Wastewater Treatment by Anaerobic Digestion Coupled with Membrane Processing

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1 Wastewater Treatment by Anaerobic Digestion Coupled with Membrane Processing Sung-Hee Roh, Young Nam Chun, Jae-Woon Nah, Hyun-Jae Shin, and Sun-Il Kim Department of Chemical Engineering, Chosun University, Gwangju 01-9, Korea Department of Environmental Engineering, Chosun University, Gwangju 01-9, Korea Department of Polymer Science and Engineering, Sunchon National University, Jeonnam 40-4, Korea Received August 0, 00; Accepted April 4, 006 Abstract: Treatment of starch waste by an anaerobic digester-membrane system was studied. The starch waste was immersed into a hollow fiber membrane unit, of area 0.1 m, with the membrane pore size varying from 0.0 to 0.1 µm. The hydraulic retention time (HRT) of the anaerobic digester ranged from 1. to days and this variation had a significant effect on the treatment efficiency; in contrast, whereas the membrane size had no significant effect. The gas production was ca. 0.4 m /kg chemical oxygen demand (COD) treated. The COD removal efficiency was ca %, depending on the HRT. The effluent from the anaerobic filter, having a total COD in the range mg/l, was treated by crossflow ultrafiltration (UF) units. Experiments performed with different membrane pore sizes indicated that a membrane with a molecular weight cut-off (MWCO) size of 6 showed a higher COD removal efficiency in the range of 8 8 %, while maintaining a steady flux of 10 L/m h. The scope of this study scope was also expanded to investigate the long term clogging effect of the membrane. Keywords: anaerobic digester, crossflow, ultrafiltration, membrane Introduction 1)Tapioca, also known as cassava [1], is grown throughout the tropical world and is one of the most important starchy root crops in the tropics. In Asia, Thailand is one of the largest producers and exporters. The main products of tapioca roots are pellets, chips, and starch. During the processing of tapioca root into tapioca starch, wastewater originates during the root washing stage and from the separators. The total wastewater from these two sources averages 0 m /ton of starch produced. As tapioca starch is a major export product with a promising future, a solution to this wastewater problem is required. Anaerobic digestion was popular in the years immediately following World War and there has been a recent resurgence of interest in its application [-]. However, this form of once-through, completely mixed, anaerobic digester, in which the solid retention time To whom all correspondence should be addressed. ( sibkim@mail.chosun.ac.kr) (SRT) equals the hydraulic retention time (HRT), is of limited value in the treatment of industrial effluent. In a digester with an SRT/HRT ratio of 1.0, wash-out of anaerobic microorganisms will pose a serious problem if high loading rates are applied with short HRT results. Hence, developments in anaerobic biological processes for industrial wastewater treatment have concentrated on ways of achieving higher SRT/HRT ratios, thus allowing high loading rates to be applied to small digester volumes. The membrane anaerobic digester system is one process that utilizes a suspended growth digester in conjunction with an external ultra/micro filtration membrane unit for solid/liquid separation [8-11]. Untreated wastewater released directly from tapioca starch factories into surrounding areas can caused environmental problems to nearby population. Experiments using a laboratory-scale anaerobic digester coupled with hollow fiber ultrafiltration (UF) were conducted using different operating parameters, such as various membrane pore sizes, organic loading rates, and detention times, to study the roles of UF in the improvement of the treat-

