INTEGRATED ANAEROBIC AND AEROBIC PROCESSES FOR TREATMENT OF MUNICIPAL WASTEWATER

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1 INTEGRATED ANAEROBIC AND AEROBIC PROCESSES FOR TREATMENT OF MUNICIPAL WASTEWATER How Yong Ng 1,*, Shih Wei Wong 1, Sing Chuan Wong 1, Kavitha Krishnan 1, Siow Woon Tiew 1, Wing Onn Kwok 2, Kian Eng Ooi 2, Yuen Long Wah 2 and Say Leong Ong 1 1 Centre for Water Research. Division of Environmental Science and Engineering, National University of Singapore, Block EA #07-23, 9 Engineering Drive 1, Singapore Public Utilities Board Singapore, 40 Scotts Road, #15-00 Environmental Building, Singapore * Corresponding Author: esenghy@nus.edu.sg ABSTRACT In this study, integrated anaerobic and aerobic treatment processes for municipal wastewater treatment were investigated for its feasibility to replace the existing conventional activated sludge process. Three types of anaerobic systems anaerobic sludge blanket, anaerobic sequencing batch reactor and anaerobic filter, as the pretreatment step were studies. High solids and organics removal efficiency was observed during the start-up period at a hydraulic retention time (HRT) of 16 h, as well as during 6 h HRT operation. The UASB was found to require the shortest start-up time (80 days) and it produced the highest amount of methane gas (0.16 L/(g tcod removed) during start-up and 0.13 L/(g tcod removed) during 6 h HRT operation). However, it was a challenge to operate the UASB due to sludge floatation problems encountered during the start-up period when the HRT was 16 h. The performance characteristics of conventional activated sludge system and membrane bioreactor were also assessed to evaluate their effectiveness for treating the effluent of the UASB system. Both aerobic systems were able to produce an excellent final effluent with quality meeting that of the secondary effluent discharge standards. However, the membrane bioreactor was noted to be a better option as it offered the advantages such as smaller footprint and higher removal efficiencies. KEYWORDS Upflow anaerobic sludge blanket, anaerobic sequencing batch reactor, anaerobic filter, municipal wastewater, conventional activated sludge, membrane bioreactor INTRODUCTION Aerobic treatment systems such as the conventional activated sludge (CAS) process are widely adopted for treating low strength wastewater (<1000 mg COD/L) like municipal wastewater. CAS process is energy intensive due to the high aeration requirement and it also produces large quantity of sludge (about 0.4 g dry weight/g COD removed) that has to be treated and disposed of. As a result, the cost of operation and maintenance of a CAS system is considerably high. On the other hand, anaerobic process for domestic wastewater treatment has presented an alternative that is potentially more economical and holistic conceptually (Mergaert et al., 1992), particularly in the sub-tropical and tropical regions where the climate is warm consistently 3205

2 throughout the year. Anaerobic process does not require aeration and it produces biogas and less sludge. Research has shown that anaerobic systems such as the Upflow Anaerobic Sludge Blanket (UASB) (Behling and Sant Anna, 1997; Barbosa et al., 1989), the Anaerobic Sequencing Batch Reactor (AnSBR) (Sung and Dague, 1992; Ng, 1989) and the Anaerobic Filter (AF) (Ng and Chin, 1986; Chernicharo and Machado, 1998) can successfully treat high-strength industrial wastewater as well as low-strength synthetic wastewater. However, there have been limited reports on using anaerobic systems as a pretreatment for municipal wastewater. The anaerobic process alone would not be able to produce an effluent of a quality that meets typical secondary effluent standards. Post-treatment will therefore be required. However, the size of the aerobic system for the integrated anaerobic and aerobic treatment processes (IAATP) will be greatly reduced because the wastewater has been pretreated by the anaerobic system. Thus, the IAATP is potentially a more cost-effective technology for treating municipal wastewater by reducing the aeration requirement and sludge production while achieving secondary effluent standards. In view of the above observations, this study was conducted to investigate the feasibility of using an IAATP for treating municipal wastewater. Three types of anaerobic system UASB, AnSBR and AF were investigated. Comparisons of the anaerobic systems were based on length of startup period, effluent quality, biogas production and ease of operation. Two aerobic systems, a CAS and a membrane bioreactor (MBR), were also used to treat the effluent from the UASB. The aerobic systems will be compared based on their effluent quality. METHODOLOGY Experimental Setup for Anaerobic Systems One 40-L UASB, one 22.5-L AnSBR and one 21-L AF laboratory-scale systems were set up. The schematic diagram of the UASB, AnSBR and AF are shown in Figures 1, 2 and 3, respectively. The three anaerobic systems were started up with a hydraulic retention time (HRT) of 16 h and subsequently, switched to 6 h (phase 2) after successful start-up. During the start-up period, there was no intentional biomass wasting for the UASB and AF while the solids retention time (SRT) of the AnSBR was maintained at 30 days by daily wasting. The anaerobic reactors were seeded with digester sludge from a local municipal wastewater treatment plant. Raw municipal wastewater was collected twice a week from the same treatment plant and stored at 4ºC before use. The UASB and AF were continuously fed with sieved (2 mm size filter) raw municipal wastewater stored in a common feed holding tank. The feed of the AnSBR was similarly sieved and fed in batch mode from a separate feed tank. The feed of all three anaerobic systems were maintained at 30ºC by a thermal controller installed in a feed transfer tank. The contents in the feed tanks and feed transfer tanks were kept homogeneous by using top mounted stirrers. 3206

