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OPEN JOURNAL OF CHEMICAL ENGINEERING AND SCIENCE

Volume 1, Number 1, MAY 2014

OPEN JOURNAL OF CHEMICAL ENGINEERING AND SCIENCE

Anaerobic Fermentation of IndustrialWastewater (Review Article)Randa M. Osman*Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Center, Egypt.

*Corresponding author : [email protected]

Abstract:Anaerobic Digestion is a series of chemical reactions during which organic material is decom-posed through the metabolic pathways of naturally occurring microorganisms in an oxygen-depleted environment. In nature this type of breakdown typically occurs in warm wet and dark

environments, such as in the digestive tracts. The microorganisms are exploited in the biotech-nological process of anaerobic digestion both to reduce the pollution caused by organic wastesand to produce methane, which can be used as a fuel. The number and types of microorganismspresent in digesters are likely to depend upon the type of digester, its operating conditions andthe waste composition. Anaerobic Digesters can be used on any carbon containing industrialwastewater i.e. food processing; pulp and paper; sugar and distillery; slaughterhouse; cheesewhey and diary units; brewing industry and municipal sludge. Also, they provide an effectivemethod for turning residues from different wastes, biogas (rich in methane which can be usedto generate heat and/or electricity); ber (this can be used as a nutrient-rich soil conditioner)and liquor (this can be used as liquid fertilizer). In the review paper, the special emphasis onsome of the environmental requirements; indicators of treatment unbalance; types of high rate

anaerobic reactors; the application of anaerobic fermentation of selected high strength industrialwastewaters; modelling of anaerobic digestion, and nally, there is a case study.

Keywords:Anaerobic Fermentation; Application; Industrial Wastewater; Types of Anaerobic Reactors;Modeling; Case Study

1. INTRODUCTION

The building of a sustainable society will require reduction of dependency on fossil fuels and lowering

of the amount of pollution that is generated. Wastewater treatment is an area in which these two goals canbe addressed simultaneously. As a result, there has been a paradigm shift recently, from disposing of wasteto using it. There are several biological processing strategies that produce bioenergy or biochemicals whiletreating industrial and agricultural wastewater, including methanogenic anaerobic digestion, biologicalhydrogen production, microbial fuel cells and fermentation for production of valuable products [ 1].

Fermentation is commonly dened as the process is which energy is formed by the process of oxidationof organic compounds like carbohydrates and sugars. This leads to conversion of these organic compoundsinto an acid or an alcohol which provides energy. It can be carried out by microorganisms with the help

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Anaerobic Fermentation of Industrial Wastewater (Review Article)

of oxygen as well as without it. When fermentation is carried out in the presence of oxygen, it is calledaerobic fermentation and when carried out without it, it is commonly known as anaerobic fermentation.

Anaerobic Fermentation or Anaerobic treatment consists of the decomposition of organic material in theabsence of free oxygen and this process produces biogas enriched in methane, carbon dioxide, ammoniaand traces of other gases and volatile fatty acids (VFA) within the reactor. The anaerobic treatment processhas been employed in several developed countries with the aim of biostabilizing fermentable organicwaste produced by rural and urban activities. The anaerobic process appears to be a promising alternativefor the treatment of organic solid waste due to the high rates of biogas production that can be achieved.However, the application of the anaerobic process for organic solid waste treatment is not widespread,mainly due to the longer time required to achieve biostabilization in comparison to the aerobic process.Decreasing the biostabilization time of organic solid waste through the use of an inoculum has givensatisfactory results [ 2]. Generally, the inoculum is digested sludge originating from sewage treatmentplants or other materials of animal origin, such as bovine manure and other wastes [3, 4].

The main difference between anaerobic wastewater treatment and aerobic treatment is that no aerationis required in the former treatment method. The absence of oxygen allows anaerobic conversion of organicpollutants to biogas which consists mainly of methane and carbon dioxide. The two main advantages of anaerobic treatment can be listed as (i) high organic loading rates (10–20 times as high as in conventionalactivated sludge treatment) and (ii) low operating costs [ 5–7].Anaerobic treatment can often be quitecost-effective in reducing the organic matter combined with the production of reusable energy in the formof biogas, which can be used for electricity production or for heating purposes. Anaerobic treatment isquite suitable for industries discharging highly concentrated wastewaters with low nitrogen content suchas food processing industry [ 7], beer breweries [8], soft drink producers [ 9] or paper processing factories[10].

2. ANAEROBIC FERMENTATION

Anaerobic fermentation of industrial wastewater and high solids wastes such as animal manure,

biological sludge, nightsoil, etc. is commonly known as “anaerobic digestion” and is carried out in airtightcontainer known as anaerobic digester (AD).

The successful operation of anaerobic digester depends on maintaining the environmental factors closeto the comfort of the microorganisms involved in the process. In this section we will discuss the differenttypes of anaerobic digestion systems and types of high rate anaerobic digesters, the microbiology of theprocess, advantages and disadvantages of anaerobic fermentation, environmental requirements such aseffect of temperature; effect of pH; effect of nutrients; effect of organic loading rate and nally there aresome indicators of treatment unbalance

2.1 Types of Anaerobic Fermentation Systems

The methods used to treat industrial wastewater can be classied into following categories;

1. Single Stage

Feed is introduced in the reactor at a rate proportional to the rate of efuent removed. Generally theretention time is 14-28 days depending on the kind of feed and operating temperature.

2. Multi Stage

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OPEN JOURNAL OF CHEMICAL ENGINEERING ANDSCIENCE

The introduction of multi-stage AD processes was to improve digestion by having separate reactorsfor the different stages of AD, providing exibility to optimize each of these reactions.

3. Batch

The batch reactors are loaded with feedstock, subjected to reaction, and then are discharged andloaded with a new batch.

Alternatively some digesters are continuously fed with automatic loading and unloading and removingof digestate. These systems are referred to as “continuous” digesters.

A single-stage anaerobic digester is susceptible to upset by rapid increase in volatile fatty acids anddecrease in the pH of bulk solution, subsequently inhibiting the methanogenesis and leading to processfailure. To eliminate such a common operational problem, two-stage anaerobic processes have beenintroduced and investigated as an alternative. The physical separation of the acidogenic and methanogenicphases increases stability as overloading of the methane reactor is prevented by proper control of theacidication step. Besides this, phase separation allows the maintenance of appropriate densities of theacid and methane formers in separate reactors and enables maximization of the rates of acidicationand methanogenesis by applying optimal operational conditions. The acidogenic reactor can also serve

as a buffer system when the composition of the wastewater is variable and can help in the removal of compounds that are toxic to methanogens. And nally, the acidogenic reactor provides a constant substratefor the methanogens, which are known to adapt slowly to varying substrate content and composition [11 ].

2.2 Types of High Rate Anaerobic Reactors

All modern high rate biomethanation processes are based on the concept of retaining high viablebiomass by some mode of bacterial sludge immobilization. These are achieved by one of the followingmethods:

1. Formation of highly settleable sludge aggregates combined with gas separation and sludge settling,

e.g. upow anaerobic sludge blanket reactor and anaerobic bafed reactor.

2. Bacterial attachment to high density particulate carrier materials, e.g. uidized bed reactors andanaerobic expanded bed reactors.

3. Entrapment of sludge aggregates between packing material supplied to the reactor, e.g. downowanaerobic lter and upow anaerobic lter.

Table 1 summarizes some of the important features of these reactors which are widely used foranaerobic digestion of industrial wastewaters.

