Sequential Production of Hydrogen and Methane from Wastewawter ...
J. Chin. Inst. Chem. Engrs., Vol. 34, No. 6, 683-687, 2003
Short communication
Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation Chi-Chung Wang [1], Chih-Wen Chang [2], Ching-Ping Chu [3], Duu-Jong Lee [4] Department of Chemical Engineering, National Taiwan University Taipei, Taiwan 106, R.O.C.
Bea-Van Chang [5] Department of Microbiology, Soochow University Taipei, Taiwan 111, R.O.C.
Abstract―This study demonstrates the feasibility of sequentially producing both hydrogen and methane from wastewater sludge using a clostridium strain, as was isolated by Wang et al. (2003a). Three commonly used pre-treatments were applied to wastewater sludge to increase the hydrogen yield. Then, the waste liquor was externally dosed with methanogenic bacteria to produce methane. The waste liquor after fermentation of hydrogen produced more methane than was directly derived without fermentation of hydrogen. The reduction of nitrogen-containing organic matter is shown to compete with the formation of hydrogen, yielding ammonium nitrogen (NH3-N) in the fermented liquor. Key Words : Anaerobic fermentation, Hydrogen, Methane, Clostridium, Pre-treatment
INTRODUCTION
Hydrogen is a clean source of energy. Bio-conversion of biomass to produce hydrogen has been demonstrated, using the anaerobic fermentation of high-strength wastewater (Bolliger et al., 1985; Liu et al., 1995; Ueno et al., 1996; Zhu et al., 1999), solid waste (Mizuno et al., 2000a; Lay et al., 1999), and some well-defined compounds in water, such as molasses (Tanisho and Ishiwata, 1994), glucose (Kataoka et al., 1997; Lin and Chang, 1999), crystalline cellulose (Lay, 2001), peptone (Bai et al., 2001), and starch (Lay, 2000). Methods for promoting hydrogen production have been reported (Tanisho and Ishiwata, 1995; Tanisho et al., 1998; Mizuno et al., 2000b; Sparling et al., 1997; Liang et al., 2001). In the literature, anaerobic fermentation has yielded around 11 mg-H2/g-dried solids (DS) from glucose solution (Kataoka et al., 1997), and 1.4 mg-H2/g-DS from peptone-containing solution (Liang et al., 2001). Only a few data on the hydrogen yield from waste[1] [2] [3] [4] [5]
王之仲 張繼文 朱敬平 李篤中, To whom all correspondence should be addressed 張碧芬
water sludge have been presented at local conferences, including the data of Huang et al. (2000) and Cheng et al. (2000). Recently, Wang et al. (2003a) conducted the first systematic study of the production of hydrogen from wastewater sludge, and found a rather high hydrogen yield from wastewater sludge using a clostridium strain isolated from the sludge sample. Later, Wang et al. (2003b) claimed that applying a filtrate to the sludge could produce more hydrogen than could be obtained using all of the particles in the sludge. Although these studies successfully established the feasibility of producing hydrogen from wastewater sludge, the hydrogen formed during the first 16-24 h of fermentation was consumed in a later stage. Wang et al. (2003a) blocked the methanogenic pathway using a pre-treatment. However, much of the produced hydrogen was still consumed. The pathway for hydrogen consumption remains unknown, but is of academic and practical interest. This study evaluated the feasibility of sequen-
J. Chin. Inst. Chem. Engrs., Vol. 34, No. 6, 2003
684
tially producing hydrogen and methane from wastewater sludge using a clostridium strain under anaerobic conditions. Based on test data, a combined process, involving two fermenters, is proposed. Additionally, the possible incorporation of nitrogen cycles, which compete with the production of hydrogen during sludge fermentation, is demonstrated.
