Scholarly article on topic 'Start-up Operation of Anaerobic Degradation Process for Palm Oil Mill Effluent in Anaerobic Bench Scale Reactor (ABSR)'

Start-up Operation of Anaerobic Degradation Process for Palm Oil Mill Effluent in Anaerobic Bench Scale Reactor (ABSR) Academic research paper on "Chemical engineering"

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{"Anaerobic degradation" / "Acclimatization phase" / "Palm Oil Mill Effluent" / "Chemical oxygen demand" / "Volatile fatty acid"}

Abstract of research paper on Chemical engineering, author of scientific article — Tjoon Tow Teng, Yee-Shian Wong, Soon-An Ong, M. Norhashimah, M. Rafatullah

Abstract The start-up operation or acclimatization process of anaerobic stabilization pond treatment for palm oil mill effluent (POME) was carried out in a laboratory anaerobic bench scale reactor (ABSR). The start-up operation of the ABSR was operated at two different organic loading rates (ORG) of 0.89 – 0.92 kg COD/m3/day and 1.34 – 1.37 kg COD/m3/day which corresponds to hydraulic retention time (HRT) of 60 and 40 days. The investigation showed that the acclimatize process for anaerobic degradation of POME was completed after 48 days of operation. In present study, the indicators that been used were pH, microbial growth, effluent of total chemical oxygen demand (COD), COD removal efficiency, alkalinity and ratio fraction between volatile fatty acid and alkalinity.

Academic research paper on topic "Start-up Operation of Anaerobic Degradation Process for Palm Oil Mill Effluent in Anaerobic Bench Scale Reactor (ABSR)"

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Procedia Environmental Sciences 18 (2013) 442 - 450

2013 International Symposium on Environmental Science and Technology (2013 ISEST)

Start-up operation of anaerobic degradation process for palm oil mill effluent in anaerobic bench scale reactor (AB SR)

Tjoon Tow Teng3-*, Yee-Shian Wong3- b, Soon-An Ongb, M. Norhashimaha,

M. Rafatullaha

aSchool of Industrial Technology, Universiti Sains Malaysia, 11600 Gelugor, Pulau Pinang, Malaysia bSchool of Environmental Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis,

Malaysia

Abstract

The start-up operation or acclimatization process of anaerobic stabilization pond treatment for palm oil mill effluent (POME) was carried out in a laboratory anaerobic bench scale reactor (ABSR). The start-up operation of the ABSR was operated at two different organic loading rates (ORG) of 0.89 - 0.92 kg COD/m3/day and 1.34 - 1.37 kg COD/m3/day which corresponds to hydraulic retention time (HRT) of 60 and 40 days. The investigation showed that the acclimatize process for anaerobic degradation of POME was completed after 48 days of operation. In present study, the indicators that been used were pH, microbial growth, effluent of total chemical oxygen demand (COD), COD removal efficiency, alkalinity and ratio fraction between volatile fatty acid and alkalinity.

© 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology.

Keywords: Anaerobic degradation; Acclimatization phase; Palm Oil Mill Effluent; Chemical oxygen demand; Volatile fatty acid

1. Introduction

Large quantities of water are required for the crude palm oil extraction process. For every ton of oil palm fresh fruit bunch, it is estimated that 0.5-0.75 tons of palm oil mill effluent (POME) will be discharged [1]. POME is a colloidal suspension, which contains 95% - 96% of water, 0.6% - 0.7% of oil and grease and 4% - 5% of total solids [2]. It is a thick brownish liquid and it's discharged at a temperature between 80°C and 90°C [3]. Meanwhile, POME is considered as one of the most polluting agro-industrial residues due to its high organic load. This highly polluting wastewater could create odor problems to the neighborhood of the mills, a nuisance to the passers-by or local residents and river pollution [4].

* Corresponding author. Tel.: +604-653-2215, Fax: +604-657-3678 E-mail address: ttteng@usm.my.