2.2.1 Fixed Film Reactor

The primary mechanism for the initial removal of TSS and therefore BOD is by biophysical ltration.The removal efciency is not affected by temperature as much as other types of digesters. The concentra-tion for the entrapped solids increases continually and then there is the major problem of removing excesssolids/biomass periodically forms the digester.

In stationary xed lm reactors Figure 1 , the reactor has a biolm support structure (media) such asactivated carbon, PVC (polyvinyl chloride) supports, hard rock particles or ceramic rings for biomassimmobilization. The wastewater is distributed from above/below the media. Fixed lm reactors offer

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Table 1. Characteristics of reactor types

Anaerobic

reactor type

Start up

period

Channeling

effectEfuent recycle

Gas solid

separation device

Carrier

packing

Typical loading

rates (kg COD/m 3 day)HRT (d)

CSTR – Not present Not required Not required Not essential 0.25-3 10-60

UASB 4-16 Low Not required Essential Not essential 10-30 0.5-7

Anaerobic lter 3-4 High Not required Benecial Essential 1-4 0.5-12AAFEB 3-4 Less Required Not required Essential 1-50 0.2-5

AFB 3-4 Non-existent Required Benecial Essential 1-100 0.2-5

Figure 1. Fixed Film Reactor (Source: http://web.deu.edu.tr/atiksu/ana58/vege06.gif)

the advantages of simplicity of construction, elimination of mechanical mixing, better stability at higherloading rates, and capability to withstand large toxic shock loads and organic shock loads. The reactorscan recover very quickly after a period of starvation. The main limitation of this design is that the reactorvolume is relatively high compared to other high rate processes due to the volume occupied by the media.Another constraint is clogging of the reactor due to increase in biolm thickness and/or high suspendedsolids concentration in the wastewater.

2.2.2 Up ow Anaerobic Sludge Blanket Reactor

This is by far the most widely studied reactor conguration for domestic wastewater treatment. Itsprimary use is for the treatment of higher strength industrial wastewaters, but it can be used for lowerstrength municipal wastewater. It is important to have a good feed inlet construction for obtaining bettercontact between the immobilised organisms and the inuent wastewater. Better contact of organisms and

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wastewater can be achieved by

1. Greater height/diameter ratio

2. The recirculation of the efuent, which results in an expanded granular sludge bed (EGSB).

The lower upward liquid velocities in the UASB reactors resulted in better entrapment of the nonsolublepollutants.

A UASB reactor Figure 2 essentially consists of gas-solids separator (to retain the anaerobic sludgewithin the reactor), an inuent distribution system and efuent draw off facilities. Efuent recycle (touidize the sludge bed) is not necessary as sufcient contact between wastewater and sludge is guaranteedeven at low organic loads with the inuent distribution system [ 12]. Also, signicantly higher loadingrates can be accommodated in granular sludge UASB reactors as compared to occulent sludge bedreactors [ 13]. In the latter, the presence of poorly degraded or nonbiodegradable suspended matter inthe wastewater results in an irreversible sharp drop in the specic methanogenic activity because thedispersed solids are trapped in the sludge. Moreover, any signicant granulation does not occur underthese conditions. The maximum loading potential of such a occulent sludge bed system is in the rangeof 1-4 kgCOD/m 3 days.

Yet another high rate digester, EGSB, is a modied form of UASB in which a slightly higher supercialliquid velocity is applied (5-10 m/h as compared to 3 m/ h for soluble wastewater and 1-1.25 m/h forpartially soluble wastewater in an UASB) [ 14]. Because of the higher upow velocities, mainly granularsludge will be retained in an EGSB system, whereas a signicant part of granular sludge bed will be in anexpanded or possibly even in a uidized state in the higher regions of the bed. As a result, the contactbetween the wastewater and sludge is excellent. Moreover, the transport of substrate into the sludgeaggregates is much better as compared to situations where the mixing intensity is much lower [ 14]. Themaximum achievable loading rate in EGSB is slightly higher than that of an UASB system, especially fora low strength V&A containing wastewater and at lower ambient temperatures.

2.2.3 Anaerobic Expanded and Fluidised Bed Reactor

Many researchers reported on the development of the anaerobic expanded bed process, which wasfound to convert dilute organic wastes to methane at low temperatures and at high organic and hydraulicloading rates. Some reactors may use sand as a carrier and others granular activated carbon (GAC). GACwill develop a biolm very quickly and have high biomass content.

In the anaerobic uidized bed Figure 3 , the media for bacterial attachment and growth is kept in theuidized state by drag forces exerted by the upowing wastewater. The media used are small particlesize sand, activated carbon, etc. Under uidized state, each media provides a large surface area forbiolm formation and growth. It enables the attainment of high reactor biomass hold-up and promotessystem efciency and stability. This provides an opportunity for higher organic loading rates and greater

resistance to inhibitors. Fluidized bed technology is more effective than anaerobic lter technology as itfavors the transport of microbial cells from the bulk to the surface and thus enhances the contact betweenthe microorganisms and the substrate [15].

These reactors have several advantages over anaerobic lters such as elimination of bed clogging; alow hydraulic head loss combined with better hydraulic circulation and a greater surface area per unit of reactor volume. Finally, the capital cost is lower due to reduced reactor volumes. However, the recyclingof efuent may be necessary to achieve bed expansion as in the case of expanded bed reactor. In theexpanded bed design, microorganisms are attached to an inert support medium such as sand, gravel or

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Figure 2. Upow anaerobic sludge blanket reactor (UASB) reactor

plastics as in uidized bed reactor. However, the diameter of the particles is slightly bigger as comparedto that used in uidized beds. The principle used for the expansion is also similar to that for the uidizedbed, i.e. by a high upow velocity and recycling.

2.3 The Microbiology of the Process

The number and types of microorganisms present in digesters are likely to depend upon the type

of digester, its operating conditions and the waste composition. The metabolic stages involved in theproduction of methane from waste in Anaerobic Digestion occur in 4 distinct processes;

1. Hydrolysis

Complex organic matter is decomposed into simple soluble organic molecules using water tosplit the chemical bonds between the substances. This is where solid complex organics, celluloseproteins, lignins, and lipids are broken down into soluble (liquid) organic fatty acids. The resultsare soluble monomers. Hydrolytic bacteria are responsible for the creation of monomers. Enzymes

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Figure 3. Schematic of the anaerobic uidized bed process (source: http://web.deu.edu.tr/atiksu/ana58/vege06.gif)

Figure 4. the Four Distinct Processes of Anaerobic Digestion

excreted from the bacteria, such as cellulase, protease, and lipase, catalyse hydrolysis. Therefore,the more complex the feedstock then the hydrolytic phase is relatively slow.

A hydrolysis reaction where organic waste is broken down into a simple sugar, in this case glucosecan be seen in the following Eq .1.

C 6 H 10 O4 + 2 H 2O → C 6 H 12 O6 + 2 H 2 (1)

2. Fermentation/Acidogenesis

The chemical decomposition of Carbohydrates, proteins and fats by enzymes, bacteria, yeasts in theabsence of oxygen. Hydrolysis is immediately followed by the acid-forming phase of Acidogenesis.Here acidogenic bacteria turn products of hydrolysis into mostly short chain (volatile) acids (e.g.

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formic or lactic), ketones (e.g. ethanol or acetone) and alcohols. The specic concentrations of products formed here vary with the type of bacteria, culture conditions, such as temperature andpH.