methane production tests, anaerobe K8 was added to the samples after bio-hydrogen tests were performed. K8 was collected from the bottom sediment at a known site of the Tam-Shui River (near Taipei). This mixed culture had high methane productivity (Chang et al., 1996). Fermentation and tests
MATERIALS AND METHOD The substrate
Waste activated sludge was extracted from a wastewater treatment plant of the Presidental Enterprise Corp., Taiwan, which daily treats 250 tons of food-processing wastewater using primary, secondary and tertiary treatments. The pH of the sludge was about 6.4. The chemical oxygen demand (COD) for the sludge was 9,600 mg/L (TCOD), as determined by directly reading a spectrometer (DR/2000, HACH, U.S.A.). The COD for the filtrate of the sludge sample after it was filtered through a 0.45 µm membrane was called the soluble COD (SCOD), and was 465 mg/L for the original sludge. The elemental composition of the dried samples was C: 34.3%, H: 5.6%, and N: 5.5%, according to an elemental analyzer (Perkin-Elmer 2400 CHN). Three pretreatments were applied to the original sludge to determine their effects on the yield of hydrogen. These pretreatments not only released insoluble organic matter into water to enhance methane production (Lee and Mueller, 2001), but also deactivated the methanogenic bacteria in the sludge to block the pathway of the conversion of hydrogen to methane. These pretreatments are summarized as follows. (1) Acidification: perchloric acid (HClO4) was mixed with the sludge sample for 10 mins to adjust the pH of the suspension to 3. Then, the sample was stored at 4°C for 6 h (Jean et al., 2000). (2) Sterilization: sludge samples were pasteurized at 121°C and 1.2 kgf/cm2 (HUXLEY AUTOCLAVE, HL-360) for 30 min. (3) Freezing and thawing: the sludge was frozen at −17°C for 24 h in a freezer and then thawed for another 12 h in a water bath at 25°C (Hung et al., 1997). The inoculum
Wang et al. (2003a) isolated the inoculum. The strain was selected and identified as Clostridium bifermentans using the polymerase chain reaction (PCR) and 16S DNA sequence analysis. In some
Batch fermentation tests were performed in 125 mL serum bottles. In each bottle, 45 mL of substrate, original or pre-treated, was mixed with 5 mL of seed bacteria suspension and anaerobically incubated at 35°C without stirring or adding any nutrients. The bottles were capped with butyl rubber stoppers and wrapped in aluminum foil to prevent photolysis of the substrate. Gas and liquor samples were collected at 8, 16, 24, 32, 40, 48, 72, and 96 h of fermentation. At each time interval, and for each substrate, the gas compositions of three serum bottles were measured and their average was reported. After the measurements were made, these samples were abandoned to prevent the introduction of any possible error associated with the sampling procedure, such as gas leakage. Tests to determine potential methane production were performed after 96 h of hydrogen fermentation. The anaerobe K8 was added to some serum bottles after the hydrogen fermentation tests were completed. The gas samples were collected at 24 h intervals up to 240 h. A GC-TCD (Shimadzu, GC-8A), equipped with a stainless column packed with Porapack Q (50/80 mesh) at 70°C and a thermal conductivity detector (TCD), was used to measure the hydrogen and methane concentrations in the gas phase. The temperature of both the injector and the detector of the GC was 100°C. Nitrogen served as the carrying gas with a flow rate of 20 mL/min. An integrator (HP3396 Series II) was used to integrate the peak area of the effluent curve, and to measure the gaseous concentrations. Repeated measurements revealed that the hydrogen and methane contents thus determined included maximum relative errors of 15% and 10%, respectively. The hydrogen content in the anaerobic glove box was also measured, and was subtracted from the hydrogen concentrations read in the serum bottles. The concentrations of ammonia-nitrogen (NH3-N) in the supernatant were measured during the fermentation test. The filtrate samples were extracted by centrifugating the sludge at 13,500 rpm for 1 min. The NH3-N concentration of the filtrate was measured spectrophotometrically (425 nm) using a mixture of the filtrate sample with two drops of mineral stabilizer, three drops of polyvinyl alcohol dispersing agent, and 1 mL of Nessler agent in a spectropho-
Chi-Chung Wang, Chih-Wen Chang, Ching-Ping Chu, Duu-Jong Lee, and Bea-Van Chang : Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation
RESULTS AND DISCUSSION Hydrogen production
Fig. 2. Time course of the pH of the suspension.
