Soluble Wastewater Treatment Using an ABR

Transcription

1 Soluble Wastewater Treatment Using an ABR Gopala Krishna G.V.T. Department of Civil Engineering, PSNA College of Engineering and Technology, Dindigul , India Abstract-- Developing countries offer a high opportunity for application of onsite, small and community scale wastewater treatment technologies, because of the great need for effective sanitation. Treatment of low strength soluble wastewater was studied using an anaerobic baffled reactor (ABR). It was operated at hydraulic retention times (HRTs) of 20, 16, 12, 8, 6, 10, 8 and 6 h. Corresponding organic loading rates (OLRs) were 0.6, 0.75, 1.0, 1.5, 2.0, 1.2, 1.5 and 2 kg COD/m 3 d. Pseudo steady state (PSS) was achieved only at 10, 8 and 6 h HRT. Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removals were observed to be at least 89% at OLRs ranging from 1.2 to 2 kg COD/m 3 d. Standard deviation for both effluent BOD and COD (total as well as soluble) at PSS varied in a narrow range from 1.5 to 5.5 mg/l. It indicated that once ABR attained PSS, performance remained quite stable in terms of effluent characteristics. Based on mass-balance calculations, more than 55% of raw wastewater COD was estimated to be recovered as CH 4 in gas phase. Compartment wise profiles indicated that most of the BOD and COD got reduced in initial compartments only. Sudden drop in ph (7.8 to 6.7) and formation of volatile fatty acids (VFA) (54 to 98 mg/l) were observed in first compartment due to acidogenesis and acetogenesis. ph increased and VFA concentration decreased longitudinally down the reactor. Residence time distribution (RTD) studies revealed that the flow pattern in ABR is neither completely plug-flow nor perfectly mixed. Keyword-- Anaerobic baffled reactor, anaerobic treatment, Mass balance, HRT. I. INTRODUCTION Developing countries offer a high opportunity for application of onsite, small and community scale wastewater treatment technologies, because of the great need for effective sanitation. Centralized collection and treatment has largely not been successful, because of a variety of reasons, but primarily due to economics. Mechanized onsite and small scale systems, used in developed countries require considerable investment, operation and maintenance costs. Therefore, there is a greater need to develop simplified, low cost technologies for the small-scale systems than the large, expensive centralized systems. Main drawback of anaerobic treatment, i.e. long retention time has been overcome by the development of a number of high-rate reactors that achieve separation between the HRT and the solids retention time (SRT). One of the innovative reactor designs developed to implement this technology is ABR. 325 It is described as a series of upflow anaerobic sludge blanket (UASB) reactors. It consists of a series of vertical baffles to force the wastewater to flow under and over them as it passes from the inlet to the outlet. The bacteria within the reactor tend to rise and settle with gas production in each compartment. They move horizontally down the reactor at a relatively slow rate. The wastewater thus gets an opportunity to come into intimate contact with a large amount of active biological mass as it passes through the ABR, and the effluent is relatively free of biological solids. The advantages of the ABR have been well documented by Barber and Stuckey (1999). Potentials of ABR have been explored for the treatment of domestic and industrial wastewaters of low strengths (COD <1000 mg/l), at laboratory and pilot- scales. Polprasert et al. (1992) used ABR to handle (pre-treated) slaughterhouse wastewater with COD ranging from 480 to 730 mg /L at OLR of 0.67 to 4.73 kg COD/m 3 d. Treatment efficiency of more than 75% was achieved. Orozco (1997) observed COD removal efficiencies more than 85% at 8 h HRT and mean water temperature of 15 o C. The start-up and performance of ABR treating a complex soluble and colloidal wastewater has been reported by Langenhoff et al. (2000). They found removal efficiencies more than 80%. Manariotis and Grigoropoulos (2002) conducted experiments on ABR to evaluate the treatment of low strength synthetic wastewater (COD of 300 to 400 mg/l) and observed COD removal ranging from 87 to 91%. It was reported that 80% of the COD removal took place in the first two compartments alone. Dama et al. (2002) monitored the performance of pilot scale ABR of 3200 L capacity fed with domestic wastewater and operated at HRTs ranging from 60 to 20 h. COD removal efficiencies up to 80% have been reported. Although, not many studies have been carried out for the treatment of low strength wastewaters using ABR, however, the performance has been rated good (COD removal 80%). Only Polprasert et al. (1992), and Manariotis and Grigoropoulos (2002) have studied compartment wise variation of COD. Changes in the concentrations of other parameters like ph, VFA, BOD, Nitrogen (N) and Phosphorus (P) etc., have not been studied as the wastewater flow horizontally. Hydrodynamics of the reactor has been evaluated by Langenhoff et al. (2000).