3. Acetogenesis

The fermentation products are converted into acetate hydrogen and carbon dioxide by so called

acetogenic bacteria. Here the Biological Oxygen Demand (BOD) and Chemical Oxygen Demand(COD) are reduced. Acetogenesis occurs through carbohydrate fermentation which acetate is themain product, and other metabolic processes. In equation 2 reaction in the acid-forming stages isshown below, glucose is converted to ethanol.

C 6 H 12 O6 ↔ 2CH 3CH 2OH + 2CO 2 (2)

4. Methanogenesis

Methane (CH 4) is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria.Most methanogenic bacteria utilize H 2 & CO 2 , but species of only two genera Methanosarcina

and Methanorthrix , can produce methane from acetic acid. The acetogenic bacteria grow in closeassociation with the methanogenic bacteria during the 4 th stage process.

2.4 Advantages of Anaerobic Fermentation

1. It can treat a wide range of organic wastes including industrial wastewater

2. No odour nuisance during the process and the reduction approximately 80% of the odour potential.

3. It is relatively small in size to the amount of waste treated.

4. A.F projects can directly boost the local rural economy through creating jobs in the A.F development

and indirectly through increasing disposable income in rural areas.

5. Reducing land and waste pollution: Poor disposal of wastewater can cause land and ground waterpollution. A.F creates integrated management system which reduces the likelihood of this happeningand reduces the likelihood of nes been imposed of such pollution.

6. A.F provides on site energy for the process

The biogas produced is a renewable energy source and used as a transport fuel or to produce electricitydisplaces fossil fuel energy and they’re by reducing the emissions of green house and acidifying gasses

1. Anaerobic digestion is also a technology that can make a signicant contribution to the management

of organic waste.2. The ber form the anaerobic digestion process can be used as a good soil conditioner and the liquor

can be used as a fertilizer

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2.5 Environmental Requirements

The methane bacteria, which are responsible for the majority of waste stabilization in anaerobictreatment, grow quite slowly compared to aerobic organisms and so a longer time is required for them toadjust to changes in organic loading, temperature or other environmental conditions.. For this reason, it isusually desirable in design and operation to strive for optimum environmental conditions so that moreefcient and rapid treatment might be obtained.

2.5.1 Effect of Temperature

Anaerobic digestion is strongly inuenced by temperature and can be grouped under one of thefollowing categories: psychrophilic (0-20 ◦ C), mesophilic (20-42 ◦ C) and thermophilic (42-75 ◦ C). Thedetails of the bacterial processes in all the three temperature ranges are well established though a largesection of the reported work deals with mesophilic operation. Changes in temperature are well resisted byanaerobic bacteria, as long as they do not exceed the upper limit as dened by the temperature at whichthe decay rate begins to exceed the growth rate. In the mesophilic range, the bacterial activity and growth

decreases by one half for each 10 ◦ C drop below 35 ◦ C [15]. Thus, for a given degree of digestion to beattained, the lower the temperature, the longer is the digestion time

The effect of temperature on the rst stage of the digestion process (hydrolysis and acidogenesis) isnot very signicant. The second and third stages of decomposition can only be performed by certainspecialized microorganisms (acetogenic and methanogenic bacteria) and thus, these are much moresensitive towards temperature change However, an important characteristic of anaerobic bacteria is thattheir decay rate is very low at temperatures below 15 ◦ C. Thus, it is possible to preserve the anaerobicsludge for long periods without losing much of its activity. This is especially useful in the anaerobictreatment of wastewater from seasonal industries such as sugar mills.

2.5.2 Effect of pH

Anaerobic reactions are highly pH dependent. The optimal pH range for methane producing bacteria is6.8-7.2 while for acid-forming bacteria, a more acid pH is desirable [ 16]. The pH of an anaerobic systemis typically maintained between methanogenic limits to prevent the predominance of the acid-formingbacteria, which may cause V&A accumulation. It is essential that the reactor contents provide enoughbuffer capacity to neutralize any eventual V&A accumulation, and thus prevent build-up of localized acidzones in the digester. In general, sodium bicarbonate is used for supplementing the alkalinity since it isthe only chemical, which gently shifts the equilibrium to the desired value without disturbing the physicaland chemical balance of the fragile microbial population [ 15].

2.5.3 Effect of Nutrients

The presence of ions in the feed is a critical parameter since it affects the granulation process andstability of reactors like USAF. The bacteria in the anaerobic digestion process requires micronutrientsand trace elements such as nitrogen, phosphorous, sulphur, potassium, calcium, magnesium, iron, nickel,cobalt, zinc, manganese and copper for optimum growth. Although these elements are needed inextremely low concentrations, the lack of these nutrients has an adverse effect upon the microbial growthand performance. Methane forming bacteria have relatively high internal concentrations of iron, nickel

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Table 2. the Elemental Composition of Methane Bacteria*

Macronutrients Element Concentration (mg/kg) Macronutrients Element Concentration (mg/kg)

N 65,000 Fe 1800

P 15,000 Ni 100

K 10,000 Co 75

S 10,000 Mo 60Ca 4000 Zn 60

Mg 3000 Mn 20

Cu 10*(Hulshoff Pol., 1995)

and cobalt. These elements may not be present in sufcient concentrations in wastewater streams fromthe processing of one single agroindustrial product like corn or potatoes or the wastewater derived fromcondensates. In such cases, the wastewater has to be supplemented with the trace elements prior totreatment [15]. The required optimum C:N:P ratio for enhanced yield of methane has been reported to be100:2.5:0.5 [ 17]. The minimum concentration of macro and micronutrients can be calculated based on the

biodegradable COD concentration of the wastewater, cell yield and nutrient concentration in bacterialcells [ 15]. Table 2 presents the elemental composition of the methane forming bacteria in the bacterialconsortium. In general, the nutrient concentration in the inuent should be adjusted to a value equal totwice the minimal nutrient concentration required in order to ensure that there is a small excess in thenutrients needed.

2.6 Indicators of Treatment Unbalance

Under normal conditions, anaerobic waste treatment proceeds with a minimum of control. However,if environmental conditions are suddenly changed, or if toxic materials are introduced to the digester,the process may become unbalanced. An ”unbalanced digester” is dened as one which is operating atless than normal efciency. In extreme cases, the efciency may decrease to almost zero, in which casea ”stuck” digester results. It is important to determine when a digester rst becomes ”unbalanced” sothat control measures can be applied before control is lost. A stuck digester is difcult to restart, and, if asupply of seed sludge containing high concentrations of methane bacteria is not available, this may takeseveral weeks. There is no single parameter which will always tell of the onset of unbalanced conditions,and several parameters must be watched for good control.

Of the many parameters, the best individual one is that for the concentration of volatile acids. Thevolatile acids are formed as intermediate compounds during the complete anaerobic treatment of complexorganic materials. The methane bacteria are responsible for destruction of the volatile acids, and if theybecome affected by adverse conditions, their rate of utilization will slow down, and the volatile acidconcentration will increase. A sudden increase in volatile acid concentration is frequently one of the rstindicators of digester unbalance and often will indicate the onset of adverse conditions long before any of the other parameters are affected. It should be noted that a high volatile acid concentration is the result of unbalanced treatment and not the cause as is sometimes believed. Thus, a high volatile acid concentrationin itself is not harmful, but indicates that some other factor is affecting the methane bacteria.

Another indicator of digester unbalance is a decreasing pH, which usually results from a high volatileacid concentration. A signicant drop in pH, however, does not usually occur until the digester is seriouslyaffected, and conditions resulting in a ”stuck” digester are near.

