24

CHAPTER 2

REVIEW OF LITERATURE

2.1 GENERAL

The steady increase in pulp and paper industry’s wastewater quantities is substantial,

which requires many facilities to provide a high degree of treatment methods. Among different

methods, anaerobic treatment is considered as one of the best treatment options for paper and

pulp industry wastewater. In this section, microbial and factors affecting anaerobic digestion,

different types of anaerobic system, pulp and paper process and its pollution potential, domestic

wastewater treatment methods and its polluting effects, specific methenogenic activity test,

sludge granulations and residence time studies reported in the literature are discussed.

2.2 ANAEROBIC BIOLOGICAL TREATMENT

2.2.1 Conversion Principles

The transformation of complex macromolecules present in wastewater into biogas

requires the mediation of several groups of microorganisms. Different steps are necessary for

the anaerobic digestion of protein, carbohydrates and lipids. In general, anaerobic digestion

takes place in three phases, hydrolysis/liquefaction, acetogenesis and methanogenesis. The

different steps involved in an anaerobic microbial interactions have been described by many

researchers (Gujer and Zehnder, (1983). In each step there are different group of bacteria

involved in the processes, which are detailed in the following topics. In Figure 2.1, the

stoichiometric substrate flow is indicated by various processes involved in the anaerobic

digestion.

25

Figure 2.1 Schematic diagram of conversion reaction in anaerobic digestion

Biomass & Organicwastes

Simple Organics

Higher organic Acids

H2, CO2Acetic Acids

CH4

26

2.2.1.1 Hydrolysis

In the first stage, complex organic compounds such as carbohydrates, fats and proteins

are hydrolyzed, resulting in a simpler compound like glucose through enzymes produced by

fermentative bacteria. In an anaerobic digestion process where organic polymers form a

substantial portion of the waste stream to be treated, the hydrolyzing bacteria and their enzymes

are of paramount importance because their activity produces the simpler substrates for the

succeeding steps in the degradation sequence. The most common extra cellular hydrolytic

enzymes are lipases, proteases, amylase and cellulases, which are excreted by hydrolytic bacteria

(Zeikus 1980). Other extra cellular enzymes which may participate in the initial hydrolysis step

of anaerobic digestion are the pectinolytic enzymes such as those elaborated by some Bacillus

and Clostridium species.

In sewage sludge, hydrolytic bacterial populations are usually high and generally

comprise between 108 and 1010 bacteria per mL of sludge (Hobson et al 1974). The general

hydrolysis reaction is shown in Equation (2.1)

C6 H10 O4 + 2 H2O C6H12 O6 + H2 ….. (Eq. 2.1)

The rate of hydrolysis is influenced by many factors like pH, temperature, HRT and other

organic compounds depending on the substrate used (Pfeffer, 1980, and Gujer and Zehnder,

1983). In general, the hydrolysis step can be considered as the rate-limiting step in the

hydrolysis of particulate substrates (Pavlostatis and Giraldo-Gomez, 1991 and Nunez and

Martinez, 1999). In the treatment of pig slaughterhouse effluent, hydrolysis of proteins appears

to be the rate limiting step (Batstone et al., 1997). Proteins are degraded slower than

carbohydrates and lipids under acidogenic conditions (Elefsiniotis and Oldham, 1994). Proteins

are generally degraded to amino acids by proteases secreted by Bacteriodes, Butyrivibrio,

Clostridium, Fusobacterium, Pseudomonas and Streptococcus species (McInerney, 1988).

27

Lipids are hydrolysed into long and short chain fatty acids and glycerol by lipases and

phospholipases (Pavlostatis and Giraldo-Gomez, 1991). Hydrolysis of cellulose by the enzyme

complex yields into a variety of monosaccharides such as glucose, galactose, xylose, arabinose

and mannose (Elefsiniotis and Oldham, 1994). Starch, glycogen and related polysaccharides are

degraded by amylase enzymes by hydrolytic cleavage of α-1, 4- and or α-1, 6-glucosidic

linkages (Stronach et al., 1986).

2.2.1.2 Acidogenesis

The second stage of the digestion process is the fermentation of amino acids and sugars,

giving rise to the intermediate products and acetate or hydrogen. Acetate is the most important

compound quantitatively produced in the fermentation of organic substrates by bacterial

populations, with the proportionate production of secondary consequence. Acidogenesis is

usually the fastest reaction and growth of acedogens is faster as well as less sensitive to pH

variation than acetogens / methanogens (Cohen et al., 1980). The conversion of single amino

acids by reductive or dehydration deaminations is carried out under anaerobic conditions by

Clostridia, Mycoplasmsas and Streptococci. The formation of end products mainly depends on

composition and nutrients present in the substrate, temperature and pH. The fermentations of

substrates yield acetone, butanol, butyric acid and iso-propanol is mainly the action of the

predominant bacteria like Clostridium and Butyribacterium. The conversion of sugars to

pyruvate via the Embeden-Meyerhof-Parnas(EMP) or glycolysis pathway initiates the butyric

acid fermentation. As a consequence of pyruvic acid fermentation, the other compounds such as

acetic, propionic, butyric, formic, lactic acids, alcohols, ketones, and aldehydes are produced

(Eastmann and Fergusan, 1981 and Pavlostatis and Giraldo-Gomez., 1991).

At steady state the main degradation pathway is through acetate, carbon dioxide and

hydrogen and the reduced fermentation intermediates can be used directly by methanogens. The

accumulation of electron sunk such as lactate, ethanol, propionate, butyrate and higher VFAs is

the response of bacteria for increased hydrogen concentration the liquid (Schink, 1997).

28

In an anaerobic digestion, the regulation of H2 partial pressure is very important. At low

partial pressure H2, the formation of organic compounds such as acetate, CO2, H2 is

thermodynamically favored. On the other hand, if the partial pressure is high, the formation of

products such as propionate and some other organic acids, lactate and ethanol occurs (Zehnder,

1978).

2.2.1.3 Acetogenesis and homeacetogenesis

Acetogenesis is achieved by syntrophic associations with hydrogen consuming

methanogens. The H2 and acetate synthesized by the metabolism of the OHPA obligate

hydrogen producing acetogenic bacteria digester population have been estimated to provide the

substrate for 54 percentage of the total CH4 produced in anaerobic reactor systems (Stronach et

al., 1986).

2.2.1.4 Methanogenesis

Methanogenesis is the final step in anaerobic digestion process in which butyrate and

propionate as well as acetate is thus converted to CH4, the most reduced organic molecule.

Methane is produced from acetate via fermentation in which the acetate molecule is cleaved and

the methyl group is reduced to methane with electrons derived from oxidation of the carboxyl

group to CO2 (Ferry 1992). The process illustration and the stoichiometric reaction for the

conversion of CO2 and H2 to CH4 are given in Equations (2.2) and (2.3), respectively.

CH3 COO - + H+ CH4 + CO2 (ΔGo = -36 kj /mol) …(Eq 2.2)

CO2 + 4 H2 CH4 + 2 H2 O (CO2 reductions) …(Eq 2.3)

Among many methanogenic genera, only two, Methanosarcina and Methanoseata are

known to grow by the acetoclastic reaction (Zinder, 1984). Methanosarcina sp. grow rapidly,

with lower substrate affinity and predominate at a low dilution rate. In a moderate dilution of

substrate and low acetate concentration both species may grow equally (Chartrain and Zeikus,

29

1986). The hydrogen consuming microorganisms are the fastest growing organisms and more

resistant to environmental changes than acetoclastic organisms. The minimum doubling time for

hetrogenotrophic methanogens has been estimated to be 6th day, whereas it is 2.6 day for

acetoclatic methanogens (Mosey and Fernandes, 1989). Other than above discussed bacteria the

sulphate reducing bacteria like Desulfovibrio, Desulfuromonas acetoxidans and

Desulfotomaculum are most commonly involved in the degradation of organic polymers to CH4

plays an important role in anaerobic digestion (Zeikus, 1980).

2.3 ANAEROBIC DEGRADATION OF ORGANIC MATTER

Under suitable conditions in an anaerobic sewage treatment system a bacterial population

will develop that is compatible with the applied hydraulic and organic loads. Among the factors

that determine the removal efficiency of biodegradable organic matter, the following are

important.

i. The nature of the anaerobic matter to be removed.

ii. The suitability of environmental factors for anaerobic digestion.

iii. The retained amount of viable bacterial matter.

iv. The intensity of contact between the influent organic matter and the bacterial

populations.

v. The design of the anaerobic reactor system, e.g. whether or not the reactor

con`sists of compartments which are operated in series.

vi. The retention time of the wastewater in the anaerobic treatment system.

The last factor is really a dependent variable in the sense that other factors determine in

the environmental and operational conditions in the system and hence the required retention time

for a particular desired removed efficiency of organic matter in the wastewater flow. Hence

wastewater characteristics, type and design of treatment systems like HUASB, UASB,

EGSB/FBR, baffled and SBR decide the efficiency and conversion of organic matter. The

interaction of these various interrelated parameters and their relation are discussed.

30

2.4 EFFECTIVE MICROORGANISMS

The technology of Effective Microorganisms (EM) was developed during the 1970’s at

the University of Ryukyus, Japan (Sangakkara, 2002). Higa et al., (1998) reported that EM is a

mixture of group of organisms that has a reviving action on humans, animals, and the natural

environment. There are three types of microorganisms which are categorized into decomposing

or degenerative, opportunistic or neutral and constructive or regenerative. EM belongs to the

regenerative category whereby they can prevent decomposition in any type of substances and

thus maintain the health of both living organisms and the environment (PSDC, 2009).

The basic purpose of EM is the restoration of healthy ecosystem in both soil and water by

using mixed cultures of beneficial and naturally-occurring microorganism. Therefore, the EM

has great potential in creating an environment most suitable for the existence, propagation, and

prosperity of life (Higa and Parr, 1994).

Maalim et al., (2009) reported that EM has shown a great power to reduce coliform in

UASB reactor under tropical conditions and it can also be used to enhance performance of

UASB reactor in order to produce better quality effluent that can be discharged into receiving

water bodies without tertiary treatment. It also produces bioactive substance and secretes various

enzymes during hydrolysis and catalyses subsequent anaerobic bioconversion resulting in the

efficient removal of COD and increasing biogas production.

2.5 ENVIRONMENTAL FACTORS

Important environmental factors affecting anaerobic sewage digestion are temperature,

pH, the presence of essential nutrients and the absence of excessive concentrations of toxic

compounds in the influent. In the case of sewage, the later three factors normally do not need

consideration. An adequate and stable pH is set by the presence of the carbonic system and no

chemicals are needed to correct the pH. Nutrients (both macronutrients, nitrogen and

phosphorous, and micronutrients) are abundantly available in sewage. Compounds that could

31

exert a distinct toxic influence on the bacterial population are generally absent in domestic

wastewater. It will be shown that the toxic effect of sulphide is not serious and that dissolved

oxygen can only constitute a problem if the design of the anaerobic treatment system is

inadequate.

2.5.1 Temperature

Anaerobic treatment process and production of methane are strongly affected by

temperature, which inhibits the microbial activity of the system. There are three different

temperature ranges available for anaerobic treatment and their growth of anaerobic bacteria

which includes psychrophilic (18-250C), mesophilic (25-400C) and thermophilic (40-700C). In

many cases, methanogenesis is the rate-limiting step in the overall degradation process,

anaerobic reactor should be operated around 37 or 550C to ensure methanogens to grow at their

optimum temperatures (Pavlostatis and Giraldo Gomez, 1991).

Psychrophilic range of temperature makes the conversion process slow and incomplete.

Several investigators have analyzed the low temperature treatment of diluted wastes (Zeeman

and Lettinga 1999 and Elmitwalli et al., (1999), Bodik et al., (2000) also observed that at low

temperature treatment, COD removal efficiency is mainly dependent on temperature and HRT.

Under low values of HRT the removal efficiency is more influenced by temperature.

Lin et al., (1987) observed that the optimum temperature for the mesophilic

methanogenesis process is 350C. They indicated that the methane production was temperature

and loading rate dependent. Bacilli were the predominant microbial species and this predominant

was independent of digestion temperature. At mesophilic range, with increasing temperature the

saturation constant (Ks) decreased, while the maximum specific substrate utilization rate (Umax)

and growth yield (Yg) increased. The reduction in operating temperature in the anaerobic

digestion process does not only retards the hydrolysis step but also leads to a significant decrease

in the maximum growth and substrate utilization rates (Lettinga et al., 2001).

At thermophilic conditions, it was observed that the digestion is more efficient in

degrading organics, increased pathogen destruction and improved mass transfer rates (Rubia et

al., 2002). However, thermophilic systems are much more sensible against the changing of the

32

organic load contents of substrate and the alteration in the environmental parameters. Moreover,

energy for heating and maintaining the temperature compensate the efficiency of the system.

Yu and Fang, (2003) studied the influences of temperature and pH in the treatment of

gelatin-rich wastewater in up flow reactor. They found that the gelatin degradation efficiency

and rate, degree of acidification, and formation rate of volatile fatty acids and alcohols slightly

increase with temperature. Temperature affected the acidogenesis of gelatin according to the

Arrhenius equation with low activation energy of 1.83 Kcal/mol due partial temperature

compensation effect.

