic waste will settle and removed passively by deposition and filtration. Both attached and

suspended microbes in the wetlands will degrade and oxidise the organic waste aerobically and anaerobically

though aerobic oxidisation will be dominant to the large amount of oxygen dissolved in the water. Coverage

provided by the submerged vegetation prevents the penetration of sunlight into the wetlands and thus limiting

the growth of algae. Besides the removal of organic carbon compounds, nitrogenous compounds are alsoremoved through nitrification and denitrification processes. The availability of dissolved oxygen allows for

efficient nitrification by aerobic microbes followed by denitrification by anaerobic microbes resulting in an

effective removal of nitrogen. Phosphorus in FWF CW systems are removed through adsorption and

absorption by surrounding soil though less effective compared to the removal of nitrogen due to the minimal

contact between soil and water (Vymazal, 2014).

In subsurface (SSF) CWs, the vegetation planted on the substrate containing soil is located above the

water surface of the wetland. This eliminates the presence of a free water surface and wastewater enters the

wetland through the porous medium under the surface of the soil supporting the vegetation (Zhang et al, 2014).

Due to the configuration of SSF CWs which does not have a free water surface available for the diffusion of

oxygen, a smaller amount of aerobic microorganisms will be present compared to anoxic and anaerobic ones,

each separated into their own zones are present in the ecosystem. Aerobic microorganisms are usually present

near the roots and rhizomes of the vegetation where dissolved oxygen is plentiful while anoxic and anaerobic


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microbial can be found near the bottom of the wetland. There are two types of SSF-CWs namely the horizontal

subsurface flow CW (HSSF-CW) and vertical subsurface flow CW (VSSF-CW) as shown in the diagrambelow.

Figure 1.2: Structures of Constructed Wetlands. (a) SF-CW; (b) HSSF-CW; (c) VSSF-CW (Li et al.,

2014).

1.1.2 Hybrid CWs

In the previous section, it is evident that each CWs have their own strengths and weaknesses in terms of

efficiency in removing different types of waste. Thus, it is a common practice to link several two different

CW systems together in series or parallel to enhance the wastewater treatment performance. Common

combinations include VF-HF CWs, HF-VF CWs, HF-FWS CWs and FWS-HF CWs. There are also multistage

CW systems which combine more than two CWs in series or parallel as mentioned in figure 1.1 (Wu et al.,

2014).

1.2Design and Operation Parameters

There are several criteria and factors that need to be considered in the design, construction and operation of aCW. One of the more trivial factor to account for is the location or site of the wetland as this will in turn

determine the type of soil and vegetation available as well as any variations in seasons or weather which canthreaten the survival of the vegetation and microbial community. Other main parameters concerning the CW

itself include the type of vegetation (plants) and substrate, depth of water, hydraulic load and retention time

(HRT) and influent feeding mode i.e. batch, continuous or intermittent (Wu et al., 2014).

1.2.1 Plant Selection

Plant vegetated on substrates in wetlands play a major role in removing nitrogen and phosphorus compounds

as well as other toxic pollutants such as antibiotics and heavy metals. Thus, it is important to select species of

plants that have a large capacity in absorbing pollutants and a high tolerance towards wastewater of highstrength. Therefore, the choice of plants used is also affected by the type of wastewater to be treated as

wastewater from different sources will contain different kinds of pollutants. For example, agricultural wastewhich contains high amount of ammonia requires plants with high tolerance towards ammonia such asZornia

latifolia. Highly saline water are tolerated by species such asArundo donax and Sarcocornia fruticosawhich

are very effective in removing organics, nitrogen and phosphorus.(Wu et al., 2014).


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1.2.2 Substrate Selection

Similar to the function of the plants as wetland vegetation, the substrate also plays a role in absorbing certain

pollutants while providing a medium for the growth of plants and facilitating the movement of wastewater. A

good substrate should be able to allow wastewater to freely percolate through it in VSSF CWs as well as a

high pollutant absorption capacity. Typical natural materials used as a substrate include sand, gravel, clay and

peat while artificial materials such as activated carbon and compost are also suitable (Wu et al., 2014).

1.2.3 Hydraulic Load and Retention Time

Hydraulic loading rate (HLR) is a quantitative indicator of the flow rate of wastewater entering into the CWwhile hydraulic retention time (HRT) indicates the amount of time the wastewater is retained inside the

wetlands. There two parameters are consistently monitored throughout the operation of the CWs. A high HLR

will cause the value of the HRT to decrease as wastewater are rapidly forced out of the wetlands due to large

amount of influent wastewater. On the other hand, a larger HRT value is usually preferred as a longer period

of retention time will ensure a more complete removal of pollutants by the microorganisms, substrates and

plants in the wetlands (Zhang et. al, 2014). However, this implies that a larger land area and cost will be

needed enlarging the area of the wetland will directly increases the HRT value. Therefore, an optimisationpoint between these two variables must be determined to enhance the performance of wetlands.

1.2.4 Influent Feeding Mode

Like many other wastewater treatment technologies, CWs can be operated in a number of different modes

namely batch, intermittent and continuous. The usage of different influent feeding modes will affect the

oxygen transfer and hence the oxidation-reduction conditions in wetlands. Overall, batch operations

performed better than continuous operations as they provide more oxidised conditions. Also, it has been

showed that a higher ammonium removal can be achieved in batch systems than in continuous systems (Zhang

et al., 2012). A similar result is observed in intermittent mode CWs where COD and nitrogenous compoundremoval are both higher than continuous mode CWs (Saeed & Sun, 2012).