With some types of toxicity, the rst indication is a decrease in total gas production. However, this

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parameter is useful as an indicator only when the daily feed is quite uniform and the daily gas productiondoes not vary too widely from day to day under normal conditions. Changes in the percentage of carbondioxide in the digester gas may sometimes indicate the onset of unbalanced condition, as unbalancedtreatment often results in decreased methane production which is accompanied by an increase in carbondioxide percentage.

Another indication of unbalanced conditions is a decrease in efciency of operation. Such a decreasein efciency may be evidenced from a drop in methane production per pound of volatile solids added,as frequently determined for municipal sludge, or may be indicated by an increase in efuent COD inthe treatment of industrial waste. Although none of the above parameters may be a sure sign of digesterunbalance when used individually, together they give a good picture of digester operation. The best andmost signicant individual parameter, however, is the volatile acids concentration, and this should alwaysbe closely followed.

3. ANAEROBIC DIGESTION OF SELECTED HIGH STRENGTH WASTEW-ATERS

3.1 Slaughterhouse and Meat Packing

Wastewater from a slaughterhouse arises from different steps of the slaughtering process such as washingof animals, bleeding out, skinning, cleaning of animal bodies, cleaning of rooms, etc. The wastewatercontains blood, particles of skin and meat, excrements and other pollutants. Typical characteristics of wastewater from slaughterhouses are given in Table 3 .

Anaerobic ponds are commonly used to achieve a high degree of BOD reduction in slaughterhousewastewater. However, this suffers from the disadvantage of odour generation from the ponds thus makingthe development of alternate designs very essential. Anaerobic contact, upow anaerobic sludge blanket,and anaerobic lter reactors have been tried for slaughterhouse wastes. All these have a higher OLRranging from 5 to 40 kgCOD/m 3 day [ 18]. The high rate anaerobic treatment systems such as UASB

and xed bed reactors are less popular for slaughterhouse wastes due to the presence of high fat oiland suspended matters in the inuent. This affects the performance and efciency of the treatmentsystems. Also, because of relatively low BOD, high rate systems which function better for higher BODconcentrations are not appropriate. Table 4 summarizes the performance data of digestors used for thetreatment of slaughterhouse wastewater.

The anaerobic contact reactor appears to be more suitable compared to UASB as the latter is constrainedby the lack of formation of granules and there is also loss of sludge due to high fat concentrations. Hence,a pre-treatment step for removal of fats and suspended solids becomes essential if an UASB is to be used.However, for a low COD load, the more efcient UASB appears to result in a high COD reduction. In astudy on sh meal processing wastewater, treatment in an upow anaerobic lter was carried out after acentrifugation step to remove the solids [ 19]. The maximum applied OLR was 5 kgCOD/m 3 day. Anincrease in the recycling ratio from 1:10 to 1:5 resulted in the accumulation of V&A, ammonia and VSS.

An anaerobic uidized bed reactor of 1.2 1 capacity has been tested in the laboratory for wastewaterfrom slaughterhouse with a COD concentration up to 4500 mg/l [ 20]. More than 94% COD reductioncould be obtained for an OLR of 27 kgCOD/m 3 day. It was reported that due to the presence of unusedacids in the reactor, it was essential to maintain the desired alkalinity.

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Table 3. Characteristics of the wastewater from the slaughterhouses*

Parameter Concentration (g/l)

pH 6.8-7.8

COD 5.2-11.4

TSS 0.57-1.69

Phosphorus 0.007-0.0283Ammoniacal nitrogen 0.019-0.074

Protein 3.25-7.86*(Ruiz I. et al. [21])

Table 4. Treatment systems for slaughterhouse wastes*

Reactor Capacity (m 3 ) OLR (kgCOD/m 3 day) Reduction (%)

UASB (granular) 33 11 85

UASB (occulated) 10 5 80-89

Anaerobic lter 21 2.3 85

Anaerobic contact 11,120 3 92.6*( Johns M.R. [18])

3.2 Cheese Whey and Dairy

The liquid waste in a dairy originates from manufacturing process, utilities and service sections. Thevarious sources of waste generation from a dairy are spilled milk, spoiled milk, skimmed milk, whey, washwater from milk cans, equipment, bottles and oor washing. Whey is the most difcult high strength wasteproduct of cheese manufacture. This contains a proportion of the milk proteins, watersoluble vitaminsand mineral salts. The characteristics of the dairy wastewater and cheese whey are given in Table 5 .

The treatment of cheese whey wastewaters by anaerobic degradation is constrained by the drop in pHthat inhibits further conversion of acids to methane. This can be taken care of with buffering action in a

hybrid reactor, which is not possible in an UASB reactor. However, with proper startup, UASB reactorscan also cope with cheese whey wastewaters at low pH of 4 even at high OLR of 6.5 kg COD/m 3 day[22].

A high treatment efciency with 90% COD reduction has been achieved in laboratory and pilot scalereactors at both mesophilic and submesophilic temperatures with a maximum OLR of 28.5 kgCOD/m 3

day and 9.5 kgCOD/m 3 day, respectively. At ambient temperature, in a 10.7 m 3 reactor, a treatmentefciency o 95% with maximum OLR of 6.5 kgCOD/m 3 day has been reported. In a study on treatment of dairy wastewater with low COD of 2.05 g/l, very high OLR of 31 kgCOD/m 3 day was possible at a HRTof 1.7 h [23]. The COD reduction of 95% dropped to 70-80% with increase in OLR to 45 kgCOD/m 3 day.This is a common problem encountered with cheese whey, that as the substrate loading is increased, theacidogenic region extends into the methanogenic. This makes the entire region acidic, ultimately resulting

in the failure of the reactor [24]. Thus, two-stage process becomes essential for improving the biogasproduction rate and methane yield. The effect of temperature and pH control on biogas production andCOD reduction has been summarized in the studies carried out by Ghaly A.E. [ 25] (Table 6 ). It is clearthat buffering is needed initially for maintaining the pH but at a later stage, the stability improves with amature microbial population.

A hybrid reactor was used with a pre-acidication step to treat three different dairy efuents-cheeses,fresh milk and butter wastewaters [ 27]. The COD reduction was found to be 91-97% for OLR rangingfrom 0.97 to 2.82 kgCOD/m 3 day. The methane yield was 0.287-0.359 m 3 /kgCOD removed. Apart

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Table 5. Characteristics of dairy efuent*

Components (mg/1) Dairy Whey Whey permeate

pH 5.6-8 ˜ ˜

COD 1120-3360 75,000 50,000

BOD 320-1750 ˜ ˜

Lactose ˜ 40,000 40,000Propionate (mmol/1) ˜ 5 4

K (mmol/1) ˜ 38 36

Ca (mmol/1) ˜ 7 2

Suspended solids 28-1900 ˜ ˜

Total solid ˜ 50,000 42,000

Oil and grease 68-240 ˜ ˜*(Central Pollution Control Board, 1992 and [26])

Table 6. Effect of temperature and pH control on the treatment of cheese whey

Temperature ( ◦ C) HRT (days) Biogas production (L/day) COD reduction (%) Methane (%)

1 2 1 2 1 2

2510 83.70 27.90 28.2 0.5 70.8 20.2

15 71.30 24.80 32.2 6.1 71.0 20.1

20 60.45 20.15 34.9 8.7 70.9 20.2

3510 156.50 58.90 28.5 10.2 70.8 20.1

15 139.50 49.60 33.6 13.4 70.9 20.2

20 125.50 41.85 36.0 15.6 70.9 20.21: With pH control, 2: Without pH control Ref: Ghaly AE. [25]

from the hybrid reactor other alternate reactor types have also been tried for the treatment of dairy-basedwastewaters ( Table 7 ). In addition, a 450 m 3 novel multiplate anaerobic reactor has been tried forcheese whey efuent in a cheese factory in place country-region Canada [ 28]. The COD of the efuent

ranged between 20 and 37 kg/m3

. The OLR uctuated between 9 and 15 kgCOD/m3

day. The maximumefciency in terms of COD removal was 92% and average methane production rate was 4 m 3 /m3 day.