3 The UASB was fed from the bottom through six evenly distributed inlet ports. The flowrate of the feed was calculated based on the intended HRT (i.e., 16 h during start-up period and 6 h during phase 2). The top of the UASB was mounted with a three-phase separator where the solids could be retained in the reactor, while the effluent could be overflown into an effluent tank and the biogas could be transferred and collected in gas collectors. The AnSBR was operated with four phases (fill, react, settle and decant) controlled by a process logic controller (PLC), as shown in Table 1. The feed wastewater was fed into the AnSBR from the bottom and the decant point was fixed at a height to achieve the calculated decant volume. Table 1. Operating parameters of the AnSBR Start-up phase Phase 2 HRT 16 h 6 h Duration 160 days 43 days No. of cycles per day 4 8 Operating protocol Fill 15 min Fill 15 min React 270 min React 90 min Settle 60 min Settle 60 min Decant 15 min Decant 15 min The AF was fed from the bottom through a single inlet port. Suspended matters at the bottom part of the reactor were kept in suspension by a top mounted stirrer. Above this, the column was filled with filter media (Sera Siprox D52518, Germany). A liter of this media would provide approximately an effective surface area of 270 m 2. The effluent would overflow from the top of the reactor into the effluent tank. The ph of the anaerobic reactors was maintained between 6.8 and 7.2 (the optimal ph range for anaerobic treatment) by a ph controller. Sodium carbonate was dosed into the anaerobic reactors through the feed line when the ph fell below this range. Experimental Setup for Aerobic Systems One conventional activated sludge (CAS) and one submerged membrane bioreactor (MBR) were start-up simultaneously to treat effluent from the UASB. Working volume for both systems was 4.5 L. The CAS system consisted of an aeration tank and a clarifier while the MBR consisted of an aeration tank with submerged membranes. The schematic diagrams of the CAS and MBR are illustrated in Figures 4 and 5, respectively. The HRTs for both systems were controlled at 4 h while the SRTs were maintained at 10 and 20 days for the CAS and MBR, respectively. Membrane module used for the MBR was Sterapore-L hollow fiber membrane from Mitsubishi Rayon Co (Japan). The membrane fiber had a nominal pore size of 0.4 μm with a total surface area of 0.03 m 2 per module. Nine membrane modules were used simultaneously in the MBR. 3207

4 Figure 1. Schematic diagram of a lab-scale UASB Figure 2. Schematic diagram of a labscale AnSBR Figure 3. Schematic diagram of a lab-scale AF ph controller Influent Pump Mechanical Stirrer ph controller Influent Pump Pressure Gauge Suction Pump Treated Effluent Diffuser Aeration Tank Air flowmeter Recycle Pump Clarifier Treated Effluent MBR Diffuser Level Sensor Submerged Membrane Module Air flowmeter Figure 4. Schematic diagram of the CAS Figure 5. Schematic diagram of the MBR 3208

