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Upgrading of anaerobic digestion by incorporating two different hydrolysis processes
JOURNAL OF BIOSCIENCE AND BIOENGINEERING Vol. 100, No. 2, 164–167. 2005 DOI: 10.1263/jbb.100.164
© 2005, The Society for Biotechnology, Japan
Upgrading of Anaerobic Digestion by Incorporating Two Different Hydrolysis Processes Chulhwan Park,1,2* Chunyeon Lee,3 Sangyong Kim,2 Yu Chen,1 and Howard A. Chase1 Cambridge Unit for Bioscience Engineering, Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK,1 Industrial Ecology National Research Laboratory, Korea Institute of Industrial Technology, ChonAn 330-825, Korea,2 and Department of Chemistry, Cleveland State University, 2399 Euclid Avenue, Cleveland, OH 44115, USA3 Received 18 February 2004/Accepted 19 March 2005
The purpose of this study was to increase the efficiency of anaerobic digestion of waste activated sludge (WAS). Either thermochemical or biological hydrolysis was used as a pretreatment and the effects of both were investigated and compared. Two different three-stage digestion systems showed improved performance, although thermochemical hydrolysis showed better results than biological hydrolysis in a bench-scale operation. After anaerobic digestion with thermochemical pretreatment, the total chemical oxygen demand (tCOD) reduction, volatile solid (VS) reduction, methane yield and methane biogas content were 88.9%, 77.5%, 0.52 m3/kg VS and 79.5%, respectively. These results should help in determining the best hydrolysis pretreatment process for anaerobic digestion and in improving the design and operation of the large-scale treatment of WAS by anaerobic digestion with hydrolysis systems. [Key words: anaerobic digestion, hydrolysis, activated sludge, methane]
The main by-product of biological wastewater treatment is waste activated sludge (WAS). The amount of WAS has been increasing worldwide as a result of an increase in the amount of wastewater being treated. WAS produced within the process must be disposed of and this may account for 60% of total plant operating costs (1). There is, therefore, considerable impetus to develop strategies for efficiently reusing the WAS produced. Anaerobic digestion has been employed for activated sludge stabilization, resulting in a reduction in the amount of sludge volatile solids (VS) with concomitant biogas production. The anaerobic digestion process generally consists of four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. These processes routinely have some disadvantages such as long retention times, low removal efficiencies of organics, and they may be unstable. Biological hydrolysis is identified as the ratelimiting step in anaerobic digestion (2–4). To reduce the impact of this rate-limiting step, pretreatment systems for WAS, such as thermal, alkaline, ultrasonic and mechanical disintegration systems, are required (2–12). These systems can accelerate the solubilization of WAS and reduce the average particle size within the sludge, which subsequently improves anaerobic digestion (2, 5). Our previous study conducted in batch experiments also emphasized the importance of pretreatments and investigated the effects of various WAS pretreatments (thermal, chemical, ultrasonic and thermochemical pretreatments) on parameters such as chem-
ical oxygen demand (COD) solubilization, particle size reduction and methane production enhancement (13). As mentioned above, many studies have investigated pretreatment by hydrolysis processes to achieve enhanced anaerobic digestion and mostly involved a comparison of the results of a system with a single pretreatment and those of a system without a pretreatment. However, few reports have been published on the comparison of two different threestage digestion systems. Accordingly, the objective of this study was to demonstrate that enhanced anaerobic digestion could be achieved by adopting two different hydrolysis pretreatments and to investigate, in a continuous operation of the resultant three-stage system, the increase in particle size reduction efficiency, the increase in soluble protein concentration, the increase in COD solubilization and the enhancement of methane production. MATERIALS AND METHODS Three-stage digestion A three-stage digestion system for methane production was operated continuously. A schematic diagram of the three-stage digestion is shown in Fig. 1. In the first stage, either thermochemical or biological hydrolysis was adopted as the hydrolysis process. For pretreatment by thermochemical hydrolysis, in the first stage, WAS was thermally treated at 121°C for 30 min. NaOH at 7 g per liter of WAS was added for chemical hydrolysis. The optimal amount of NaOH per liter of WAS was determined in our previous study (13). WAS treated thermochemically was cooled in a storage tank for about 2 h, the pH of the sludge was adjusted to pH 6.7 by HCl and then it was transferred to the second stage. During pretreatment by biological hydrolysis, in the first stage, WAS was stirred in a 5-l reactor at 150 rpm and hydro-
* Corresponding author. e-mail: [email protected]; [email protected] phone: +82-41-589-8426 fax: +82-41-589-8580 164
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FIG. 1. Flow diagram of the three-stage digestion system for treating waste activated sludge.
