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
36
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
57
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).
59
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
62
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)
66
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
67
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.