5 The aeration rates for the CAS and MBR were maintained at 1.0 and 5.0 L/min, respectively. High aeration rate was provided in the MBR to achieve effective scouring of the membranes to minimize cake formation on the membrane surface. In addition, the membrane modules were operated intermittently (8 min on, 2 min off). The ph was maintained between 6.8 to 7.2 and the temperature ranged between 25 and 32 o C. Sample Collection and Analysis Feed and effluent samples were collected regularly from their respective sampling tubes. The soluble portion of the feed and effluent were collected by filtering through a 0.45 μm filter disc (Pall GN-6 Metricel Grid, US) after centrifuging for 10 minutes at 9,000 rpm. Several parameters were used to assess the performance of the systems. Solids removal was determined by suspended and volatile suspended solids (SS & VSS) concentrations, and organics removal by total chemical oxygen demand (tcod) and total biochemical oxygen demand (tbod 5 ). The soluble portion of the feed and effluent were also tested for soluble COD (scod) and soluble BOD (sbod 5 ). Biogas production by the anaerobic systems was monitored by the volume of gas produced daily and the methane gas composition was determined using a gas chromatograph (Shimadzu GC-17A, Japan) equipped with a thermal conductivity detector and a 2 m x 1/8 in stainless steel Porapa Q 80/100 mesh column. All tests were conducted in accordance with the Standard Methods (APHA, 1998). RESULTS AND DISCUSSIONS Anaerobic Systems Start-up Performance It is well known that the start-up of anaerobic systems can be relatively long compared with aerobic systems because of the slow growth rate of anaerobic microorganisms. Therefore, in order to allow the biomass to acclimatize, the HRT used for the start-up period was slightly longer (i.e., 16 h). This could indirectly help to increase the SRT of the system by preventing biomass washout and retaining the biomass in the reactor. Accumulation of volatile fatty acids, which can suppress the metabolic activity of methanogens, may be an issue during the start-up phase. This could be effectively resolved by effective ph control. For an anaerobic system, once the start-up phase is successful, it can be left dormant for extended periods without severe deterioration in biomass properties since it is rather robust. Therefore, it is critical that the startup be monitored closely. The UASB, AnSBR and AF were operated at a HRT of 16 h for 250 days, 160 days and 237 days, respectively. The start-up performance data shown in Table 2 was compiled based on experimental results obtained after the anaerobic reactors had achieved rather stable performance. A summary of the results is shown in Table

6 Table 2. Summary of start-up performance of the UASB, AnSBR and AF Reactor UASB AnSBR AF Effective Vol (L) HRT (h) Days of Operation Influent Effluent Removal % Influent Effluent Removal % Influent Effluent Removal % SS (mg/l) VSS (mg/l) tcod (mg/l) scod (mg/l) tbod (mg/l) sbod (mg/l) Biogas production (L/d) Methane composition (%) Specific methane production (L/g tcod removed) (460) (323) (525) (88) (197) (38) (94) (69) (155) (47) (46) 9 38 (16) (81) (80) (70) (46) (77) (56) (411) (281) (369) (51) (198) (13) (61) (52) (93) 5 36 (25) (43) (85) (85) (75) (48) (78) 7 10 (8) (39) (363) (316) (514) (85) (213) 9 38 (27) (50) (36) (88) 7 74 (42) (41) 2 25 (13) (4.3) (1.0) (0.76) (78) (61) (51) (0.16) (0.04) (0.032) (87) (88) (83) (50) (80) (56) Note: Data Format: minimum maximum (average) UASB. The average SS and VSS removal efficiencies were 81 and 80%, respectively. The average tcod and scod removal efficiencies achieved were 70 and 46%, respectively. The average tbod 5 and sbod 5 removal efficiencies were found to be 77 and 56%, respectively. The average biogas production was 4.3 L/d with 78% of it comprised of methane. This is equivalent to about 0.16 L/(g tcod removed). AnSBR. The average SS and VSS removal efficiencies were both 85%. The average tcod, scod, tbod 5 and sbod 5 removal efficiencies were 75, 48, 78 and 39%, respectively. The average biogas production was 1.0 L/d with 61% of it comprised of methane. This is equivalent to about 0.04 L/(g tcod removed). AF. The average SS and VSS removal efficiency were 87 and 88%, respectively. The tcod, scod, tbod 5 and sbod 5 removal efficiencies were 83 %, 50 %, 80 % and 56 %, respectively. The average biogas production was 0.76 L/d with 51% of it as methane. This is equivalent to about L/(g tcod removed). The results of the start-up phase showed that at a relatively long HRT (16 h), the anaerobic systems were capable of producing effluents of high quality. The first sign of stability was observed at Day 80 for the UASB, Day 110 for the AnSBR and Day 115 for the AF. Stability was judged based on rather consistent effluent quality and steady biogas (methane) production. The UASB seemed to be able to achieve a successful start-up much earlier than the AnSBR and AF. This presented UASB with an obvious advantage. During the early stage of the start-up period, too low an organic loading rate (OLR) (i.e., 0.55 g COD/L.d) posed a problem for the AnSBR. Biogas production was very low and soluble organic concentration in the effluent was high. This was due to the low specific microbial 3210