200
(g/kg-DS)
Figure 1 shows the hydrogen yield for one gram of dried solid (DS). In contrast to the 66 h time lag reported by Cheng et al. (2000), this study found negligible time lag for hydrogen production, perhaps because the inoculum was directly derived from the substrate sludge. The hydrogen concentration in the gas phase yielded an increasing-decreasing curve, with a peak at about 16-24 h (indicated in Fig. 1 by the arrow). A specific quantity of produced hydrogen was, thus, “consumed.” As Fig. 1 reveals, the specific hydrogen yield reached 5 g/kg-DS for the original sludge, much higher than that reported by Huang et al. (2000) (0.16 g-H2/kg-DS). Meanwhile, sterilization increased the specific hydrogen yield to about 21.5 g-H2/kg-DS. The acidification, freezing and thawing treatments enhanced the yield to about 9-10 g/kg-DS. This observation shows that the yield of hydrogen from wastewater sludge is similar to that from glucose solution if the former is subjected to suitable pretreatment. Figure 2 plots the variation in the pH of the solution during fermentation. Clearly, during hydrogen production, the solution’s pH increased with time until the yield peaked at 16-24 h, after which time the pH dropped or did not increase further in the case of the acidified sludge. The formation of hydrogen produced a by-product acidic in nature.
Figure 3 plots the time evolution of the concentration of ammonium nitrogen (NH3-N) in the suspension. In the case of the original, sterilized and freeze/thawed sludges, the NH3-N concentration increases with the amount of hydrogen produced until it reached a peak at about 24 h, after which time it leveled off in the hydrogen consumption phase. The nitrogen-containing compounds were reduced to ammonium nitrogen when hydrogen was formed. The acidified sludge, on the other hand, produced the least NH3-N of all the tests. Hence, although the nitrogen cycles competed with hydrogen formation, they are not preferred in acidic environment.
NH3-N (mg/L)
tometer (DR/2000, HACH, U.S.A.). Standard solutions of NH3-N were used for calibration.
685
Time (h)
Fig. 3. Time course of the ammonium nitrogen concentration during hydrogen and methane producing phases.
Methane production Fig. 1. Time course of the hydrogen yield in the gas phase. Clostridium strain was added at hour 0, and K8 was added at hour 96. Each data point is the average of triplicate tests.
In the hydrogen fermentation tests, the amount of methane produced was negligible (0-96 h as shown in Fig. 4). Hence, the consumption of hydrogen shown in Fig. 1 was not associated with methane
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J. Chin. Inst. Chem. Engrs., Vol. 34, No. 6, 2003 H2 Solid-Liquid Separation Fermenter
filtrate
Raw sludge Pretreatment
CH4 Liquor Waste Liquor
Cake
Fermenter Mixer
Fig. 5. A proposed sequential process that produces hydrogen and methane from wastewater sludge.
CONCLUSION Fig. 4. Time course of the methane yield. The anaerobe K8 was added at hour 96.
production. Since only a small proportion of organic matter was converted into hydrogen, the potential for using the exhausted liquor to produce methane was of interest. Anaerobe K8 was added to the serum bottles after 96 h of hydrogen fermentation. After K8 was added, as Fig. 4 shows, the amount of methane accumulated in the serum bottles increased monotonically with time. The amount of methane produced followed the order freeze/thawed > original > sterilization >> acidification. Although the acidification produced hydrogen at a yield two times that of the original sludge, it produced less methane. After K8 was added, the solution’s pH increased again, corresponding to the formation of methane. The acidified sludge, however, also did not undergo an efficient increase in pH, and so generated least methane.
The clostridium strain used herein could ferment wastewater sludge into hydrogen at a much higher rate than heretofore reported in the literature. Acidification, sterilization and freezing and thawing could increase the hydrogen yield. After hydrogen fermentation, adding a methanogenic culture to the fermented liquor accelerated the production of methane, such that the rate of production was higher than that of unfermented samples. A combined process that can produce hydrogen and methane sequentially has, thus, been proposed. The reduction in the amount of nitrogen-containing matter can reduce the hydrogen yield, the effect of which is less profound in an acidic environment.
ACKNOWLEDGEMENT
Support for this work by the National Science Council, R.O.C., is gratefully appreciated.