At low temperatures, the low hydrolysis rate and a decrease in the degradable organic

matter fraction were found to cause the deterioration of the overall anaerobic reactor

performance (Elmitwalli et al., 2001). One possible way to improve the performance of a UASB

reactor at low temperatures is to provide surface area for biomass attachment and growth in the

reactor volume above the sludge blanket (Tilche and Vieira, 1991). This can be accomplished by

replacing the typical gas/solids separator of the classical UASB reactor with filter media.

Beni Lew et al.,(2011) reported that the under temperate climate conditions the integrated

UASB digester system can be a good alternative for raw domestic wastewater treatment. During

cooler periods below 20 °C, influent wastewater particulate matter accumulates in the top part of

the UASB reactor sludge and can be removed and treated in a small, heated (30 °C) digester and

returned to the UASB reactor. The digester can be designed with an operational volume 33per

cent smaller than the UASB (operated at 6h HRT) and with a retention time of 3.20 days. The

methane produced in the digester is sufficient to warm the solids influent to the desired operating

temperature.

Elmitwalli et al., (1999) compared the performances of a hybrid UASB-filter and a

classical UASB reactor at 13°C. Beni Lew et al.,(2011) reported that both reactor designs gave

similar performance. At summer conditions (20–28°C) COD removal rates above 72per cent can

be obtained in both reactors. At lower temperatures (14–10°C) when the bacterial activity is

33

lower, solids accumulation in the reactor is more pronounced with better solids retention in the

classical UASB. In both reactors, the accumulated sludge from the winter is subsequently

digested in the following summer, which is evidenced by a large gas production at the beginning

of the new warm season.

The toxic effects of high VFA concentrations on the anaerobic digestion process have

been studied and reported by Ahring et al.,(1988) that the resulting drop in pH is generally

considered to be the main cause of the toxicity. The rate of methane production from hydrogen

was lowered by long chain fatty acids. The methane production of acetate was inhibited so

strongly that a long lag period appeared (Wang et al., (1999). Hence it is necessary to

investigate the optimum conditions and efficiencies of digesters by examining VFAs.

Kripa and Viraraghavan (2000) studied the effect of temperature on performance,

microbiological, and hydrodynamic aspects of UASB reactors treating municipal wastewater.

Two reactors were started-up at 20°C and subsequently operated at temperatures of 32, 20, 15,

11, and 6°C. COD removal efficiency ranged from 70 to 90 percent up to an HRT of 6 h and at

11°C. The performance of the reactor was not very satisfactory during 6°C operation with an

average COD removal 40 percent. Digital image analysis and scanning electron microscopic

observations of sludge samples revealed aggregation of biomass in the form of irregular shaped

granules and concluded the hydraulic regime was impacted by the change in reactor operating

temperature.

Sreekanth et al., (2009) developed a 7L bench scale hybrid up flow anaerobic sludge

blanket (HUASB) at five different hydraulic retention times (HRTs) under thermophilic

conditions (55±3°C). The result showed that all three phenolic compounds from synthetic

wastewater could be treated effectively by the hybrid UASB reactor at different HRT’s varying

between 8 and 30 h under thermophilic conditions.

Turan yilmaz et al., (2008) studied the performance of mesophilic (35°C) and

thermophilic (55°C) anaerobic filters treating paper mill wastewater. The hydraulic retention

34

time (HRT) ranged from 6 to 24 h with organic loading rates (OLR) 1.07–12.25 g/L per day. The

result of the study showed that the higher organic loading rate, the SCOD removal performance

of thermophilic digester was slightly better compared to mesophilic digester.

2.5.2 pH

Anaerobic reactions are highly pH dependent each of the microbial groups involved in

the reactions has a specific pH range for optimal growth. The control of pH is fundamental to the

maintenance of optimal bacterial growth and or conversion process in anaerobic microbial

systems. Higher organic load cause to accumulate VFA at the anaerobic part of reactor resulted

in reduction of pH and a consequence reduction of methanogens, reducing removal efficiency in

this part (Moosavi et al., 2005).

The optimum pH for acidogenic bacteria is 5.2 to 6.5, and specific growth rate is over 2

days, whereas the optimum pH environment for methanogens is within the range 7.5 to 85.

Solera et al.,( 2002), Angelidaki and Ahring (1997) found that most methanogens often have a

lower optimum pH. Stronach et al., (1986) observes that the pH value less than 6.8 and greater

than 8.3 would cause souring of the reactor during anaerobic digestion.

Kwong and Fang (1996) reported that the reduced trend of pH value in the treatment of

cornstarch wastewater in the upflow reactors. Upto an OLR of 60 g COD/L.d the pH was in

between 7.3-7.9 and beyond that the increase in loading rates the pH drop was observed due to

the accumulation of VFA and finally reached at 6.8, Kalyuzhnyi et al., (1997) observed that the

feeding of UASB and HUASB reactors with higher pH values of soft drink influent led to their

failure because of alkali and the recovery period was about 1.5-2 months.

Bhatti et al., (1996) studied the feasibility of methanolic waste treatment in an UASB

reactor operated continuously for a period of more than 400 days. Optimum pH was found to be

between 7.0-7.3. In this pH range, there was no excessive build-up of volatile fatty acids and no

upset of reactor failure was observed. Elementary pathways of degradation of methanol to

methane were shown to be governed by the operating pH. In a UASB reactor, because of the

35

partial separation of the phases along the height of the sludge bed, the acidogenic phase

dominates in the lower part of the bed leading to an increase in VFA which reduces the H2CO3

alkalinity and pH. In the upper part of the bed the VFA are converted to CH4 and CO2.

2.5.3 Nutrients Requirements

It has been well established that the availability of nutrients (both macro and micro-

nutrients) in a form in which they are easily assimilated, is an important factor related to the

good growth and sufficient building up of biomass in the reactor, (Kaul and Nandy, 1994;

Speece, 1987; Jarrell and Kalmhoff, 1988). In order to have good microbial growth activity

these elements must be present and available in the required ratio. Their absence or scarcity can

in fact, be rate limiting resulting in poor quality of accumulated sludge.

The theoretical minimum COD/N ratio has been seen to be about 50. A value of around

60 may be regarded as reasonably for highly loaded anaerobic processes (OLR: 0.8-12 g COD/g

VSS.day).

2.5.4 C/N. Ratio

Under ideal condition, a small portion of Volatile Solids (V.S.) of 2 – 5% is destroyed

into cell mass. The major factor interfering with biodegradability is the presence of non

degradable compound (recalcitrant) such as lignin. Lignin biodegradable in aerobic environment

however, under anaerobic condition lignin is fairly recalcitrant. Delignification of the waste by

chemical means increase biodegradability. The C:N ratio plays a vital role in anaerobic

digestion with an optimum of 20 – 30 and hence necessitated to substitute with other substrates.

Many substrates do not possess the ideal C:N ratio namely, wheat straw (180), tomato (120),

urine (0.8), sugar cane tops (500).

The deficiency of Phosphorus (P) is shown to reduce the methanogenic activity in the

UASB reactors by 50per cent when compared to the one with the right concentration of

Phosporus and this reduction in efficiency could be reversed by higher dosage of Phosporus. A

complete recovery of maximal methanogenic activity has been observed with the

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supplementation of P dosage of 0.5 g /L. Over-dosage of the sulphate, which in practice is

related to high effluent phosphate concentration, has been found to be unprofitable (Alphenaar et

al., 1993). Dold et al., (1987) have treated apple processing wastewater in a UASB process and

after determining the concentrations of the nutrients in the influent and effluent.

The ratios of COD: N:P arrived by several investigators for various types of waste waters

have been presented in Table 2.1.

Table 2.1 Working ratios of COD:N:P for various types of wastewaters

Wastewater COD:N:P Investigators

Beet sugar 800-2000:30:5-10 Heertzes and Meer, (1978)

Carbohydrates (general) 35 : 5: 1 Lettinga et al., (1982)

Food processing 300 : 5: 1 Berg and Lentz, (1978)

Synthetic substrate 280 : 18 : 1 Frostell, (1981)

Petro-chemical effluent 145 : 5. 5 : 1 Nel et al., (1984)

Paper and board mill effluent 350 : 5 : 1 Habets and Knelissan, (1985)

Apple processing 100 : 8 : 1 Dold et al., (1987)

Sugar Mill 300 : 10 : 1 Manjunath et al., (1990)

Distillery waste water 350 : 5 :1 Souza et al., (1986)

Sewage simulated water 100 : 5 : 4 Agrawal et al., (1997)

2.5.5 Volatile Fatty Acids(VFA)

Volatile fatty acids are important intermediate compounds in the metabolic pathway of

methane fermentation and cause microbial stress when it is present in high concentrations. This

results in a decrease of pH, ultimately leading to failure of the digester. On the other hand at low

pH values the inhibition effects are more severe, due to the high un-ionized acetic acid

37

concentrations. The acetic acid level in excess of 800mg/L or a propionic acid to acetic acid

greater than 1.4 indicated digester failure.

In the treatment of pulp and paper mill effluent in upflow anaerobic filter, it was observed

that VFA concentration was decreased with increase of detention time which was due to

biomethanation of volatile fatty acids (Sharma et al., 1994). The role of UASB and AF regions

of the hybrid reactor of bio-degradation of synthetic molasses based organic compounds using

VFA as an operational parameter. Buyukkamaci, (2004) reported that the average acetic acid

concentration in the upper end and sludge bed regions were 150 and 345.9 mg/L respectively,

which shows that the higher methanogenic activity was in the upper part and higher acidogenic

activity was in the lower part of the reactor.

2.5.6. Organic Loading Rate (OLR)

The Organic loading rate and the hydraulic capacity are the most critical design criteria

for up flow sludge bed reactors. While the reactor design for treating low strength effluents is

mostly hydraulically limited, for treating high strength effluent, the system is generally limited

by its organic loading capacity. Higher organic loading rates serve to optimize volumetric

methane productivity, while lower organic loading rates maximize treatment efficiency.

Organic loading rate (OLR) is an important parameter significantly affecting microbial

ecology and characteristics of UASB systems. It is defined as the quantity of the load given to

the unit area of a reactor in a day. Numerous studies reveal that the treatment efficiency of

complex wastewaters like slaughterhouse wastewater increases with increase in OLR up to a

certain limit. Operating well above certain OLR resulted in sludge bed floatation, excess

foaming in the GLS region leading to clogging of gas outlet pipe and accumulation of

undigested constituents at the top of the reactor affecting the overall treatment efficiency (Sayed,

1987; Ruiz et al., 1997; Kalyuzhnyi et al., 1998).

While starting the reactor for high strength wastewater, the loading rates should be

increased step by step by initially diluting the wastewater and once the granulation starts the

38

loading rate can be increased (Makarand, 2000). Lettinga and Hulshoff, (1997) optimized the

loading rate for reactors operating in temperature between 15 and 35°C at 1.5 to 18 kg COD m-3

day-1. The OLR can be varied either by varying the flow rate or by varying the concentration of

wastewater ultimately altering the HRT and upflow velocity.

Boopathy and Tilche (1991), conducted tests on high strength molasses wastewater in a

hybrid anaerobic blanket reactor (HABR) and found that a COD removal in excess of 70% was

achieved at an organic loading rate of 20Kg.COD/m3.day.

Manual on Sewage and Sewerage Treatment (1993), report that organic matter of

wastewater is expressed in terms of biochemical oxygen demand (BOD) or chemical oxygen

demand (COD) in anaerobic treatment systems. The COD value is finding greater usage, which

sends itself directly to mass balance calculations. Reduction in COD for domestic and municipal

wastewater would normally correspond to an equivalent amount of ultimate BOD.

Uyanik (2002), conducted pilot plant studies for the granule formation in an anaerobic

baffled reactor (ABR) of 24g/L of colloids at OLR of 0.62 – 14.4 kg.COD/m3 and HRT of 0.43 –

10 days with the ice cream industrial waste of effluent concentration of 0.47g.VSS/L of a mixed

culture of the reactor performance.

Kishore and Kansal et al., (1999), concluded that anaerobic digestion is the most suitable

option for the treatment of high strength organic effluents. The presence of a biodegradable

component in the effluents coupled with the advantages of anaerobic process over the other

treatment methods is an attractive option. Another modification can be made to improve the

efficiency.

Young and Mc Carty (1969), conducted pilot plant studies on protein and carbohydrates

wastes and concluded that in the case of both the wastes for loading rates ranging from 0.42 –

3.4Kg.COD/m3.day COD reduction was 60 – 98%.

The efficiency of treatment decreased with an increase in organic loading rate according

to Khan and Siddiqui (2003) and while the total gas production and methane content increased

39

(Roy et al, 1996). The work done by Khageshan (1996), showed a gradual decline of methane

content in sago wastewater. At high organic loading rates the sintered glass media was more

effective than the glass media filter that had a greater stability and better performance (Anderson

et al., 1992). The suspended biomass in the filter is lifted up with the wastewater flow and the

suspended biomass present in the liquid column of 30cm to the top of the media, acted as a

trapping medium for the suspended biomass besides providing some COD removal at higher

hydraulic loading rates (Subramanyam et al., 1989).