1.3Removal of Organics

In terms of classification, wetlands can be considered as a biological secondary treatment of wastewater. Dueto its rich content of microorganisms and absorbing plants and substrates, wetlands are popularly used to

remove organics in terms of BOD as well as nutrients such as nitrogen and phosphorus compounds. Thefollowing diagram shows the efficiency in the removal of BOD as well as other pollutants.


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Figure 1.3: Average Removal of Pollutants in Various CW Systems (Zhang et al., 2014)

From the figure above, it can be seen that CWs are most effective in removing BOD and COD as alltypes of systems plot a higher bar in average removal of BOD and COD compared to total suspended solids

(TSS), total phosphorus (TP) and nitrogen. Wetlands are also preferred in areas with large lands and highamount of vegetation. Although the wetland treatment technology is less energy intensive and more

environmentally friendly, it is unsuitable in treating large continuous amount of wastewater and is limited to

treating relatively low strength wastewater.

Present Technologies

2. Submerged Membrane Bioreactor (SMBR)

Membrane Bioreactor (MBR) can be evaluated as one of the most promising technology for water and

wastewater treatment especially the submerged (SMBR) which uses both physical and biological processes in

wastewater organic removal. SMBR has been acquiring attention on wastewater treatment for its preferable

effluent quality and lower production of sludge.

There are mainly three types of MBR models. The first is the biomass kinetic model. It is mainly based

on activated sludge models (ASMs) where they were modified to account for the formation and degradation

of soluble microbial product (SMPs) in the membrane. The second is membrane fouling. This MBR is based

on solid-liquid separation and the simulation filtration processes as ideal settler with unitary efficiency on

fouling phenomenon. The third category considers the formation and degradation of SMP and integrated with

the first and second models.

2.1 Membrane Module Characteristics and Comparison with an Activated Sludge System

The MBR membrane can be placed either inside or outside the bioreactor. Membranes placed inside thebioreactor is known as SMBR while outside the bioreactor is known as external membrane bioreactor. In

municipal applications, hollow fibre SMBR configuration would be useful for medium to large size

wastewater treatment plants. For a smaller plant size, plate and frame technologies would have more

advantages where it could be designed with secondary or tertiary treatment followed by membrane filtration


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and ultrafiltration. In SMBR configurations, membrane in aeration basin acts as filters and improve the

biological treatment of effluent. The permeate flux is removed by suction at around 0.5 pressure bar. Whilefor external configuration, the amount of permeate flux varies and the pressure bar in the range of 14 bar.

Thus, the submerged configuration appears to be more economical based on energy consumption.

The following are the advantages of SMBR over the traditional activated sludge system.

i) Energy consumption and capital cost for fabrication and maintenance are lower for submerged

system.

ii) The traditional secondary clarifier is replaced by a membrane module which is better in compact and

quality of rejected water is independent of the variations of sludge settling velocity.

iii)

The process can be run at higher solid retention time, which favour the development of slow growing

micro-organism and ultimately lead to better removal of refractory organic matter and making the

system robust to load variation and toxic shock.

The figure and table below show the pilot plant layout as well as the main features of the membrane and the

general operating conditions respectively.

Figure 2.1: A Zenon Zeeweed (ZW 10) Pilot Plant in Palermo, Italy Fed with Municipal Wastewaterfrom the Acqua dei Corsari (Palermo) WWTP.

Table 2.1: General Operational Parameters of a Pilot MBR

Parameter Unit Average Minimum Maximum

HRT h 11.9 11.4 12.4

VLR kg COD m3day1 1.1 1 1.2

Permeate flux L m2h1 21.5 20.5 22

Suction period min 9

Backwash period min 1 O.D. mg L1 3.9 0.5 7.7

Temperature C 22 15 30

pH 8 7.3 8.2


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2.2

Operation Process

Pre-screen of wastewater through a 2 millimetres screen is used to remove small substances debris, hair, rags,

sand and others. It is then enter the aeration tank where organic matter, phosphorus and nitrogen were

removed. The permeate was draw from the membrane module and pass through in a tank and the concentrated

sludge was remain in the aeration tank.

The SMBR influent was stored in a tank (volume of 1500 L) where the influent was diluted in order

to maintain a constant influent COD concentration to operate with a constant volumetric loading rate (VLR)

of 1.1 Kg COD m3day1and an average hydraulic retention time (HRT) of 11.9 h. Table 2 shows the influent

characteristics.

The permeate was extracted by imposing an average flux of 21.5 L m2h1on the membrane surface

and a maximum negative pressure of 0.3 bar. In particular, the membrane was periodically backwashed (every

9 min) by pumping a fraction of permeate back through the membrane for a period of 1 min. The effluent

effective mean flux resulted of 19.3 L m2h1. More specifically, the bioreactor was indicated by an operatingvolume of approximately 190 L and it was equipped with a level controller and a pump to regulate the feeding

and to discharge the effluent, respectively.

Table 2.2 Influent Characteristics

COD

(mg/L)

BOD

(mg/L)

TSS

(mg/L)

VSS

(mg/L)

NNH4(mg/L)

NNO2(mg/L)

NNO3

(mg/L)

NNTOT

(mg/L)

Average 510 255 274 262 35.8 0.07 0.78 53.9

Minimum 310 150 122 180 18.5


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2.3 Membrane Modules Fouling

Even with the expansion in physical and biological models improve the wastewater management, fouling

mechanisms is still a complication in wastewater treatment. Fouling in MBR can be internal as blocking in

pore of membrane or externally deposition of cake for both organic and inorganic substances.