2 Need of a detailed integrated study covering different aspects like performance, stability of reactor performance, compartment wise variation of parameters, hydrodynamics of the reactor, and ability to resist organic shock loads was felt. The present study aims to integrate these aspects in a single study. Anaerobic wastewater treatment technology has been successfully used in India as large number of UASB based sewage treatment plants (STPs) (> 25 with total installed capacity 1000 X 10 3 m 3 /d) are currently working. Tropical Indian climate has favored its use. With the background of success of UASBRs and supportive climate, ABR too has potentials of application in India. II. Experimental Setup and Feed MATERIALS AND METHODS A laboratory scale ABR was fabricated using transparent plexi glass sheets (Fig.1). The lower portion of the hanging baffles was bent at 45 0 to route the flow to the center of the upflow chamber, to achieve better contact of feed and biosolids. The effective liquid volume of the ABR was 10 L. The reactor was housed in a temperature controlled wooden chamber maintained at 30 ± 1 C. A peristaltic pump was used to feed the synthetic wastewater of 500 mg/l COD to the reactor. The wastewater was composed of 450 mg sucrose COD/L, and 50 mg peptone COD/L. The compositions of the buffers and trace elements used were adopted as per (Singh et al., 1996). Seed Sludge ABR was seeded with flocculent anaerobic sewage sludge collected from the UASB reactor working at 38 ML/d capacity UASB based STP located at Saharanpur, India. Seed was introduced uniformly into all eight compartments of the ABR. Each 1.25 L compartment initially contained L sludge having a solid concentration of 39 g suspended solids(ss)/l and 19 g volatile SS(VSS)/L giving a total of 95 g VSS in the reactor. The remaining portion of each compartment was initially filled with soluble wastewater. After seeding, the reactor was sealed and the head space above each compartment was flushed with oxygen-free nitrogen gas in order to displace residual air from the system. The reactor was allowed to stabilize for 72 h before starting the continuous feeding. Fig.1 Experimental set-up of anaerobic baffled reactor Reactor Operation Barber and Stuckey (1998) had suggested to initially use long HRT (80 h) to start-up the ABR and to reduce it in stepwise fashion while maintaining a constant incog substrate concentration. They suggested that it provides greater reactor stability and superior reactor performance. Accordingly, during the entire length of the study, the concentration of the feed was kept constant at 500 mg COD/L and OLR was varied by decreasing HRT. PSS condition at each OLR was believed to have been achieved when variation in effluent COD values was found to be insignificant. Detailed analysis was carried out after attaining PSS, and results discussed are derived from the data collected at PSS. Analyses The reactor was monitored daily for ph, temperature, COD, and biogas. VFA, ammonical nitrogen, total phosphorus, BOD (3 day at 27 C), SS, and VSS were measured weekly. Samples were also collected for analysis from different compartments of ABR on a regular basis after attaining PSS at all the HRTs. All the parameters were detered according to Standard Methods (APHA 1998), except VFA. VFA was detered as per the procedure suggested by DiLallo and Albertson (1961). III. HYDRODYNAMIC FLOW CHARACTERISTICS Residence time distribution (RTD) studies were carried out to analyze the hydrodynamics of the reactor after attaining the PSS at HRTs of 10, 8 and 6 h. 326