The OLR to be used for the design of UASB reactor for different temperature is provided

by Lettinga and Hulshoff, (1991). For COD concentration in the range 2 to 5 g/L, the

performance of the reactor depends upon the loading rate and is independent of influent substrate

concentration. For COD concentration greater than 5 g/L, it is recommended to dilute the

wastewater to about 2 g. COD/ L during primary start-up of the reactor. Once, the primary start-

up of the reactor is over with granulation of sludge, loading rates can be increased in steps to

bring the actual COD concentration of the wastewater. The loading above 1–2 Kg.COD/m3.day

is essential for proper functioning of the reactor.

In general, for temperature between 15° C and 35°C, the reactor can be designed for

loading between 1.5 to 18 Kg.COD/m3.day. Lower OLR should be preferred for low temperature

and higher OLR can be adopted for high temperature.

Chaisri et al., (2007) treated the palm oil mill effluent through the UASB reactor to find

out the effect of OLR on the performance of the reactor. During experimental operation, the

OLR was gradually increased from 2.50 to 17.5 g COD/L day-1 in the UASB reactor,

consequently the HRT variation ranged from 20.0 to 2.90 days. The total volatile fatty acids and

acetic acid production in the UASB reactor reached 5.50 g / L and 4.90 g/ L, respectively at

OLR of 17.5 g COD/ L day-1 and HRT of 2.90 days before washout was observed. The result

showed that the COD removal efficiency observed was greater than 60 percent. At the HRT of

2.90 days, washout was observed. Also the methane production significantly decreased when

OLR increased up to 10.0 g COD / L day-1. Finally they optimized the OLR in the laboratory-

scale UASB was 15.5 g COD / L day-1.

40

Torkian et al., (2003) showed the treatment of slaughter house wastewater treated using

UASB reactor. The reactor was operated with the OLR ranges from 13 – 30 kg COD m-3 day-1

and 300 litre of methane per kg of COD removed. There was no sludge washout even at OLR

values above 30 kg COD m-3 day-1 at HRT as low as 2.3 h.

Ruiz et al., (1997) treated the slaughter house wastewater through UASB and AF

reactors. The UASB reactor was operated at OLR by 1 - 6.5 kg COD m-3 day-1. The COD

removal was 90 percent for OLR up to 5 kg COD m-3 day-1 and 60 per cent for OLR of 6.5

kg COD m-3 day-1. At higher OLR sludge, flotation occurred and consequently the active

biomass was washed out of the filter. The results indicated that anaerobic treatment systems

are applicable to slaughterhouse wastewater and that the UASB reactor shows a better

performance, giving higher COD removal efficiencies than the AF.

Lettinga et al., (1980) and Pol et al., (1983) have shown that a UASB reactor can take

exceptionally high organic and hydraulic loading rates and may also accommodate hydraulic and

organic shock loads, temperature fluctuations and low influent pH values fairly well, provided

hat the digester pH remains well above 6.0 and that the sludge load applied is below the

maximum specific COD removal rate of the sludge at the temperature prevailing in the digester.

Determination of maximum loading rate of a UASB system is not yet completely

understood which may be due to non-understanding of the theoretical failure phenomena.

Apparently the failure is defined empirically. For example, the failure is assumed when the

propionate, and, in turn, the soluble COD start accumulating and reaches an under level

(Wentzel et al. ,1994). The maximum loading rate appears to be higher if the maximum rate is

attained by using smaller increments of load (Samsoon et al., 1990). At daily increments of 0.6

and 0.38 g COD/L sludge bed. day, the maximum loadings reached were about 44-70 g COD/ L

sludge bed. day, for apple juice waste (Dold et al., 1987) while Samsoon et al., (1990) have

concluded that the maximum loading appears to be independent of the influent COD

concentration in flow through system.

41

Fang and Chuti (1995) have determined the efficiency of a UASB reactor over a wide

range of OLR (18-260 g COD/ L.day) with high strength wastewater composed of milk and

sucrose as substrate at 370C. They reported a soluble COD and the total COD removal

efficiency of 94-98% and 75-90%, respectively at loading rates up to 160 g COD/ L.day

corresponding to a HRT of 1.8 h and the feed COD level concentration of 12 g/L. The

efficiency of COD removal declined at loading rates higher than 160 g COD/ L.day.

2.5.7 Hydraulic Retention Time (HRT)

Hydraulic Retention Time is considered as very essential parameter to maintain an

adequate up flow velocity to assure good mixing. The hydraulic retention time (HRT), which

depends on wastewater characteristics and environmental conditions, must be long enough to

allow metabolism by anaerobic bacteria in digesters.

Nadais et al., (2005) observed that the raising of HRT 6 to 12 hrs increased the COD

removal efficiency and methane production rate, during the treatment of dairy wastewater in an

UASB reactor using flocculent biomass. Further, the raising of HRT from 12 to 16 hrs the

differences are not meaningful. The biomass washout was heavier in the reactors operated with 6

h and 8 h HRT (85 and 80 % of biomass washout, respectively) compared to the reactors

operated over 12 and 16 hrs HRT (45 and 35 % biomass washout, respectively).

Tawfik et al., (2008) studied on dairy wastewater and used a flow rate of 5 L/d and at 1

day HRT while reactor height was also sufficient to run experiments at higher HRTs.

Rajakumar and Meenambal (2008) carried out an experiment in a poultry slaughter house

wastewater using a HUASB and AF reactors in order to compare the startup time and optimum

HRT required for the treatment of poultry slaughterhouse wastewater under similar loading

conditions. The reactors were operated at OLR from 0.77 to 3.43 kg COD m-3 day-1 with HRT

from 36 h to 8 h. HUASB reactor showed the TCOD and SCOD removal efficiencies of 80 and 86

per cent respectively at an optimum HRT of 10 h, whereas the AF reactor showed the removal

efficiencies of 70 and 79 per cent respectively at optimum HRT of 12 h. Reducing the HRT

42

below 10 h in HUASB reactor shows the sludge washout and lower COD removal efficiencies of

less than 80 percent. The study revealed that the HUASB reactor has very good removal

efficiency and less start-up time compared to that of the AF reactor for the treatment of poultry

slaughterhouse wastewater.

Asif Latif et al., (2011), reported that the up flow anaerobic sludge blanket reactor is an

efficient waste water treatment technology that connects anticipated anaerobic decomposition to

lessen the waste volume and produce biogas. On spot and time to time analysis of incoming

wastewater stream and biomass within the reactor are important for both lab and large-scale

UASB reactors.

Gupta Sunil Kumar et al., (2007), reported that the anaerobic hybrid reactor is superior

and a promising technology as compared to an UASB reactor for the treatment of distillery spent

wash. The hybrid reactor is more efficient in terms of COD removal and biogas production as

compared to UASB reactor. At optimum HRT of 5 days and OLR of 8.7 kg COD/m3.d, the COD

removal efficiency and methane yield in hybrid reactor were approximately 5% more than an

UASB reactor. The rate of sludge washout is a major drawback of UASB reactor, which can be

reduced by 25% in hybrid reactor.

Zimi and Zamanzadeh (2004) carried out an anaerobic sewage treatment using UASB

reactor with the HRT varied from 2 to 10 h with various OLR ranging from 0.95 to 5.70 kg COD

m-3 day-1 for colder period and from 1.35 to kg COD m-3 day-1 for warmer period. Based on the

result they concluded the optimum HRT for a warmer period with a 2.20 kg COD m-3 day-1.

OLR was 6 h with BOD5, COD and TSS removal efficiency were 71, 63 and 65 per cent

respectively. While the colder period removal ratio of BOD5, COD and TSS with an optimal

HRT time of 8 h and OLR of 1.22 kg COD m-3 day-1 were 54, 46 and 53 per cent respectively.

Lowered HRT is accompanied by high upflow velocity leading to wash out of influent solids and

high concentrated biomass (Mahmoud et al., 2003). The required HRT decides the sludge

concentration in the reactor which further decides the sludge retention efficiency by phase

separator (Cavalcanti et al., 2001).

43

2.5.8 Effect of Upflow Velocity/HRT

A reasonably increased upflow velocity (decreased HRT) is shown to favor the

granulation process (Lettinga et al., 1980; Alphenaar et al., 1993). The advantages of

maintaining the sludge bed in the fluidized condition with a high upflow velocity have been

studied by Guiot et al., (1992) who have reported that the minimum superficial velocity for

fluidization of carbohydrate-fed anaerobic granular sludge is around 2 m/h. The mean geometric

diameter of individual granules and the specific activities were also found to be affected by the

superficial velocity. The optimum upflow velocity for the starting up of a UASB reactor has

been suggested to be in the range of 0.72 to 0.96 m/day (Lettinga et al., 1984).

The upflow velocity is one of the main factors affecting the efficiency of upflow reactors.

Goncalves et al., (1994) examined the treatment of sewage anaerobically at 200C in an upflow

anaerobic sludge blanket reactor without gas liquid separator operated at upflow velocities of

3.2, 1.7, 1.6, 0.9, 0.75 and 0.6 m/h corresponding to HRTs of 1.1, 2.1, 2.3, 2.8 3.3 and 4.3 h,

respectively. It was observed that the suspended solids removal efficiency decreased from 70 to

51% when up flow velocity increased from 0.75 to 3.2 m/h. The upflow velocity should be high

enough to provide good contact between substrate and biomass, as it should be enough to disturb

the gas pockets gathered in the sludge bed. The higher V up is believed to facilitate the separation

of gas bubbles from the surface of biomass.

The upflow velocity has two opposing effects in the upflow anaerobic reactors. On one

hand, increasing upflow velocity increase the rate of collisions between suspended particles and

the sludge and thus enhance the removal efficiency. On the other hand, increasing the upflow

velocity could increase the hydraulic shearing force, which counteracts the removal mechanism

through exceeding the settling velocity of more particles and detachment of the captured solids

and consequently deteriorates the removal efficiency (Mahmoud et al., 2003).

Gangagni Rao et al (1997) found that sludge wash out was observed during the stepped

increase of flow rate from 5000 to 6000 mL/h in the treatment of synthetic wastewater using

UASB reactor. The corresponding linear velocity was 0.339 m/h at a HRT of 4.1h.

44

Ghangrekar et al., (2002), suggests that the maximum liquid up-flow velocity allowed in

design should not exceed 1.2 – 1.5m/h. Up-flow velocities as 0.25 – 0.8m/h are favorable for

granule growth and accumulation, during normal operation of the reactor and maximum up-flow

velocity up to 1.5m/h at peak flow conditions for short duration can be used in design.

Panesar et al., (1999), concluded an up-flow velocity of 0.2 – 2m/h has been considered

as optimum for formation of sludge blanket in the ABR process.

2.5.9 Inoculum-Substrate Ratio

Lopes et al., (2004) affirmed that the inoculum used in the process, substantially

improved the performance of the process. For this study Lopes used bovine rumen fluid as an

inoculum for the organic fraction of solid waste. Results clearly indicated that the better

performance of the inoculated reactors might be related to the potential increase in number of

indigenous anaerobic microorganisms of rumen that contributed substantially to degradation of

the organic material in the reactor.

Deshmukh et al., (2009) report the number and types of organo-chlorine compounds

degraded by UAF are comparatively higher. Even the operating cost of UAF is much lower

since UAF does not require aeration or agitation unlike aerobic biological methods. Hence single

step treatment in the form of UAF supplemented with electron donors is a better option over the

physical, chemical or other bio-logical processes to degrade AOX from pulp and paper mill

wastewater.

2.6 EFFECT OF SHOCK LOADING

Sudden increase or decrease of loading (shock loading) causes adverse effect on the

performance of a UASB reactor. After a shock loading of the UASB reactor with the wet milling

operation wastewater, it was observed that the granules lost their ability to settle down and hence

began to float on the top of the reactor; a hollow core within these granules has been found

(Blaszezyk et al., 1994). It has been postulated that starvation of the granules and partial

45

autolysation of biomass was the main reason for this hollow core condition. The produced gas

was entrapped within the granules in the empty space and could not be released as the granules

moved from the bottom of the reactor (Blaszezyk et al., 1994). This problem can be overcome

by adding nutrients; otherwise, the granules get reduced in number and size. Earlier the

phenomenon of granules losing their ability to settle and beginning to float to the top had been

observed by Kosaric et al., (1990) with synthetic VFA as the substrate.

Eng et al., (1996) have reported the effect of shock loading as an accumulation of lactic

acid and the resultant acidity thereby reducing the pH (7.2 to 4.7) while treating diluted land fill

leachate. This drop, in turn, inhibited the methanogenesis. Shock loading also caused major

disturbances in the composition of the biogas. Experimental results have indicated that complete

sludge washout from a conventional reactor is likely to occur within 2-8 hours if the system is

overloaded with an influent containing more than 100 mg carbon/L. (Rizema et al., 1989).

However, Dold et al., (1987) are of the opinion that a UASB reactor is reasonably robust to

shock loading provided the peak loading rate during the shock does not exceed the maximum

system capacity.

