TREATMENT OF SEWAGE USING UASB PROCESS

Abstract

With the advent of several high-rate treatment systems, anaerobic process has emerged as a promising alternative for the treatment of different types of wastewaters. Among high rate reactors, upflow anaerobic sludge blanket (UASB) process has found wide application. Recent experience with full scale UASB reactors treating domestic wastewater has demonstrated the suitability ofthe process for the treatment of low strength wastewater in tropical regions. As a partially soluble wastewater, raw domestic sewage contains 30-70% suspended COD. Small amount ofsuspended solids (SS) have been reported to enhance the attachment of methanosacta and to favour granulation. However, high concentration of SS in the sewage can impair the UASB process: • by reducing the specific methanogenic activity ofthe sludge, • bythe formation of scum layers, • by sudden washout ofthe sludge due to prolonged entrapment ofvoluminous suspended solids by the granular sludge Information on the systematic evaluation ofeffects ofSS or particulate COD on the CH, generation and substrate reduction could not be found in the literature. With this in view, the objectives ofthe work embodied in the thesis are given as under: • to study the effect ofparticulate COD on biogas/CH, generation in batch tests. • to investigate the effect of solubilization of particulate matter in wastewater on the performance ofUASB reactors. To meet out these, investigations have been carried out to study (i) the effect of particulate COD (CODP) on anaerobic biodegradability of wastewater, (ii) the performance of UASB rectors at different levels of bed height (sludge occupancy) and sludge loading rate (SLR) when fed with soluble wastewater and wastewater containing 40% particulate COD. (iii) the solubilization of low strength complexwastewater and (iv) the performance of diphase UASB process for the treatment of complex wastewater at different levels of sludge bed height (sludge occupancy). For the ease of presentation the subject matter ofthe thesis has been divided into following chapters. 1) Introduction 2) Literature Review 3) Materials and Methods 4) Results and Discussion 5) Conclusion References are appended at the end. Information and data relevant to the work have been incorporated as appendices. The objectives of the work along with the justification of the problem undertaken form the subject matterof Chapter-1. Chapter-2 deals with the literature review on sewage treatment by UASB process in laboratory, pilot and full scale plants. Anumber offull scale plants which are in operation in India have been emphasized. Evaluation of BMP and SMA is summarized. Waste solubilization along with diphasic treatment ofsewage has also been incorporated. Chapter-3 describes the materials and methods employed in the investigations. The experiments were conducted in four concurrent / sequential phases as given below to evaluate (i) Anaerobic degradability of wastewaters and (ii) Performance of UASB reactors treating low strength synthetic wastewater (a) with and without CODP and (b) with solubilization as pretreatment. Following are the four phases of experimental work: Phase-1 Batch study (Anaerobic biodegradability): In order to investigate the effect of particulate organics on methane generation, a series ofbatch experiments were carried out using serum bottle technique. Experiments were conducted with wastewater containing varying fractions of suspended and soluble COD. The synthetically prepared wastewater contained particulate COD fraction (CODPF) of 0, 0.25, 0.50, 0.75 and l.The digester iv sludge was used as inoculum. Biotransformation of organics to biogas was carried out at seven values of the ratio of food to microorganism (F/M) i.e. 0.18, 0.25, 0.57, 0.68, 0.89, 1.10 and 2.0. With each set a blank was kept. All the experiments were performed in duplicate. Atotal of (6x7) x 2 serum bottles were monitored and analysed for gas generation for 60 days. Initially, for 30 days the gas was analysed at an interval of5days. In another experiment, a set of serum bottles inoculated with UASB sludge as inoculum was monitored at F/M ratio of 0.89. Phase-2 Continuous study (Single phase UASB treatment process): Two UASB reactors R, and R2 of 56.8 and 10.8L were operated. Reactor R, was fed with low strength (COD * 500 mg L"1) synthetic wastewater containing both soluble (*300 mg L"1) and particulate COD (*200 mg I/1) and the other reactor R2 was fed with wastewater containing only soluble COD (*500 mg I/1). Experiments were conducted (i) to start-up R, and R2 and (ii) to evaluate subsequent performance of R, and R2 at different levels of bed height (10-41 cm) or sludge occupancy (6-44%) and sludge loading rates (0.2-2.2 kg COD kg'VSS d"1). The reactors R, and R2 were run for 424 and 252 days respectively. Phase-3 Continuous study (Solubilization): This study was aimed to determine the minimum HRT required for maximum solubilization of the synthetic wastewater of 500 mg COD L*1 containing suspended (cellulose) and soluble (sucrose) COD. The reactor was acclimated at 8hHRT. HRT was subsequently decreased to 6, 4, 3, 2and lh. HRT of 2 h was found to be optimum. Reactor configuration was modified and studies were repeated at HRT of2 h. Phase-4 Continuous study (Diphase UASB treatment process): The performance of the UASB reactors Rld and R2d (HRT=6h) treating solubilized wastewater at different bed heights or sludge occupancy was evaluated. Reactor Rld contained granular, fluffy and light sludge developed from wastewater containing particulate COD (SWi) and R2 contained sludge grown on soluble wastewater (SW2). At every stage specific methanogenic activity (SMA) ofthe sludge was determined. Results and discussion pertaining to different experiments performed have been organized in the Fourth Chapter of the thesis. Results of batch bioassay test evaluating anaerobic degradability of the wastewater have been first presented and discussed. Cumulative methane generation was found to exhibit decreasing trend with increasing particulate fraction in the wastewater. Temporal variation of CH4 generation conformed to BOD progression type of relationship [(H= Mo (1-e **)]. k was calculated by taking average of six k values (kaVg), calculated from above equation by substituting M60 for Mo and M5, M10, M15, ...for Mt. The Mt values computed from kaVg were found in conformity with the experimental observations, k estimated by Thomas method gave values of Mt which were not in agreement with the experimental observations. At all F/M values kaVg decreased linearly with increase in CODPF (particulate fraction of COD). Substrate biodegradability 0v= CH4-COD/COD) was found to be maximum (0.95-1) for soluble wastewater at F/M ranging from 0.57-0.89 and minimum (0.26) for wastewater containing particulate COD at F/M=2. Anaerobic degradability measured as BMP5 and BMP30 (cumulative CH4 generated in mL from 1 g COD after 5 and 30 days) also exhibited linear declining trend with CODPF. Maximum value of BMP30 was found for 100% soluble wastewater at F/M=0.57. Decrease in cumulative methane per unit particulate COD (-A mL CH, g'1 CODP) was also found to be minimum at F/M = 0.57. Methane generation was not found to be decreased significantly provided wastewater contained up to around 25% particulate COD. Substrate Activity, A (ratio of biogas or methane from wastewater containing varying fractions of organics in particulate form to biogas or methane from soluble wastewater) was correlated to CODPF through a polynomial of second order. Substrate activity also decreased with increase in CODPF. Sludge activity due to entrapment of SS is adversely affected by the presence of SS in wastewater. The percent sludge activity [{(mL CH4 g'1 VSS) / (350 mL CH, g1 VSS)}* 100] varied linearly with CODPF. Sensitivity of the biodegradability parameters towards F/M decreased with CODPF. In most of the cases, optimum value of F/M ranged from 0.57-0.68. The kavgJ BMP5, BMP30, substrate activity, percent sludge activity have been found to decrease linearly with increase in CODP/VSS up to a value of 0.9. vi Comparative performance of single phase UASB reactors fed with complex (reactor R^ and soluble (reactor R2) wastewaters of 500±200 mg COD L"1 (range) at different levels ofsludge bed (sludge occupancy) and SLR was studied in phase-2. In the first part ofthe study, efforts were made to start-up and develop granular sludges in Ri and R2. Ri attained pseudo steady-state earlier than R2 (86 days compared to 116). In Ri although the sludge developed was granular (diameter 1-4 mm) but granules were light and fluffy and used to float while in R2 the sludge was of better settling characteristics although smaller and nongranular in size and shape. Subsequently, performance ofRi was evaluated for 294 days at successive sludge occupancies of23, 13, 10, 12, 10, 7, 6, 16, 12, 13 and 7%. Sludge occupancies tested in R2 over a period of 115 days were 35, 27, 31, 19, 13 and 9%. Particulate matter influenced wastewater treatment by UASBR in terms of gas generation, CODs removal and sludge lifting. The biogas production in general was found to be more from soluble wastewater (0.35-0.40 m3 kg1 CODs removed) at different bed heights than wastewater made up of 40% particulate COD (0.21-0.27 m3 kg"1 CODs removed). However, at bed height of 10 cm, the gas collection from Ri and R2 was reduced. Percentage soluble COD removal efficiencies were also ofhigher order in case of 100% soluble wastewater. The sludge was invariably lifted at a bed height of 10cm (7% sludge occupancy) in case of wastewater having 40% particulate COD. While sludge lifting was not observed in the reactor fed with 100% soluble wastewater. Another difference was in terms ofm3 ofbiogas collected per kg COD fed. The gas collection from SWi (40% CODP) is 60-67% of the gas collected from SW2 (100% CODs). The gas generation from continuous reactor operation is close to BMP5 values from batch serum bottles. The BMP5 ofwastewater containing 40% CODP is also found to be 65-68% ofgas from 100%) CODs at an optimum range