1878-0296 © 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology.

doi: 10.1016/j.proenv.2013.04.059

Currently, the treatments employed for treating POME in Malaysia are follows to a large extent and the principles of biochemical operations. Three different types of treatment systems are adopted and these include: a) open tank digester and extended aeration system [5], b) closed anaerobic digester and land application system [6], and c) pond treatment system [5]. The choice of the treatment systems are depends to a large extent on the company's preference, location of the mill and availability of useable land. However, the pond treatment system is the most popular as it has been adopted by more than 85% of the palm oil mills in Malaysia nowadays [6]. Most of the pond systems that been applied for the treatment of POME in Malaysia are classified as waste stabilization pond. Besides, waste stabilization ponds are been widely used as a method of sewage disposal since the ancient times [7]. According to Arceivala [8], stabilization pond is similar to an activated sludge process but be different in the following ways: Stabilization ponds have i) long retention period, ii) low loading rate, iii) less active microbial biomass, and iv) less mixing and agitation where the particulate solids are settled and formed sludge layer in which the anaerobic process breakdown occurs. The configuration of the pond system consists of essentially a number of ponds of different functions such as anaerobic, facultative and aerobic ponds, which are made up of earthen structures with no lining at the bottom [6].

Anaerobic pond system for POME treatment consist of at least two ponds connected in series. The raw POME is channeled into the anaerobic pond from the sludge recovery tank. Anaerobic pond system is very effective in the treatment of wastewater with high strength, biodegradable organic contents (BOD>500) generated in large quantities by agricultural and food industries [7]. Anaerobic ponds system are usually designed with deeper basins than the other ponds in order to reduce the surface area to volume ratio thereby minimizing re-aeration (since oxygen transfer through the air-water interface is undesirable) and heat loss [7]. The anaerobic ponds system for POME treatments in Malaysia are usually 5-7 meters in depth [9]. Three zones can be identified in the pond, which includes: the scum layer, the supernatant layer and the sludge layer [10]. The anaerobic reaction takes place in the sediment includes solubilization of biodegradable particulate matter followed by acidogenesis, acidogenesis and methanogenesis process [11].

Generally, anaerobic ponds system are been used as preliminary treatment for high strength organic wastes and for partial stabilization of the waste before the secondary treatment system. The organic loading is considerably reduced and the retention time needed is generally long [12]. The organic loading for POME treatment is varies from 0.2-0.35 kg BOD/m3/day with a minimum of 30 days HRT [6]. Therefore, the start-up operation or acclimatization process of the anaerobic digestion process must be done with extreme caution to allow the anaerobe bacteria present in the mixed liquor could entirely acclimatized to a new environment.

The aim of this work is focused on the start-up operation or acclimatization process of anaerobic degradation process for POME. Therefore, an anaerobic bench scale reactor (ABSR) was operated at two different organic loading rates (ORG) of 0.89 - 0.92 kg COD/m3/day and 1.34 - 1.37 kg COD/m3/day which corresponds to hydraulic retention time (HRT) of 60 and 40 days to treat the POME.

2. Materials and Methods

2.1. Wastewater Sampling and Characterization

The raw and anaerobic digested POME that been used for this research project was taken from the recovery tank and anaerobic pond at MALPOM Industries Bhd, Nibong Tebal, Penang. Both samples were separately collected in 10 liters high-density polyethylene (HDPE) bottles. The wastewater was then stored in the refrigerator at 4°C until further detailed analysis could be conducted. Meanwhile, the sampling time was fixed between 2-3 pm for every batch of sampling. The characteristic of POME that been used in the experiment is shown in Table 1.

Table 1. Characteristics of palm oil mill effluent (POME).