2.7 EFFECT OF SLUDGE LOADING RATE (SLR)

Granulation is reported to have been observed when the SLR just exceeds 0.6 g COD/ g

VSS.day (Lettinga et al., 1980) and granules could not be developed at a SLR of 0.3 g COD/ g

VSS.day while running a reactor with VFA feed (Pol et al., 1983). The flocculent sludge

prevailed representing long filamentous bacteria, presumably Methanothrix, while treating sugar

mill waste (aqueous molasses solution). Manjunath, (1987), has reported granulation at a SLR

of 0.3 g COD/ g VSS.day. Ghangrekar et al., (2003), have reported that the COD removal

efficiency at steady-state is profoundly influenced by SLR and that, about 50% of the total

removal takes place at a SLR of 0.6 g COD /g VSS.day, while 90% removal could be observed

at 0.3 g COD/g VSS.day.

Experiments have shown that for SLR control, the ratio of the volume of sludge bed to

the reactor volume has to be controlled in the range of 0.398-0.469 and the reactor has to be de-

46

sludged when this ratio is exceeded (Khan and Menrotra, 1990). The excess sludge production in

anaerobic treatment of wastewater having the soluble matter is very low, particularly for VFA

wastewaters. In the case of bi-phasic process, the volume of excess sludge obtained from the

methanogenic reactor is extremely small because it is generally very thick (TSS concentration

above 100 g/L) while the volume of excess sludge from the acidogenic reactor can be

substantially larger as it rather be voluminous and the sludge yield is also larger for which

aerobic post-treatment would be necessary (Lettinga and Pol, 1991). According to Yan and Tay,

(1997), the SLR maintained at 80per cent of the specific methanogenic activity (SMA) during

start-up is shown to enhance the sludge growth and reduce the granulation time to one month

and this sludge got stabilized in four months.

2.8 INHIBITORS OF ANAEROBIC DIGESTION

2.8.1 Toxic Substances

The incorporation of heavy metals has a profound effect on the microorganisms

degrading the anaerobic digester performance. Similarly, the chlorine analogues methane is

inhibitory to the methanogenic activity, namely, carbon tetrachloride (CCl4) and

chloroform(CHCl3). The overproduction of VFA (upto 2000 mg/L VFA) will reduced to pH

(6.4 to 7.5) (Kugelman, and Chin, 1971) and inhibit the methane production for which alkaline

control of pH is required. The high concentration of ammonia, antibiotics, pesticides,

detergents, heavy metals such as chromium, copper, nickel, zinc, etc. are toxic to the

microorganisms involved in biogas production. A low C/N ratio of the slurry leads to high

concentrations of ammonia. Heavy metals are mostly present in industrial was concentrationtes.

The maximum allowable concentration of toxic materials are presented in Table 2.2.

With respect to sustainability and cost-effectiveness, anaerobic treatment has a much

better score than many treatment processes (Jules van Lier, 1996), principally in the energy

conservation aspect, besides energy is reclaimed from the organic waste constituents in the form

of biogas. Anaerobic treatment is presently the lowest cost wastewater treatment option for

highly polluted industrial wastewater in tropical countries (Deepak et al., 1998).

47

Contrary to the aerobic treatment, the anaerobic treatment extracts the energy available in

the wastewater without air or oxygen with the help of microorganisms. The increasing cost of

energy associated with the aerobic treatment provides the opportunity for anaerobic treatment to

gain an upper hand considerably (Deepak and Chongrak, 1998).

Anaerobic digestion with the help of biological microorganisms is the technique for the

treatment of organic matter under absence of oxygen to produce a blend of gas containing

methane and carbon dioxide predominantly. It is a well proven technology for treating different

sorts of industrial wastewaters like dairy wastewater (Anderson et al., 1992), slaughter house

wastewater (Borja et al., 1995), Coffee wastewater (Bello-Mendoza and Castillo-Rivera, 1998)

and pulp and paper mill effluents ( Ali and Sreekrishnan, 2001).

Borja et al., (1998) carried out an experiment in a laboratory-scale anaerobic hybrid

reactor, in which the bottom two-thirds were occupied by a sludge blanket and the upper one-

third by submerged small cubes of polyurethane foam was evaluated for the treatment of

slaughterhouse wastewater. The reactor was operated at 35°C during the two experimental

studies. In the first study, the chemical oxygen demand (COD) concentration of the wastewater

was increased from 3.74 to 10.41 g /L, whilst maintaining a constant HRTof 1.5 days. In the

second, the HRT was decreased from 1.35 to 0.50 days, whilst maintaining a constant influent

COD concentration of 10.41 g /L. These results showed that this type of reactor was suitable for

the anaerobic treatment of this wastewater and demonstrated a high COD removal of between

90.2 and 93.4 per cent at organic loading rates between 2.49 and 20.82 g COD/L. day−1 at an

HRT of 0.5 day, the reactor achieved a methane yield of 0.345 L CH4 /g of COD removed at

standard temperature and pressure.

Pulavendran et al., (2005) carried out the anaerobic digestion of animal glue industry

solid wastes (residues from neutralized fleshing after glue extraction) in a semi-continuous

bench scale digester in order to determine the extent of the conversion into biogas. Solid wastes

and wastewater from a local glue manufacturer were used as substrate for the anaerobic

digestion. HRT was maintained at 40 days at an ambient temperature of 28.2°C. The average

48

specific biogas productions were 0.281 g-1 of volatile solids added and 0.481 g-1 of volatile solids

removed respectively.

Bench-scale and pilot scale reactor systems have demonstrated that the anaerobic

wastewater treatment is applied in a very wide temperature range, i.e. between 10 and 80°C

(Van Lier et al.,2007), whereas COD concentrations as low as 100–200 mg.l-1 (Kato et al., 1994)

and as high as 100,000 mg.l-1 can be applied. Zeeman and Lettinga, (1999) reported that

anaerobic treatment of industrial wastewater is suitable for on-site treatment owing to its low

energy consumption, small space requirement and relatively simple reactor design.

A higher retention of biomass inside the reactor is the key to successful anaerobic

treatment of wastewater for recovering energy, besides the ratio of bacteria involved is much

more vital since the entire biodegradation process depend on this parameter significantly.

Bouallagui et al., (2003) suggested that the most appropriate HRT for the anaerobic digestion of

fruit and vegetable wastes varies in the range of 10–20 days, although this should be slightly

higher when the wastes are not mixed with the other substrates to be digested.

2.9 ADVANCED ANAEROBIC TREATMENT SYSTEMS

During the last few decades, a new generation of advanced anaerobic reactors has been

developed capable of retaining high concentration of active biomass and consequently of treating

wastewaters at high loading rates. The development and application of advanced or high rate

anaerobic reactor for domestic and industrial wastewaters are mainly based on the following key

concepts (Bodik et al., 2000).

Accumulation of biomass by means of settling, attachment to solid (fixed or mobile) or

by recirculation. Such systems allow the retention of slowly growing microorganisms by

ensuring that the means SRT become much longer than the mean HRT.

Improved contact between biomass and wastewater, overcoming the problems of

diffusion of substrates and products from the bulk liquid to the biofilm of granules.

Enhanced activity of the biomass, due to adaptation and growth.

The various types of advanced anaerobic reactors are used for the different wastewaters

are discussed in the following sections.

49

Table 2.2 Maximum Allowable Concentration of Toxic Materials

S.No. Heavy metalConcentration in

mg/L

1 Sulphate SO4 5000

2 Sodium chloride (NaCl) 40,000

3 Copper (Cu) 100

4 Chromium (Cr) 200

5 Nickel(Ni) 200-500

6 Cyanide <25

7 Detergent (ABS) 40 ppm

8 Ammonia (NH3) 3000

9 Sodium (Na) 5500

10 Potassium (K) 4500

11 Calcium(Ca) 4500

12 Magnesium(Mg) 1500

50

2.9.1 Anaerobic Contact Reactor

The contact or recycled flocs process comprises a continuous fed completely mixed

reactor stage followed by solid/liquid separation. A degasification step is frequently included in

system design (Stronach et al., 1986). The effluent is discharged from the settling device and the

settled biomass returned to the digester vessel, where it is mixed with the incoming feed. Re-

inoculation of a well acclimatized sludge can maintain optimum stabilization of industrial

wastewaters which, unlike sewage sludge for example, do not generally contain high proportion

of microflora.

The COD removal efficiency varied from 65-98% depending upon the type and

characteristics of wastewater (Nahle, 1991). The anaerobic contact reactor appears to be more

suitable compared to UASB reactor. However, there is paucity of formation of sludge and there

is loss of sludge due to high fat concentration observed in UASB reactors (Rajeswari et al.,

2000).

2.9.2 Anaerobic Filters (AF)

Anaerobic filters were first introduced by Young and McCarty in 1969 and have grown to

represent an advanced technology that has been used effectively for treating a variety of

industrial wastewaters (Young, 1991). AF with random support has been successfully used for

the treatment of municipal sewage and different industrial wastewaters (Chernicharo and

Machado, 1998 and Sharma et al., 1994). A variety of natural materials such as smooth quartzite

pebbles, shells, granite stones, cinder, brick ballast and synthetic materials like polyvinyl

chloride sheets, needle punched polyester, glass ranching ring and other materials have been

used for attachment and growth of anaerobic biomass.

There are two types of AF; one is upflow anaerobic filter (UAF) and another is down

flow anaerobic filter (DAF). UAF may lead to clogging due to the excessive growth of biomass

on the support media and biomass gets accumulated in the bottom portion of the reactor. To

overcome the problems of clogging in UAF, the mode of feeding was changed from up flow to

51

down flow and hence the variant was named as DAF (Jawed and Tare, 2000). In the DAF,

sloughed biomass is expected to come out of the filter along with the effluent.

In fixed film system such as the anaerobic filter, an initial biofilm attachment to the

carrier media is the primary and perhaps most difficult aspect of startup. The biomass

concentration in the reactor which obviously is distributed into two fractions; one immobilized

in the biofilm, the other being suspended in the void volume of the reactor. In upflow filters

packing materials has the major role to keep the suspended biomass inside the reactor. The

COD removal in upflow filters being mainly dependent on the suspended biomass, the reactor

efficiency is not proportional to the specific media surface but on their biomass entrapping

capacity (Tilche and Vieira, 1991).

Sharma et al., (1994), investigated the treatment of pulp and paper mill effluent by UAF

with a capacity of 5.77 L. Wastewater was fed at COD concentration of 1000, 2000, 4000, 6000

mg/L and the filter performance with respect to COD reduction and gas production was studied

for various hydraulic loading conditions. Maximum COD removal of 84.38% was observed for

an influent COD concentration of 4182.5 mg/L operated at a HLR of 129.92 L/d/m2 and the

corresponding OLR was 0.431 kg COD/m3.d. The maximum yield co efficient of methane was

0.425 L/g of COD removed.

Prasertsan et al., (1994) studied the treatment of fishery wastewater at an OLR of 0.3 –

1.8 kg COD/m3 .d and HRT ranging from 36 to 6 d. More than 75% COD reduction took place

up to an OLR of 1 kg COD/m3.d with an HRT of 11d. An OLR of 1.3 kg COD m3/.d

corresponding to a HRT of 6.6 days gave maximum bio gas productivity of 1.5 m3/m3.d and

65% COD reduction.

Joo-Hwa Tay and Kuan-Yeon Show, (1998) studied the influence of media related

factors such as porosity, specific surface and pore size on performance of up flow anaerobic bio

reactors. Three 15 L outflow biofilters, each packed with different support media, were

subjected to identical synthetic protein carbohydrate substrate with COD concentration ranging

from 2500 to 10000 mg/L and HRT from 15 to 30 h, corresponding OLR varies from 2 to 16 g

52

COD/ L.d. Waste treatment performance indicates that the biofilter associated with the media of

the largest pore size and porosity consistently demonstrated the highest COD removal from

96.73 % at loading varying from 2 to 16 kg COD/m3.d.

Swaminathan and Subrahmanyam, (2002) evaluated the biodegradation of P-Nitro Phenol

(PNP) in upflow anaerobic fixed film bed reactor. The studies showed that PNP was not

degraded as a carbon source in the reactor. Addition of glucose as co-substrate increased the

degradation of PNP. A ratio of greater than 1 in terms of glucose to PNP could achieve 90%

PNP degradation. However, the total organic carbon (TOC) removal was 76.35% indicating the

possibility of biotransformation of PNP.

Gangagni Rao et al., (2005) investigated the treatment of bulk drug industry wastewater

with high suspended solids using fixed film reactor by using different start up procedure. The

seed sludge was taken from UASB reactor treating slaughterhouse wastewater which was

acclimatized with bulk drug industry wastewater for three weeks in anaerobic conditions. This

acclimatized sludge was inoculated and reactor was started with batch mode in 10 days with an

OLR of 0.5 kg COD/m3.d. Then this was continued in continuous mode and increased stepwise

to 1.0 kg COD/ m3.d. This process reduced the start up period to 30 days. The COD and BOD

reduction of 60-70% and 80-90% were observed, respectively at an optimum OLR of 10 kg

COD/ m3.d. The biogas production was consistent in the range of 0.3 – 0.5 mL/mg COD

reduction irrespective of the operated OLR. The CH4 value varied from 65-70% and CO2 varied

from 30-35%.