lyzed by aerobic bacteria (Cellulomonas uda KCCM 12156 and C. biazotea KCCM 40760) at 30°C. In the second stage, Clostridium butyricum KCCM 35433 was used in an acidogenic process for volatile fatty acid (VFA) production in a 10-l stirred reactor operated at 100 rpm and 37°C (14). In the third stage, rumen microorganisms for methane production were obtained from cattle dung (13, 15) and used in a 20-l stirred reactor operated at 50 rpm and 41°C. The retention times in each stage were 3, 6 and 12 d, respectively. Analysis Standard methods were used for the estimation of total chemical oxygen demand (tCOD), soluble COD (SCOD), VS and pH (16). After the digestion of the samples in the COD reactor (model 45600; HACH, Loveland, CO, USA), the level of COD in a sample was measured by a colorimetric method. SCOD was measured after the centrifugation of the sample at 10,000 rpm for 10 min, whereas tCOD was measured without prior centrifugation. The degree of COD solubilization was calculated by the following equation: SCOD- × 100 (%) COD solubilization (%) = -----------------tCOD The amount of soluble protein was determined by the Bradford method using bovine serum albumin (BSA) as the standard (17). The particle size of sludge samples was measured using a laser particle size analyzer (MAF5001; Malvern, Worcestershire, UK). The biogas composition and VFAs were analyzed using a gas chromatograph (6890N; Agilent, Palo Alto, CA, USA). The tCOD, SCOD, VS, soluble protein and pH of WAS obtained from a sewage sludge treatment facility were measured to be 27,700 mg/l, 2250 mg/l, 26 g/l, 30 mg/l and 6.7, respectively.
RESULTS AND DISCUSSION In this work, two different hydrolysis processes for increasing the efficiency of anaerobic digestion (thermochemical or biological pretreatment) were compared using benchscale experiments. The effects of thermochemical and biological hydrolysis on particle size distribution, soluble protein and COD solubilization under various conditions were compared, and the results are shown in Fig. 2. The control sample comprised nonpretreated WAS. In the case of thermochemical hydrolysis, the 10%, 50% and 90% accumulated values indicate that 10%, 50% and 90% of particles were of sizes below 2, 29 and 144 µm, respectively. In the case of biological hydrolysis, 10%, 50% and 90% of the particles were of sizes below 15, 60 and 230 µm, respectively. After thermochemical and biological hydrolysis, the soluble protein concentration increased from 30 to 1983 mg/l and from 30 to 625
FIG. 2. Comparison of soluble protein, COD solubilization (A) and particle size distribution (B) after thermochemical or biological hydrolysis.
mg/l, respectively. These two different hydrolysis methods decreased the particle size and increased the level of soluble protein. It is expected that pretreatment should be able to improve the performance of anaerobic digestion substantially. In addition, the control sample showed 8.1% COD solubilization (SCOD = 2250 mg/l). On the other hand, 88% COD solubilization (SCOD = 24,380 mg/l) and 24% COD solubilization (SCOD = 6620 mg/l) were achieved after thermochemical and biological hydrolysis, respectively. The turbidity of the supernatant obtained following settling also increased after hydrolysis, showing that the levels of WAS colloidal and dissolved solids increased after hydrolysis, remained in the supernatant, and were not removed by settling. As the levels of soluble protein and COD solubilization increased, the efficiency of anaerobic digestion was also expected to be improved. To investigate the effects of pretreatment on the major parameters of anaerobic digestion, namely, VFA reduction, tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content, experiments were performed using a three-stage digestion process. In a comparison of the concentrations of VFA and SCOD in the control and the two systems incorporating pretreatment, the results showed that both pretreatment systems resulted in higher values than the control (Fig. 3). In the case of the control sample, the total
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FIG. 3. Changes in levels of VFA and SCOD in the three-stage digester incorporating either thermochemical (A) or biological hydrolysis (B), and in the two-stage digester (control) (C).