7 activity within the bioflocs. In a lab-scale study of AnSBR by Zhou (1999), the optimum OLR in terms of performance was reported to be 2.5 g COD/L d. However, it is undesirable to decrease the start-up HRT because of the high possibility of a substantial loss in biomass. This is why the start-up period for the AnSBR was observed to be relatively long in this study. The AF also required a relatively long start-up period because biofilms would need time to develop on the surface of the filter media and to mature. In this study, biofilm cells tended to slough off and caused the effluent quality to deteriorate during the early stage of the start-up period. Anaerobic Systems Performance at HRT of 6 h The UASB, AnSBR and AF were subsequently operated at a HRT of 6 h after successful startups. Data was collected over a period of 164, 140 and 141 days for the UASB, AnSBR and AF, respectively. Table 3 shows a summary of the performance of the UASB, AnSBR and AF when operating at 6 h HRT. UASB. The average SS and VSS removal efficiencies were 58 and 56%, respectively. The average tcod and scod removal efficiencies achieved were 57 and 38%, respectively. The average tbod 5 and sbod 5 removal efficiencies were found to be 68 and 48%, respectively. The average biogas production was 6.9 L/d with 73% of it comprised of methane. This is equivalent to about 0.13 L/(g tcod removed). AnSBR. The average SS and VSS removal efficiencies were 63 and 65%, respectively. The average tcod, scod, tbod 5 and sbod 5 removal efficiencies were 57, 56, 72 and 47%, respectively. The average biogas production was 3.7 L/d with 74% of it comprised of methane. This is equivalent to about 0.12 L/(g tcod removed). AF. The average SS and VSS removal efficiencies were 86 and 84%, respectively. The average tcod, scod, tbod 5 and sbod 5 removal efficiencies were 78, 63, 82 and 54%, respectively. The average biogas production was 1.4 L/d with 49% of it comprised of methane. This is equivalent to about L/(g tcod removed). Comparing the performance of the three anaerobic systems, it could be observed that the solids removal efficiency of the UASB seemed to be lower than those of the AnSBR and AF. This was because as the UASB was operated at a maximum sludge hold-up (no intentional sludge wasting), it was less tolerant towards a sudden increase in feed solids concentration. Excessive solids beyond what the reactor could retain would overflow out of the reactor with the effluent. However, this happened only periodically, causing the average solids removal efficiency to be relatively lower. A lower solids removal efficiency would inevitably decrease the tcod and tbod 5 removal efficiencies, as shown in Table 3. On the other hand, the filter medium in the AF was able to trap a substantial amount of SS, which explained the relatively higher solid removal efficiency obtained. Nevertheless, flow blockage within the filter column due to solids accumulation was not detected over the 180 days of operation. In the case of AnSBR, it was able to achieve organic removal efficiencies comparable to the UASB despite having a lower MLVSS concentration (MLVSS concentration of 4.5 g/l in the AnSBR during the react phase compared 3211

8 with 13.3 g/l at the mid level of the UASB column). This could be because the AnSBR has the advantage of providing a high food-to-microorganisms (F/M) ratio during the fill and react phases that encourages metabolic activity, and a low F/M ratio during the settle phase that is ideal for biomass flocculation and phase separation (Dague and Pidaparti, 1991). Table 3. Summary of performance of the UASB, AnSBR and AF Reactor UASB AnSBR AF Effective Vol (L) HRT (h) Days of Operation Influent Effluent Removal % Influent Effluent Average Removal % Influent Effluent SS (mg/l) (58) (63) (437) (322) (488) (170) (550) (71) VSS (mg/l) tcod (mg/l) scod (mg/l) tbod (mg/l) sbod (mg/l) Biogas production (L/d) Methane composition (%) Specific methane production (L/g tcod removed) (322) (544) (93) (229) (38) (134) (226.7) (57) (79) (19) (56) (57) (38) (68) (48) Note: Data Format: minimum maximum (average) (390) (467) (122) (191) (15) (124) (198) (56) (54) (65) (57) (56) (72) 6 12 (8) (47) (448) (632) (98) (243) (28) (71) (132) 8 48 (33) (44) 5 29 (13) (6.9) (3.7) (1.4) (73) (74) (49) (0.13) (0.12) (0.021) Average Removal % (86) (84) (78) (63) (82) (54) Despite the relatively low solids removal efficiency, the UASB has the highest biogas production among the three anaerobic systems investigated. This was probably due to a well structured microbial community along the height of the UASB column. In addition, the UASB operating at its maximum sludge hold-up was also able to maintain a higher concentration of biomass inside the reactor with a MLVSS concentration of 14.7 g/l at the base and 13.3 g/l at the mid level of the UASB column (compared with the MLVSS concentration of 4.5 g/l in the AnSBR during the react phase). Operational Challenges of UASB, AnSBR and AF In all biological wastewater treatment systems, the variability in the raw wastewater will impose an uncontrolled selective pressure on the microbial population in the reactor. If not counteracted with appropriate operating strategies, it may affect the performance of the reactors. In this study, the three types of anaerobic systems were found to be rather easy to operate and maintain. Occasionally, the UASB would experience sludge floatation problem due to biogas being trapped in the biomass, which was at very high concentration. The low upflow velocity at a HRT of 16 h could not provide the sufficient shear force needed for dislodging the small gas bubbles away from the biomass. As a result, gas bubbles tended to be trapped within the dense biomass. As the amount of small gas bubbles accumulated, an uplift force that was large enough to lift the sludge blanket was created. This phenomenon caused the effluent quality to deteriorate 3212