Chemical nature of membrane and the membrane operational parameters such as bubbling and suctionare the influences of membrane fouling. Hollow-fibre microfiltration membrane induces transmembrane

pressure gradient that will have an impact on flux rates. The flux depends on the design of hollow-fiber and

also the properties of the cake. The structure of the membrane pores also play an important role on fouling,

the rougher the surface of the membrane, the faster the fouling by attachment of colloid and particulate matters

on the membrane surface.

The limiting factor for further process development is membrane fouling resulting are the:

i. Formation of a layer or cake on the membrane or the intrusion of the molecules, colloids and particles

in the porous structureii.

Preferential adsorption on the membrane surface. Fouling induces transmembrane flux reduction,

when the flux is increased by biological activity, and the progression in this field is relatively slow.

3. Adsorption Technology

Adsorption technology has gained importance as an effective purification and separation method in water and

wastewater treatment over the last few decades. It provides no sludge handling problem and discharges a high

quality effluent. Moreover, it has advantages over the other methods because of simple design and can involve

low investment in term of both initial cost and land required. Hence, the adsorption technology is extensively

used for treatment of industrial wastewater from organic and inorganic pollutants and meet the great attention

from the researchers. Latterly, the exploration for low-cost adsorbents that have pollutantbinding capacitieshas escalated. Materials locally available likes natural materials, agricultural wastes and industrial wastes can

be utilized as low-cost adsorbents. Activated carbon manufactured from these materials can be used as

adsorbent for wastewater treatment (Crini, 2005).

3.1

Adsorption Phenomenon

Adsorption is a surface phenomenon with conventional mechanism for both organic and inorganic pollutants

removal in water and wastewater treatment. It take place when a solution containing absorbable solute comesinto contact with a solid having a highly porous surface structure. The liquid-solid intermolecular forces of

attraction cause some of the solute molecules from the solution to be concentrated and deposited at the solidsurface. The solute which retained on the solid surface in adsorption processes is called adsorbate, while, the

solid on which it is retained is called as an adsorbent. This surface accumulation of adsorbate on adsorbent is

called adsorption. At last, the creation of an adsorbed phase with a composition different from that of the bulk

fluid phase forms the cornerstone of separation by adsorption technology.

In a bulk material, all the bonding requirements such as ionic, covalent, or metallic of the constituentatoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent

are not wholly surrounded by other adsorbent atoms and hence they can attract adsorbates. In fact, the exactnature of the bonding depends on the details of the species involved. But, in general, the adsorption process

can be classified as physisorption (characteristic of weak Van Der Waals forces) and chemisorption(characteristic of covalent bonding). Somehow, it may also occur due to electrostatic attraction. Besides,

adsorption is nearly always an exothermic process.


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Physical adsorption takes place quickly and may be mono-molecular (unimolecular) layer or

monolayer, or 2, 3 or more layers thick (multi-molecular). As physical adsorption occurs, it begins as amonolayer. It can then become multi-layer, and then, if the pores are close to the size of the molecules, more

adsorption occurs until the pores are filled with adsorbate. Duly, the maximum capacity of a porous adsorbent

can be more related to the pore volume than to the surface area.

On the other hand, chemisorption involves the formation of chemical bonds between the adsorbate and

adsorbent is a monolayer, often with a release of heat much larger than the heat of condensation.

Chemisorption from a gas generally takes place only at temperatures greater than 300C, and may be slow

and irreversible. Most commercial adsorbents rely on physical adsorption; while catalysis relies on

chemisorption.

As the adsorption progress, an equilibrium of adsorption of the solute between the solution and

adsorbent is attained (where the adsorption of solute is from the bulk onto the adsorbent is minimum). Theadsorption amount (qe, mmol g-1) of the molecules at the equilibrium step was determined according to the

following equation:

qe=( )

Where,

Vis the solution volume (L);

Mis the mass of monolithic adsorbents (g); and

C0and Ceare the initial and equilibrium adsorbate concentrations, respectively.

In the other words, adsorption process is a mass transfer process by which a substance is transferred from the

liquid phase to the surface of a solid, and becomes bound by physical and/or chemical interactions. Largesurface area leads to high adsorption capacity and surface reactivity (Kurniawan and Lo, 2009).

3.2

Adsorption Isotherm and Models

An adsorption isotherm is the demonstration of the amount of solute adsorbed per unit weight of adsorbent as

a function of the equilibrium concentration in the bulk solution at constant temperature. Langmuir and

Freundlich adsorption isotherms are commonly used for the description of adsorption data.

The Langmuir equation is expressed as:

qe=

1

+

Where,

Ceis the equilibrium concentration of solute (mmol L-1),

qe is the amount of solute adsorbed per unit weight of adsorbent (mmol g-1of clay),

Xmis the adsorption capacity (mmol g-1), or monolayer capacity, and

bis a constant (L mmol-1)


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On the other hand, the Freundlich isotherm illustrates heterogeneous surface adsorption. The energy

distribution for adsorptive sites (in Freundlich isotherm) follows an exponential type function which is close

to the real situation. The rate of adsorption/desorption changes with the strength of the energy at the adsorptive

sites. The Freundlich equation is expressed as:

logqe= log+1

log

Where,

k(mmol g1) and

1/nare the constant characteristics of the system

3.3 Types of adsorbents

Adsorbents are grouped into natural adsorbents and synthetic adsorbents. Natural adsorbents include charcoal,clays, clay minerals, zeolites, and ores. These natural materials, in many instances are relatively cheap,

abundant in supply and have significant potential for modification and ultimately enhancement of theiradsorption capabilities. Synthetic adsorbents are adsorbents prepared from agricultural products and wastes,

house hold wastes, industrial wastes, sewage sludge and polymeric adsorbents. Each adsorbent has its own

characteristics such as porosity, pore structure and nature of its adsorbing surfaces. Many waste materials used

include fruit wastes, coconut shell, scrap tyres, bark and other tannin-rich materials, sawdust, rice husk,

petroleum wastes, fertilizer wastes, fly ash, sugar industry wastes blast furnace slag, chitosan and seafood

processing wastes, seaweed and algae, peat moss, clays, red mud, zeolites, sediment and soil, ore minerals etc.