2.10 UP-FLOW ANAEROBIC SLUDGE BLANKET REACTOR

One of the most distinguished developments in anaerobic treatment process technology is

the UASB developed in the Netherlands. The distinguished characteristic of this reactor is the

presence of active biomass at the bottom of the reactor operating on suspended growth method.

The microbes affix themselves to each other or small particle of suspended matter to form

granules that have excellent settling properties.

53

The UASB reactor system, which was developed by Lettinga and his coworkers in

1970’s, has received widespread acceptance and has been used successfully in the treatment of

several types of wastewaters (Hickey et al., 1991). In the UASB reactor the microorganisms are

kept in the reactor due to the production of the highly flocculated, well settling, compact sludge

granules which develops during the process. UASB reactor exhibits positive features such as

high organic loadings, short HRT and has a low energy demand (Lettinga et al., 1980)

UASB processes have found a variety of applications in recent years in the treatment of

high, low and medium strength wastewater and a variety of other substances (Lettinga and

Vinken, 1980 and Elefsiniotis and Oldham, 1994 and Ramasamy, 2004). In UASB reactor,

anaerobic bacteria form dense granules and settle and remain as a bed at the bottom of the

reactor. When the wastewater flows upward through a blanket of biologically formed granules,

they consume the waste and produced methane and carbon dioxide. Hence it is essential, the

formation of granules in UASB reactors for efficient operation of the reactor.

However, some authors concluded that the development of granules does not maintain

the reactors become more efficient for the treatment of domestic sewage (Kalogo and Verstraete,

1999), because flocculent sludge is more effective in the entrapment of the suspended solids than

the granular sludge (Mulia, 2002).

Fang et al., (1995), reported that the UASB process consistently removed 97-99% of

COD from wastewater containing concentrated mixed VFA concentration at 37°C at loading

rates upto 24 kg COD/ m3.d corresponding to a food/microorganism (F/M) ratio of 0.78 g

COD/g VSS.d. It was suggested that, pre acidification, the UASB process can be effective for a

wide variety of wastewater. The COD removal efficiency deteriorated at higher loading rates,

there was no butyrate in the effluent, suggesting that butyrate degradation was not a rate limiting

step. Of the COD removed, 92.6% was converted to methane; the rest was converted to granular

biomass with an average yield of 0.054 g /VSS/g COD. The granules had the size of 1-2 mm

54

and settled satisfactorily. Each gram of granule in the reactor was capable of converting a daily

maximum of 0.86 g of COD into methane.

Gangagni Rao et al., (1997) studied the performance of 29 L UASB reactor using a high

strength synthetic wastewater consisting of glucose, acetate, propionate and butyrate with a COD

of 11 g/L yielding 90-95% COD reduction. The maximum OLR achieved was 47 kg COD/

m3.d. The methane content was 72% with a methane yield of 0.29 m3/kg COD removed at

lowest HRT of 4.9 h. The reactor took a maximum of only 10 days to reach its earlier levels of

productivity, when the plant was shut down for one month.

Ghangrekar et al., (2002), evaluated the loading capacity of lab scale UASB reactor of

the treatment of synthetic wastewater to represent simple bio degradable industrial wastewater.

It has been observed that, once proper primary start up of the reactor is achieved with the

development of good granular sludge, the reactor is capable to handle higher loading rates with

HRT as low as 4 h and efficiency more than 90% up to OLR of 19.28 kg COD/ m3.d and SLR of

0.88 kg COD/kg VSS.d. Further increase in loading has resulted in decrease in efficiency of the

reactor and at OLR of 29.9 kg COD/ m3.d.The efficiency observed was 78%. It has been

observed that under high loading rate, gas loading should be less than 80 m3/m2.d for proper

operation of gas solid separator (GSS) device. In addition, SRT greater than 50 days is

recommended for getting COD removal efficiency greater than 90%.

Ramasamy et al.,(2004), evaluated the feasibility of UASB rectors for the treatment of

dairy wastewaters. Two types of UASBs were used, one operating on an anaerobic sludge

granules developed from digested cow dung slurry (DCDS) and the other, the granules obtained

from the reactor of sugar treating industry wastewaters. The reactors were operated at HRT of 3

and 12 h and on COD loading rates ranging from 2.4 to 13.5 kg COD/ m3.d. At the 3h HRT, the

maximum COD reduction in the DCDS seeded and the industrial sludge seeded reactors was

95.6 and 96.3% respectively, better than at 12 h HRT (90 and 92 % respectively). In both the

reactors, the maximum, the second best, and the third best COD reduction occurred at the

55

loading rates of 10.8, 8.6 and 7.2 kg/ m3.d respectively. At loading rates higher than 1.8 kg.

m3.d., the reactors performance dropped precipitously.

The GLS constitutes an essential component of a UASB-reactor system. The dispersed

sludge aggregates generally can be retained sufficiently well in the reactor by separating the

biogas using this GLS collector assembly, which is placed at the upper part of the reactor. In

UASB reactor the SRT is always longer than the hydraulic retention time. The difference

becomes larger, as the phase separator is more efficient. By maintaining a long SRT, the sludge

mass present in the system is large and this augments the efficient removal of biodegradable

organic material present in the wastewater. Cavalcanti et al., (2001) proved that the sludge age is

strongly dependent on the efficiency of the sludge retention device of a UASB reactor.

Beni Lew et al., (2011), discusses the performance of an UASB reactor for treating raw

domestic wastewater under temperate climates conditions and concluded that the decrease in

temperature, the COD removal decreased from 78% at 28°C to 42% at 10°C for the UASB

reactor operating alone at a HRT of 6h.

Selvamurgan et al., (2011), discusses the performance of a 650m3 full-scale upflow

anaerobic sludge blanket (UASB) reactor treating in distillery spent-wash. This study concluded

that the COD, BOD5 and TS removal efficiencies were stabilized to the range of 62.19–66.59,

72.42–77.11, and 58.47–60.46%, respectively at an organic loading rate of 2.15–4.60 kg COD

m-3 day-1. The biogas production was stabilized to the range of 48, 290–135, 115 m3 week-1 with

60% methane content.

Tawfik et al., (2008), developed an UASB reactor followed by activated sludge for

treating combined dairy and domestic wastewater and concluded that the combined system

achieved an overall removal efficiency of 98.9% for COD, 99.6%for BOD.

Ghangrekar et al., (2003) states the UASB reactor as a system in which substrate passes

first through an expanded sludge bed containing a high concentration of biomass. The sludge in

56

the reactor may exist in granular or flocculent form, but the granular sludge offers advantages

over flocculent sludge. Most of the substrate removal takes place in sludge bed. The remaining

portion of the substrate passes through a less dense biomass, called the sludge blanket.

Miranda et al., (2005) discusses the performance of an 800m3 full-scale UASB reactor in

treating meat-packing plant and slaughterhouse effluents containing high concentrations of oil

and grease (O&G) (413-645 mg/L), resulting in a COD/O and G ratio of 26-32%. Those

macromolecules were considered responsible for the unbalance of the system resulting in a total

washout of the biomass. The removal of O&G from the influent using a physicochemical system

(coagulation-flocculation) improved the physical characteristics of anaerobic sludge, controlling

the biomass washout.

Reactor performance was significantly improved when the COD/O andG ratio influent

was maintained in the 10%. The COD and O and G removal rates obtained after implantation of

the physicochemical system were 70-92% and 27-58%, respectively. The specific methanogenic

activity (SMA) of the biomass shows towards a tendency stabilization and adaptation to the

substrate influent. Pre-treatment of the influent allowed the maximum organic load to be

increased (1.46 to 2.43 Kg COD/m3.d) and improved the quality of the effluent.

Habeeb et al .,(2011) reported that the microorganism’s tolerance as well as development

was slightly rapid due to the suitability of Palm Oil Mill effluent (POME) as treated wastewater

in the entire treatment. On the other hand, palm oil shell as filter packing media showed the

optimization case of the conducted study by using the mesophilic temperature of 37°C for the

treatment and diluted POME can be efficiently treated with HUASB technology.

Rajakumar et al ., (2008) reported that HUASB reactor was needed less time for start-up

and showed better removal efficiencies as compared to AF reactor using the same substrate of

poultry slaughterhouse wastewater. The TCOD and SCOD removal efficiencies were as high as

80% and 86% in HUASB as compared with AF of 70%and 78%, respectively. The optimum

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HRT was found to be 10 and 12h, at loading rates of 2.74 and 2.27 Kg COD/m3.d for HUASB

and AF reactors respectively.

Nandy and Kaul, (2001), reported that the start-up period of an anaerobic hybrid UASB

reactor is directly proportional to the concentration of the microbial population. Rate depends on

the type of inoculums, the type, strength of waste, level of volatile acids.

2.10.1 Fluidized Bed Reactor

In the anaerobic fluidized bed reactor, the media for bacterial attachment and the growth

is kept in the fluidized state by drag forces exerted by the up flowing wastewater. The

advantages of these systems are larger surface area due to fluidization of small media like sand

and activated carbon, higher reactor biomass holdup, better system efficiency, opportunity for

higher loading rates and better resistance to inhibitors (Rajeshwari et al., 2000).

The start up of anaerobic fluidized bed process is initiated by the development of bio film

due to the various influencing parameters such as liquid flux rate; scale up of the rector, gas flux

and organic loading rate and subsequent attachment to the carriers. Fluidized bed technology is

more effective than anaerobic filter technology as it favours the transport of microbial cells from

the bulk to the surface and thus enhances the contact between the micro organisms and the

substrate (Hickey et al., 1991).

Saravanane et al., (2001) studied the treatment of sago effluent in a continuous flow

anaerobic fluidized bed reactor using activated carbon as a carrier material. The start up of the

reactor was carried out using a mixture of digested supernatant sewage sludge and cow dung

slurry at different proportions. The bed expansion was maintained at 25% during the entire start

up period by adjusting the upflow velocity from 20-25 m/h. The COD values were varied

between 250 to 4000 mg/L and efficiency of 82% was obtained at a maximum applied OLR of

60.5 kg COD/ m3.d. The biogas yield was found 0.2 – 0.25 m3 / kg COD.d and the maximum

rate of generation were 59-66.3 L/d. The methane percentage was varied between 55-65%.

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Borja and Banks, (1995) studied the comparison of an AF and anaerobic FBR treating

POME. Sand of 0.3 – 0.5 mm diameter was used as carrier materials. Start up of AF was

achieved with 1.5 – 4.5 d residence times and FBR residence times were maintained at 6 h.

After acclimatization, COD removal higher than 90% were reached in both reactors at 6 h

residence time, equivalent to a loading of 10g COD / L.d At higher loading the FBR gave a

better performance, even at 40 g COD/L.d, with 6h residence times, 78% COD was degraded.

The AF could not be operated above 20g COD/L.d without clogging. The AF and FBR

performed similar at reactor concentrations upto 1 g COD/L. while above 2.2 g COD/L the AF

showed a maximum removal rate of 17g COD/L.d compared to 31.2 g COD/L.d for the FBR.

These differences were probably due to diffusion limitations and a less active biomass in the AF.

2.10.2 Expanded Granular Sludge Bed (EGSB) Reactor

The EGSB reactor is a modified form of UASB reactor and this type of reactor was

developed to overcome the potential problems such as preferential flows, hydraulic short cuts

and dead zones that can occur in UASB reactors. In this type of reactor, the superficial liquid up

flow velocity is applied up to 10m/h, in contrast with the 0.5 – 1.5 m/h in UASB reactors. Due

to this very high up flow velocity, special attention has to be made to the design of GLS design

in order to prevent the bio mass washout in the effluent, which may result in the drop in reactor

efficiency (Kato et al., 1999).

As a result of high velocity, granular sludge bed will be in an expanded or possibly in a

fluidized state in the higher regions of the bed. This has resulted, excellent contact between the

wastewater and the sludge as well as increase in substrate transport into the sludge aggregates

(Rajeshwari et al., 2000). The diameter of the particles is slightly bigger as compared to that

used in fluidized beds. Compared to UASB reactors, higher organic loading rates can be

accommodated in EGSB systems. Consequently, the gas production is also higher, improving

even more than the mixing of gas inside the reactor (Seghezzo et al., 1998).

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Collins et al., (1998), evaluated the treatment of primary clarifier effluent of wastewater

treatment plants in anaerobic expanded bed reactor. Before feeding the wastewater to Expanded

Bed Reactor (EBR) batch assays showed that this wastewater has anaerobic biodegradability of

90% or more. Reactor performed well over wide range of influent COD’s and temperatures,

especially under severe conditions (5°C and 50 mg/L influent COD). The most efficient

treatment was obtained with HRT of 3 – 6 h and influent COD approximately 150 mg/L. Start

up of the reactor took about 60 days at 30°C and 80 days at 20°C.

Kato et al., (2003), proved better performance and stable operation of EGSB reactor

using flocculent sludge for the post treatment of effluent from UASB reactor treating domestic

sewage. The reactor was seeded with flocculent sludge from a full scale UASB reactor. Even

using a flocculent sludge good mixing conditions and high retention of bio mass was achieved.