9 as the solids were washed out of the UASB and a corresponding decrease in the biogas production due to the upset of the microbial community structure along the height of the reactor. When the said phenomenon occurred, internal recycling of biomass from the top to the base of the UASB was carried out for about two days and the performance of the UASB would require about two weeks for recovery. However, this problem was resolved when the HRT was decreased to 6 h. A higher upflow velocity created a sufficient shear force to free the gas bubbles, allowing them to rise to the top of the UASB. For the AF, gas bubbles could also be trapped in the filter media. Although it did not cause sludge floatation problems as in the UASB, it caused biogas production to be unstable. The AnSBR did not experience problems with gas bubbles because it had a stirrer which kept the contents suspended during the react phase. Lettinga et al. (1984) reported a significant fraction of biomass becoming dispersed in the liquid above biomass bed because of high turbulence caused by biogas, especially during the settling phase. The OLR in this case was very low, even at a short HRT of 6 h, because the organic strength of the municipal wastewater was weak. Therefore, gassing problems were not encountered in the AnSBR in this study. One important operation parameter that directly affected the quality of the treated effluent as well as the biogas production was ph, as anaerobic microorganisms are exceptionally sensitive to ph changes. The ph within a reactor must be maintained in the range of 6.4 to 7.8. Outside this range, methanogens would be inhibited and volatile organic acids would then be accumulated in the bioreactor, causing the ph to drop further. The alkalinity in the feed wastewater normally provides sufficient buffering capacity. Nevertheless, it is crucial to check and maintain the ph level within the optimal range at all times, particularly during the critical start-up period. This was achieved in this study using ph controllers for the UASB, AnSBR and AF. Aerobic Post-Treatment System Performance at HRT of 4 h Table 4 shows a summary of the performance of the two aerobic systems the CAS and MBR. Both the CAS and MBR performed excellently, especially the MBR, in terms of suspended solids, organic and ammonia removals, which achieved the typical secondary effluent discharge standards. The average tcod for the CAS and MBR effluents were 42.7 and 23.2 mg/l, respectively. Effluent tbod 5 concentrations were 6.3 and 1.3 mg/l for the CAS and MBR, respectively. Both systems were able to meet the typical secondary effluent discharge standards. Comparing the MBR and the CAS, the MBR could achieve a higher organic removal due to effective solid-liquid separation provided by the membrane. In addition, the MBR could support a higher SRT than CAS. This enabled the enrichment of slow growing bacteria such that a more diverse microbial community with broader physiological capabilities could be established. Effluent SS for CAS ranged from 4 to 41 mg/l, with an average of 18 mg/l. High effluent SS that exceeded the discharged limit of 30 mg/l was observed on certain days. It has been reported that irregular shaped floc particles of low density and pin point flocs were present in 3213