2 490 Sung-Hee Roh, Young Nam Chun, Jae-Woon Nah, Hyun-Jae Shin, and Sun-Il Kim Table 1. Composition of Synthetic Starch Wastewater Components Starch (NH 4) SO 4 MgSO 4 H O FeCl 6H O CaCl KH PO 4 K HPO 4 MnSO 4 H O NaHCO Concentration (mg/l) Table. Experimental Runs with Anaerobic Digester Coupled with Hollow Fiber UF Module Run Membrane pore size Detention time Organic loading rate No. (µm) (days) (kg COD/m d) Figure 1. Experimental set-up for the anaerobic digester coupled with a hollow fiber ultrafiltration (UF) membrane. ment efficiency and biogas production. This study also looked into the possibility of using a membrane for post treatment of the effluent of tapioca starch wastewater from an anaerobic filter (AF). Materials and Methods The experimental set-up of the anaerobic digester coupled with a hollow fiber UF is shown in Figure 1. The volume of the digester was 1 L and the synthetic tapioca starch wastewater was fed twice a day using a peristaltic pump. The feeding time for each digester was ca. 0 min. During this period, the filtrate was withdrawn at a constant rate. After every feed, the synthetic starch wastewater was maintained at room temperature. The digesters were installed in a room in which the temperature maintained at 0 o C. The composition of the synthetic starch wastewater [1,1] is shown in Table 1. The experimental runs conducted with the anaerobic digester coupled with a hollow fiber module are shown in Table. In the experimental study, the temperature, ph, alkalinity, volatile acids, chemical oxygen demand (COD) removal, and gas production were all determined. After experimental start-up, the HRT decreased for all units (i.e., organic loading rate increased). When the units reached the steady state after a number of days of operation, the organic loading rate was again increased, andsoon. Figure. Experimental set-up of the laboratory-scale crossflow ultrafiltration (UF) module. The treatment of tapioca starch wastewater using AF alone was not sufficient because the effluent continued to have high BOD and COD levels. In this study, the tapioca wastewater effluent treated through AF underwent further treatment using the laboratory-scale crossflow UF module. A schematic diagram of the laboratory-scale UF unit is shown in Figure. In this case, a flat geometry UF was used for post-treatment after AF. The AF effluent was pumped from the base of the stock tank and fed into the membrane filter. A small portion of the feed was filtered out, while the remainder was recycled back to the stock tank. The by-pass system was necessary for controlling the operating conditions at different pressures and crossflow velocities, and the cooling system for keeping the temperature constant. These experiments were conducted to observe the filtration performance at different membrane pore sizes. Furthermore, a long-term membrane filtration study was also conducted to investigate the degree of recovery of flux in the membranes, as shown in Table. The temperature, crossflow velocity

3 Wastewater Treatment by Anaerobic Digestion Coupled with Membrane Processing 491 Table. Experimental Runs with Laboratory-scale Crossflow Ultrafiltration Module Run No. 1 4 P (kpa) Velocity (m/s) Membrane pore size (molecular weight cut-off) Table 4. Average COD Removal with Organic Loading Rate HRT Membrane pore size (days) (µm) , 0.0, 0., Organic loading rate (kg COD/m d) Average COD removal At steady state (%) Figure. Average COD removal efficiency at steady-state with respect to the organic loading rate (influent COD=0,10 mg/l). and applied pressure were fixed at 0 o C, m/s, and 18 kpa, respectively. Results and Discussion Anaerobic Digester Coupled with Membrane Process For the start-up of the digesters, four laboratory-scale anaerobic digesters coupled with hollow fiber UF membranes were seeded with the active digester sludge taken from the Huay Kwang Treatment Plant in Bangkok. All of the digesters were fed with synthetic starch wastewater at the rate of 0.44 L/d (HRT=0 days). The effluent concentration had an average COD of 010 mg/l. Thus, the average organic loading rate for the startup was 0.88 kg COD/m d. The acclimatization was complete after 8 days of operation. After the start-up stage, the HRT was decreased from 0 to days. COD Removal Efficiency The COD removal efficiency increased upon increasingthe HRT, but decreased upon increasing the organic loading rate. The results obtained are summarized in Table 4. An HRT of days was conducted with four different membrane pore sizes and the COD removal from these digesters remained was nearly unchanged. Figure shows the average COD removal efficiency at the steady-state, which was reduced with decreasing HRT from 9. % at an HRT of days (organic loading rateofkgcod/m d) to 81.1 % at an HRT of 1. days. An empirical relationship between the average COD removal efficiency and the organic loading rate was devel- Figure 4. Biogas and methane gas production with respect to the organic loading rate. oped [14] as given by equation (1). Y = 9.6 X (-0.09) (1) (R-squared = 0.84) where Y is the COD removal efficiency (%), and X is the organic loading rate (kg COD/ m d), < X < 11. Note that R-squared was obtained from the logarithmic value of the organic loading rate and COD removal. Biogas Production Table shows the average biogas and methane gas production at the steady-state. Figure 4 shows plots of the biogas and methane gas production at different organic loading rates. The maximum gas production rate was equal to 0.4 m /kg COD utilized at 9. % COD removal.