3.4

Activated Carbon

Activated carbon can be produced from carbonaceous material, including coal (bituminous, subbituminous,and lignite), peat, wood, or nutshells (i.e. coconut). The manufacturing process consists of two phases,

carbonization and activation. The carbonization process includes drying and then heating to separate by-

products, including tars and other hydrocarbons, from the raw material, as well as to drive off any gases

generated. The carbonization process is finish made by heating the material at 400600C in an oxygen-

deficient atmosphere that cannot support combustion.

Activated carbons (AC) (both granular activated carbon (GAC) and powdered activated carbons

(PAC)) are common adsorbents used for the removal of undesirable odour, colour, taste, and other organicand inorganic impurities from domestic and industrial waste water owing to their large surface area, micro

porous structure nonpolar character and due to its economic viability. Powdered activated carbon is made upof crushed or ground carbon particles, 95100% of which will pass through a designated mesh sieve or sieves.

Granular activated carbon can be either in the granular form or extruded. In addition, active carbons contain

other hetero atoms such as hydrogen, nitrogen, sulfur, and oxygen.

3.5

Adsorption on Activated Carbon

Methyl tert-butyl ether (MTBE) is an organic pollutants used commonly as a fuel component in fuel in

gasoline engine and also as a solvent. The adsorption of methyl tert-butyl ether (MTBE) by granular activatedcarbon (GAC) was studied, the maximum adsorption capacity of MTBE on granular activated carbon was

204.1 mg/g. Results demonstrate that granular activated carbon (GAC) is an effective adsorbent for methyl

tert-butyl ether and also provide specific guidance into adsorption of methyl tert-butyl ether on granular

activated carbon (GAC) in contaminated groundwater (Chen, Zhang and Chen, 2010).


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Five commercially available types of activated carbon (GAC 1240, GCN 1240, RB 1, pK 1-3, ROW

0.8 SUPRA ) are prepared and used to remove organic chlorinated compounds from wastewater of a chemical

plant. The various types of activated carbon were tested on the basis of Freundlich adsorption isotherms for

14 pure organic chlorinated compounds, of molecular weight ranging from that of dichloromethane (MW

84.93 g mol-1) to hexachlorobenzene (MW 284.78 g mol-1). The best adsorbent (GAC 1240 granulated

activated carbon) was selected and used in a laboratory fixed bed column to assess its removal efficiency withrespect to the tested organic chlorinated compounds. Removal efficiency was always higher than 90% (Pavoni

et al., 2006).

Table 3.1: Removal Efficiency (%) of Chlorinated Compounds from Wastewater by Five

Commercially Available Types of Activated Carbon (Pavoni et al., 2006).

Substance Percentage of Adsorption Efficiency (%)

Dichloromethane

Trichloromethane1,1,1- Trichloromethane

Carbon tetrachloride1,2-Dichloroethane

Trichloroethylene

1,1,2- Trichloroethane

Tetrachloroethylene

1,1,1,2- Tetrachloroethane

Trans 1,4-dichloro-2-butene

1,2,4-Trichlorobenzene

1,2,3-TrichlorobenzeneHexachloro-1,3-butadiene

Hexachlorobenzene

98.3

98.899.0

99.082.8

94.7

86.3

91.6

87.3

94.2

99.2

90.599.4

95.1

Bottom ash is a kind of waste material generated from thermal coal-fired power plants. It is commonly

used in road bases and building materials. Bottom ash was used to remove the organic pollutants in cokingwastewater and papermaking wastewater. Particular attention was paid on the effect of bottom ash particle

size and dosage on the removal of chemical oxygen demand (COD). The results indicate that the COD removalefficiencies increase with decreasing particle sizes of bottom ash, and the COD removal efficiency for coking

wastewater is much higher than that for papermaking wastewater due to its high percentage of particle organic

carbon (POC). Different trends of COD removal efficiency with bottom ash dosage are also observed forcoking and papermaking wastewaters because of their different POC concentrations (Sun et al., 2008).


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4. Chemical Oxidation Technologies

Advanced chemical oxidation processes make use of oxidants to remove both organic and oxidizable inorganic

components, which also lowers down COD and BOD levels (Lennech, n.d.). The processes can completely

oxidize organic materials to carbon dioxide and water. Nowadays, the conventional biological methods cannot

be used for complete treatment of the effluent due to the increasing presence of molecules, refractory to the

microorganisms in the wastewater streams. Newer technology such as chemical oxidation technology which

uses chemical oxidant (H2O2, O3, ClO2, K2MnO4, K2FeO4and so on as shown in Table 1) has been introduced

to oxidize pollutant to slightly toxic, harmless substances or transform it into manageable form (Zheng et al.,

2013).

Table 4.1: Relative oxidation activity of some oxidizing agents (Munter, 2001; US Peroxide, n.d.).