In the study carried out by Guitonas et al. [ 29], a xed bed reactor of 10.7 L volume with cellsimmobilized on rice straw was used for the treatment of milk based synthetic organic waste. Theadvantage of the system was the lower adaptation time with change in the OLR.

3.3 Pulp and Paper

The manufacture of pulp and paper broadly involves the following steps:

1. Pulping process, involving the pulping of cellulosic materials by mechanical, chemical or chemo-

mechanical means.

2. Bleaching process, wherein the colour on pulp due to lignin is removed by using chlorine or otheroxidising agents.

3. Paper making involving the blending of pulp with water in desired proportion and further processingin paper machine.

In the pulp and paper industry, there are various points of wastewater generation, some wastewaterresults from leaks and spills from digester. Pulp washing and bleaching gives wastewaters of various

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Table 7. Performance study of different types of anaerobic reactors for the treatment of cheese whey and dairywastewater*

Reactor HRT (days) Inuent conc.(g/l) OLR (kgCOD/m 3 day) COD reduction (%)

UASB 2.3-11.6 5-77 1-28.5 95-99

UASB 5.4-6.8 47-55 7-9.5 90-94

UASB 3.3-12.8 16.50 1-6.7 90-95

UASB (dairy) 17 h 2.05 31 90

UASB (cheese whey) 5 4.5-38.1 – –

2-stage (cheese whey) 10-20 72.2 – 36

UFFLR 5 79 14 95

DSFFR 5 13 2.6 88

FBR 0.4 7 7.7 90

FBR 0.1-0.4 0.8-10 6-.40 63-87

AAFEB 0.6-0.7 5-15 8.2-22 61-92

AnRBC 5 64 10.2 96

SDFA – 69.8 16.1 99

UASB 1.5 11 7.1 94

UASB 5 5-28.7 0.9-6 97-99

DUHR 7 68 10 7UASB (whey permeate) 5-0.4 10.4 – –

An-RBC (cheese whey and dairy manure) – – – 46

characteristics depending on the bleaching sequence. Bleaching section results in wastewater andchlorolignins. Wastewater is also generated from paper machine section, caustic chlorine manufactureand black liquor recovery. There are variations in the COD, inhibitors and the degradability dependingupon the source of the wastewaters ( Table 8 ).

Chlorine bleaching efuents are not suitable for anaerobic treatment due to their low biodegradabilityand presence of toxic substances that affects the methanogens. Some of the alternate chlorine bleachingprocesses currently being adopted are elemental chlorine free and total chlorine free bleaching. In thestudy by Vidal et al., [22], the toxicity and degradability of the above bleaching efuents were comparedwith that of chlorine bleaching efuents. The effect of the elemental chlorine free and chlorine bleachingefuents were similar but the total chlorine free efuents were found to be less toxic. This can be attributedto the fact that apart from elemental chlorine, other components such as wood resin compounds producedduring extraction processes are toxic. The COD reduction was found to be 75% in case of efuentgenerated from total chlorine free bleaching process whereas the reduction is 67% for chlorine bleachingefuent. The application of biological granular activated carbon process for the treatment of bleach plantefuent is evident from the study carried out by Jackson-Moss et al. [ 30]. It was observed that 50% of the COD and colour could be removed and that there was improvement in the adsorptive capacity due tomicrobial activity.

A laboratory scale study was carried out by Korczak et al. [ 31], for the anaerobic treatment of efuentsfrom acid hydrolysis of wood from sulfate cellulose production and from the sulte cellulose berswashing. The efciency was about 80% in terms of COD reduction and the methane production was 0.34m3 /kgCOD removed for the high strength efuent (63,000 mg/1) from acid hydrolysis. However, forthe efuent from cellulose washings, the COD reduction was only 20-30% and the methane yield was0.27-0.36 m 3 /kg COD removed. This was due to the fact that the efuent contained refractory compoundssuch as lignin derivatives, resins and tannins apart from sugars. An attempt has been made to purify thethermomechanical pulp efuent by combining a nanoltration method to anaerobic digestion [32]. Thisnovel process was found to result in a very clean water that could be reused in the water circulation system

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Table 8. Characteristics of wastewater generated from pulp and paper industry*

Wastewater COD (mg/1) Degradation (%) Inhibitors

Wet debarking 1300-4100 44-78 Tannins, resin acids

Pulping 1000-5600 60-87 Resin acids

Thermomechanical

Chemithermomechanical 2500-13,000 40-60 Resin acids, fatty acids, sulfurChemical pulping 7000 ˜ Sulfur, ammonia

Sulte condensate

Chlorine bleaching 900-2000 30-50 Chlorinated phenols, resin acids

Sulte spent l iquor 120,000-220,000 ˜

Kraft condensate 1000-33,600 83-92 resin acids, fatty acids, terpenes

Sulte condensate 7500-50,000 50-90 Sulfur, organic sulfur*(Rintala JA, Puhakka JA. [ 34])

Table 9. Comparison of treatment efciency of various reactors for wastewaters from different streams of paper andpulp

Reactor type Wastewater COD removed (%) OLR

UASB Debarking 40 40 kgCOD/m 3 day

Fluidized bed Debarking 50 (BOD) 0.66 m 3 / m3 day

UASB Mechanical pulping ˜

Mesophilic Thermomechanical 60-70 12-31 kgCOD/m 3 day

55-70 ◦ C 60 80 and 13 kgCOD/m 3 day

Chemithermomechanical 60 4 and 20 kgCOD/m 3 day

35-55 4.7-22 kgCOD/m 3 day

Contact process Sulte condensate 30-50 5 kgCOD/m 3 day

of the plant. In the case of paper and pulp mill efuent, a four stage treatment process ˜ pretreatment,anaerobic treatment using an UASB, aerobic treatment and tertiary otation was found to be successful.

This had resulted in an average COD reduction of 82% [ 33].Table 9 summarizes the use of different types of reactors for the treatment of paper and pulp efuent.

3.4 Sugar and Distillery Waste

The manufacturing process in a distillery involves dilution of molasses with water followed by fermen-tation. The product is then distilled to obtain rectied spirit or neutral alcohol. The distillation processresults in the generation of a strong organic efuent ( Table 10 ). The source of other wastes is fromoor washings, recovery units of yeast and other byproducts. The sugar manufacturing process broadlyinvolves the extraction, clarication and concentration of sugarcane juice. Finally, the concentrated juice

is crystallized and dried. The manufacturing process primarily produces bagasse and press mud as waste.In addition, the process generates wastewater, with the typical characteristics as summarized in Table 10 .In the case of efuent from a cane sugar factory, the buffering capacity is low and the alkali requirement

is high leading to high operational cost. An increased growth rate of the methanogens at higher tempera-tures makes the thermophilic anaerobic digestion process a suitable alternative to mesophilic digestion.With synthetic sugar waste in a 5.75 l UASB reactor, more than 85% conversion of glucose could beachieved up to 49.3 kgCOD/m 3 day within a period of 92 days. The maximum methane production was14.1 m 3 CH4 /m3 day. The granules were well formed and the sludge was maintained in the granular state,

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Table 10. Characteristics of sugar cane and distillery efuents*

Components Concentration (mg/l)

Sugar cane Distillery

pH 8.14 3.8-4.4

COD 276 70,000-98,000

BOD 54 45,000-60,000Na 4.05 150-200

K 1.64 5000-12,000

Fe 10.83 -

Cu 0.72 -

Mn 0.06 -

Total solids - 60,000-90,000

Total suspended solids - 2000-14,000[26], [36])

starting from 48 days after the feeding was started [ 35].A diphasic xed lm reactor with GAC as support media has been used for treatment of distillery

spentwash. Though the COD reduction is only 67.1%, the gas yield is high at 0.45 m 3 /kgCOD removedwith a methane content of 70%. The HRT is reported to be 4 days corresponding to an OLR of 21.3kgCOD/m 3 day. In the acid phase, the optimum condition is at an HRT of 1.2 days, corresponding to anOLR of 54-72 kgCOD/m 3 day [36].