4 49 Sung-Hee Roh, Young Nam Chun, Jae-Woon Nah, Hyun-Jae Shin, and Sun-Il Kim Table. Average Biogas and CH 4 Production HRT (days) 1. Organic loading rate (kg COD/m d) Biogas production (ml/d) CH 4 production (ml/d) CH 4 content (%) Figure 6. Long-term filtration study: effluent of tapioca starch wastewater from AF (molecular weight cut-off size=1,000,000; velocity= m/s; pressure=18 kpa; temperature=0 o C). Note that after backwash the membrane was submerge in distilled water overnight. Figure. Effect of membrane pore size: effluent of tapioca starch wastewater from AF (velocity= m/s; pressure=18 kpa; temperature=0 o C). Crossflow Ultrafiltration (UF) as Post Treatment to Anaerobic Filter (AF) The AF effluent, having a total COD in the range mg/l, was treated using crossflow UF units. These results suggest the alternative of using a UF membrane for post-treatment, after the biological process, of this wastewater. A long-term membrane filtration study was also conducted to investigate the degree of recovery of the membrane flux. Effect of Membrane Pore Size The effect of the membrane pore size was studied by performing a series of experiments using flat geometry membranes with four different pore sizes (molecular weight cut-off, MWCO): 000, 0000, 0000, and Figure shows that a higher filtrate rate was achieved upon increasing the membrane pore size. A microscopic investigation revealed that most of the fibers in tapioca starch wastewater were decomposed by biological and physical processes in the AF. The COD removal at the steady state was also high, at ca. 8 %, at a steady state flux of 10 L/m h. Long-Term Filtration Study A long-term membrane filtration study was conducted to investigate the degree of recovery of the filtrate rate in the membrane. The temperature, crossflow velocity, and applied pressure were fixed at 0 o C, m/s, and 18 kpa, respectively. Figure 6 shows the performance of membrane filtration in terms of the filtrate rate recovery and the COD removal. After each steady state was reached, the membrane was backwashed using a Gooch crucible apparatus and distilled water. The filtrate rate recovery was ca. 80 % of the previous cycle. The steady state filtrate rate after the first backwash was higher than that from the new membrane, possibly because the filtration mechanism at the beginning of this experiment was an intermediate blocking. For the second backwash, the filtrate rate recovery at the steady state was lower than that after the first backwash, possibly because of the progressive standard blocking that occurred. The COD

5 Wastewater Treatment by Anaerobic Digestion Coupled with Membrane Processing 49 removal was high, at ca. 80 %, and increased with increasing time, possibly due to the gel formation and standard blocking mechanism that occurred on the membrane surface. Conclusion This study clearly demonstrates the satisfactory performance of the four digesters, thereby confirming the potential for the treatment of tapioca starch wastewater using an anaerobic digester coupled with a hollow fiber UF membrane and achieving COD removal of over 80 %. The HRT, but not the membrane size, exerted a significant effect on treatment efficiency within the range used. The COD removal efficiency was ca % over the HRT range of 1. to days. The performance of the digester operating at an organic loading rate of kg COD/m d was the best among the four digesters in terms of the daily gas production. The maximum gas yield was 0.4 m /kg COD utilized at 9. % COD removal. The methane gas composition was about %. The theoretical methane gas production was ca. 0. m /kg COD utilized. Stable operation was maintained over an operational period of months at an average net flux of m /(m s). Intermittent operation was necessary to reduce membrane clogging. Post-treatment of the effluent of the tapioca starch wastewater from an AF was achieved using UF membrane filtration. The COD removal efficiency was ca. 8 % (from ca. 000 to 600 mg /L). Acknowledgment This study was supported by grant No. R from the Basic Research Program of the Korea Science and Engineering Foundation. References 1. M. Tanticharoen, Biogas Production from Tapioca Starch Wastewater, in Proc. of UNEP/TISTER, Bangkok Mircen Regional Workshop on Upgrading of Cassava, Cassava Wastes by Appropriate Biotechnologies, pp. 4-4, Bangkok, Thailand (1989).. G. K. Anderson and T. Donnelly, J. Water Pollut. Control, 8, 491 (1984).. C. C. Forster, J. Water Pollut. Control, 8, 484 (1984). 4. M. Henz and P. Harremoes, Water Sci. Tech., 1, 1 (198).. X. A. Ferandes, A. D. Contwell, and F. E. Mosey, J. Water Pollut. Control, 84, 99 (198). 6. D. Bhattachary, J. WPCF, 0, 846 (198).. J. S. Jeris, Water Sci. Tech., 1, 169 (198). 8. S. Hong and M. Elimelech, J. Membr. Sci., 1, 19 (199). 9. I. C. Kim, Y. H. Ka, J. Y. Park, and K. H. Lee, J. Ind. Eng. Chem.,, 11 (004).. Y. Watanabe and T. Itonaga, J. Ind. Eng. Chem.,, 1 (004). 11. S.J.Kim,J. Ind. Eng. Chem., 11, 4 (00). 1. T. Manickam and A. F. Gaudy, J. WPCF, 1, 0 (199). 1. B. C. Saha and S. Ueda, J. Appl. Microbio. Biotech., 19, 41 (1984). 14. L. Den Berg and K. J. Kennedy, Effect of Type of Waste of Performance of Anaerobic Fixed Film and Upflow Sludge Bed Reactor, National Research Council of Canada, KIA OR6, pp , Ottawa, Ontario, Canada (198).