By applying a 4 h HRT and Vup values upto 3.75 m/h, effluent COD concentrations in EGSB

were below 87 and 55 mg/L, for total and filtered samples respectively. SS concentrations in the

effluent were below 32 mg/L.

2.10.3 Anaerobic Baffled / Anaerobic Migrating Blanket Reactor

Anaerobic Baffled Reactor (ABR) is considered as compartmentalized series of baffles in

which the wastewater is forced to flow over and then under them as it travels through the reactor

and creating conditions approaching plug flow (Bachmann et al., 1985). Angenent and Dague

(1996), developed the Anaerobic Migrating Blanket Reactor (AMBR) with gentle mechanical

mixing to accomplish sufficient contact between substrate and biomass. In this reactor, a higher

migration rate of flocculent biomass relative to the granular biomass, to the final compartment

was observed. This kind of reactor has very long HRT and biomass washout can be prevented.

Furthermore, the compartmentalization of the bacteria may provide the ability to separate

acidogenesis and methanogenesis longitudinally down the reactor, allowing the different

bacterial groups to operate at their preferred conditions (Barber and Stuckey, 1999).

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Hutnan et al., (1999), compared the start up performance of UASB hybrid and baffled

reactors for the treatment of synthetic wastewater using digested sludge from municipal

wastewater treatment plant as seed. From the comparison studies, the lowest biomass washout

was observed in the baffled reactor compared to other two reactors. The efficiency of COD

removal was comparable for all three reactors were 80-90%. Due to an intense acidification in

the first compartment of ABR a doubled dosage of NaHCO3, as compared to the dosage used in

the other two reactors, was necessary to maintain the pH in the range suitable for methanization.

Kennedy and Brarriault, (2005) treated the aircraft de-icing wastewater of COD 7000

mg/L, successfully in ABRs varying between 4 and 11 g COD/L.d. The ABR operated without

recycle achieved a minimum HRT of 27h with an acceptable COD removal efficiency of 89%.

The ABR operated with 6:1 recycle ratio achieved a minimum HRT in 17 with an acceptable

COD removal efficiency of 93% at an OLR of 9.9 g COD/L.d.

2.10.4 Hybrid Upflow Anaerobic Sludge Blanket Reactor

Many attempts have been made to overcome the negative aspects of high rate anaerobic

reactors. One among is called a hybrid upflow anaerobic sludge blanket (HUASB) reactor in

which the upper 20-30per cent of the reactor is filled with either floating or stationary materials,

such as polyurethane foam, polymer balls or random packed plastic rings, to retain some of the

escaping biomass. This design was investigated by Guiot and Van den berg (1984) and applied

by other researchers. The HUASB reactor is a combination of upflow anaerobic sludge blanket

unit at the lower part and upflow fixed film unit at the upper portion. In the hybrid reactor, the

upper packed section performs a dual function. It serves to retain the suspended sludge within

the reactor while also exerting a polishing effect on the wastewater through the activity of the

biofilm developed on the packing material. Another advantage of this kind of design is its

ability, even without granular sludge it retains high amount of biomass inside the reactor. This

kind of reactor is referred by different nomencuture namely, Upflow Blanket Filter (UBF),

Upflow Anaerobic Sludge Fixed Film (UASFF), Sludge Blanket Filter (SBF), Sludge Blanket

Anaerobic Filter (SBAF), Upflow Sludge Bed Filter (USBF) or simply a hybrid reactor.

61

Until recently, numerous hybrid designs has been proposed for the treatment of different

wastewaters. Among them, Lettinga et al., (1981), modified the design by providing gas

collector of similar one which is provided in typical UASB reactor below the filter layer to

improve the removal of suspended solids from municipal wastewater where it is very difficult to

maintain the stable granulation. Another simpler design is filter materials, located in the upper

part of the reactor without gas solid liquid separation device. The later one is normally being

used for most of the laboratory and full scale anaerobic hybrid reactors. Figure2.2 shows

different configuration of anaerobic hybrid reactor (Lettinga et al., 1981). The optimum amount

of filter media provided in the hybrid reactor is very much useful for both retention of active

biomass and economic consideration.

Suspended and colloidal components of wastewaters in the form of fat, protein and

cellulose have adverse impact on UASB reactor performance and can cause deterioration of

microbial activities and wash out of active biomass (Torkian et al., 2001). Modification of

UASB reactor was needed in order to overcome the above said impediments.

Particle removal in anaerobic filter media involves two distinct steps that are transport

and attachment. The particle is firstly transported to the filter media by mechanisms such as

diffusion, interception and sedimentation, before attachment takes place (Prasanthi, 1996).

The packing medium in the UASB reactor is intended to increase solids retention by

dampening short circuiting, improving gas-liquid-solid separation, and providing more surface

area for biomass attachment (Suraruk et al., 1998). Internal packing creates a suitable

environment to accelerate biogranule formation by particles recirculation. The biogranules are

dense microbial consortia packed with several bacterial species and typically contain millions of

organisms per gram of biomass (Liu et al., 2003).

One of the most efficient and flexible anaerobic high rate reactors are anaerobic hybrid

reactors (AHR) which combines the properties of both anaerobic filter (AF) and UASB reactor.

In order to prevent the wash out of the active biomass through effluent, AHR is provided with

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packing media. The factors affecting the performance of the AHR includes specific surface area,

porosity, surface roughness, pore size and the orientation of the packing media.

Borja et al., (1995) demonstrated a hybrid reactor treating virgin olive oil wash water

with COD removal efficiency of higher than 89 per cent at an OLR of 8 kg COD m-3 d-1 with

HRT of 18 h.

Gomes et al.,(2011) discusses the enzyme pretreatment on the stability and efficiency of a

hybrid up-flow anaerobic sludge blanket reactor treating dairy effluent, concluded that the

HUASB reactor fed with dairy wastewater operated with great stability until an OLR of 8.9

Kg.m−3.d−1. The reactor achieved COD removal of 93%.

Anushuya Ramakrishnan (2008) reported that the feasibility of anaerobic treatment of

complex phenolic mixture from a simulated synthetic coal wastewater using bench scale hybrid

up-flow anaerobic sludge blanket (HUASB). The result of this study showed that the above

wastewater could be treated effectively by hybrid UASB reactor at different HRTs varying

between 18 and 36 h. COD removal efficiencies decreased from 93% to 83%.

Sreekanth et al., (2009) developed a hybrid UASB reactor for treatment of

pharmaceutical wastewater under thermophilic conditions and concluded that the hybrid UASB

reactor has a capacity to handle overloading, which is an added advantage for industrial

application where both the quantity and the quality of the wastewater vary considerably. COD

removal efficiency of 65-70 per cent and BOD reduction of 80-90 per cent were observed while

operating the reactor at OLR of 9 kg COD m-3 d-1.

Gupta Sunil Kumar et al., (2007) reported that anaerobic hybrid reactor is superior and a

promising technology as compared to UASB reactor for the treatment of distillery spent wash.

The hybrid reactor is more efficient in terms of COD removal and biogas production as

compared to UASB reactor. At optimum HRT, 5 days and OLR, 8.7 kg COD/m3.d, the COD

removal efficiency and methane yield in hybrid reactor were approximately 5% more than

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UASB reactor. The rate of sludge washout, which is a major drawback of UASB reactor, can be

reduced by 25% in hybrid reactor.

Rajesh Banu et al., (2006) treated the effluent emanating from sago industry using Hybrid

Upflow Anaerobic Sludge Blanket reactor which offers the advantages of both fixed film and up

flow anaerobic sludge blanket treatment. The reactor was operated at OLR varying from 10.7 to

24.7 kg COD m-3 d-1. The COD removal varied from 87-91 percent, at the end of experiment

they concluded the ideal OLR for the reactor treating sago effluent was 23.5 kg COD m-3 d-1.

Sumi Lea Mathew, (2009) developed a hybrid reactor for treatment of synthetic

wastewater. The filter media used in the reactor was polyurethane foam (PUF) and COD

removal efficiency of 90.50% and BOD removal efficiency of 93.27%.

Oktem et al., (2008) evaluated the performance of a lab-scale hybrid up-flow anaerobic

sludge blanket (UASB) reactor, treating a chemical synthesis-based pharmaceutical wastewater

under different operating conditions. Initially, the carbon source in the reactor feed came entirely

from glucose, applied at an OLR 1 kg COD/m3 d. The hybrid UASB reactor was found to be far

more effective at an OLR of 8 kg COD/m3 d with a COD removal efficiency of 72%. At this

point, specific methanogenic activities (SMA) value was 200 mL CH4/g TVS d.

Tran et al., (2003) developed a laboratory scale Anaerobic Hybrid (AH) reactor for

treatment of domestic wastewater. The hybrid reactor has a capacity of 5.5L and concluded that

the domestic wastewater with 130 mg/L of BOD (350 mg/L of COD) was continuously fed by

up-flow to the reactor. During the operating period of 65 days, the OLR in the AH reactor

increased from 0.16 to 3.5 g COD/L.d, COD removal efficiency of 54%.

Bello-Mendoza and Cartillo-Rivere, (1998) tested the quick start-up of anaerobic hybrid

reactor using coffee processing wastewater at pilot scale with working volume of 10.5m3. The

top third of the reactor was packed with volcanic rocks ranging in diameter from 5 to 8 cm. After

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few days’ operations, the reactor achieved COD removal of 77.2% at an OLR of 1.89 kg

COD/m3.d and a HRT of 22h.

From visual observations done in transparent laboratory hybrid reactors, Tilche and

Vieira, (1991) proposed the following model of Gas / solid Separation (GSS) as shown in Figure

2.3.

The biomass flocs (or granules) are pushed upwards by the gas bubbles to which they are

physically bound.

The impact between the support material and the bubble helps the separation of gfas from

the solids that can fall back into the blanket or be temporarily entrapped within the filter.

Higher is the impact velocity, more efficient in the gas release.

Lo et al., (1994), achieved the treatment efficiency of 57% COD removal and 0.71 L

CH4 / in hybrid UASB reactors treating wine wastewater, in the absence of granulated seeding

sludge in the startup process. The organic loading rates were moderate (i.e. 3.5 g COD / L.d).

Through the sludge wash out occurred in this study, the methane production rates increased and

the concentrations of VFA in the effluents were kept below 100 mg/L.

Borja et al., (1996) studied the treatment performance of wastewater derived from the

purification of virgin olive oil using a hybrid reactor, the bottom one third of which was

occupied by a sludge blanket, the upper two thirds by submerged clay rings. The reactor was

operated under mesophilic conditions and different HRT ranged from 0.20 – 1.02 d under

normal operating conditions after starting up. COD removal efficiencies of more than 89% were

achieved at an organic loading rate of 8.0kg COD/ m3 .d and OLR was gradually increased from

2.6 to 7.1 kg COD/ m3.d within 16 days but the anaerobic reactor performance did not change

significantly. The reactor was operated varying influent COD concentrations to test the response

of the system to both high and low strength wash waters. The system can tolerate OLRs as high

as 17.8 kg COD/ m3.d with an average COD removal efficiency of 76.2% Although the reactor

was fed by diluted influent, with an average COD of 1030 mg.L, at every hydraulic loadings

(HRT – 4.8h) COD removals over 75%.

65

Figure 2.2 Different configurations of anaerobic hybrid reactors (lettinga et al., 1981)

Figure2.3 Visual model of the gas / solid separation effect of the filter in a hybrid

reactor (Tilche and Vierira, 1991)

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Klyuzhnyi et al., (1997) evaluated the performance of soft drink wastewater in 1.8 L

UASB reactor and 3 L hybrid reactor in which upper part packed with polyurethane materials.

Treatment efficiencies of 80% were achieved in both reactors when loaded upto a higher OLR of

13 and 16.5 g COD/L.d for hybrid and UASB reactors, respectively. Hybrid reactor seems to be

preferential than UASB with the OLR higher than 10g COD/L.d. And the HRT shorter than 1 d.

Both reactors treated the wastewater with an influent pH upto 11.

Tur Mao-Yuan and Haung Ju-Chang (1997) evaluated the treatment of phthalic waste by

anaerobic hybrid reactor where the packing media used was plastic rings. In the study, phthalate

degradation was started after 3 months when sucrose used as a carbon supplement and sewage

sludge used as seed. At 35°C and a phthalic loading of 20 g COD/L.d, the COD removal

efficiency was nearly 95%. About 89.5% of the removed phthalic COD were converted to

methane. When phthalic loadings were increased to 26.7, 33.0, 39.7 and 46.3 g COD/L.d, the

COD removal efficiencies were progressively reduced to 78, 65, 58 and 47.7% respectively.

More than 95% of the residual effluent COD was composed of non decomposed phthalic acid.

In the hybrid reactor, 86% of the biomass were found in the UASB section while the remaining

14% was found in the biofilter section. At 35°C and a phthalic loading of 26 g COD/Ld the

overall removal rate was 0.81 – 0.85 g COD/g VSS.d and the corresponding methane production

rate was 0.24 – 0.26 L CH4 / g VSS.d.