VFA concentration was 212 mg/l and comprised 133 mg/l acetic acid, 65 mg/l butyric acid and 14 mg/l propionic acid. For the system after thermochemical pretreatment, the VFA concentrations in the pretreatment and acidogenic stages were 1145 and 3053 mg/l, respectively. In the acidogenic process, the major components of VFA were 1572 mg/l acetic acid, 1051 mg/l butyric acid and 430 mg/l propionic acid. In the case of biological pretreatment, the VFA concentrations in the pretreatment and acidogenic stages were 762 and 1035 mg/l, respectively. The major components of VFA in the acidogenic process were 406 mg/l acetic acid, 420 mg/l butyric acid and 209 mg/l propionic acid. VFA concentrations after the methanogenic process (stage III) decreased substantially from 3053 to 134 mg/l with thermochemical pretreatment and from 1035 to 38 mg/l with biological pretreatment. These results indicated that VFAs are important substrates that are readily used by methanogenic microorganisms (3). The SCOD reduction was greatly increased by hydrolysis processes. In the control, two-stage digestion process, the level of SCOD decreased from 2250 to 1001 mg/l (55.5%) following the sequential acidogenic and methanogenic processes. When three-stage digestion was performed by incorporating either thermochemical or biological hydrolysis, there were increases in the reduction of SCOD: 91.6% (from 24,380 to 2055 mg/l) for thermo-
J. BIOSCI. BIOENG.,
chemical hydrolysis and 73.2% (from 7870 to 2110 mg/l) for biological hydrolysis. Although the final levels of SCOD were only slightly different, the degree of reduction showed a considerable difference. In the three different processes, namely, two-stage digestion (control), three-stage digestion after thermochemical hydrolysis, and three-stage digestion after biological hydrolysis, the final levels of tCOD were 16,564, 3074 and 9113 mg/l, respectively. The overall reductions in tCOD were 40.2%, 88.9% and 67.1%, respectively. These observations clearly indicate that the treatment of WAS can be efficiently improved by incorporating either of the two hydrolysis processes studied here. Complex substances in WAS were solubilized into readily biodegradable substances and these substances were easily converted into methane together with significant increases in the amounts of VFA produced. Thus, the solubilization of WAS by hydrolysis processes plays an important role in enhancing anaerobic digestion and the efficiency of the destruction of organics. Improved accessibility to soluble organic substances resulted in higher, more extensive rates of VFA and methane generation (3). The three-stage digestion system involving pretreatment using thermochemical hydrolysis showed that the tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content were 88.9%, 91.6%, 77.5%, 0.52 m3/kg VS and 79.5%, respectively. Alternatively, the adoption of biological hydrolysis as the pretreatment process showed that the tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content were 67.1%, 73.2%, 75.0%, 0.43 m3/kg VS and 75.3%, respectively. Anaerobic digestion including either of the two hydrolysis pretreatment processes showed improved performance, although the levels of improvement in the performance of three-stage digestion incorporating thermochemical hydrolysis were more pronounced than those achieved by incorporating biological hydrolysis in a bench-scale operation. In the three-stage digester, the residual levels of substrates from the second- and third-stage digesters could be reduced by using a longer retention time. For readily fermentable wastes, a three-stage reactor can have a lower overall retention than a singlestage system (18). The three-stage digestion system developed in this study was compared with those described in other studies (Table 1). One three-stage operation described previously comprised biological hydrolysis, an acidogenic process and a methanogenic process (14), and was similar to our three-stage digestion processes. Four different two-stage systems (2, 11, 19), namely, an acidogenic process-methanogenic process, ultrasonic disintegration-anaerobic digestion, mechanical pretreatment-anaerobic digestion and a continuously stirred tank reactor-upflow anaerobic filter, showed relatively low efficiencies in comparison with three-stage digestion systems, although four different two-stage systems showed good performances. In addition, the use of mechanical pretreatment-anaerobic digestion resulted in higher methane yield (11), but this system showed a relatively lower tCOD reduction, VS reduction and methane content in comparison with our systems. A single-stage system resulted in a very low performance and it was considered that the development of the process should be required (13).