Sequential process
The data shown in Fig. 4 reveal that the sludge after hydrogen fermentation was more readily digestable than the unfermented sample. This observation implies that some products of the fermentation test promoted methane production. As Wang et al. (2003b) concluded, fermenting the filtrate of the sludge alone could produce more hydrogen than fermenting all of the sludge (including solids). Hence, Fig. 5 shows a proposed combined process that yields hydrogen and methane sequentially. Restated, the filtrate is first separated from the raw sludge (with or without pretreatment) and is sent to fermenter #1 to produce hydrogen. Then, the waste liquor is mixed with the cake from the solid-liquid separator and is sent to fermenter #2 to produce methane. This sequential arrangement effectively produces hydrogen and methane from wastewater sludge.
REFERENCES Bai, M. D., S. S. Cheng, and T. C. Tseng, “Biohydrogen Produced due to Peptone Degradation by Pretreated Seed Sludge,” The IWA Asia-Pacific Regional Conference (WaterQual 2001), Fukuoka, Japan (2001). Bolliger, R., H. Zürrer, and R. Bachofen, “Photoproduction of Molecular Hydrogen from Wastewater of a Sugar Refinery by Photosynthetic Bacteria,” Appl. Microbiol. Biotechnol., 23, 147 (1985). Chang, B. V., J. X. Zheng, and S. Y. Yuan, “Effects of Alternative Electron Donors, Acceptors and Inhibitors on PCP Dechlorination in Soil,” Chemosphere, 33, 313 (1996). Cheng, S. S., M. D. Bai, S. M. Chang, K. L. Wu, and W. C. Chen, “Studies on the Feasibility of Hydrogen Production Hydrolyzed Sludge by Anaerobic Microorganisms,” The 25th Wastewater Tech. Conference, Yunlin, Taiwan (in Chinese) (2000).
Chi-Chung Wang, Chih-Wen Chang, Ching-Ping Chu, Duu-Jong Lee, and Bea-Van Chang : Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation
Huang, C. S., C. Y. Lin, Y. Y. Tsai, and Y. G. Shieh, “Preliminary Study of Anaerobic Hydrogen Production Using Various Substrates and Incubation Methods,” The 25th Wastewater Tech. Conference, Yunlin, Taiwan (in Chinese) (2000). Hung, W. T., W. H. Feng, I. H. Tsai, D. J. Lee, and S. G. Hong, “Unidirectional Freezing of Waste Activated Sludge: Radial Freezing Versus Vertical Freezing,” Water Res., 31, 2219 (1997). Jean, D. S., B. V. Chang, G. S. Liao, G. W. Tsou, and D. J. Lee, “Reduction of Microbial Density Level in Sewage Sludge through pH Adjustment and Ultrasonic Treatment,” Water Sci. Technol., 42(9), 97 (2000). Kataoka, N., A. Miya, and K. Kiriyama, “Studies on Hydrogen Production by Continuous Culture System of Hydrogen-Producing Anaerobic Bacteria,” Water Sci. Technol., 36(6-7), 41 (1997). Lay, J. J., Y. J. Lee, and T. Noike, “Feasibility of Biological Hydrogen Production from Organic Fraction of Municipal Solid Waste,” Water Res., 33, 2579 (1999). Lay, J. J., “Modeling and Optimization of Anaerobic Digested Sludge Converting Starch to Hydrogen,” Biotechnol. Bioeng., 68, 269 (2000). Lay, J. J., “Biohydrogen Generation by Mesophilic Anaerobic Fermentation of Microcrystalline Cellulose,” Biotechnol. Bioeng., 74, 280 (2001). Lee, D. J. and J. A. Mueller, “Preliminary Treatments,” Sludge into Biosolids, L. Spinosa and P. A. Vesilind, Ed., International Water Association, U.K. (2001). Liang, T. M., K. L. Wu, and S. S. Cheng, “Hydrogen Production of Chloroform Inhibited Granular Sludge,” The IWA Asia-Pacific Regional Conference (WaterQual 2001), Fukuoka, Japan (2001). Lin, C. Y. and R. C. Chang, “Hydrogen Production during the Anaerobic Acidogenic Conversion of Glucose,” J. Chem. Technol. Biotechnol., 74, 498 (1999). Liu, S. J., W. F. Yang, and P. Q. Zhou, “The Research on Hydrogen Production from the Treatment of Bean Products Wastewater by Immobilized Photosynthetic Bacteria,” Environ. Sci., 16, 42 (1995).