3 Influent COD (mg/l) International Journal of Emerging Technology and Advanced Engineering A pulse of an inert tracer (lithium chloride) was added to the reactor at a concentration of 10 mg Li + /L of reactor volume. Samples were taken from the outlet of the reactor for at least 3 HRTs after the addition of the pulse and were analyzed for lithium concentration by using a microprocessor based flame photometer (model TMF 45, Toshniwal, India). Effluent samples were collected at regular intervals of 1 h except between time ranging from 0.6 to 1.0 HRT. During this period sampling interval was reduced to 0.5 h. IV. RESULTS AND DISCUSSION The reactor was started with a soluble feed concentration of 500 mg/l at a HRT of 20 h (OLR = 0.6 kg COD/m 3 d). In spite of wastewater being purely synthetic, feed COD could not be maintained initially upto around day 160 in a narrow range. Accordingly, OLR to ABR also varied considerably. However, from around day 160 till the end of experiments (day 678), variations in feed COD were not significant. Box-plot of influent COD from day is shown in Fig. 2. The influent ph was around 7.5 and the effluent ph was close to 7. From the very beginning COD removal efficiencies were in the range of 50 to 60% and within 50 days of start-up it improved to 70 to 80%. After 54 and 73 days the flow rates were increased by 1.25 and 1.67 times, to decrease HRTs to 16 and 12 h respectively. COD removal observed was around 80%. Efforts were not made to achieve PSS at 20, 16 and 12 h HRTs. HRT was further decreased to 8 h on day 96. It resulted in decrease in COD removal from day 96 to around day 130. Thereafter, reactor performance gradually improved and ABR attained PSS (at 8 h HRT) around day 251. ABR was operated for a fairly long time (day 251 to 469) at PSS. HRT was further lowered to 6 h on day 469. It resulted in escape of solids and decrease in COD removal efficiency. Therefore, it was decided to increase HRT to 10 h on day 485. ABR attained PSS at 10 h HRT on day 490. It was operated at PSS from day 490 to 572. HRT was once again lowered to 8 h on day 572 and it attained PSS quickly on day 579. It was operated at PSS upto day 621. Finally, HRT was further lowered to 6 h and it attained PSS on day 632. It was operated at PSS upto day Fig. 2 Box-plot of influent COD (day ) Treatment processes appeared quite stable from the fluctuations in concentrations of various parameters in effluent during PSS conditions at HRTs of 8 h (day 251 to 469 and 579 to 621), 10 h (day 490 to 572) and 6 h (day 632 to 678). The corresponding average characteristics of influent and effluent (at PSS) at different HRTs (or OLRs) are summarized in Table 1. The average total and soluble effluent COD values at PSS at 10, 8 and 6 h HRTs were found to be around 47 and 35 mg/l, 50 and 40 mg/l and 53 and 41 mg/l respectively. Correlation between HRT and effluent COD values is quite clear [Fig. 3(a) and (b)]. At all the HRTs total and soluble COD removals averaged 90% indicating fairly good amount of removal [Table 1]. At PSS, day to day fluctuations in effluent COD and BOD (total as well as soluble) values were found to be very low. Reactor was operated for 82 days (197 cycles), 260 days (780 cycles), and 46 days (184 cycles) at PSS at HRTs of 10, 8, and 6 h respectively. Standard deviation for both effluent BOD and COD (total as well as soluble) at PSS only ranged from 1.5 to 5.5 mg/l at three HRTs. This indicated that once reactor attained PSS (and if parameters like OLR or HRT etc. were not changed) performance remained quite stable in terms of effluent characteristics. Average effluent total BOD values were found to be 24, 30 and 33 mg/l at HRTs of 10, 8 and 6 h respectively [Table 1]. Indian standards prescribe that treated sewage should not have BOD > 30 mg/l. Average effluent total BOD was found to be 30 mg/l at HRTs of 10 and 8 h. However, on further lowering HRT to 6 h or increasing OLR > 1.5 kg COD/m 3 d, BOD of the effluent exceeded > 30 mg/l. 327