Oxidizing Agent Relative Oxidation Activity

Positively charged hole on titanium dioxide, TiO2 2.35

Fluorine 2.23

Hydroxyl radical 2.05Atomic oxygen 1.78

Ozone 1.52Hydrogen peroxide 1.31

Perhydroxyl radical 1.21

Permanganate 1.24

Hypobromous acid 1.17

Chlorine dioxide 1.15

Hypochlorous acid 1.10

Hypiodus acid 1.07

Chlorine 1.00

Bromine 0.80Iodine 0.54

Some AOPs are enhanced with ultrasound, ultraviolet and catalysts to improve the oxidation

performance. They are capable to remove organics and inorganics substances, biologically toxic or non-

degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in

waste water (AST Clean Water Technologies, n.d.). In the chemical oxidation technologies, some oxidizing

agents such as ozone and hydrogen peroxide exhibit lower rates of degradation. Therefore, advanced oxidation

processes (AOPs) are introduced with the capability of exploiting the high reactivity of hydroxyl radicals in

driving the oxidation reaction that have today emerged a promising technology for the treatment of

wastewaters containing refractory organic compounds.

External energy sources such as electric power, ultraviolet radiation (UV) or solar light are required

for AOPs, leading these processes to cost more expensive than conventional biological wastewater treatment

(Mazille & Spuhler, n.d.). Besides overall COD reduction, AOPs are also able to provide sludge treatment,

specific pollutant destruction, increased bioavailability of recalcitrant organics and color and odor reduction.

This technology does not produce toxic or hazardous waste as the end products are stable inorganic compounds

such as water, carbon dioxide and salts (AST Clean Water Technologies, n.d.).

There are a few AOPs that are introduced today for the removal of organics in the wastewater treatment

plant. This includes ozone water processes, photolysis, UV/ozone, UV/H2O2 and UV/O3/H2O2 processes,

photocatalysis, Fentons processes, photoassisted Fenton processes, electro-Fenton processes, and wet


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oxidation process such as wet peroxide process, wet air oxidation, catalytic wet air oxidation and supercritical

wet air oxidation (Heponiemi et al., 2012). The main difference of these AOPs is the source of radicals.

AOPs today can be applied in the food industry, agricultural field, oil milling production, paper milling

industries, textile wastewater, wastewater containing pharmaceutical, pesticides and herbicides and many

industrial waste streams.

Past technologies started with Fentons processes which were reported at the year 1894, currently the

AOPs have been advanced to more catalytic treatments such as photocatalysis, catalytic ozonation, wet air

catalytic oxidation (WACO) and wet hydrogen peroxide catalytic oxidation (WHPCO) (Centi et al,. 2012). In

the future, these AOPs can be integrated with other treatments such as in biological and in physical treatment.

Some proposed to combine with electrochemical process and this lead to an improvement on the AOPs to

EAOPs (Electrochemical Advanced Oxidation Processes), which is reported of its capability to remove a

wider range of organic substances in the wastewater (Oturan et al., 2007).

4.1 Hydrogen Peroxide

Hydrogen peroxide (H2O2) is an environment friendly oxidant which could oxidize organic pollutants

efficiently and economically. The standard reduction potentials (1.77V, 0.88V) of hydrogen peroxide imply

that it is a strong oxidant in both acidic and basic solutions to oxidize many kinds of organic contaminants in

wastewater directly (Zheng et al., 2013).

H2O2+2H++2e 2H2O E

= 1.77V

HO2+H2O + 2e

3OHE= 0.88V

Hydrogen peroxide has been used for detoxification of cyanide, nitrite and hypochlorite, for the

destruction of phenol aromatics, formaldehyde, and removal of sulfite, thiosulfate and sulfide compounds.

However, the application of hydrogen peroxide alone present major problems such as very low rates for

applications involving complex materials, stability of H2O2 and mass transfer limitations. Hence, use of

hydrogen peroxide alone is not likely a recommendable option for industrial wastewater treatment. Today, it

can be enhanced by homogeneous and/or heterogeneous catalysts, the progress named wet hydrogen peroxide

catalytic oxidation (WHPCO). WHPCO operates at temperatures in the 20-80 range and atmospheric

pressure (Zheng et al., 2013). It is an enhanced technology from Wet Peroxide Oxidation (WPO), adapted

from Fentons Process. WPO has the same oxidation mechanism as Fentons process and the only differencebetween WPO and Fenton is the operating reaction temperature. WPO operates at higher temperature

(>100oC) so that more efficient TOC removal can be obtained (Heponiemi & Lassi, 2012).

4.1.1 WHPCO

WHPCO process has been proposed for a variety of agro-food and industrial effluents: removal of dyestuffs

from textile, treat sewage sludge, purifies wastewater from pharmaceutical and chemical production, dumping

site, or from cellulose production and pre-treat water streams from food-processing industries (olive oil mills,

distilleries, sugar refineries, coffee production, tanneries, etc.) (Zheng et al., 2013). It can also be used in

cosmetic industries that present a high organic load on their wastewater (Bautista et al., 2010).


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Introduction of hydrogen peroxide into the waste stream is critical due to lower stability of hydrogen

peroxide. H2O2 in the pollutant stream should be given a larger residence time, but due to the practicalconstraints and poor mixing conditions, injection of H2O2 in line is not always possible and an additional

holding tank is required. The simplest, faster and cheapest method for injection of hydrogen peroxide is gravity

feed system. Pump feed systems can also be used, but it requires regular attention.

Figure 4.1: Typical Reactor Used for WHPCO Technology (Zheng et al., 2013).

In figure 4.1, the WHPCO is used for the treatment of olive oil milling waste water using Fe-ZSM-5

solid catalysts. In the flow diagram, there is a fixed bed reactor and a main vessel for WHPCO (Zheng et al,

2013). H2O2is added progressively at the top of a fixed bed catalytic reactor before a static mixer to maximizeits local concentration. Iron solution is added on the top of the reactor to maintain catalyst activity constant.