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TABLE 1. Comparison of the performances of digestion processes VS tCOD reduction reduction (%) (%) Three-stagea WAS 88.9 77.5 WAS 67.1 75.0 Three-stageb Food wastes 94.7 38.0 Three-stageb WAS 40.2 32.3 Two-stagec WAS – 33.7 Two-staged WAS – 29.0 Two-stagee Organic wastes 79.8 79.6 Two-stagef WAS 15.2 20.5 Single-stageg a Thermochemical hydrolysis process–acidogenic process–methanogenic process. b Biological hydrolysis process–acidogenic process–methanogenic process. c Acidogenic process–methanogenic process. d Ultrasonic disintegration–anaerobic digestion. e Mechanical pretreatment–anaerobic digestion. f Continuously stirred tank reactor–upflow anaerobic filter. g Anaerobic digestion.
Stage
Feed
It is concluded that the three-stage digestion described here is competitive in comparison with other similar systems for enhanced anaerobic digestion. Thermochemical hydrolysis provides faster treatment and a higher efficiency than biological hydrolysis, but it will be expensive to apply it in an actual process because of its additional costs in terms of adding large quantities of chemical reagents and maintaining a high temperature. The rate of hydrolysis is relatively low in biological hydrolysis, but this method involves relatively cheaper maintenance costs. For this reason, thermochemical hydrolysis would be preferred when rapid treatment of a small quantity of WAS is required, whereas biological hydrolysis would be preferred when cost is of importance and large-scale treatment of WAS is required. The above results show that the use of a three-stage digester including hydrolysis pretreatment is a good method for increasing the process performance in anaerobic digestion and the digestion efficiency of the WAS. ACKNOWLEDGMENTS This work was supported by the post-doctoral fellowship program of the Korea Science Engineering Foundation (KOSEF), and by the national research laboratory program of the Korea Ministry of Science and Technology (MOST).
REFERENCES 1. Horan, N. J.: Biological wastewater treatment systems. Wiley, Chichester (1990). 2. Tiehm, A., Nickel, K., Zellhorn, M., and Neis, U.: Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization. Water Res., 35, 2003–2009 (2001). 3. Wang, Q., Kuninobu, M., Kakimoto, K., Ogawa, H. I., and Kato, Y.: Upgrading of anaerobic digestion of waste activated sludge by ultrasonic pretreatment. Bioresour. Technol., 68, 309–313 (1999). 4. Li, Y. Y. and Noike, T.: Upgrading of anaerobic digestion of waste activated sludge by thermal pretreatment. Water Sci. Technol., 26, 857–866 (1992). 5. Tanaka, S., Kobayashi, T., Kamiyama, K. I., and Bildan, M. L. N. S.: Effects of thermochemical pretreatment on the anaerobic digestion of waste activated sludge. Water Sci.