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Mizuno, O., R. Dinsdale, F. R. Hawkes, D. L. Hawkes, and T. Noike, “Enhancement of Hydrogen Production from Glucose by Nitrogen Gas Sparging,” Bioresource Technol., 73, 59 (2000a). Mizuno, O., T. Ohara, M. Shinya, and T. Noike, “Characteristics of Hydrogen Production from Bean Curd Manufacturing Waste by Anaerobic Microflora,” Water Sci. Technol., 42(3-4), 345 (2000b). Sparling, R., D. Risbey, and H. M. Poggi-Varaldo, “Hydrogen Production from Inhibited Anaerobic Composters,” Int. J. Hydrogen Energy, 22, 563 (1997). Tanisho, S. and Y. Ishiwata, “Continuous Hydrogen Production from Molasses by the Bacterium Enterobacter Aerogenes,” Int. J. Hydrogen Energy, 19, 807 (1994). Tanisho, S. and Y. Ishiwata, “Continuous Hydrogen Production from Molasses by Fermentation Using Urethane Foam as a Support of Flocks,” Int. J. Hydrogen Energy, 20, 541 (1995). Tanisho, S., M. Kuromoto, and N. Kadokura, “Effect of CO2 Removal on Hydrogen Production by Fermentation,” Int. J. Hydrogen Energy, 23, 559 (1998). Ueno, Y., S. Otauka, and M. Morimoto, “Hydrogen Production from Industrial Wastewater by Anaerobic Microflora in Chemostat Culture,” J. Fermentation Bioeng., 82, 194 (1996). Wang, C. C., C. W. Chang, C. P. Chu, D. J. Lee, B. V. Chang, and C. S. Liao, “Producing Hydrogen from Wastewater Sludge by Clostridium Bifermentans,” J. Biotechnol., 102, 83 (2003a). Wang, C. C., C. W. Chang, C. P. Chu, D. J. Lee, B. V. Chang, C. S. Liao, and J. H. Tay, “Using Filtrate to Produce Bio-Hydrogen by Anaerobic Fermentation,” Water Res., 37, 2789 (2003b). Zhu, H., T. Suzuki, A. A. Tsygankov, Y. Asada, and J. Miyake, “Hydrogen Production from Tofu Wastewater by Rhodobacter sphaerodies Immobilized in Agar Gels,” Int. J. Hydrogen Energy, 24, 305 (1999). (Manuscript received Dec. 17, 2002, and accepted Jul. 4, 2003)
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ᅶ 東吳大學微生物學系
ᄔ! ! ा 本研究顯示以 Wang et al. (2003a) 分離出之雙酶梭菌可以串聯厭氧發酵方式由廢水污泥中產生氫氣及甲烷。本文採用 三種常用之前處理技術增加氫氣產量，接下來再在醱酵液中加入甲烷產生菌以產生甲烷。產氫後之醱酵液可較原始污泥產 生更多甲烷，含氮化合物之還原反應則會與產氫機制競爭而產生氨氮。
Short communication
Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation Chi-Chung Wang [1], Chih-Wen Chang [2], Ching-Ping Chu [3], Duu-Jong Lee [4] Department of Chemical Engineering, National Taiwan University Taipei, Taiwan 106, R.O.C.
Bea-Van Chang [5] Department of Microbiology, Soochow University Taipei, Taiwan 111, R.O.C.