For the treatment of stillage from sugarcane molasses using an UASB reactor, the dilution had asignicant effect on the loading rate. In a 100 l reactor for stillages with COD ranging from 35 to 100g/l, an OLR of 24 kgCOD/m 3 day resulted in 75% COD removal and a biogas production of 9 l/l daywith methane content of 58%. Feeding with undiluted stillage resulted in a tremendous increase in theconcentrations of acetic and propionic acids, thus affecting the stability of the reactor [ 37]. Malt whiskydistillery potale, a liquid waste product from the malt whisky industry, treated in a laboratory scale UASBreactor [ 26] indicated the importance of dilution and pH control in attaining a high COD reduction.

There is normally a rise in the pH due to ammonia production during the process of digestion. Themaximum loading rate for a stable operation was 15 kgCOD/m 3 day at a retention time of 2.1 days. Thefeasibility of UASB for distillery wastewater at a high temperature of 55 ◦ C was investigated by Harada etal. [38]. In a 140 l capacity UASB reactor for an inuent concentration of 10 g COD/l, an OLR of 28kgCOD/m 3 day could be attained. However, the COD reduction was very low at approximately 65%.Application of UASB for the treatment of simulated distillery waste was studied in a 29 l UASB reactor byRao et al. [39]. The maximum organic loading rate achieved was 47 kgCOD/m 3 day. The minimum HRTwas 4.9 h and a methane yield of 0.29 m 3 CH 4 /kgCOD removed was obtained. A short period of 10 dayswas sufcient for the reactor to recover after a shutdown for one month. The performance of the reactoris currently being studied with the efuent from a local distillery. Thermophilic anaerobic digestion forvinasse, the wastewater of alcohol distilleries has also been carried out with the adapted sludge [ 35]. After

adaptation of the sludge for 4 months, an organic load of 86.4 kgCOD/m 3 day could be accommodated.The methane generation rate is 26 m 3 CH4/m 3 day. The high concentration of vinasse was found to affectthe size of the sludge granules though the overall reactor performance was not affected. In certain cases,supplementing the efuent with the nutrients such as nitrogen and phosphorus has proved effective. In thecase of anaerobic digestion of wood ethanol stillage using an UASB reactor [40], supplementation withnitrogen, phosphorus and alkalinity resulted in a stable reactor performance at an organic loading rate of 16 kgCOD/m 3 day. The soluble COD and BOD removal was 86% and 93%, respectively. However, thecolor removal was just 40%. The methane yield at this loading rate was 0.302 m 3 CH4 / kg COD removed.

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4. MODELLING OF ANAEROBIC FERMENTATION

4.1 Theoretical aspects

Fermentation is an enzyme-catalyzed reaction. The objective of this section is to develop suitablemathematical expressions for the rates of the substrate consumption. A brief outline of some thesemathematical expressions that used in the fermentation processes has been discussed.

4.1.1 The reaction rate

If the reaction is

S → P (3)

Reaction rate (V) is, in the quasi-steady state approximation, is dened by

V = − (ds / dt ) = ( d p / dt ) (4)

4.1.2 Michaelis–Menten kinetics

In biochemistry, Michaelis–Menten kinetics is one of the simplest and best-known models of kinetics.The model takes the form of an equation describing the rate of enzymatic reactions, by relating reactionrate r s to Cs, the concentration of a substrate S . Its formula is given by

r s = − d (C s)/ d (t ) = ( r sm · C s)/ (k m + C s) (5)

Here, r sm represents the maximum rate achieved by the system, at maximum (saturating) substrate

concentrations. The Michaelis constant k m is the substrate concentration at which the reaction rate is half of rm. Biochemical reactions involving a single substrate are often assumed to follow Michaelis–Mentenkinetics, without regard to the model’s underlying assumptions [ 41].

4.1.3 Inhibition

A major contribution of the Michaelis–Menten approach to enzyme kinetics is accounting quantitativelyfor the inuence of modulators.Competitive Inhibition

The following sequence reasonably approximates the interactions of a totally competitive inhibitor and

substrate with an enzyme. Here E, S, I and EI denote the free enzyme, the substrate, the inhibitor and theenzyme-inhibitor respectively.

If more than one substrate is present, they can compete with each other for the limited number of available active enzyme sites and this form of competition is referred to as competitive inhibition .

E + S ↔ Es (6)

E + I ↔ EI (7)66

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ES ↔ E + P (8)

The maximum rate of the substrate consumption r sm is unaffected, but the apparent Michael’s constantis increased by the presence of the competitive inhibition. Stated in different terms, the rate of reductioncaused by a competitive inhibitor can be completely offset by increasing the substrate concentration

sufciently.Non-Competitive Inhibition

The simple type of that enzymatic reaction can be described in the following equation:

E + S ↔ ES ↔ E + P (9)

Whereas in competitive inhibition these two chains reaction proceed simultaneously and generallythe inhibitor bears a structural similarity with the ’natural’ substrate, but in non-competitive inhibitionthe interfering material can combine equally with free or complex enzyme. Since it attacks the enzymemolecule, whole its effect is not inuenced by substrate concentration but only, by that of the inhibitor.

EI + S ↔ EIS (10)

ES + S ↔ EIS (11)

Uncompetitive InhibitionThis is a specialized variation of non-competitive inhibition systems, in which the inhibitor does not

combine with the free enzyme but has an afnity with the enzyme substrate complex.

E + S ↔ ES ↔ P + E (12)

⇓

+ I

⇓

ESI

Inhibition by Excess SubstrateIn the idealized picture of an enzyme molecule it was assumed that all active centers were equally

attractive to molecules of the ’natural’ substrate and it could be interred that each center combined withone molecule of substrate. A more complex mechanism is clearly operative in a number of cases, when itis found that the reaction velocity rises to a maximum at a certain substrate-enzyme ratio and then fallssteeply as the substrate concentration is raised.

The mechanism of substrate inhibition can be described as follow, when substrate concentration isincreased above the threshold limit; the specic growth rate is proportional to an increase in the substratelevel and approaches a maximum value. A subsequent increase in the substrate concentration willultimately lead to a decrease in the specic growth rate. This well-known phenomenon is termed ”substrateinhibition” and it is frequently observed in industrial fermentation and biological waste treatment.