CH4 yield (m3/kg VS) 0.52 0.43 0.48 0.29 0.30 0.85 0.31 0.07
CH4 content in off-gas (%) 79.5 75.3 72.0 70.2 68.9 70.0 61.0 68.6
Ref. This study This study 14 This study 2 11 19 13
Technol., 35, 209–215 (1997). 6. Sawayama, S., Inoue, S., Tsukahara, K., and Ogi, T.: Thermochemical liquidization of anaerobically digested and dewatered sludge and anaerobic retreatment. Bioresour. Technol., 55, 141–144 (1996). 7. Penaud, V., Delgenès, J. P., and Moletta, R.: Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability. Enzyme Microb. Technol., 25, 258–263 (1999). 8. Rajan, R. V., Lin, J. G., and Ray, B. T.: Low-level chemical pretreatment for enhanced sludge solubilization. Res. J. Water Pollut. Control Fed., 61, 1678–1683 (1989). 9. Ray, B. T., Rajan, R. V., and Lin, J. G.: Low-level alkaline solubilization for enhanced anaerobic digestion. Res. J. Water Pollut. Control Fed., 62, 81–87 (1990). 10. Lin, J.-G., Chang, C.-N., and Chang, S.-C.: Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol., 62, 85–90 (1997). 11. Nah, I. W., Kang, Y. W., Hwang, K. Y., and Song, W. K.: Mechanical pretreatment of waste activated sludge for anaerobic digestion process. Water Res., 34, 2362–2368 (2000). 12. Choi, H. B., Hwang, K. Y., and Shin, E. B.: Effect on anaerobic digestion of sewage sludge pretreatment. Water Sci. Technol., 35, 207–211 (1997). 13. Kim, J., Park, C., Kim, T.-H., Lee, M., Kim, S., Kim, S.-W., and Lee, J.: Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng., 95, 271–275 (2003). 14. Kim, S. W., Park, J. Y., Kim, J. K., Cho, J. H., Chun, Y. N., Lee, I. H., Lee, J. S., Park, J. S., and Park, D.-H.: Development of a modified three-stage methane production process using food wastes. Appl. Biochem. Biotechnol., 84, 731–741 (2000). 15. Shapton, D. A. and Board, R. G.: Isolation of anaerobes. Academic Press, London, New York (1971). 16. APHA, AWWA, and WEF: Standard methods for the examination of water and wastewater, 19th ed. American Public Health Association, Washington, D.C. (1995). 17. Bradford, M. M.: A rapid sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. Tech., 72, 248–254 (1976). 18. Gunaseelan, V. N.: Anaerobic digestion of biomass for methane production: a review. Biomass Bioenerg., 13, 83–114 (1997). 19. Held, C., Wellacher, M., Robra, K. H., and Gübitz, G. M.: Two-stage anaerobic fermentation of organic waste in CSTR and UFAF-reactors. Bioresour. Technol., 81, 19–24 (2002).
© 2005, The Society for Biotechnology, Japan
Upgrading of Anaerobic Digestion by Incorporating Two Different Hydrolysis Processes Chulhwan Park,1,2* Chunyeon Lee,3 Sangyong Kim,2 Yu Chen,1 and Howard A. Chase1 Cambridge Unit for Bioscience Engineering, Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK,1 Industrial Ecology National Research Laboratory, Korea Institute of Industrial Technology, ChonAn 330-825, Korea,2 and Department of Chemistry, Cleveland State University, 2399 Euclid Avenue, Cleveland, OH 44115, USA3 Received 18 February 2004/Accepted 19 March 2005
The purpose of this study was to increase the efficiency of anaerobic digestion of waste activated sludge (WAS). Either thermochemical or biological hydrolysis was used as a pretreatment and the effects of both were investigated and compared. Two different three-stage digestion systems showed improved performance, although thermochemical hydrolysis showed better results than biological hydrolysis in a bench-scale operation. After anaerobic digestion with thermochemical pretreatment, the total chemical oxygen demand (tCOD) reduction, volatile solid (VS) reduction, methane yield and methane biogas content were 88.9%, 77.5%, 0.52 m3/kg VS and 79.5%, respectively. These results should help in determining the best hydrolysis pretreatment process for anaerobic digestion and in improving the design and operation of the large-scale treatment of WAS by anaerobic digestion with hydrolysis systems. [Key words: anaerobic digestion, hydrolysis, activated sludge, methane]