4 Effluent COD S at PSS (mg/l) Effluent COD T at PSS (mg/l) Methane (m 3 /kg COD T removed) International Journal of Emerging Technology and Advanced Engineering Average SS and VSS in the effluent were found to be around 32 and 17 mg/l, 35 and 23 mg/l, and 37 and 29 mg/l respectively. Shorter HRT and higher OLR resulted in higher liquid upflow velocity and higher rate of gas production. As anaerobic granules are acted upon by the hydraulic and buoyant forces in the ABR they can be carried through the ABR with the wastewater flow and escape the reactor. Relatively low concentrations of VFA, usually below 40 mg/l, indicate that the incog COD was almost completely degraded to CH 4 and CO 2 or taken up by biomass for growth. Every step changes in HRT resulted in disturbance in the VFA concentrations for a short period like any other parameter (a) Gas collection improved on replacing it on day 249 by a combined gas collection manifold. At PSS, average biogas production rates in case of 10, 8 and 6 h HRTs were 0.38, 0.39 and 0.40 m 3 /kg COD removed respectively. Correlation between OLR and CH 4 in terms of m 3 of CH 4 produced/kg COD removed is shown in Fig OLR Fig. 4 Box-plot of methane at different OLRs (b) HRT (h) HRT (h) Fig. 3 (a) and (b) Effect of HRT on effluent PSS COD values Initially arrangements were made to collect the biogas separately from each compartment of the ABR. Cumulative gas production was initially low. The reason for low gas production can be ascribed to the low temperature start-up compared with the conventional start-up at a temperature of o C, dilute nature of the wastewater and leakage in complicated network of gas collection installed to collect/measure gas separately from each compartment of reactor. Some fraction of the gas produced solubilized in reactor effluent and escaped from collection. V. COMPARTMENT WISE PROFILES Average profiles at PSS of (a) COD, (b) BOD, (c) ph, and (d) VFA are shown in Fig. 5(a-d) to follow the spatial changes as wastewater passed through the ABR at different OLRs. Maximum COD reduction took place in first compartment [Fig. 5. (a)]. Later compartments played little role in reduction of organics. As OLR was increased from 1.2 to 1.5 to 2.0 kg COD/m 3 d, compartments which followed i.e. compartments 2, 3 received higher and higher COD concentrations and organic loading. If only first compartment is considered, at OLRs of 1.2, 1.5 and 2 kg COD/m 3 d, average COD concentrations were observed as 157, 271 and 286 mg/l respectively while average effluent COD concentrations recorded were 47, 51 and 53 mg/l respectively. It can be generalized that higher the OLR, higher the number of compartments (and microorganisms housed in them) participated in reduction of organics. BOD profiles for ABR at OLR of 1.2, 1.5 and 2 kg COD/m 3 d are shown in Fig. 5 (b). From the bar diagrams it is quite clear that most of the BOD removal also took place in the first compartment at lower OLR (1.2 kg COD/m 3 d). When the OLR was increased to higher value (2 kg COD/m 3 d) it shifted to the next few compartments like COD. Although concentrations of COD and BOD decreased in every successive compartment, however, compartments 4 to 8 seem to play very little or insignificant role. 328

5 VFA(mg/L) BOD: 3 d at 27 o C (mg/l) COD (mg/l) International Journal of Emerging Technology and Advanced Engineering ph profiles for the ABR at 10, 8 and 6 h HRTs are shown in Fig. 5(c). Sudden drop in the ph in the first compartment is quite noticeable. It gradually increases as wastewater moves towards the later compartments. The ph in compartment 1 was lowest. Hutnan et al. (1999) also reported that ph in a four chamber ABR increases from the first to the last compartment as 6.3; 6.6; 6.7; 6.8 respectively. The ph of the ABR effluent was reported as 7.0. Dama et al. (2002) had also found that the earlier compartments have a lower ph as the acidogenesis and acetogenesis takes place in these compartments OLR = 1.2 kg/m3d OLR = 1.5 kg/m3d OLR = 2 kg/m3d (a) (b) The VFA concentration also decreased longitudinally down the reactor [Fig. 5 (c)]. The highest VFA concentrations were found in the first compartment with average values of 54, 85 and 98 mg/l at OLRs of 1.2, 1.5 and 2 kg COD/m 3 d. Results indicated that VFA concentration in every compartment increased on increasing OLR from 1.2 to 2 kg COD/m 3 d. The VFA profile