Parameter Concentration range (mg/l) Mean Standard Deviation

pH 4.15 - 4.45 4.3 ± 0.11

COD 45500 - 65000 55250 ± 7534

SCOD 20500 - 24500 22500 ± 1436

BOD 21500 - 28500 25000 ± 2607

TS 33790 - 37230 35515 ± 767

TVS 27300 - 3015 28697 ± 1054

SS 15660 - 2350 19610 ± 2908

O & G 1077 - 7582 3822 ± 2483

TN 300 - 410 355 ± 37

NH3-N 8.3 -8.5 8.4 ± 0.08

* Unit for all parameter is mg/l except pH

2.2. Experimental set-up

The schematic diagram of the laboratory anaerobic bench-scale reactor (ABSR) is shown in Figure 1. ABSR was fabricated of flexi glass with the dimension of 60 cm length, 30 cm height and 30 cm width. The reactor was divided into two parts, which are anaerobic tank with dimensions of 55 cm length, 28 cm height and 30 cm width and the overflow collection tank of 5 cm length, 28 cm height and 30 cm width. Two panes of flexi glass with dimension of 10 cm in width and 55 cm in length were fixed at an angle of 45o at the bottom of the anaerobic tank as shown in Fig. 1. The total volume of the ABSR was 54 liters and the actual working volume was 38 liters.

Influent Open Top Reactor A A

: Scum Layer : .

Effluent

Anaerobic Reactor

Fig. 1. Schematic diagram of laboratory anaerobic bench-scale reactor (ABSR).

2.3. ABSR Sampling Procedure

The ABSR was manually been controlled and monitored. ABSR was operated under the mesospheric conditions. Master flex peristaltic pump (Model 7520-35) was used to pump the raw POME wastewater into the anaerobic tank. Treated POME was streamed into the collection tank. In present study, the treated

POME was taken from the collection tank after 24 hours for the purpose of analysis at each batch of hydraulic retention time (HRT). The ABSR was monitored constantly to ensure smooth operation. The samples were subjected to the analysis of the following parameters such as pH, COD, SCOD, MLSS, MLVSS, alkalinity, volatile fatty acids and others based on American Public Health and Association (APHA) standard methods for water and wastewater analysis [13].

2.4. Acclimatization of ABSR

Throughout this present study, the seeding that required for starting up the ABSR was taken from anaerobic pond of MALPOM Industries Bhd wastewater treatment plant. About 38 liters of the anaerobic digested POME was used to acclimatize the laboratory ABSR. Throughout this acclimatization period, the start-up of the ABSR involved stepped increases in influent organic volumetric loading rates from 0.89 - 0.92 kg COD/m3/day to 1.34 - 1.37 kg COD/m3/day. This acclimatization phase was continued for about 50 days to allow the microorganisms present in the mixed liquor perfectly acclimatized to the new environment [4].

3. Results and discussion

3.1. Acclimatization phase of ABSR

The acclimatization process or better known as start-up strategy of the anaerobic digestion process has to be done with extreme caution. This process could allow the anaerobe bacteria present in the mixed liquor entirely acclimatized to a new environment. Therefore, the start-up strategy was been done by monitoring a number of indicators that would supply complimentary information for the anaerobic degradation process. This information would provide an early warning of the degradation process so that remedial actions could be taken before the degradation process failure occurred [14]. The indicators that been used for the acclimatization process are pH, microbial growth, total COD of effluent, COD removal efficiency, alkalinity and ratio faction of VFA:Alk.