The feed solution is recirculated to and from a tank in order to have good turbulence in the catalyst bed, but

also to guarantee the necessary total residence time to obtain the required level of removal of phytotoxic

chemicals.

The performance of WHPCO depends on the catalysts used. The main catalyst or the active phase used

for WHPCO is iron but there are also many researches include different catalyst in this chemical oxidationprocess, including Fe/-Al2O3, Fe/AC, TiO2-CeO2, Fe/TiO2-CeO2, Cu-zeolite, Cu-pillared clay, CuO

impregnated with activated carbon (CuAC), Au/AC, Al-Fe and Zr-Fe pillared clays and other many other

catalysts that is under research. Today, several supports that reported can be used for the catalyst are alumina,pillared clay, silica, zeolites and activated carbon (AC) (Bautista et al, 2010).

In the experiment reported by Bautista et al. in 2010, two samples of wastewater with significantly

different values of COD (4730 and 2300 mg/L) and total organic carbon (TOC) (1220 and 686 mg/L) were

used. In the authors, study, aerobic biodegradability of the wastewaters was assessed by the BOD5/COD ratio.

This ratio is a well widespread biodegradability index for industrial wastewaters. The BOD5/COD ratio was

0.085 and 0.220, respectively, showing low biodegradability. The calcination temperatures were at 350oC and

400oC for the Fe/-Al2O3catalyst. At the calcination temperature of 300oC, two catalysts were prepared with

4% and 8% of Fe content respectively. The operating conditions were: pHo= 3, T= 5585 C, and P= 1 atmfor the experiments. The results are as follows:


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Figure 4.2: Effect of the calcination temperature of the catalyst (Al4%Fe; COD o= 2070 mg/L;

TOCo= 617 mg/L; [H2O2]o= 4400 mg/L; T= 85 C) (Bautista et al., 2010).

Table 4.1: Effect of the Fe load of the alumina-supported catalysts (CODo= 2070 mg/L;

TOCo= 617 mg/L; [H2O2]o= 4400 mg/L; T= 85 C) (Bautista et al., 2010).

Catalyst XCOD(%) XTOC(%) XH2O2(%) Feleached(%)

Al4%FeT300 83.5 55.9 69.5 0.26

Al8%FeT300 75.5 52.0 76.1 0.28

Table 4.2: Effect of the temperature in the CWPO of cosmetic wastewaters with the Al4%FeT300

catalyst (CODo= 2070 mg/L; TOCo= 617 mg/L; [H2O2]o= 4400 mg/L) (Bautista et al., 2010).

Temperature (C) XCOD(%) XTOC(%) XH2O2(%) Feleached(%)

55 23.6 16.6 12.3 0.05

70 44.1 15.2 37.4 0.1685 83.5 55.9 69.5 0.26

Figure 4.2 shows the use of Fe/alumina catalyst calcined at 300oC and 450oC. The graph shows that theremoval of COD and TOC increases over time but at higher calcination temperature, the COD removal and

TOC reduction on the wastewater sample is lower. Table 2 shows the use of catalysts calcined at 300 oC with4% and 8% Fe content. From the table, lower Fe content at 4% shows a higher COD and TOC reduction with

lower H2O2dosage and lower leaching of Fe content from the catalyst. In Table 3, higher temperature displaysa higher removal of COD and TOC, but also with higher dosage of hydrogen peroxide and higher leaching of

Fe content from the catalyst. According to the authors, temperature about 70 to 85oC will be the optimum

temperature for COD to be reduced to an allowable COD limit for the industrial wastewater discharge. The

biodegradability of these wastewaters was also found to be improved from an initial BOD5/COD ratio of 0.22

to 0.53 after 4 h reaction time at 85 C.


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4.1.2 Fenton

The Fenton's process has its origin in the discovery reported in 1894 by H. J. H. Fenton that ferrous ion strongly

promotes the hydrogen peroxide oxidation of tartaric acid (Lenntech, n.d.). It is sometimes known as Fentonsreaction and its oxidation mechanism can be summarized by the following step: a mixture of H2O2and ferrous

iron in acidic solution in the presence of FeSO4 catalyst generates the hydroxyl radicals which will

subsequently attack the organic compounds present in the solution (Herney-Ramirez et al., 2010).

Fe2++H2O2Fe3++HO+HO

As iron (II) acts as a catalyst, it has to be regenerated by decomposing H2O2 again, which seems to occurthrough the following scheme:

Fe3++H2O2Fe-OOH2++H+

Fe-OOH2+Fe2++HO2

The important mechanistic feature of the Fentons reaction is the electron transfer from Fe2+to H2O2

and generates hydroxyl radicals and hydroxide anions. Hydroxyl radicals (as shown in Table 1) are extremely

powerful oxidizing species to abstract one electron from an electron rich organic substrate or any other speciespresent in the medium to form hydroxide anion. The oxidation potential of hydroxyl radicals has been

estimated as +2.8 and +2.0V at pH 0 and 14, respectively. The high reactivity of hydroxyl radical (HO) iscapable to react with a wide range of organic compounds. Fentons reaction gives rise to CO2 and the

heteroatoms also form the corresponding oxygenated species such as NOx, SOx and POx, indicating that the

carbons and heteroatoms of the organic substrate are converted to inorganic species (Zheng et al., 2013).