demonstrated that hydrolysis and acidogensis were the main biochemical activities occurring in the first few compartments (Akunna and Clark, 2000; Baloch and Akunna, 2003). The methanogensis appears to be doant in the last few compartments. These observations suggest that the ABR structure promotes systematic selection in a manner which brings out phase separation. VI. RESIDENCE TIME DISTRIBUTION STUDIES Residence time distribution (RTD) studies were carried out at 10, 8 and 6 h HRTs. The data was analyzed with a two-phase dispersion model (Levenspiel, 1999; Hutnan et al., 1999; Grobicki and Stuckey, 1992) and the results are shown in Table 2. Dead space consists of both hydraulic and biological dead spaces. Hydraulic dead space is a function of the flow rate and the number of compartments in the reactor and the biological dead space is a function of the biomass concentration and activity (Grobicki and Stuckey, 1992). An increase in the (hydraulic) dead space was expected with a decrease in HRT because of more channeling in the reactor bed. However, at lower HRT, the organic loading to the reactor increased and this resulted in higher rate of gas production. Therefore, results show that the volume of dead space decreased with decrease in HRT upto 8 h and increased once again after that Influent Compartments Effluent Fig. 5 (a-d) Spatial or compartment wise variation of (a) COD, (b) BOD, and (d) VFA along with respective influent and effluent concentrations at different OLRs Table 2 Results of the residence time distribution studies HRT(h) Dead space (%) Dispersion number A large dispersion number, D/µL shows a perfectly mixed system, whereas a small dispersion number, D/µL 0 relates to a plug flow system, with D/µL = 0.02 being defined as an intermediate and D/µL= 0.2 large degree of dispersion (Grobicki and Stuckey, 1992). Using the calculated dispersion numbers in the ABR it can be concluded that the flow pattern within the reactor was intermediate between plug flows and perfectly mixed. 329

6 VII. ORGANIC SHOCK LOADS Due to the variable nature of domestic wastewaters in terms of flow and organic load, the reactor stability to hydraulic and organic shock loads is one of the most important aspects of reactor design. Shock loads can manifest themselves in two ways: (a) either as short term transient which only lasts a few hours, or (b) as a long term step change of days to weeks before reverting back to original operating conditions. In sewage treatment, shortterm changes in hydraulic and organic loads are quite common. To study the stability of the reactor due to short duration organic shock loads the influent COD was increased by 100% (i.e. 500 to 1000 mg/l) and 200% (i.e. 500 to 1500 mg/l) for two durations of 4 and 8 h in each case. It is apparent that imum disturbances in characteristics of wastewater occurred in Compartment 1. In general, appreciable changes in different parameters were observed only in first three to four compartments. For increase in COD from 500 to 1500 mg/l over a period of 8 h, imum COD values in Compartments 1, 2, 3, and 4 were recorded after 8, 10, 10 and 12 h respectively. Similarly, peak values of VFA in Compartments 3, 4, 5 were found at a later time than earlier compartment. During and after the organic shock load, characteristics in Compartments 5 to 8 almost remained unchanged. It appears that because of the number of compartments, ABR can sustain the type of organic shock loads generally experienced at a STP. VIII. CONCLUSIONS COD and BOD removals were observed to be at least 89% at OLRs ranging from 1.2 to 2 kg COD/m 3 d. Standard deviation for both effluent BOD and COD (total as well as soluble) at PSS varied in a narrow range from 1.5 to 5.5 mg/l. It indicated that once ABR attained PSS, performance remained quite stable in terms of effluent characteristics. Maximum COD reduction took place in the first compartment of the ABR. Although in every compartment of ABR, COD and BOD concentrations decreased compared to earlier compartment, compartments 4 to 8 seem to have played very little role in reduction of organic load. In first compartment, average COD concentrations increased from 157 mg/l at an OLR of 1.2 kg COD/m 3 d to 286 mg/l at an OLR of 2 kg COD/m 3 d. It indicated that at low OLR most of the organics were removed in the first compartment itself. It was observed that greater the organic loading to ABR, greater the number of compartments and bacterial cultures housed in them play proent role in removal of organics. 