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At high substrate level, there is an increase in adsorption or complexity between enzymes and substrateswhich in turn reduces the enzyme activity. From a biological viewpoint, an increase in the substrateconcentration could cause an alteration in the cell metabolism such as an overproduction of a molecule byone pathway which results in the feedback inhibition of a second related pathway. The enzyme inhibitionkinetic model for describing microbial inhibition kinetics is Haldene equation . [42]

r s = ( r sm · S )/ (K S + S + S 2/ K I ) (13)

Inhibition by the ProductNegative effect of fermentation end products on microbial activity constitutes problems of fundamental

biological interest. The accumulation of end products such as ethanol, butanol, lactic acid, etc. resultsin a continuous decrease in specic growth rate and product formation rate; at sufciently high productconcentration, all cell metabolic activities completely cease.

The mechanism of alcohol inhibition has been used to explain the product inhibition phenomenon.Alkanols such as ethanol, propanol, and butanol inhibit the transport of sugars, ammonium, and aminoacids. All these inhibitors are noncompetitive in nature and act within the hydrophobic region of the plasma membrane . Alkanols also affect the cell membrane potential and proton extrusion from the cell or

enhancing the passive proton inux into the cell. They can be expected to inhibit and / or inactive if notall the enzymes.

4.2 Application of Monod Kinetic for UASB Reactor

Monod model is widely applied for describing the relationship between reaction rate and substrateconcentration in methane fermentation process. According to literature, the UASB reactor has two distinctcharacteristics: the sludge bed and blanket which can be described as a combination of a completelymixed region and the ow characteristics in the setting zone which can be described as plug ow, takingaccount of the effect of rising gas bubbles from the sludge bed and blanket zone, the UASB reactor is

assumed to be completely mixed ow.For an UASB reactor without biomass recycle, the rate of change of biomass and substrate in the systemcan be expressed respectively as Eqs. (14) and (15):

dX / dt = ( Q/ V b)∗ X o − − (Q/ V b)∗ X e + µ ∗ X − − K d ∗ X (14)

dS / dt = ( Q/ V b)∗S o − − (Q/ V b)∗S e − − (µ ∗ X )/ Y (15)

The ratio of total biomass in the reactor to biomass wasted per given time correspond to the overagetime called SRT or referred as mean cell residence time ( θ c) and calculated from Eq. ( 16)

θ c = ( V b∗ X )/ (Q∗ X e) (16)

The relationship between specic growth rate and limiting substrate concentration can be expressed byMonod Eq. ( 17) as

µ = ( µ m∗S e)/ (K s + S e) (17)

The constant µ m indicates maximum growth rate of microorganisms when the substrate is being used atits maximum rate, and K s indicates the level of substrate concentration at one half the specic substrate

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utilization rate. If it is presumed that biomass concentration of inuent wastewater, X o is negligible and atsteady-state conditions (when dX / dt = 0, and dS / dt = 0), then the equations biomass derived from Eqs.(14)- (17) are as

X = [Q∗Y ∗θ c∗(S o − S e)]/ [V b∗(1 + K d ∗θ c)] (18)

S e = [K s∗(1 + K d ∗θ c)]/ [θ c∗(µ m − K d ) − 1] (19)

Eqs. (18) and (19) are nonlinear in nature; hence, it is indispensable to transform them to equivalentlinearized forms so that linear regression can be used for estimating the values of kinetic constants. Twodifferent linearized equations can be framed to obtain values Y and K d , which are as

[Q∗(S o − S e)]/ (V b . X ) = [( 1/ Y ∗1/ θ c)+ ( 1/ Y ∗K d )] (20)

1/ θ c = [Y ∗Q∗(S o − S e)]/ [(V b∗ X ) − K d ] (21)

To obtain the estimates of µ m and K s, linear regression is commonly applied on the linearized equationderived from Eq. (19). A review of literature reveals that Eq. (22) has been widely used for estimating µ mand K s [43] and [44].

[V b∗S e∗ X ]/ [Q∗(S o − S e)] = [(Y ∗S e)/ µ m + ( Y ∗K s)/ µ m] (22)

The other forms of linearized equation reported in the literature for estimation of µ m and K s are asfollows:

[( X ∗V b)/ Q∗(S o − S e)]∗1/ Y = [(K s/ µ m)∗(1/ Se)+ ( 1/ µ m) (23)

[Q∗(S o − S e)∗Y ]/ ( X ∗V b) = [ µ m − − [K s∗Q∗(S o − S e)∗Y ]/ ( X ∗V b∗S e)] (24)

4.3 Grau second-order multi-component substrate removal model

The general equation of a second-order kinetic model is exemplied in Eq.( 25) [45].

dS / dt = K S 2∗ X ∗(S e / S o) (25)

If Eq. (25) is integrated (boundary conditions: S = S o to Se ; and t = 0 to θ H ) and then linearized, Eq.(26) will be formed:

(S o∗θ H )/ (S o − − S e) = [ θ H + S o/ (K S 2∗ X )] (26)

If the second term of the right part of Eq. (26) is acknowledged as a constant, Eq. (27) will be obtained(Buyukkamaci and Filibeli,2002)

(S o∗θ H )/ (S o − − S e) = ( b∗θ H + a ) (27)

(So – Se) / So expresses the substrate removal efciency and is symbolized as E. Therefore, Eq. (27)can be written as

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(θ H / E ) = ( a + b∗θ H ) (28)

The coefcient ’b’ in Eq. (28) is close to one and generally reects the impracticality of attaining a zerovalue of COD. The substrate removal kinetic constant K S 2 = So / (a*X). This indicates substrate removalrate for each unit of microorganism depending on second-order substrate removal kinetics. If the value of

KS 2 in terms of ’a’ is replaced in Eq. ( 26) then Eq. (29) can be obtained as

S e)/ S o = 1/ (1 + θ H / a ) (29)

5. CASE STUDY

5.1 Kinetic Analysis and Simulation of UASB Anaerobic Treatment of a SyntheticFruit Wastewater

An Upow Anaerobic Sludge Bed (UASB) reactor was used by Diamantis and Aivasidis [46] to

evaluate mesophilic anaerobic treatment of a pre-acidied fruit wastewater. The system was operatedat increasing volumetric loading rates by sequentially increasing wastewater owrate. The operationaltemperature was maintained initially at 37 ◦ C and consequently decreased to 30 and 25 ◦ C. For thevolumetric loading rates examined i.e. 5-35 Kg COD m − 3d− 1).

In the present paper the rst-order kinetic model was applied on a pilot-scale UASB reactor fed with apre-acidied fruit wastewater, since relevant data were not detected in the literature. The kinetic constantwas determined at three different operational temperatures (37, 30 and 25 ◦ C). Aim of the paper wasto examine the efciency of the rst-order kinetic model to predict UASB reactor performance undercontinuous operation (in terms of COD removal and methane production).

5.1.1 Wastewater Characteristics

The wastewater was prepared daily by diluting 20 ml of peach nectar and pulp per L of tap water; total(COD TOT ) and soluble-COD (COD SOL ) were maintained at 3420 ( ± 100) and 3170 ( ± 130) mg COD L− 1

respectively. The wastewater was stored into a plastic tank at 4 ◦ C. Nutrients and trace metals were addedinto the storage tank to ensure that no limitation occurs (concentration in mg L − 1 : N=170; P=30; S=20;K=40; Ca=20; Mg=10; Fe=5; Cu=0.10; Zn=0.20; Mn=0.10; Ni=0.07; Co=0.02; Mo=0.01; Se=0.07;B=0.05).

5.1.2 Experimental Setup

The pilot plant facility ( Figure 5 ) comprised of a CSTR for wastewater acidication with variableworking volume (2-10 L) and a sequential UASB reactor having an operational volume equal to 2 L.