Abstract―This study demonstrates the feasibility of sequentially producing both hydrogen and methane from wastewater sludge using a clostridium strain, as was isolated by Wang et al. (2003a). Three commonly used pre-treatments were applied to wastewater sludge to increase the hydrogen yield. Then, the waste liquor was externally dosed with methanogenic bacteria to produce methane. The waste liquor after fermentation of hydrogen produced more methane than was directly derived without fermentation of hydrogen. The reduction of nitrogen-containing organic matter is shown to compete with the formation of hydrogen, yielding ammonium nitrogen (NH3-N) in the fermented liquor. Key Words : Anaerobic fermentation, Hydrogen, Methane, Clostridium, Pre-treatment
INTRODUCTION
Hydrogen is a clean source of energy. Bio-conversion of biomass to produce hydrogen has been demonstrated, using the anaerobic fermentation of high-strength wastewater (Bolliger et al., 1985; Liu et al., 1995; Ueno et al., 1996; Zhu et al., 1999), solid waste (Mizuno et al., 2000a; Lay et al., 1999), and some well-defined compounds in water, such as molasses (Tanisho and Ishiwata, 1994), glucose (Kataoka et al., 1997; Lin and Chang, 1999), crystalline cellulose (Lay, 2001), peptone (Bai et al., 2001), and starch (Lay, 2000). Methods for promoting hydrogen production have been reported (Tanisho and Ishiwata, 1995; Tanisho et al., 1998; Mizuno et al., 2000b; Sparling et al., 1997; Liang et al., 2001). In the literature, anaerobic fermentation has yielded around 11 mg-H2/g-dried solids (DS) from glucose solution (Kataoka et al., 1997), and 1.4 mg-H2/g-DS from peptone-containing solution (Liang et al., 2001). Only a few data on the hydrogen yield from waste[1] [2] [3] [4] [5]
王之仲 張繼文 朱敬平 李篤中, To whom all correspondence should be addressed 張碧芬
water sludge have been presented at local conferences, including the data of Huang et al. (2000) and Cheng et al. (2000). Recently, Wang et al. (2003a) conducted the first systematic study of the production of hydrogen from wastewater sludge, and found a rather high hydrogen yield from wastewater sludge using a clostridium strain isolated from the sludge sample. Later, Wang et al. (2003b) claimed that applying a filtrate to the sludge could produce more hydrogen than could be obtained using all of the particles in the sludge. Although these studies successfully established the feasibility of producing hydrogen from wastewater sludge, the hydrogen formed during the first 16-24 h of fermentation was consumed in a later stage. Wang et al. (2003a) blocked the methanogenic pathway using a pre-treatment. However, much of the produced hydrogen was still consumed. The pathway for hydrogen consumption remains unknown, but is of academic and practical interest. This study evaluated the feasibility of sequen-
J. Chin. Inst. Chem. Engrs., Vol. 34, No. 6, 2003
684
tially producing hydrogen and methane from wastewater sludge using a clostridium strain under anaerobic conditions. Based on test data, a combined process, involving two fermenters, is proposed. Additionally, the possible incorporation of nitrogen cycles, which compete with the production of hydrogen during sludge fermentation, is demonstrated.
methane production tests, anaerobe K8 was added to the samples after bio-hydrogen tests were performed. K8 was collected from the bottom sediment at a known site of the Tam-Shui River (near Taipei). This mixed culture had high methane productivity (Chang et al., 1996). Fermentation and tests
MATERIALS AND METHOD The substrate
Waste activated sludge was extracted from a wastewater treatment plant of the Presidental Enterprise Corp., Taiwan, which daily treats 250 tons of food-processing wastewater using primary, secondary and tertiary treatments. The pH of the sludge was about 6.4. The chemical oxygen demand (COD) for the sludge was 9,600 mg/L (TCOD), as determined by directly reading a spectrometer (DR/2000, HACH, U.S.A.). The COD for the filtrate of the sludge sample after it was filtered through a 0.45 µm membrane was called the soluble COD (SCOD), and was 465 mg/L for the original sludge. The elemental composition of the dried samples was C: 34.3%, H: 5.6%, and N: 5.5%, according to an elemental analyzer (Perkin-Elmer 2400 CHN). Three pretreatments were applied to the original sludge to determine their effects on the yield of hydrogen. These pretreatments not only released insoluble organic matter into water to enhance methane production (Lee and Mueller, 2001), but also deactivated the methanogenic bacteria in the sludge to block the pathway of the conversion of hydrogen to methane. These pretreatments are summarized as follows. (1) Acidification: perchloric acid (HClO4) was mixed with the sludge sample for 10 mins to adjust the pH of the suspension to 3. Then, the sample was stored at 4°C for 6 h (Jean et al., 2000). (2) Sterilization: sludge samples were pasteurized at 121°C and 1.2 kgf/cm2 (HUXLEY AUTOCLAVE, HL-360) for 30 min. (3) Freezing and thawing: the sludge was frozen at −17°C for 24 h in a freezer and then thawed for another 12 h in a water bath at 25°C (Hung et al., 1997). The inoculum