The main by-product of biological wastewater treatment is waste activated sludge (WAS). The amount of WAS has been increasing worldwide as a result of an increase in the amount of wastewater being treated. WAS produced within the process must be disposed of and this may account for 60% of total plant operating costs (1). There is, therefore, considerable impetus to develop strategies for efficiently reusing the WAS produced. Anaerobic digestion has been employed for activated sludge stabilization, resulting in a reduction in the amount of sludge volatile solids (VS) with concomitant biogas production. The anaerobic digestion process generally consists of four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. These processes routinely have some disadvantages such as long retention times, low removal efficiencies of organics, and they may be unstable. Biological hydrolysis is identified as the ratelimiting step in anaerobic digestion (2–4). To reduce the impact of this rate-limiting step, pretreatment systems for WAS, such as thermal, alkaline, ultrasonic and mechanical disintegration systems, are required (2–12). These systems can accelerate the solubilization of WAS and reduce the average particle size within the sludge, which subsequently improves anaerobic digestion (2, 5). Our previous study conducted in batch experiments also emphasized the importance of pretreatments and investigated the effects of various WAS pretreatments (thermal, chemical, ultrasonic and thermochemical pretreatments) on parameters such as chem-
ical oxygen demand (COD) solubilization, particle size reduction and methane production enhancement (13). As mentioned above, many studies have investigated pretreatment by hydrolysis processes to achieve enhanced anaerobic digestion and mostly involved a comparison of the results of a system with a single pretreatment and those of a system without a pretreatment. However, few reports have been published on the comparison of two different threestage digestion systems. Accordingly, the objective of this study was to demonstrate that enhanced anaerobic digestion could be achieved by adopting two different hydrolysis pretreatments and to investigate, in a continuous operation of the resultant three-stage system, the increase in particle size reduction efficiency, the increase in soluble protein concentration, the increase in COD solubilization and the enhancement of methane production. MATERIALS AND METHODS Three-stage digestion A three-stage digestion system for methane production was operated continuously. A schematic diagram of the three-stage digestion is shown in Fig. 1. In the first stage, either thermochemical or biological hydrolysis was adopted as the hydrolysis process. For pretreatment by thermochemical hydrolysis, in the first stage, WAS was thermally treated at 121°C for 30 min. NaOH at 7 g per liter of WAS was added for chemical hydrolysis. The optimal amount of NaOH per liter of WAS was determined in our previous study (13). WAS treated thermochemically was cooled in a storage tank for about 2 h, the pH of the sludge was adjusted to pH 6.7 by HCl and then it was transferred to the second stage. During pretreatment by biological hydrolysis, in the first stage, WAS was stirred in a 5-l reactor at 150 rpm and hydro-
* Corresponding author. e-mail: [email protected]; [email protected] phone: +82-41-589-8426 fax: +82-41-589-8580 164
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FIG. 1. Flow diagram of the three-stage digestion system for treating waste activated sludge.
lyzed by aerobic bacteria (Cellulomonas uda KCCM 12156 and C. biazotea KCCM 40760) at 30°C. In the second stage, Clostridium butyricum KCCM 35433 was used in an acidogenic process for volatile fatty acid (VFA) production in a 10-l stirred reactor operated at 100 rpm and 37°C (14). In the third stage, rumen microorganisms for methane production were obtained from cattle dung (13, 15) and used in a 20-l stirred reactor operated at 50 rpm and 41°C. The retention times in each stage were 3, 6 and 12 d, respectively. Analysis Standard methods were used for the estimation of total chemical oxygen demand (tCOD), soluble COD (SCOD), VS and pH (16). After the digestion of the samples in the COD reactor (model 45600; HACH, Loveland, CO, USA), the level of COD in a sample was measured by a colorimetric method. SCOD was measured after the centrifugation of the sample at 10,000 rpm for 10 min, whereas tCOD was measured without prior centrifugation. The degree of COD solubilization was calculated by the following equation: SCOD- × 100 (%) COD solubilization (%) = -----------------tCOD The amount of soluble protein was determined by the Bradford method using bovine serum albumin (BSA) as the standard (17). The particle size of sludge samples was measured using a laser particle size analyzer (MAF5001; Malvern, Worcestershire, UK). The biogas composition and VFAs were analyzed using a gas chromatograph (6890N; Agilent, Palo Alto, CA, USA). The tCOD, SCOD, VS, soluble protein and pH of WAS obtained from a sewage sludge treatment facility were measured to be 27,700 mg/l, 2250 mg/l, 26 g/l, 30 mg/l and 6.7, respectively.