3.2. pH variation during the acclimation process of ABSR

The pH variations of the anaerobic bench scale reactor (ABSR) are presented in Fig. 2. The initial pH of the anaerobic digested POME from anaerobic pond of MALPOM Industries Bhd. wastewater treatment plant as seeded sludge for the ABSR is obtained at 7.07. ABSR is initially loaded at 0.89 - 0.92 kg COD/m3/day, which corresponds to hydraulic retention time (HRT) of 60 days. Gonzalez et al., [14] stated that the acclimatization process should be started with a low loading rate because the upsets of the most systems are caused by both hydraulic and organic overload. As shown in Fig. 2, the pH profiles of ABSR tend to reduce gradually from 7.07 to 6.24 on the operation days between 0 and 10 days. This is largely due to an increased concentration of acetic or propionic acid that indicates the acidogenesis phase had occurred in the ABSR [15]. For the next 17 days, the pH level leisurely increased from 6.24 to 7.75 as the methanogenic activity started to proceed optimally in a narrow pH value range between 6.3 and 7.8 [16]. On the 29th day of operation, the pH reached 8.33 because the previous loading rate was increased to 1.34 - 1.37 kg COD/m3/day which corresponds to HRT of 40 days. According to Yacob et al. [1], the performance of anaerobic degradation process warranted to increase with the use of stepped increment of raw POME loading rate. Moreover, the sharp increase of pH from 7.75 to 8.33 was due to the rise of methanogenic population with the organic loading rate, since more organic matter may support a larger population [17]. The ABSR continued operating at the same loading rate to monitor the stability of the reactor. The pH values showed fluctuation between 8.33 and 8.36 between the 29th to 38th days of

operation. Besides, the pH slightly increased about 0.10 to 8.46 in the next two days and fluctuated between 8.46 and 8.48 for the next eight days. It was indicated that the ABSR had already reached stability of the anaerobic digestion process.

Fig. 2. pH variation of anaerobic bench scale reactor (ABSR) during acclimation process.

3.3. Microbial growth variations during acclimation process of ABSR

The microbial growth or biomass concentration during acclimation process of ABSR was measured as mixed liquor volatile suspended solids (MLVSS). Meanwhile, the mixed liquor suspended solids (MLSS) was also measured for investigation of fraction ratio between MLVSS and MLSS. The microbial growth variations of ABSR during the acclimation process are shown in Fig. 3. During the initial period of operation, the MLVSS concentration was at 15100 mg/l while the VSS:SS ratio is 0.7. The MLVSS concentration suddenly increased to 21900 mg/l after two days of operation. Besides, the MLVSS concentration continued to fluctuate between 21820 and 20320 mg/l for next 8 days since the pH of the ABSR is reduce. At this stage, it is believed that the colony of non-methanogenic (acidogenesis) bacteria was more than the methanogenic bacteria to convert the raw waste into organic acids, carbon dioxide and hydrogen gas [18]. Thus, the MLVSS shows increase up to 23280 mg/l on the operation days of 13 but gradually decreased from 21900 to 17200 mg/l for the next eight days. This situation shows that the ABSR had proceeded from acidogenesis to methanogenesis phase because the pH of ABSR was started to increase. Here, the biomass needed an extension of time required for the stabilization of organic fraction of the waste to stabilize since methanogenic anaerobes are known to be very sensitive to pH [19]. The ratio faction of VSS: SS still remained about 0.73 in the reactor.

60000 =■ 50000

f 40000 ^ 30000 | 20000 Ü5 10000 0

MLSS MLVSS

0 6 13 17 24 29 34 38 43 48

Operation days

Fig. 3. Microbial growth of the ABSR during acclimation process.

After 17 days of operation, the biomass started to increase from 21620 mg/l and reached a peak of 31220 mg/l on the next twelve days. On the other hand, the ratio faction of VSS: SS is decrease from 0.73 to 0.63. The dramatic increase of biomass is caused by the step increment of organic loading rate from 0.89 - 0.92 kg COD/m3/day to 1.34 - 1.37 kg COD/m3/day. Moreover, the steep increase of pH from 7.18 to 8.33 also provided high amounts of active methanogenic bacteria in the ABSR [20]. The ML VSS concentration reduced to 26640 mg/l on the 31st day of operation. The stationery phase of microbial growth quickly reached on the 31st until 48th of operation day. At this period, the MLVSS concentration almost remained constant between 26600 and 25600 mg/l. Besides, the ratio faction of VSS:SS is always staying at 0.63. The constant concentration of biomass in the reactor demonstrated that ABSR is capable of holding high organic strength of POME and produce less amount of sludge even the ABSR had been operated for 48 days [21].