330 The dead space in the reactor was found to range from 19 to 25%. Based on dispersion numbers found out, the flow pattern in the reactor could be classified as intermediate between plug flow and perfectly mixed flow. REFERENCES [1 ] Akunna, J.C., and Clark, M. (2000). Performance of a granular-bed anaerobic baffled reactor (GRABBR) treating whisky distillery wastewater. Bior. Technol., 74(3), [2 ] APHA. (1998). Standard methods for the exaation of water and wastewater. 20 th ed., American Public Health Association, Washington, DC. [3 ] Baloch, M.I., and Akunna, J.C. (2003). Granular bed baffled reactor (Grabbr): solution to a two-phase anaerobic digestion system. J. Envir. Engrg., 129(11), [4 ] Barber, W.P., and Stuckey, D.C. (1998). The influence of start-up strategies on the performance of an anaerobic baffled reactor. Envir. Technol., 19(5), [5 ] Barber, W.P., and Stuckey, D.C. (1999). The use of the anaerobic baffled reactor (ABR) for wastewater treatment: a review. Water Res., 33(7), [6 ] Dama, P., Bell, J., Foxon, K.M., Brouckaert, C.J., Huang, T., Buckley, C.A, Naidoo, V., and Stuckey, D.C. (2002). Pilot-scale study of an anaerobic baffled reactor for the treatment of domestic wastewater. Water Sci. Technol., 46(9), [7 ] DiLallo, R., and Albertson, O. E. (1961). Volatile acids by direct titration. J.WPCF., 33(3), [8 ] Grobicki, A., and Stuckey, D.C. (1992). Hydrodynamic characteristics of the anaerobic baffled reactor. Water Res., 26(3), [9 ] Harada, H., Uemura, S., and Momonoi, K. (1994). Interaction between sulfate-reducing bacteria and methane-producing bacteria in UASB reactors fed with low strength wastes containing different levels of sulfate. Water Res., 28(2), [10 ] Hutňan, M., Drtil, M., Mrafková, L., Derco, J., and Buday, J. (1999). Comparison of start-up and anaerobic wastewater treatment in UASB, hybrid and baffled reactor. Bioproc. Engrg., 21(5), [11 ] Langenhoff, A.A.M., Intrachandra, N., and Stuckey, D.C. (2000). Treatment of dilute soluble and colloidal wastewater using an anaerobic baffled reactor: influence of hydraulic retention time. Water Res., 34(4), [12 ] Levenspiel, O. (1999). Chemical Reaction Engineering. 3 rd Ed., Wiley, New York. [13 ] Manariotis, I.D., and Grigoropoulos, S.G. (2002). Low-strength wastewater treatment using an anaerobic baffled reactor. Water Environ. Res., 74(2), [14 ] Orozco, A. (1997). Pilot and full-scale anaerobic treatment of lowstrength wastewater at sub-optimal temperature (15 C) with a hybrid plug flow reactor. Proc. 8 th Int. Conf. on Anaerobic Digestion, Sendai, Japan vol. 2, [15 ] Polprasert, C., Kemmadamrong, P., and Tran, F.T. (1992). Anaerobic baffle reactor (ABR) process for treating a slaughterhouse wastewater. Envir. Technol., 13, [16 ] Singh, K.S., and Viraraghavan, T. (1998). Start-up and operation of UASB reactors at 20 ºC for municipal wastewater treatment. J. Ferment. Bioengrg., 85(6), [17 ] Singh, K.S., Harada, H., and Viraraghavan, T. (1996). Low-strength wastewater treatment by a UASB reactor. Bior. Technol., 55(3),

7 Table 1 Average characteristic of influent and effluent (at PSS) at different HRTs. Parameter Influent Effluent OLR (kg COD/m 3 d) ± ± ± 0.03 HRT (h) ph 7.74 ± ± ± ± 0.02 COD (mg/l) 502 ± ± ± ± 3.8 COD S (mg/l) ± ± ± 2.99 %COD removal ± ± ± 0.65 %COD S removal ± ± ± 0.55 BOD (mg/l) 317 ± ± ± ± 1.96 BOD S (mg/l) ± ± ± 1.67 %BOD removal ± ± ± 0.9 %BOD S removal ± ± ± 0.8 VFA (mg/l) ± ± ± 5.87 Biogas(m 3 /kg COD removed) ± ± ±