RESULTS AND DISCUSSION In this work, two different hydrolysis processes for increasing the efficiency of anaerobic digestion (thermochemical or biological pretreatment) were compared using benchscale experiments. The effects of thermochemical and biological hydrolysis on particle size distribution, soluble protein and COD solubilization under various conditions were compared, and the results are shown in Fig. 2. The control sample comprised nonpretreated WAS. In the case of thermochemical hydrolysis, the 10%, 50% and 90% accumulated values indicate that 10%, 50% and 90% of particles were of sizes below 2, 29 and 144 µm, respectively. In the case of biological hydrolysis, 10%, 50% and 90% of the particles were of sizes below 15, 60 and 230 µm, respectively. After thermochemical and biological hydrolysis, the soluble protein concentration increased from 30 to 1983 mg/l and from 30 to 625
FIG. 2. Comparison of soluble protein, COD solubilization (A) and particle size distribution (B) after thermochemical or biological hydrolysis.
mg/l, respectively. These two different hydrolysis methods decreased the particle size and increased the level of soluble protein. It is expected that pretreatment should be able to improve the performance of anaerobic digestion substantially. In addition, the control sample showed 8.1% COD solubilization (SCOD = 2250 mg/l). On the other hand, 88% COD solubilization (SCOD = 24,380 mg/l) and 24% COD solubilization (SCOD = 6620 mg/l) were achieved after thermochemical and biological hydrolysis, respectively. The turbidity of the supernatant obtained following settling also increased after hydrolysis, showing that the levels of WAS colloidal and dissolved solids increased after hydrolysis, remained in the supernatant, and were not removed by settling. As the levels of soluble protein and COD solubilization increased, the efficiency of anaerobic digestion was also expected to be improved. To investigate the effects of pretreatment on the major parameters of anaerobic digestion, namely, VFA reduction, tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content, experiments were performed using a three-stage digestion process. In a comparison of the concentrations of VFA and SCOD in the control and the two systems incorporating pretreatment, the results showed that both pretreatment systems resulted in higher values than the control (Fig. 3). In the case of the control sample, the total
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FIG. 3. Changes in levels of VFA and SCOD in the three-stage digester incorporating either thermochemical (A) or biological hydrolysis (B), and in the two-stage digester (control) (C).
VFA concentration was 212 mg/l and comprised 133 mg/l acetic acid, 65 mg/l butyric acid and 14 mg/l propionic acid. For the system after thermochemical pretreatment, the VFA concentrations in the pretreatment and acidogenic stages were 1145 and 3053 mg/l, respectively. In the acidogenic process, the major components of VFA were 1572 mg/l acetic acid, 1051 mg/l butyric acid and 430 mg/l propionic acid. In the case of biological pretreatment, the VFA concentrations in the pretreatment and acidogenic stages were 762 and 1035 mg/l, respectively. The major components of VFA in the acidogenic process were 406 mg/l acetic acid, 420 mg/l butyric acid and 209 mg/l propionic acid. VFA concentrations after the methanogenic process (stage III) decreased substantially from 3053 to 134 mg/l with thermochemical pretreatment and from 1035 to 38 mg/l with biological pretreatment. These results indicated that VFAs are important substrates that are readily used by methanogenic microorganisms (3). The SCOD reduction was greatly increased by hydrolysis processes. In the control, two-stage digestion process, the level of SCOD decreased from 2250 to 1001 mg/l (55.5%) following the sequential acidogenic and methanogenic processes. When three-stage digestion was performed by incorporating either thermochemical or biological hydrolysis, there were increases in the reduction of SCOD: 91.6% (from 24,380 to 2055 mg/l) for thermo-
J. BIOSCI. BIOENG.,
chemical hydrolysis and 73.2% (from 7870 to 2110 mg/l) for biological hydrolysis. Although the final levels of SCOD were only slightly different, the degree of reduction showed a considerable difference. In the three different processes, namely, two-stage digestion (control), three-stage digestion after thermochemical hydrolysis, and three-stage digestion after biological hydrolysis, the final levels of tCOD were 16,564, 3074 and 9113 mg/l, respectively. The overall reductions in tCOD were 40.2%, 88.9% and 67.1%, respectively. These observations clearly indicate that the treatment of WAS can be efficiently improved by incorporating either of the two hydrolysis processes studied here. Complex substances in WAS were solubilized into readily biodegradable substances