ofF/M (0.57-0.68). Sludge bed height is one of the factors which influence short circuiting flow through the bed and reactor performance. However, the bed height of 15-33 cm and 10-41 cm did not influence the performance of reactors Ri and R2 operating at OLR 1.7-2.2 kg COD m"3 d"1 and 2.3-3.6 kg COD m*3 d"1. UASBR can be operated at sludge occupancy as low as 10-15%. At low sludge occupancy or bed height, sludge washout is minimized. In the case of wastewater containing particulate COD, the net vn increase in sludge (VSS) height was due to entrapment of incoming solids, biomass growth, decay, washout and transformation of entrapped or sorbed solids. On the other hand, with soluble wastewater it is due to sludge growth, decay and washout. Higher value ofSMA ofsludge ofRi than SMA ofsludge ofR2 may be due to entrapment ofsolids and subsequent release of substrate in Ri. Solubilization-fermentation of total organic matter (both soluble and particulate fractions) has been studied in phase-3 in a simple tank. Retention of SS, fermentation and clarification offermented effluent was achieved in a single reactor. As a primary settler, the fermenter was able to consistently produce an effluent with low SS (< 100 mg SS L'1) under all operating conditions tested (HRT values of 6 h to 1 h). HRT of 2 h was found to be optimum for maximum retention of CODP (60%) and minimum reduction of CODs (*7.5%). The results obtained also showed that under the optimum operation conditions, with aHRT of 2h, an average of 0.17 mg HAc mg1 CODT was produced. The relatively low HRT (2 h) may result in compact reactor volume with good solids retention, VFA production and clarification characteristics. The phase-4 study comprised of the solubilization followed by the treatment of solubilized wastewater from solubilization tank by UASB reactors Rld and R2d. Rld was operated at bed height of8-21.5 cm (sludge occupancy of5-15%) and OLR of 1.09-1.48 kg COD m"3 d-1. The OLR of 1.18-1.45 kg COD m"3 d"1 and bed heights of 9-31 cm (initial sludge occupancy of 9-28%) were maintained in R2d. The efficiency of R2d was more than Rid. Improved performance of R2d was mainly due to sludge type as well as low SLR. Placement of solubilization reactor ahead of Rld and R2d resulted in reduction of OLR to reactors. At low loading of1-1.45 kg COD m"3 d"1, the reactor performance has been found to be. sensitive to bed height. The biogas (m3 m"3 d"1) collected increased linearly with increase in bed height up to 31 cm (investigated herein). This observation is in contrast to the observation made during single phase operation of R, and R2 as OLRs to R, and R2 were greater than OLRs to Ridand R2d. The performance ofsingle and diphase treatment of complex wastewater was compared. The methane recovery from diphase was more than single phase. Another advantage of diphase system was its relatively better performance in terms of single washout and lifting. Vlll The conclusion and limitations derived from the present study and scope for future investigation form the subject matter of Chapter-5. From the investigations on the treatment of synthetic wastewater similar to sewage, the following conclusion has been drawn: • Anaerobic biotransformation process was found to depend on CODPF and F/M. Biodegradability rate constant (k), substrate biodegradability (jib), anaerobic degradability measured as BMP5 and BMP30, substrate activity (A), and percent potential sludge activity were found to be maximum for soluble substrate and minimum for wastewater made up of 100% particulate organics. Up to about 25% particulate COD, methane generation was not found to decrease significantly. In general, optimum value ofF/M for anaerobic digestion ranged from 0.57-0.68. Decrease in cumulative methane per unit particulate COD was also found to be minimum at F/M = 0.57. • Presence of organics in wastewater made up of 40% particulate COD matter influenced biotransformation process in UASBR in terms of biogas collection, CODs removal and sludge lifting. The main difference was due to entrapment and subsequent transformation of entrapped or sorbed incoming solids in case ofwastewater having CODP. UASBR could be operated at bed height as low as 15-20 cm. At lowbed height, possibility of sludge washout is minimized. • In the solubilization reactor, retention, solubilization and fermentation of solids (CODP) were achieved. HRT of 2 h was found to be optimum for maximum retention of CODP (*60%) and minimum reduction of CODs («7.5%). VFA production was 0.17 mg HAc mg'1 CODT. • Pre-solubilization of complex wastewater resulted in lower OLR of 1-1.45 kg COD m"3 d'1 to Rid and R2d. At low OLR, the reactor performance has been found to be sensitive to bed height. The biogas increased linearly with increase in bed height. The performance of diphase was better than single phase. The methane recovery in diphase UASB reactor (R2d) was more than single phase UASB (Ri) Use of synthetic wastewater similar to sewage is one of the unavoidable limitations of bench scale investigations carried out. IX