10 systems operating at a SRT between 9 to 12 days (Bisogni and Lawrence, 1971). In this study, pin flocs were also found in the clarifier of the CAS (SRT of 10 days). Table 4. Summary of performance of the CAS and MBR CAS MBR SS (mg/l) 4 41 (18) N.D VSS (mg/l) 3 28 (12) N.D tcod (mg/l) (42.7) (23.2) scod (mg/l) (21.2) tbod 5 (mg/l) (6.3) (0.8) sbod 5 (mg/l) (1.3) NH + 4 -N (mg/l) N.D N.D NO 3 N (mg/l) (41.3) (33.9) N.D non detectable No ammonia was detected in the effluent from both CAS and MBR systems, which showed that complete nitrification could be readily achieved. Membrane fouling was observed from Day 80 onwards and the membranes finally reached the allowable transmembrane pressure of -20 kpa on Day 112. Average MLVSS concentration was 10,500 mg/l. CONCLUSIONS The results have shown that the IAATP has a great potential to replace existing conventional activated sludge systems for municipal wastewater treatment in sub-tropical and tropical regions. The cost of treating municipal wastewater could be potentially reduced through biogas production, reduction of aeration requirements and decrease in the amount of sludge needed to be disposed of. The anaerobic systems were able to achieve more than 80% SS and VSS removal, more than 70% tcod removal and more than 77% tbod 5 removal during the start-up period. The UASB was found to require the shortest start-up time of 80 days and it achieved the highest rate of methane production (0.16 L/g tcod removed). When the UASB was operated at a long HRT of 16 h without intentional sludge wasting, sludge floatation phenomenon was encountered which would reduce removal efficiencies and require recirculation of the biomass from the top to the base of the UASB. At a HRT of 6 h, the UASB achieved lower SS and VSS removal efficiencies than those of the AnSBR and AF, due to period washouts of biosolids when no intentional sludge wasting was carried out. The UASB, however, had the highest biogas production (0.13 L/g tcod removed), which could be attributed to its well structured microbial community along the height of the UASB and higher mixed liquor concentration. Both the aerobic post-treatment systems, CAS and MBR, treating the effluent of the UASB were able to achieve high quality effluents that could readily meet secondary effluent discharge standards in terms of solids, organics and ammonia concentrations. The effluents may be further treated with advanced treatment process such as reverse osmosis and disinfection for direct non- 3214

11 potable or indirect portable use. On comparison, the MBR seemed to be a more favorable aerobic system because it required a smaller footprint and could achieve higher organic removal efficiencies. REFERENCES APHA. (1998) Standard Methods for the Examination of Water and Wastewater, 20 th ed., Washington, D.C., U.S.A. Barbosa R.A., G.L. Sant Anna Jr, (1989) Treatment of Raw Domestic Sewage in an UASB Reactor, Wat.Res. 23 (12) Behling E., A. Diaz, G. Colina, M. Herrera, E. Gutierrez, E. Chacin, N. Fernandez & C. F. Forster. (1997) Domestic Wastewater Treatment Using A UASB Reactor, Bioresource Technology Bisogni, James J. Jr. and Alonzo Wm. Lawrence, (1971). Relationships between biological solids retention time and settling characteristics of activated sludge. Water Research Chernicharo C. A. L. and R. M. G. Machado. (1998) Feasibility of the UASB/AF system for domestic sewage treatment in developing countries. Water Science & Technology 38 (8-9) Dague, R. R. and Pidaparti, S. R. (1991) Anaerobic Sequencing Batch Reactor Treatment of Swine Wastes. In Proceedings of the 46 th Industrial Waste Conference, Purdue university, West Lafayette, Indiana, Lettinga, G., Hulshoff Pol, L.W., Koster, I.W., Wiegant, W.M.,Zeeuw, W.J.de, Rinzema A., Grin, D.C., Roersma, R.E. and Hobma, S.W. High rate anaerobic wastewater treatment using the UASB reactor under a wide range of temperature conditions. Biotech. Genetic Eng. Rev., 2, pp Mergaert, K., B. Vanderhaegen & W. Verstraete. (1992) Applicability and Trends of anaerobic pre-treatment of municipal wastewater. Wat. Res. 26 (8) Metcalf & Eddy. (2003) Wastewater Engineering Treatment and Reuse, 4th ed., McGraw-Hill Companies, Inc. 582, 991, Ng W. J. and K. K. Chin. (1986) Random-packed anaerobic filter in piggery wastewater treatment. Biological Wastes Ng, W. J. (1989) A Sequencing Batch Anaerobic Reactor for Treating Piggery Wastewater. Biological Wastes

12 Sung, S. and Dague, R. R. (1995) Laboratory Studies on the Anaerobic Sequencing Batch Reactor. Water Environment Research. 67 (3) Zhou, X. F. The effects of ammonia-n on AnSBR performance. M.Eng. Thesis, National University of Singapore, pp