and these substances were easily converted into methane together with significant increases in the amounts of VFA produced. Thus, the solubilization of WAS by hydrolysis processes plays an important role in enhancing anaerobic digestion and the efficiency of the destruction of organics. Improved accessibility to soluble organic substances resulted in higher, more extensive rates of VFA and methane generation (3). The three-stage digestion system involving pretreatment using thermochemical hydrolysis showed that the tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content were 88.9%, 91.6%, 77.5%, 0.52 m3/kg VS and 79.5%, respectively. Alternatively, the adoption of biological hydrolysis as the pretreatment process showed that the tCOD reduction, SCOD reduction, VS reduction, methane yield and methane content were 67.1%, 73.2%, 75.0%, 0.43 m3/kg VS and 75.3%, respectively. Anaerobic digestion including either of the two hydrolysis pretreatment processes showed improved performance, although the levels of improvement in the performance of three-stage digestion incorporating thermochemical hydrolysis were more pronounced than those achieved by incorporating biological hydrolysis in a bench-scale operation. In the three-stage digester, the residual levels of substrates from the second- and third-stage digesters could be reduced by using a longer retention time. For readily fermentable wastes, a three-stage reactor can have a lower overall retention than a singlestage system (18). The three-stage digestion system developed in this study was compared with those described in other studies (Table 1). One three-stage operation described previously comprised biological hydrolysis, an acidogenic process and a methanogenic process (14), and was similar to our three-stage digestion processes. Four different two-stage systems (2, 11, 19), namely, an acidogenic process-methanogenic process, ultrasonic disintegration-anaerobic digestion, mechanical pretreatment-anaerobic digestion and a continuously stirred tank reactor-upflow anaerobic filter, showed relatively low efficiencies in comparison with three-stage digestion systems, although four different two-stage systems showed good performances. In addition, the use of mechanical pretreatment-anaerobic digestion resulted in higher methane yield (11), but this system showed a relatively lower tCOD reduction, VS reduction and methane content in comparison with our systems. A single-stage system resulted in a very low performance and it was considered that the development of the process should be required (13).
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TABLE 1. Comparison of the performances of digestion processes VS tCOD reduction reduction (%) (%) Three-stagea WAS 88.9 77.5 WAS 67.1 75.0 Three-stageb Food wastes 94.7 38.0 Three-stageb WAS 40.2 32.3 Two-stagec WAS – 33.7 Two-staged WAS – 29.0 Two-stagee Organic wastes 79.8 79.6 Two-stagef WAS 15.2 20.5 Single-stageg a Thermochemical hydrolysis process–acidogenic process–methanogenic process. b Biological hydrolysis process–acidogenic process–methanogenic process. c Acidogenic process–methanogenic process. d Ultrasonic disintegration–anaerobic digestion. e Mechanical pretreatment–anaerobic digestion. f Continuously stirred tank reactor–upflow anaerobic filter. g Anaerobic digestion.
Stage
Feed
It is concluded that the three-stage digestion described here is competitive in comparison with other similar systems for enhanced anaerobic digestion. Thermochemical hydrolysis provides faster treatment and a higher efficiency than biological hydrolysis, but it will be expensive to apply it in an actual process because of its additional costs in terms of adding large quantities of chemical reagents and maintaining a high temperature. The rate of hydrolysis is relatively low in biological hydrolysis, but this method involves relatively cheaper maintenance costs. For this reason, thermochemical hydrolysis would be preferred when rapid treatment of a small quantity of WAS is required, whereas biological hydrolysis would be preferred when cost is of importance and large-scale treatment of WAS is required. The above results show that the use of a three-stage digester including hydrolysis pretreatment is a good method for increasing the process performance in anaerobic digestion and the digestion efficiency of the WAS. ACKNOWLEDGMENTS This work was supported by the post-doctoral fellowship program of the Korea Science Engineering Foundation (KOSEF), and by the national research laboratory program of the Korea Ministry of Science and Technology (MOST).
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CH4 yield (m3/kg VS) 0.52 0.43 0.48 0.29 0.30 0.85 0.31 0.07
CH4 content in off-gas (%) 79.5 75.3 72.0 70.2 68.9 70.0 61.0 68.6
Ref. This study This study 14 This study 2 11 19 13
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