Wang et al. (2003a) isolated the inoculum. The strain was selected and identified as Clostridium bifermentans using the polymerase chain reaction (PCR) and 16S DNA sequence analysis. In some
Batch fermentation tests were performed in 125 mL serum bottles. In each bottle, 45 mL of substrate, original or pre-treated, was mixed with 5 mL of seed bacteria suspension and anaerobically incubated at 35°C without stirring or adding any nutrients. The bottles were capped with butyl rubber stoppers and wrapped in aluminum foil to prevent photolysis of the substrate. Gas and liquor samples were collected at 8, 16, 24, 32, 40, 48, 72, and 96 h of fermentation. At each time interval, and for each substrate, the gas compositions of three serum bottles were measured and their average was reported. After the measurements were made, these samples were abandoned to prevent the introduction of any possible error associated with the sampling procedure, such as gas leakage. Tests to determine potential methane production were performed after 96 h of hydrogen fermentation. The anaerobe K8 was added to some serum bottles after the hydrogen fermentation tests were completed. The gas samples were collected at 24 h intervals up to 240 h. A GC-TCD (Shimadzu, GC-8A), equipped with a stainless column packed with Porapack Q (50/80 mesh) at 70°C and a thermal conductivity detector (TCD), was used to measure the hydrogen and methane concentrations in the gas phase. The temperature of both the injector and the detector of the GC was 100°C. Nitrogen served as the carrying gas with a flow rate of 20 mL/min. An integrator (HP3396 Series II) was used to integrate the peak area of the effluent curve, and to measure the gaseous concentrations. Repeated measurements revealed that the hydrogen and methane contents thus determined included maximum relative errors of 15% and 10%, respectively. The hydrogen content in the anaerobic glove box was also measured, and was subtracted from the hydrogen concentrations read in the serum bottles. The concentrations of ammonia-nitrogen (NH3-N) in the supernatant were measured during the fermentation test. The filtrate samples were extracted by centrifugating the sludge at 13,500 rpm for 1 min. The NH3-N concentration of the filtrate was measured spectrophotometrically (425 nm) using a mixture of the filtrate sample with two drops of mineral stabilizer, three drops of polyvinyl alcohol dispersing agent, and 1 mL of Nessler agent in a spectropho-
Chi-Chung Wang, Chih-Wen Chang, Ching-Ping Chu, Duu-Jong Lee, and Bea-Van Chang : Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation
RESULTS AND DISCUSSION Hydrogen production
Fig. 2. Time course of the pH of the suspension.
200
(g/kg-DS)
Figure 1 shows the hydrogen yield for one gram of dried solid (DS). In contrast to the 66 h time lag reported by Cheng et al. (2000), this study found negligible time lag for hydrogen production, perhaps because the inoculum was directly derived from the substrate sludge. The hydrogen concentration in the gas phase yielded an increasing-decreasing curve, with a peak at about 16-24 h (indicated in Fig. 1 by the arrow). A specific quantity of produced hydrogen was, thus, “consumed.” As Fig. 1 reveals, the specific hydrogen yield reached 5 g/kg-DS for the original sludge, much higher than that reported by Huang et al. (2000) (0.16 g-H2/kg-DS). Meanwhile, sterilization increased the specific hydrogen yield to about 21.5 g-H2/kg-DS. The acidification, freezing and thawing treatments enhanced the yield to about 9-10 g/kg-DS. This observation shows that the yield of hydrogen from wastewater sludge is similar to that from glucose solution if the former is subjected to suitable pretreatment. Figure 2 plots the variation in the pH of the solution during fermentation. Clearly, during hydrogen production, the solution’s pH increased with time until the yield peaked at 16-24 h, after which time the pH dropped or did not increase further in the case of the acidified sludge. The formation of hydrogen produced a by-product acidic in nature.
Figure 3 plots the time evolution of the concentration of ammonium nitrogen (NH3-N) in the suspension. In the case of the original, sterilized and freeze/thawed sludges, the NH3-N concentration increases with the amount of hydrogen produced until it reached a peak at about 24 h, after which time it leveled off in the hydrogen consumption phase. The nitrogen-containing compounds were reduced to ammonium nitrogen when hydrogen was formed. The acidified sludge, on the other hand, produced the least NH3-N of all the tests. Hence, although the nitrogen cycles competed with hydrogen formation, they are not preferred in acidic environment.