Fernandez et al., (2001) studied the treatment performance of fibre board manufacturing

wastewaters in pilot hybrid Sludge Bed filter (USBF) reactors. The effective volume of the

reactor is 1.1 m3 in which top 26.1 % were filled with packing media of PVC corrugated rings

(diameter and height of 50 mm). COD removal efficiencies of 90-93% were attained in the

anaerobic reactor operating at 37°C at OLR 6.5 – 8.5 kg COD/m3 d. When the wastewater was

subjected to pretreatment in coagulation – flocculation unit.

Buyukkamaci and Filibeli, (2002) investigated the performance of a hybrid reactor using

three different substrates namely, synthetic wastewater, baker’s yeast and meat processing waste

water. The media used to be PVC hose pieces of 3 cm long. The achieved COD removal

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efficiencies were 77-90% at an OLR varied 1-10 kg COD/m3 .d when synthetic wastewater was

used as substrate for 2 years. The methane content was 58%. Subsequently, baker’s yeast and

meat processing industry wastewater were fed to the model reactor. COD removal efficiencies

of 78 and 75% were achieved at an HRT, OLR of 2d, 9 kg COD/.m3.d and 2 d, 1 kg COD/m3 d

for yeast and meat processing wastewater, respectively. The methane contents were 58 and 70%

respectively.

Najafpour et al., (2006) studied the treatment of Palm oil mill effluent in upflow

anaerobic sludge fixed film (UASFF) reactor with tabular flow behavior, to shorten the start up

at low HRT. The reactor was operated at 38°C and HRT of 1.5 and 3 days and OLR was

increased from 2.63 to 23.15 g CODS/L.d Granulation was observed within 20 days and the size

of granules was reached to 2 mm. High COD removal efficiencies of 89 and 97% of HRT of 1.5

and 3 days were achieved respectively. A methane yield of 0.346 L CH4 /g COD removed when

the highest OLR was obtained. The SVI at 15, 35 and 55 cm were 16.9 37.9 and 117 mL/.g

respectively.

Oktem et al., (2008) evaluated the performance of a lab scale hybrid UASB reactor,

treating a chemical synthesis based pharmaceutical wastewater under different operating

conditions. Initially the carbon source in the reactor feed came entirely from glucose, applied at

an OLR 1 kg. COD/m3,.d. The OLR was gradually step increased to 3 kg COD/m3.d at which

point the fed to the hybrid UASB reactor was progressively modified by introducing the

pharmaceutical wastewater in blends with glucose, so that the wastewater contributed was

approximately 10,30,70 and ultimately, 100% of the carbon COD to be treated. At the

acclimation OLR of 3 kg COD /m3.d, the HRT was 2 days. During this period of feed

modification, the COD removal efficiencies of the anaerobic reactor were 99, 96, 91 and 85%

and specific methanogenic activities (SMA) were measured as 240, 230 225 and 231 mL CH4/g

TSS d, respectively. Following the acclimation period, the hybrid UASB reactor was fed with

100% (w/v) pharmaceutical wastewater upto an OLR of 9 kg COD/m3.d, in order to determine

the maximum loading capacity achievable before reactor failure. At this OLR, the COD removal

efficiency was 28% and the SMA was measured as 170 mL CH4 /g VSS.d. The hybrid UASB

68

reactor was found to be far more effective at an OLR of 8 kg COD/m3 d with a COD removal

efficiency of 72%. At this point, SMA value was 200 mL CH4/g VSS.d.

2.11 ANAEROBIC SLUDGE GRANULATION

Various types of conglomerates of microbes have been described, such as granules,

pellets, flocs, and flocculent sludge. The diameter of sludge granules varies from 14 to 5 mm,

depending upon the wastewater used, operational conditions and the analytical methods.

Granules cultivated on acidified substrates, such as acetate is generally smaller than granules

grown on subseries like glucose. The granules vary widely in shape, depending on the

conditions in the reactor, but they usually have a spherical form. Granules with different

volumes and densities can be present in a reactor at a given linear flow rate. Both small granules

with high densities and large granules with low and high densities will be present.

Zhou et al., (2006) examined the theophilic granulation process with synthetic

wastewaters and compared it with the parallel mesophilic ones. The results indicated that

granules could be formed well under compatible thermophilic surroundings on both

carbohydrate and protein rich substrates. Compared with the mesophilic proceeds, thermophilic

granulation took relative longer time, but thermosphilic UASB reactors had much higher

treatment capacity and efficiency than the mesophilic ones. Under the same running conditions

thermophlic sludge had lower contents of extracellular polymer (ECP) than mesophilic ones,

which might be one of the reasons for the longer period needed for thermophilic granulation.

The granular sludge occurs mainly in the lower regions of the reactor and forms a sludge

bed with a solid control of about 100 to 150 g/L at the bottom and about 5 to 10 g/L above it

(Pethe and Versprille, 1982). The growth of propionate utilizing bacteria would be responsible

for the increase in the VSS content of the granular sludge while acetocleatic microflora

production will be less (Guiot et al., 1992). Further, the strength of granules is shown to have a

bearing on the operating conditions (Ghangrekar et al., 2003).

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Due to the upflow pattern in the reactor, granulation is developed preferentially in the

lower part of their reaction zone and accumulated there. This sludge is gradually pushed

upwards to the whole sludge. In addition to this, biologically induced granular growth, the

physical shear stress and selective bio particle washout also contribute favourably to the

granulation process (Yan and Tay,1997). In a well granulated UASB, the reaction zone is

stratified into a static dense granular bed and a suspended, constantly moving thin sludge

blanket. Flocculent sludge predominates the upper part of the reactor forming a blanket

depending on the loading rates. Scanning electron micrographs indicate bacteria as the main

constituents of the granules. This observation is supported by the fact that the granular sludge

contains about 80 % volatile mater and 12 % nitrogen while the seed sludge (digested sewage

sludge) contains generally about 50% VSS and 5 to 7% nitrogen (Lettinga et al., 1980).

Start up is often considered to be the most unstable and difficult phase of anaerobic

digestion. Its main task is to develop a highly active settleable granular sludge as quickly as

possible. The formation of anaerobic granular sludge can be considered as the main reason of the

successful introduction of the USAB reactor concept for anaerobic treatment of industrial

effluents. There is a close correlation between efficiency of an USAB reactor and development

of granular sludge. The microbiology of these granular ecosystems has been studied by a number

of researchers during the past decade (Grotenhuis et al., 1991, Liu et al., 2003 and Hulshoff Pol

et al.2004).

2.11.1 Granulation Process

Now more than two decades later, numerous researchers from all over the world have

studied the granulation process. However, there is still no consensus about the determining

mechanism triggering granulation. Liu et al., (2003), described the four step general model for

anaerobic sludge granulations as follows.

First step, physical movement to initiate bacterium -to- bacterium contact or bacterial

attachment onto nuclei by various forces like hydrodynamic, diffusion, gravity, thermodynamic

70

(e.g. Brownian motion) forces and cell mobility. In the second step, initial attractive forces

involved are ; physical, chemical and biochemical forces. Physical forces include, van der waals

forces, opposite charge attraction, thermodynamic forces including free energy of surface

(surface tension), hydrophobicity and filamentous bacteria grasp individual cells together.

Chemical forces are hydrogen liaison formation of ionic pairs, formation of ionic triplet and inter

particulate bridges and so on. The various biochemical forces are cellular surface dehydration,

cellular membrane fusion, signaling and collective action in bacterial community. In the third

step, microbial forces to make cell aggregation by means of production of extracellular polymer

of bacteria, growth of cellular cluster, metabolic change and genetic competence induced by

environment to form a highly organized microbial structure by facilitating cell-to-cell

interaction.

In the final step, steady state three dimensional structure of microbial aggregate shaped

by hydrodynamic shear forces. The outer shape and size of microbial aggregates are determined

by the interactive strength / pattern between aggregates and of hydrodynamic shear forces,

microbial species and substrate loading rate.

2.11.2 Theories on Sludge Granulation

Hulshoff pol et al., (2004) reviewed the different theories of anaerobic sludge granulation

in USAB reactors and most researchers concluded that Methanosaeta concilii is a key organism

in granulation. Only the Cape Town hypothesis presumes that an autotrophic hydrogenotrophic

organism (i.e.) Methano bacterium AZ, growing under conditions of high H2-pressures, is the

key organism in granulation. Different theories developed during granulation the process has

been classified into; (i) Physical (ii) microbial and (iii) thermodynamic approaches.

Physical theories are considered based on physical phenomena such as gas and up flow

velocity, suspended solids in the effluent or seed sludge, attrition and removal of excess sludge

from the reactor are considered as the factors responsible for granulation.

Microbial theories are developed based on the characteristics of certain microorganisms.

The observation of granular characteristics, namely granule structure and corresponding

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microbiology, coupled with the conditions prevailing in the reactor (hydrodynamics, substrate

and intermediates concentration profiles along the reactor, etc., are the basis of the theories

presented. Physical approaches are also integrated. The physical factors are also integrated.

In thermodynamic theories granulation mechanisms based on aspects like physio-

chemical interactions between cell walls or between cell wall and alien surfaces, hydrophobicity,

electrophoretic mobility, proton translocating activity across the bacterial surface causing its

energisation are considered for sludge granulation.

Bacterial granulation is a complex process and is related to many factors. Among them

the formation of granules is mainly the result of microbial and hydraulic selection pressures.

Microbial selection depends on substrate concentration, substrate type and various other

environmental factors. Hydraulic selection involves the physical separation of dispersed

micrograms from aggregate-forming microorganism using shear forces (mixing) and differences

in settling characteristics (Hulshoff Pol et al., 1983) and Joo-Hwa Tay Tay and Yue Gen Yan,

1996). After physical separation, dispersed microorganisms are washed out as a result of their

low settling characteristics (poorly settling biomass), whereas the aggregate forcing

microorganisms (well settling biomass) are used as nuclei for granule formation.

According to the multi layer model as proposed by MacLeod et al., (1990) and Guiot,

(1992), the microbiological composition of granules is different in each layer. The inner layer

mainly consists of methanogens that may act nucleation centers for the initiation of granule

development. H2 producing and H2 utilizing bacteria are dominant species in the middle layer,

and a mixed species including rods, cocci and filamentous bacteria takes predominant position in

the outermost layer. To convert a target organic to methane, the spatial organizations of

methanogens and other species in UASB granules are essential.

Granulation of methanogenic consortium is essential to the stable operation of anaerobic

high rate biological systems. The formation and stability of the granules are essential for

successful operation and which are closely related to nutritional and environmental factors such

as trace metal ions (particularly calcium,), temperature, seed sludge, wastewater characteristics

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and hydraulic properties of the reactor environment (Grotenhuis et al., 1991 and Guiot et al.,

1984), the treated effluent stream, they reported a minimum ratio of COD:N:P = 100:8:1.

2.11.3 Flocculent / Granular Sludge

Howgrave – Graham et al., (1990), the bacteria present in the granules were found to be

capable of removing 93% of COD (600mg/L) from the feed. It can be concluded that the widely

accepted four stage model for the microbiology of anaerobic digestion comprising of hydrolyte,

acidogenic, acetogenic and methanogenic stages applies to granular sludge. It should however

be noted that the nature of the digester feed would probably play a significant role in the

selection of the specific microorganism within the granules.

Sandrine M.L. Salvi et al., (1996), according to their studies the treatment of low strength

wastewater using ABR, the sludge microbial products (SMP’s) probably formed due to

enhanced biomass decay at high HRT and also decreasing the temperature of ABR process

perform satisfactory up to organic loading rate of 20Kg.COD/m3.d and the flocculants sludge

was capable of removing the coarse suspended solids and the colloidal.

Dolfing, (1986) has been observed that granular methanogenic sludge in a variety of up

flow reactors, which is an agreement with the hypothesis that these systems select for well

settling sludge and also suggests that microscopic and kinetic evidence gave methanothrix like

organisms play an important role in determining which type of sludge will develop under

methanogenic conditions.

Anushuya Ramakrishnan et al., (2008), reported that morphological examination of the

sludge showed that the granules contained diverse groups of micro-organisms, where rod-type,

Methanothrix like, cells were dominant on the surface. As the HRT decreased from 36 to 18h, a

change in the surface morphology of the granules could be observed.

D.R.S.Gomes et al., (2011), reported that the observed morphologies were cocci, bacilli

and vibrios, which are commonly found in anaerobic reactors. In the samples from the superior

foam bed of the reactor, Methanosarcina species like morphologies were constantly present.

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Azhari Samsu Baharuddin et al., (2011), reported that SEM, FT-IR and NMR analysis

revealed that the pressed-shredded EFB and POME anaerobic sludge was suitable substrate

for the co-composting process based on its physicochemical properties as compared to the

shredded EFB and raw POME. FTIR analysis also proved that the pressed-shredded EFB-

POME anaerobic sludge based compost was stabilized after 40 days of treatment and the

result corresponds to the final C/N ratio of 12.4.

Subramanyam Revanuru et al ., (2011) reported that relatively higher percentage of iron

and calcium and a lower percentage of other inorganic components including sodium, potassium,

etc., got incorporated in the granules during the treatment of phenolic wastewater. The results

indicate that the calcium and iron at increased levels in the granules play a very important role in

the treatment of the mixed feed of catechol and resorcinol in a catechol acclimated UASB

reactor.