The efuent from the acidication stage with a pH of 3.75 was introduced continuously (after removalof suspended solids in a sedimentation tank) into the methane reactor. The pH of the UASB inuent wasregulated at 6.6 by aqueous solutions of NaOH and HCl, which were dosed into a 0.1 L conditioning tank.The latter was installed on the UASB recycle stream and was equipped with a magnetic stirrer. UASBrecycle ow and the substrate was fed into the conditioning tank using peristaltic pumps.

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Figure 5. UASB reactor with separated acidication for the anaerobic treatment of synthetic fruit wastewater.

The study was conducted at mesophilic conditions at 36.5 ( ± 0.6), 29.8 ( ± 0.3) and 24.4 ( ± 0.3) ◦ C. Ateach operational temperature, the performance of the pilot plant was assessed at sequentially increasingvolumetric loading rates from 5 to 35 Kg COD m − 3d− 1). The biomass concentration inside the UASBwas maintained at 14.7 ( ± 1.4) Kg V SS m− 3 reactor volume by regularly removing excess sludge.

5.1.3 Determination of Kinetic Constants

The rst-order kinetics is represented by the following equation:

P RS = dS / dt = K Se (30)

Where: R RS = volumetric substrate removal rate (Kg COD r m− 3d− 1), K = rst-order kinetic constant ( d − 1), S e =

efuent substrate concentration (Kg COD m− 3).Using equation ( 30) it is possible to determine the kinetic constant, K, as shown below:

K = P RS / S e = ( S 0 − S e)/ τ .(1/ S e) = ( S 0 − S e).Q/ (V .S e (31)

Where:S o = inuent substrate concentration (Kg COD m− 3), τ = hydraulic retention time (d), V = reactor

volume ( m3), Q = wastewater owrate ( m3d − 1).

In practice, the kinetic constant, K, is derived from the slope of the line of R RS versus S eusingexperimental data from different steady-state conditions. As shown in Figure 6(a) the values of K weredetermined equal to 23, 21 and 19 d-1 at 37, 30 and 25 ◦ C respectively. These values are signicantlyhigher than those reported by Borja and Banks [20] (0.9-4.7 d − 1) and this can be attributed to the differenttesting conditions (wastewater pre-acidication, higher biomass concentration).

The volumetric methane production rate [R CH 4 , (m3CH 4 m− 3d− 1)] is given by equation ( 32):

RCH 4 = Y CH 4 . R RS = Y CH 4 . L RS .U S (32)

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Where:Y CH 4 = methane selectivity coefcient ( m3CH 4 kg− 1CODr ), L RS = volumetric COD loading rate (kg CODo

m- 3d− 1) and U S = COD removal (-). The Y CH 4 coefcient is determined from the slope of the line of RCH 4 versus R RS using experimental data from different steady-state conditions.

According to Figure 6(b) the methane selectivity coefcient is equal to 0.324, 0.302 and 0.287 m 3CH4kg− 1CODr at 37, 30 and 25 ◦ C respectively. Among these values, especially the one obtained at 37 ◦ C, isclose to the theoretical (0.35 m 3CH 4kg− 1CODr ) and reveals the high degradability of the substrate.

5.1.4 Process Simulation

In full scale anaerobic digesters, the volumetric COD loading rate ( Figure 7(a) ) is determined by theraw wastewater COD concentration (kg m − 3) and the hydraulic retention time (d) or wastewater owrate (m 3 d− 1). After determining the kinetic constant K, using experimental data at different operationalconditions (e.g. owrate, temperature, biomass concentration, and wastewater strength), the reactorefuent COD (Se) and COD removal (U S ) can be calculated as follows:

S e = ( S 0 · Q)/ (Q + K · V ) (33)

U S = ( K · τ )/ (1 + K · τ ) (34)

In Figure 7(b) and Figure 7(c) the estimated and measured values of efuent COD and CODremoval are presented. It is evident that using the rst-order kinetic model it is possible to predict reactorperformance in terms of substrate removal. Furthermore, using equation (32) it is possible to estimate thevolumetric methane production rate from the experimentally determined volumetric COD loading rate,the YCH 4coefcient and the simulated COD removal values. In Figure 8 the experimental and simulatedvalues of R CH 4 during continuous reactor operation are presented. As shown, a slight deviation from theactual values occurs especially during reactor start-up period (initial 7 d). This is attributed to the gradualincrease of biomass concentration, which was under starvation for 14 months before the beginning of theexperiments. The quantity of VSS per L of settled sludge was 34 ( ± 4) and 62 ( ± 19) gVSS L− 1 beforestart-up and at the end of the experimental period respectively.

From this study we can conclude that prediction of efuent COD and methane production rate duringcontinuous reactor operation was possible using the rst-order kinetic model. The degradation of apre-acidied fruit wastewater was studied on the basis of UASB reactor. The UASB attained CODremoval levels higher than 70%. The kinetic constant decreased from 23 to 21 and 19 d − 1 at 37, 30 and25 ◦ C respectively.

6. CONCLUSION

Although most of the high rate reactors have proved their applicability for different high strengthwastewaters over a range of organic loading rates, there exist certain differences in the preference of aparticular type of digester over others in terms of various factors such as requirement of pre-treatment,dilution, control of operating conditions, etc. In the case of slaughterhouse wastewater, an anaerobiccontact reactor can be used without pre-treatment whereas for the usage of high rate digester such asUASB, a pre-treatment step for removal of the suspended solids and fats is essential prior to anaerobictreatment. Two phase digestion with pH and temperature control results in a higher biogas production rate

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(a)

(b)

Figure 6. Volumetric substrate removal rate as a function of efuent substrate concentration (a) and volumetricmethane production rate as a function of the volumetric COD removal rate (b) at 37 ( ), 30 ( ) and 25◦ C ( × )

with cheese whey wastewater digestion. An aerobic post-treatment is necessary to attain the permissibleCOD and BOD level before discharge. Due to the generation of wastewater from various sections of pulp and paper industry, there are variations in the composition and the treatability of efuents. Hence, itis preferable to treat the efuents from each section separately depending on their biodegradability and

suitability to the digestion process rather than treating the combined efuent. Advanced methods such ascoupling of reactors for suitable pre-treatment and post-treatment can result in complete treatment of theefuents with the acceptable limits.

It is clear from the review that there are no governing factors that dictate the suitability of any particularreactor design for a specic efuent. By suitable modications in the reactor designs and by altering theefuent characteristics, the existing high rate digesters can be accommodated for treatment of organicefuents. However, based on the characteristics of the different reactors such as efciency based onloading rate and COD reduction, biomass retention and other factors like cost, operation and maintenance

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(a)

(b)

(c)

Figure 7. Actual volumetric COD loading rate (a) efuent COD (b) and COD removal (c) during continuous reactoroperation [Estimated (—), measured values ( ♦ , ∆ ,)]74

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Figure 8. Actual( ) and simulated (–) volumetric methane production rate during continuous reactor operation

requirements, UASB and xed lm congurations appear to be the most suitableAn understanding of the process kinetics is vital in the design, development and operation of anaerobic

reactors. Based on the biochemistry and microbiology of anaerobic process, kinetics provides a judiciousbasis for process analysis, control, and design. In addition to quantitative description of the substrateutilization rates, process kinetics also deals with operational and environmental factors affecting theserates.

Bacterial growth kinetics are based on two fundamental relationships, i.e., growth rate and substrateutilization rate. Various kinetic models are reported for anaerobic processes predominantly based onMonod’s equation or its modications.

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