3.4. COD variations during acclimation process of ABSR

In present study, the influent of COD concentration of raw POME was fixed in the range of 52000 -55000 mg/l. Fig.4 illustrates the ABSR effluent COD concentration and removal efficiency during the acclimation process. ABSR is initially loaded at 0.89 - 0.92 kg COD/m3/day, which corresponds to hydraulic retention time (HRT) of 60 days. As shown in Fig. 4, the initial COD concentration of the seeded supernatant was at 11800 mg/l. After two operation days, the effluent concentration of COD increased to 12550 mg/l and keeps constant at around 12500 mg/l after a week of operation before increased about 8% to 13650 mg/l at 10th operation days. Meanwhile, the COD removal efficiency was steadily reduced from 77.03% to 74.61% by 2nd until 13th of operation days. At this stage, it was believed that the methanogenic bacterial growth slowly to adapt to the new environment since the pH of ABSR was reduced as mention at the section 3.2.

c 16000

o 14000

fó 12000

c V 10000

o c 8000

o u 6000

o O 4000

O 2000

J 100%

-- 90% 80% -- 70% -- 60% 50% ■f 40% 30% 20% 10% 0%

0 2 6 10 13 15 17 21 24 27 29 31 34 36 38 40 43 45 48 Operation days

Fig. 4. COD concentration of effluent and removal efficiency of ABSR during acclimation process.

On the other hand, the effluent COD kept increasing until it reached its peak of 17700 mg/l during the operation days of 10th to 21st. Thus, the COD removal efficiency also steeply decreased from 74.61% to 67.67% between 13th and 21st operation days. This is largely due to unbalance of anaerobic digestion process because the rate of acid formation exceeds the rate of breakdown to methane [22]. For the next three days, the effluent COD sharply dropped to 11100 mg/l but the COD removal efficiency was on the opposite trend as it dramatically increased to 79.78%. Here, it is a clearly noted that the methanogenesis phase completely occurred in the ABSR, since the pH of ABSR is at 7.53 on the 24 days of operation. Damirel and Yenigun [20] have stated that the pH of about 7.5 seemed to provide high amounts of active

methanogenic bacteria in the system. Reading of effluent COD remained between 11100 and 11500 mg/1 for the next three days. On the 29th day of operation, the COD effluent reduced about 19% to 9300 mg/1 while the COD removal efficiency increased up to 83.06%, where the organic loading rate increased to 1.34 - 1.37 kg COD/m3/day. According to Toprak [23], this situation is expected that COD removal efficiency will increase with increasing of organic loading rate until a maximum condition at which the anaerobic biomass is optimum. Therefore, the effluent COD fluctuated between 9375 and 9200 mg/l while the removal efficiency also fluctuated between 82.28% and 83.2% for the remaining days of the operation. As a result, the acclimatization period of ABSR is already completed.

3.5. Alkalinity and VFA: Alk variations during acclimation process

The indicator of alkalinity and concentration ratio of volatile fatty acids (as acetic acids) to the alkalinity, VFA: Alk play an important role to tell us about the buffering capacity of anaerobic degradation system [18]. Fig. 5 describes the alkalinity concentration and VFA: Alk ratio of the ABSR during the acclimation process. The initial concentration of alkalinity and VFA: Alk ratio was 6370 mg CaCO3/l and 0.44. It was depicted that the initial VFA: Alk was greater than 0.4 indicating that the anaerobic degradation was not stable in the ABSR [22]. After two days of loading at 0.89 - 0.92 kg COD/m3/day, the alkalinity sharply increased up to 10290 mg CaCO3/l. Meanwhile, the VFA: Alk ratio also steeply reduced to 0.21. The acidogenesis phase occurred in the ABSR with the reduction of pH due to VFA accumulation proving an optimal environment for the acidogenesis bacteria [24]. For the next 4 days of operation, the alkalinity slightly reduced to 10040 mg CaCO3/l and the VFA: Alk ratio also reduced to 0.18. The alkalinity gradually increased from 11550 to 14190 mg CaCO3/l from 10th until 21st days of operation while the VFA: Alk ratio reduced from 0.14 to 0.09. The increase of alkalinity level indicated that the bacterial groups were in equilibrium within the alkalinity range of 12500 - 14000 mg CaCO3/l [25]. Throughout the 24th operation day, the alkalinity kept increasing to 15470 mg CaCO3/l. The alkalinity remained at 15500 mg CaCO3/l on the next three days. On the other hand, the VFA: Alk reduced to 0.08 and fluctuated between 0.08 and 0.09 at the period of 24th until 31st operation day. The VFA: Alk ratio was within the optimum range after a month of operation. This is largely due to the usage of seed sludge from the same type of waste and hence reducing the acclimatization period by sustaining