The performance of Fentons oxidation application to wastewater treatment was based on the followingparameters: operating pH, amount of ferrous ions, concentration of hydrogen peroxide, initial concentration

of the pollutant, type of buffer used for pH adjustment, operating temperature and chemical coagulation

(Zheng et al. 2013; US Peroxide, n.d.). Parameter pH is the major factor affecting the performance of Fentons

reaction. Optimum pH has been observed to be 3 in the majority of the cases, but it could also be in the range

of 3 to 6 (Lennech, n.d.). At higher pH, the iron precipitate Fe(OH)3will form and decompose H2O2 into

oxygen and water molecules (US Peroxide, n.d.). The pollutant removal efficiency increases with an increase

in the dosage of ferrous ions and hydrogen peroxide but higher dosage will lead to environmental problem

and high treatment cost. In commercial application, most Fentons reaction operates at temperature between

20 to 40oC. Higher temperature will decrease the oxidation performance as H2O2 could decompose into O2and water molecule (US Peroxide, n.d.).

The conventional Fentons reaction to produce hydroxyl radicals is a homogeneous catalytic reaction.

Major problems encountered for homogeneous catalyst include catalyst separation, regeneration, etc. Today,

there are heterogeneous catalysts Fentons reaction, i.e., solids containing transition metal cations (mostly iron

ions) being developed and tested (Navalon et al., 2010).

A batch Fentons reactor consists of a non-pressurized stirred reactor with metering pumps for theaddition of acid, base, a ferrous sulfate catalyst solution and industrial strength (35-50%) hydrogen peroxide.

The reactor vessel should be coated with an acid-resistant material, because the Fentons reagent is very


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aggressive and corrosion can be a serious problem as it contains some residual H 2SO4(US Peroxide, n.d.).

The pH of the solution must be adjusted at 6, usually iron hydroxide is formed. For many organic pollutants,the ideal pH for the Fenton reaction is between 3 and 4, and the optimum catalyst to peroxide ratio is usually

1:5 wt/wt (Zheng et al., 2013). Addition of reactants is done in the following sequence: dilute sulfuric acid

catalyst in acidic solutions, pH adjusting agent (adjustment of pH at 3-4) and lastly added hydrogen peroxide

slowly (Lennech, n.d.). Effluent of the Fentons reactor (oxidation tank) is fed into a neutralizing tank for

adjusting the pH (adjustment of pH at 9), then the stream followed by a flocculation tank and a solid-liquid

separation tank for removing the precipitate. A schematic representation of the Fentons oxidation treatmenthas been shown in Figure 3 (Gogate & Pandit, 2004).

Figure 4.3: Typical Reactor used for Fentons Oxidation (Gogate & Pandit, 2004).

The main advantage of this process is its simplicity of design, implementation and its operation at nearambient condition (Bautista et al, 2010). It also gains large attention for its ability to remove wide variety of

compounds. Fentons reagent has been used quite effectively for the treatment and pre-treatment of leachate

from composting of different wastes. Reported COD removal efficiencies range from 45% to 85%, and

reported final BOD5/COD ratio can be increased from less than 0.10 initially to values ranging from 0.14 to

more than 0.60, depending on leachate characteristic and dosages of Fenton reagents (Deng & Englehardt,2006). The optimal conditions for Fentons reaction were found at a ratio [Fe2+]/[COD] equal to 0.1. Both

leachates were significantly oxidized under these conditions in terms of COD removal 77-75% and BOD 5

removal 90-98% (Trujillo et al., 2006). Fentons oxidation was also used to degrade complexing agents such

as N-bis[2-(1,2-dicarboxyethoxy) ethyl)] glysine (BCA5), N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid

(BCA6) and EDTA from bleaching wastewater (Pirkanniemi et al., 2007). It was reported that an almost

complete removal of EDTA was attained at its concentration of 76 mM. There are also other compounds suchas m- and p-xylenes concentration being reduced significantly. It was reported that there is a 81.8% reduction

in the organic contaminant mass using Fentons reaction, and it was also found to be effectively mineralizing

methyl tert-butyl ether (MTBE) in the wastewater (Bergendahl & OShaughnessy, n.d.).

Figure 4.4: Reduction in COD of an Industrial WW with Fentons Oxidation.


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Figure 4.4 shows the result of a bench-scale experiment using Fentons reaction for the evaluation on

the potential in COD reduction using Fentons reagent. It was found that the process was able to reduce theCOD up to 96% from its initial concentration (Bergendahl & OShaughnessy, n.d.).

Fentons processes can be applied in many industries today, such as chemical, pharmaceutical, pulp

and paper, textile, food, cork processing, and landfill leachates (Bautista et al., 2008). Chemical industries are

major contributors to nowadays problems of industrial wastes, in terms of discharge volume and the hazardous

pollutants in the effluent. Phenolic resin manufacturing industry is one of the chemical industries that generatehigh concentration of phenol and derivatives, which are extremely toxic and refractory. Kavitha and

Palanivelu evaluated the efficiency of different Fenton-related processes, such as Fenton and solar-Fenton, for

the degradation of phenol in industrial wastewaters. The effluent was taken from a resin-manufacturing

industry in India containing 2904 mg/L COD, 933 mg/L dissolved organic carbon (DOC) and 1215 mg/L

phenol. The reaction is run at pH of 3, H2O2/COD weight ratio of 2.2, Fe2+/H2O2molar ratio of 0.026 for

Fenton and 0.013 for solar-Fenton. It was reported that phenol was completely removed in 5 min reactiontime. In Fentons reaction, COD removal and DOC mineralization is 82 and 41% respectively, whereas in

solar-Fentons reaction, almost complete COD reduction and 97% mineralization of DOC were achieved in 2

hours of reaction time.

Future Technologies

5. Ultrasound (US) and Advanced Oxidation Processes (AOP)

As mentioned previously, AOP are the usage of chemicals to decompose and remove organic or inorganic

pollutants, using light to catalyse the process. These treatments have long been known, but are not widespread

in industry due to their high costs or difficulties in the process. However, recently, the finding that AOP may

utilize ultrasound as well instead of just UV light became popular among researches, and such process is

termed as sonolysis.