Gary Chinga-Carrasco, (2010) reported that wood pulp fibres are bio-degradable and are

potentially an important raw material in a wide range of application areas. Quantitative

microscopy is a major advantage, as several structural characteristic details can be quantified

efficiently and objectively. Such advances will provide a comprehensive understanding of fibre

and fibril structures and thus contribute to the development of novel bio-based materials.

2.11.4 Characteristics of Wastewater in Anaerobic Sludge Granulation

Jhung et al., (1995) compared the operating characteristics of laboratory UASB and fixed

film reactors with various waste namely. Complex carbohydrate vs simple volatile waste, and

concentrated vs diluted wastes. The result showed that characteristics of waste plays a major

role than that of reactor type and complex carbohydrate waste having a higher COD/VFA ratio,

produced filamentous microbes and these appeared to promote granulation. It was concluded

that fixed film reactor generally had a consistent treatment capability with various waste. Fang

et al., (1995) also reported that biogranules bacterial populations and microstructures were

strongly dependent on the nature of the substrate.

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2.12 ROLE OF EXTRACELLULAR POLYMERIC SUBSTANCES (EPS)

Extracellular Polymeric Substances (EPS) have played crucial role in the formation of

granules as well as maintenance. Especially, the surface of microorganisms was negative

consistently so that it needs some positive charges or other means such as EPS and polymers in

order to develop granules. The loosely adhered bacterial aggregates are strengthened by

extracellular polymers secreted by bacteria to form firmly attached initial granules based on the

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in a UASB reactor. EPS contained

organic debris, phages, lysed cells and consists of polysaccharides, proteins, lipids, phenols and

nucleic acid (Zhou et al., 2006 and Stal et al., (1989).

2.13 GRANULATION IN THE TREATMENT OF DIFFERENT WASTEWATERS

Lin et al., (1987) studied the microbial population when synthetic substrate consisting of

VFA and inorganic nutrients was used a substrate. Bacilli were the predominant microbial

species and coccid and sarcinae were observed at shorter retention times. This predominance is

unaffected by temperature changes over mesophilic and thermophilic ranges. It has been

reported that sarcina were the predominant microbial species at low temperature (25°C) and

short SRT (4.5 days) when using acetate as the substrate (Lawrence and McCarty, 1969).

Kosaric et al., (1990) evaluated hydrodynamic behavior which influences granule activity

and characteristics using 4.2 L UASB reactors at constant temperature (35°C) and volumetric

loading rate (6.2 g COD/L.d). The substrate revealed that at low up flow velocities (0.25 and 0.5

m/h), granules accumulate on the bed, they enlarge and the fraction of granules with high

settling rate increases. At higher up flow velocities ( 1 and 1.5m/h) granules are partly

disintegrated and washed out of the bed.

Fang et al., (1995) reported formation of granules during the treatment of volatile fatty

acid concentration at 37°C at loading rates upto 24 g COD/L.d The granules had a fluffy

surface mostly composed of inter wound filamentous Methanothrix like bacteria. Syntrophic

associations between Methanothrix, Methanospirillum hungatei, and Syntrophobacter like

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bacteria were prevalent in the granule interior. The syntrophic relation between these species

was elucidated by thermodynamics.

Yue-Gen Yan and Joo-Hwa Tay, (1997) examined the granulation process using a

synthetic feed composed of glucose, peptone and meat extract. At the end of operation period,

Methanothrix like species had a diameter of 2.6mm was observed and found highly active with

well settable. Imai et al., (1998) studied the formation of granules and treatment characteristics

in an UAHB reactor, adding water absorbing polymer (WAP) for treating a fermentation process

wastewater consisting high sulfide and ammonia. The author observed that sulfide inhibition did

not occur when free hydrogen sulfide concentration was less than 200 mg/L. To enhance the

granulation WAP ST 500D was added to the digester and granules of 1.5mm were observed after

75 days of operation.

Angenent et al., (2000) found that granules of Methanoseata and Methanosarcina species

in the treatment of synthetic wastewater treatment in AMBR using flocculent anaerobic digester

sludge as seed material. It was concluded that this finding was very important because it was

previously believed that a hydraulic outflow pattern was necessary to select for a granular

blanket.

Picanco et al., (2001) investigated the influence of material porosity on the anaerobic

biomass adhesion on four different inert matrices: polyurethane foam, PVC, refractory brick and

special ceramic using anaerobic filters. The substrate used was synthetic wastewater containing

protein, lipids and carbohydrates. Polyurethane foam and special ceramic were found to present

better retentive properties than the PVC and refractory bricks. The large specific surface area,

directly related to materials porosity, is fundamental to provide a large amount of attached

biomass. However, different support can provide specific conditions for the adherence of

distinct microorganism types. Microbiological archaism resembling Methanosaeta was observed

both in the refractory brick and the special ceramic Methanosarcina like microorganisms were

predominant in the PVC and the polyurethane foam matrices.

Pender et al., (2004) studied diversity, population dynamics and activity profiles of

methanogens in anaerobic granular sludge from two anaerobic hybrid reactors treating a

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molasses wastewater both mesophilically (37°C) and thermophilically (55°C) during a 108 day

trial with the addition of sulphate. During mesophilic operation both in the presence and

absence of sulphate, the biomass characterized by a predominance of Methanosaeta sp. During

temperature elevation, the methanogens found in sulphate supplemented and non supplemented

reactors were Methanocarpusculum parvum and Methanobacterium thermoautotrophicum,

respectively. During mesophilic operation, both reactors shown better COD removal

efficiencies of over 90% irrespective of sulphate supplementation, but in contrast at thermophilic

the COD removal efficiency reduced due to the inhibition of sulphide.

Gupta and Gupta, (2005) studied the morphology of granules formed during anaerobic

digestion of distillery wastewater in two laboratory scale bioreactors namely, upflow anaerobic

sludge blanket reactor and hybrid reactor. In this study two different inoculums were used for

startup and results revealed that earlier start up and granulation of biomass could be achieved

using mixed sludge of anaerobic digested sludge, cow dung and aerobic sludge than

anaerobically digested sludge. The predominant microorganisms observed were Methanosarcina

and Methanothrix types of species on the surface of granules. Granules of layer diameter

observed in lower active zones and smaller granules observed in top and middle zones of the

sludge bed.

In the treatment of Palm Oil Effluent, Najafpour et al., (2006) observed Methanosaeta

like organisms in the granules. They supported that the dense granules in spherical shape were

developed due to the hydrodynamic shear force caused by the upflow liquid and biogas formed.

2.14 RESIDENCE TIME DISTRIBUTION STUDIES

The hydraulic characteristics of the reactor may be measured through mixing studies

using a impulse or stepped addition of a tracer material (e.g. Lithium, Tritium) to obtain a tracer

curve of retention times within the reactor. The notion of using the RTD was first proposed by

MacMullin and Weber as stated by Fogler, (1986). The objectives of that RTD measurement are

as follows (Missen et al., 1999 and Tilche and Vieira, 1991).

To use as a diagnostic tool for detecting and characterizing flow behavior

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To estimate values of parameters for non ideal flow models, such as tanks in series model

and the diffusion model

To assess performance of the reactor

To design reactors with optimal environmental and process conditions

The system is characterized by the dispersion number D//µL) and the theoretical number of

ideally mixed reactors (N) equal size reactors connected in series which as for performance

correspond to the investigated system. Levenspiel, (1991) suggested a model to evaluate the

values of D/µL and N, which is described below.

Mixing characteristics of anaerobic reactors depend on the reactor geometry, the type of

medium, inlet and outlet design, the amount of biogas produced, the hydraulic aspects and flow

velocities of the feed and the effluent recycle and any other artificial mixing provided. Under

plug-flow conditions, incoming substrate remains in the reactor for one retention time, allowing

maximum time for conversion. However, the high substrate concentrations resulting from back

of dispersion may inhibit bacterial activity. On the other hand, excessive dispersion may result

in short circuiting of the substrate and would not be ideal for granule formation in some

anaerobic reactor configurations (Uyanik, 2003). Consequently, an intermediate degree of

mixing appears to be optimal for substrate conversion (Smith et al., 1996).

Most laboratory scale filters operate at liquid velocities between 1 and 8m/d. Thirumurti,

(1998) observed that poor reactor performance below 16m/d velocity due to inadequate mixing

and also greater than 163 m/d excessive velocity excessive biomass shear and loss in the

effluent. Smith et al., (1996) concluded that most mixing takes place in the space below the

filter media because the origin of the majority of the gas production is in the sludge bed zone

which exists below the media.

In the comparative study of MFR with SFR, RTD showed that the reactors behaved like

ideal CSTR and the working volume of MFR was higher than 85% while only 65% of SFR

remained active after 410 days of operation (Punal et al., 1998).

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Kuan-yeon Show and Joo-Hwa Tay, (1999) studied the tracer curves under different

media types,namely, glass open pored; PVC smooth with surface holes a PVC smooth media

without surface holes at identical volumetric loading rates and hydraulic retention times. They

observed early peak in tracer curves, during the tracer studies in anaerobic filters, indicating a

reduction in effective volume of the reactor due to less pore size and porosity. The tracer

subsided after the peaks and gradually reduced to insignificant levels at a period of almost twice

the HRT of 15h. The pattern of tracer curves reflects more closely to the response of a mixed

flow reactor than that of a plg flow unit. This suggests that there is significant short circuiting in

the AFs operating at the higher COD loading rates (16 g COD/L.d).

Langenhoff et al, (2000) observed that the flow pattern was intermediate between plug

flow and perfectly mixed in anaerobic baffled reactor under all the conditions tested when semi

skimmed pasteurized milk mixed with tap water used as feed. Any deviation from ideality could

have been caused by channeling of the liquid through the biomass bed, or by creation of dead

spaces in the reactor.

Taking all the above points into consideration, it is proposed to study and compare the

performance of HUASB reactors for the treatment of paper and pulp mill wastewater with and

without effective microorganism with co-substrates like domestic wastewater and EM as

inoculums.

2.15 REGRESSION AND ANN MODELLING

A correlation study among ground water quality parameters have been reported in

literature (Nagarajan et al., 1993, Klavins et al., 1996, Rajasekaran et al., 2004). Similar type

of correlation study has been reported for industrial wastewaters (Tiwari et al., 2009).

The physicochemical parameters and their correlation regression analysis for the

industries, such as textile, dyeing and printing, distillery, pulp and paper have been reported

(Nemade and Shrivastava, 1997, Kulkarni and Shrivastava, 2001, Malaviya and Rathore, 2001).

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Divya et al., (2008) prepared a correlation analysis for the various chemical parameters

such as pH, electrical conductivity, total hardness, calcium, magnesium, total alkalinity,

carbonate, bicarbonate of sodium, potassium, chloride, phosphate, fluoride and nitrate

significant linear relationships among some water quality parameters have been obtained and

found maximum between dissolved solids and electrical conductivity (0.9999) and between total

hardness and magnesium (1.000) which can be used for rapid monitoring of water quality

parameters.

Venkatesh et al., (2009) reported that the r-value varies in the range of 0.0608 to 0.9969

depending on the set of parameters considered for analysis. The correlation values above 0.94

were selected for analysis. The highest correlation was between EC and TDS. High positive

correlations between turbidity and TSS, BOD and COD, EC and chlorides were also observed.

Among the different neural network structures, back propagation neural Network

(BPNN) introduced by Rumelhart et al., (1996) and most popular because of their applicability

in many different areas (Wasser man, 1989). In principle, a BPNN may have several hidden

layers, but in practice, only one or two layers were used. The number of mode in the hidden

layer was determined mainly by trial and error. Several attempts have been made to arrive at

some kind of optimal structure of a BPNN model (Fahlman and Lebiere, 1990, Lim and Hong,

1993).

According to Principle et al., (1999) and Haykin, (1999) the initial weights were

randomly generated between 0 and 1 with a random number generator. Selection of successful

network geometry was dependent on the problem domain. Various researches gave guidance to

how many hidden layer modes should be used (Baum and Haussler, 1989).

The literature studied reveals that paper and pulp effluents released into the environment

are toxic to both flora and fauna. The paper mill effluent is being treated by various physic

chemical and biological methods but the biological method is ecofriendly and cost effective.

The anaerobic process has been reported as the most economical of waste disposal compared to

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aerobic process. The research review various stages involved in anaerobic degradation process

and has also discussed the features of the UASB reactors and several bio reactors such as SBR,

FBR, CSTR, PBR, UASBR, were reported to have been used for treatment of paper and pulp

mill wastewater. These reactors are found to be effective in removal of degradation of organics.

Role of various operating parameters also discussed. No study contains tractability of pulp and

paper mill wastewater with and without cosubstrates like domestic wastewater and EM as

inoculum incorporating the CETP concept of the paper and pulp mill wastewater .

Keeping this fact in mind an attempt has been made to study the treatability of combined

wastewater of paper and pulp mill as a substrate and cosubstrate like domestic wastewater using

EM as inoculum. In order to predict the optimal proportion of the substrate and cosubstrate,

with and without EM, the maximum COD removal efficiency, gas productivity, extensive

investigation is to be carried out under different organic loading rates and hydraulic retention

time.