0.1 0.05 0

j 18000 ^ ^ ♦—+—< ►

-- 16000 -- 14000 -- 12000 -- 10000 8000 6000

4000 2000 0

0 2 6 10 13 15 17 21 24 27 29 31 34 36 38 40 43 45 48

Fig. 5. Alkalinity and VFA:Alk of ABSR during acclimation process.

high microbial activity [1]. This condition warranted the organic loading rate can be increased from 0.89 - 0.92 kg COD/m3/day to 1.34 - 1.37 kg COD/m3/day. The anaerobic degradation system received remedial of load to increase alkalinity to 17100 mg CaCO3/l on the 29th day of operation. The alkalinity decreased from 16760 to 16310 mg CaCO3/l for the next five days of operation. Meanwhile, the VFA: Alk ratio slightly increased from 0.09 to 0.10 in the same period. The alkalinity increased back to the level of 16850 mg CaCO3/l by the 36th day of operation. Besides, the VFA: Alk ratio loop increased to 0.14 from 36th to 38th day of operation. The system regained its stability until the 48th day of operation with the fluctuating of alkalinity between 16830 and 16630 mg CaCO3/l while the VFA: Alk ratio remained at 0.13. As a result, the VFA: Alk ratio was still less than 0.4 which meant that the ABSR had reach stability condition.

4. Conclusion

In present study, it can be concluded that the start-up operation for anaerobic digestion process of anaerobic bench scale reactor (ABSR) has provide useful information for treating the high strength wastewater of POME. Hence, an early precaution could be taken before the anaerobic degradation process failure occurred. Therefore, the indicators such as pH, microbial growth, total chemical oxygen demand (COD) of effluent, COD removal efficiency, alkalinity and ratio fraction between volatile fatty acid and alkalinity were applicable for anaerobic digestion process of POME.

Acknowledgements

The authors would like to express their sincere gratitude to University Sains Malaysia Research University (RU) grant: 1001/PTEKIND/814160 of financial support, School of Environmental Engineering of University Malaysia Perlis (UniMAP) and MALPOM Industries Sdn Bhd.

References

[1] Yacob Shahrakbah, Shirai Yoshihito and Hassan Mohd Ali. Start up operation of semi commercial closed anaerobic digester for palm oil mill effluent treatment. Process Biochemistry, 2006, 41: 962-964.

[2] Ma, A. N (2000) Environment management for the palm oil industry.Palm Oil. 30: 1-10.

[3] Ahmad A.L, Sumathi S., Hameed B.H. Residual Oil and Suspended Solid Removal Using Natural Adsorbents Chitosan, Bentonite and Activated Carbon: A Comparative Study, Chemical Engineering, 2005, 108: 179-185.

[4] Wong Yee Shian, Mohd Omar A.B. Kadir and Teng Tjoon Tow. Biological kinetics evaluation of anaerobic stabilization pond treatment of palm oil mill effluent. Bioresource Technology, 2009, 100: 4969-4975.

[5] Lim, K. H., Quah, S. K., Gillies, D., and Wood, B. J. Palm oil mill effluent treatment and utilization in Sime Derby Plantations- the current position. PORIM Worksp. Proc. Palm oil Res. Inst. Malaysia, 1984, 9: 42-52.