Figure 5.1: Schematic Representations of (a) Effective Reaction Zone in Cavitation Bubbles (b)

Catalytic Surface.


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Sonolysis uses the compression and expansion cycles of ultrasonic waves to pull water molecules away

from each other, causing voids, which are known as acoustic cavitations. Figure 2 shows diagram of cavitationbubbles. The cavitation bubbles entrap vapors from the medium, eventually reaching its critical size of several

hundred micrometers before implosion of bubble occurs. The implosion of cavitation generates high

temperature and pressure. The timescale of the collapse is less than 0.1 microseconds, and the collapse of

cavitation bubbles may generate high-speed microjets of 100 m/s, causing direct damage to other nearby micro

particles, which hinders their agglomeration, allowing higher active surface area for subsequent reaction. As

explained, since only vapors are entrapped in the cavitation bubble, this process is only suitable for destroyinghigh volatility compounds that are hydrophobic. Hydrophilic compounds or non-volatile compounds are

decomposed by another mechanism, which will be explained below, through the generation of OH radicals.

Aside from using sonolysis to directly decompose pollutants, sonolysis have good affinity with manyAOPs. One example is the the combination of sonolysis in Fentons Process, in the in-situ production of

Hydrogen Peroxide. In aqueous medium, the ultrasound waves provoke the formation and collapse ofcavitation bubbles. Cavitation produces local spots of high temperature and pressure for a short time, which

also causes the splitting of water molecules into OH-and H+radicals during the collapse. Though, an estimated

80% of them recombines back into water before they decompose pollutants, and cannot be improved withultrasound. Regardless, investigations have found that the combination of utilizing ultrasound improved

mineralization in the presence of O3from 30% with methyl orange to 80% with ultrasound, caused by the

increased yield of OH-radicals from O3molecules under ultrasound. Aside from that, the activity of catalytic

processes also improved by the synergized effect induced by ultrasound waves, which is cause by increased

active surface area and increased concentration of OH-radicals.

A second example is the addition of ultrasound into TiO2/ZnO/UV photocatalyst systems. With

ultrasound, complete degradation was achieved in 4 hours while in the absence of ultrasound, only 79%

degradation was achieved. A third example is the use of ultrasound with copper oxide (CuO) catalyst. Guo et

al. found that the degradation rate of 2,4-dinitrophenol after 4 hours of ultrasonic irradiation increased from14% to 96% in the presence of CuO/H2O2/air.

The economics study of sonolysis concluded that sonolysis alone is not sufficiently cost effective for

water treatments for both organic pollutants and inorganic pollutants. However, sonolysis can be used as an

auxiliary tool for AOPs such as increasing OH-radicals production in Fentons Process explained above toallow more economical and more efficient AOPs.

6.

Electrochemical Treatment

Electrochemical has been found to be more effective than conventional treatments in removing organic

compounds as well as nitrates. Organic compounds are removed by electrochemical oxidation at the anode.

Oxidation can occur directly on the anode surface, as well as indirectly when electrochemical method isincorporated with oxidizing agents.

One example is by oxidizing chlorine into hyprochlorous acid, which then oxidizes organic matter.

However oxidation of organic compounds by hyprochlorous acid produces organochlorine compounds, which

are carcinogenic in nature. Another effective method is the incorporation of electrochemical treatment withFentons method. In this method, the ferrous ions can be continuously regenerated, hence allowing economical

and efficient removal of organic compounds when compared to the conventional Fentons process.


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For the performance of direct anodic oxidation process, it was found to have high COD and BOD

removal of 92.4% and 78% respectively from pond water. However, this method is extremely expensive dueto the use of Ti/RuO2-TiO2anode. Several performance of direct and indirect anodic oxidation obtained from

experiments are listed below.

Table 6.1: Organic Compound Removal from Different Wastewater Types using Different Anode and

Cathode Materials.

Anode/Cathode Material Type of Wastewater Results

Ti / Ti Aquaculture Wastewater 97.3% Organic Compound removal

BDD / BDD Seawater containing chlorine ion 88% COD removal

Other than the above, electrochemical method may also be added to biological treatment systems, and

it is known as bio-electrochemical technology. These systems use microorganisms as catalyst for

electrochemical reactions. These systems are divided into two major groups. One which is microbial fuel cell

(MFC), which produces electrical energy from organic material in waste water, and the other one is microbial

electrolysis cell (MEC), which utilizes external electricity to convert organic materials into other products.These systems are considered as clean technology as it produces much less sludge than conventional systems.

The following table shows the performance of bio-electrochemical oxidation of organic matter.

Table 6.2: Performance of Bio-electrochemical Oxidation of Organic Matter from Synthetic

Wastewater for Different Anode and Cathode Materials.

Anode / Cathode Material Type of Wastewater Results

Graphite / Graphite

Synthetic Wastewater

93.6% Organic Matters Removal

Graphite / Graphite 74.2% COD removal

Graphite / Bio-Cathode 98.8% COD removal

Conclusion

As a conclusion, the treatment technologies for organic wastewater of past, present and future were reviewed

and evaluated. This includes a variety of technologies such as biological treatment, chemical oxidationtechnologies, adsorption technology and the others were introduced. There are many more other technologies

that cannot be covered within the scope of this report. Furthermore, each of these technology, old or new, arestill being researched and studied upon until this day. This shows that no single technology is perfect and there

is still ample room for improvement in the future.


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overview of treatment technology in organic waste removal

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