NH3-N (mg/L)
tometer (DR/2000, HACH, U.S.A.). Standard solutions of NH3-N were used for calibration.
685
Time (h)
Fig. 3. Time course of the ammonium nitrogen concentration during hydrogen and methane producing phases.
Methane production Fig. 1. Time course of the hydrogen yield in the gas phase. Clostridium strain was added at hour 0, and K8 was added at hour 96. Each data point is the average of triplicate tests.
In the hydrogen fermentation tests, the amount of methane produced was negligible (0-96 h as shown in Fig. 4). Hence, the consumption of hydrogen shown in Fig. 1 was not associated with methane
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J. Chin. Inst. Chem. Engrs., Vol. 34, No. 6, 2003 H2 Solid-Liquid Separation Fermenter
filtrate
Raw sludge Pretreatment
CH4 Liquor Waste Liquor
Cake
Fermenter Mixer
Fig. 5. A proposed sequential process that produces hydrogen and methane from wastewater sludge.
CONCLUSION Fig. 4. Time course of the methane yield. The anaerobe K8 was added at hour 96.
production. Since only a small proportion of organic matter was converted into hydrogen, the potential for using the exhausted liquor to produce methane was of interest. Anaerobe K8 was added to the serum bottles after 96 h of hydrogen fermentation. After K8 was added, as Fig. 4 shows, the amount of methane accumulated in the serum bottles increased monotonically with time. The amount of methane produced followed the order freeze/thawed > original > sterilization >> acidification. Although the acidification produced hydrogen at a yield two times that of the original sludge, it produced less methane. After K8 was added, the solution’s pH increased again, corresponding to the formation of methane. The acidified sludge, however, also did not undergo an efficient increase in pH, and so generated least methane.
The clostridium strain used herein could ferment wastewater sludge into hydrogen at a much higher rate than heretofore reported in the literature. Acidification, sterilization and freezing and thawing could increase the hydrogen yield. After hydrogen fermentation, adding a methanogenic culture to the fermented liquor accelerated the production of methane, such that the rate of production was higher than that of unfermented samples. A combined process that can produce hydrogen and methane sequentially has, thus, been proposed. The reduction in the amount of nitrogen-containing matter can reduce the hydrogen yield, the effect of which is less profound in an acidic environment.
ACKNOWLEDGEMENT
Support for this work by the National Science Council, R.O.C., is gratefully appreciated.
Sequential process
The data shown in Fig. 4 reveal that the sludge after hydrogen fermentation was more readily digestable than the unfermented sample. This observation implies that some products of the fermentation test promoted methane production. As Wang et al. (2003b) concluded, fermenting the filtrate of the sludge alone could produce more hydrogen than fermenting all of the sludge (including solids). Hence, Fig. 5 shows a proposed combined process that yields hydrogen and methane sequentially. Restated, the filtrate is first separated from the raw sludge (with or without pretreatment) and is sent to fermenter #1 to produce hydrogen. Then, the waste liquor is mixed with the cake from the solid-liquid separator and is sent to fermenter #2 to produce methane. This sequential arrangement effectively produces hydrogen and methane from wastewater sludge.
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Chi-Chung Wang, Chih-Wen Chang, Ching-Ping Chu, Duu-Jong Lee, and Bea-Van Chang : Sequential Production of Hydrogen and Methane from Wastewawter Sludge Using Anaerobic Fermentation
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аՍᖄჇ਼วሇҗቲНԦݝౢғణϷҘ∔ ФώҶ! ᝧЌ! ԗཡѱ! ׳ᑏϋ 國立台灣大學化學工程學系
ᅶ 東吳大學微生物學系
ᄔ! ! ा 本研究顯示以 Wang et al. (2003a) 分離出之雙酶梭菌可以串聯厭氧發酵方式由廢水污泥中產生氫氣及甲烷。本文採用 三種常用之前處理技術增加氫氣產量，接下來再在醱酵液中加入甲烷產生菌以產生甲烷。產氫後之醱酵液可較原始污泥產 生更多甲烷，含氮化合物之還原反應則會與產氫機制競爭而產生氨氮。