[6] Ma, A. N. Treatment off palm oil mill effluent. Oil Palm and the Environment: Malaysian perspective. Malaysia oil palm growers' council. 1999, 277

[7] Gray, N. F. Biology Of Wastewater Treatment. Oxford University Press. Oxford University Press. Oxford. 1992, Pp 828.

[8] Arceivala Soli J. Wastewater Treatment for Pollution Control. 2nd edn. Tata McGraw-Hill, New Delhi. 1998, Pp 82-83, 210220.

[9] Chooi, C. F. Ponding system for palm oil mill effluent treatment. PORIM. 1984, 9: 53-62.

[10] Kosaric, N. Treatment of industrial wastewaters by anaerobic processes new developments, In: Recent advances in Biotechnology. Kleuver academic Publishers. The Netherlands, 1992, 285-313.

[11] Rajbhandari, B. K. and Annachhatre, A. P. Anaerobic ponds treatment of starch wastewater: Case study in Thailand. Bioresource Technology, 2004, 95:135-143.

[12] Ashhuby, B. A., Idrees, M. and Mustapha, S. Upflow anaerobic filtration of palm oil mill effluent. The l2h symp. Malaysia Chem. Engin. Bangi., 1996.

[13] APHA Standard methods for the Examination of Water and Wastewater. Washington DC, 2005

[14] Gonzalez, G., Urrutia, H., Roeckel, M. and Aspe, E. Protein hydrolysis under anaerobic, saline condition in presence of acetic acid. Chemical Technology and Biotechnology, 2005, 80: 151-157.

[15] Kus, F. and Wiesmann, U. Degradation kinetics of acetate and propionate by immobilized anaerobic mixed cultures. Water Resource Journal. 1994, 29: 1437-1443.

[16] Leitao Renato Carrha, Van Haandel Adrianus Cornelius, Zeeman Grietje and Lettinga Gatze. The effect of operation and environmental variations on anaerobic wastewater treatment systems: A review. Bioresource Technology, 2006, 97: 11051118.

[17] Zhang, T. C. and Noike, T. Influence of retention time on reactor performance and bacterial trophic populations in anaerobic digestion processes. Water Resource, 1994, 28: 27-36.

[18] Grady CPL and Lim H C. Biological wastewater treatment: theory and applications. Marcel Dekker, INC, New York. 1980, Pp 220-222, 870-876.

[19] Bolzonella, D., Battistoni, P., Mata-Alvarez, J. and Cecchi, F. Anaerobic digestion of solid wastes: process behavior in transient conditions. Water Science Technology. 2003, 48: 1-8.

[20] Demirel Burak and Yenigun Orhan. Changes in microbial ecology in an anaerobic reactor. Bioresource Technology, 2006, 97: 1201-1208.

[21] Savant, D. V., Abdul-Rahman, R. and Ranade, D. R. Anaerobic degradation of absorbable organic halides (AOX) from pulp and paper industry wastewater. Bioresource Technology, 2006, 97: 1092-1104.

[22] Isik Mustafa and Sponza Delia Teresa. Substrate removal kinetics in an up-flow anaerobic sludge blanket reactor decolorizing simulated textile wastewater. Process Biochemistry, 2005, 40: 1189-1198.

[23] Toprak Hikmet. Temperature and organic loading dependency of methane and carbon dioxide emission rates of a full scale anaerobic waste stabilization pond. Water Resource, 1994, 29: 1111-1119.

[24] Azbar Nuri, Ursillo Pepi and Speece Ricard, E. Effect of process configuration and substrate complexity on the performance of anaerobic processes. Water Resource, 2000, 35: 817-829.

[25] Rubia, M. A. de la, Perez, M., Romero, L. I. and Sales, D. Effect of solids retention time (SRT) on the pilot scale anaerobic thermophilic sludge digestion. Process Biochemistry, 2006, 41: 79-86.