- Email: [email protected]
Effects of pH conditions on the biological conversion of carbon dioxide to methane in a hollow-fiber membrane biofilm reactor (Hf–MBfR)
Desalination 234 (2008) 409–415
Effects of pH conditions on the biological conversion of carbon dioxide to methane in a hollow-fiber membrane biofilm reactor (Hf–MBfR) Dong-Hun Jua, Jeong-Hoon Shina, Hong-Kyun Leeb, Sung-Ho Kongb, Jong-Il Kimc, Byoung-In Sanga* a
Center for Environmental Technology Research, Korea Institute of Science and Technology, Seoul, Korea Tel. þ82(2)9586751; Fax þ82(2)9586858; email: [email protected] b Department of Chemical Engineering, Hanyang University, Seoul, Korea c Hansol EME Co. Ltd., Sungnam, Korea Received 14 August 2007; accepted revised 28 September 2007
Abstract Carbon dioxide is one of the global warming gases. Utilization of a sustainable energy is one of the effective technologies for the mitigation of CO2 accumulation in the atmosphere. If renewable energy can be used for methane synthesis, H2 is converted to methane by reducing CO2. We investigated the conversion of CO2 to CH4 using a novel hollow-fiber membrane biofilm reactor. We have converted CO2 to CH4 with autotrophic methanogens using CO2 and H2. All the gases were diffused into water through the membrane without bubbles. We have successfully operated the Hf–MBfR for stable methane production from CO2 and H2 under continuous operations for 60–70 days at acidic and neutral pH. The methane ratio of the gas produced depended on the pH condition and reached about 60% at neutral pH and 80–90% at acidic pH. The produced methane contents were 751 mL day1 on average from 20 to 58 days at neutral pH and 135 mL day1 on average from 36 to 43 days, 247 mL day1 on average from 44 to 70 days at acidic pH. At neutral pH, during the operating periods, acetic acid was continuously produced to 4000–7000 mg/L so that produced methane was considered to have been produced by hydrogenotrophic methanogens and acetoclastic methanogens. At acidic pH, during the initial operating periods, pH was maintained to 5.9–6.6 so that acetic acid was produced by acetogens. After adjusting to less than pH 5.5, however, it was decreased. At the same time, the methane contents produced were considered to have increased by acetoclastic methanogens. Keywords: Carbon dioxide; Hydrogen; Methane; Hydrogenotrophic methanogen; Hf–MBfR
*Corresponding author. Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.111
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D.-H. Ju et al. / Desalination 234 (2008) 409–415
1. Introduction One of the major global environmental problems facing mankind is the global warming of the atmosphere. The increasing consumption of fossil fuels since the industrial revolution (i.e., since 1750) has caused a marked increase in the global concentration of carbon dioxide, which now amounts to 360 ppm [1]. In 1990, the intergovernmental panel on climate change (IPCC) issued a statement declaring that more than 60% of CO2 emissions must be decreased to retain a level of long-life greenhouse gases [2]. Mitigation of these greenhouse gas emissions to the atmosphere can be achieved by various control technologies (capture, disposal or chemical recycling, immobilization). There are several ways of immobilizing carbon dioxide: first, production of CO, HCOO, CH3OH, CH4 through reduction of carbon dioxide by sun light; second, production of CO, HCOO and H2 by electrical reduction of water and carbon dioxide using the energy obtained from solar-powered battery; third, production of methanol, formic acid, formamide, and so on through hydrogenation of carbon dioxide by regular or irregular catalyst; and fourth, production of hydrogen and immobilization of carbon dioxide by genetically engineered photosynthetic bacteria and algae. However, in the disposal and digest process of organic waste, CO2 and CH4 are reproduced at the level of 40–50%, 50–60%, while the yield of usable methane gas as energy is relatively low. If the occurrence of CO2 is minimized (10–20%) while the yield of methane is maximized (80–90%), we may expect that the reduction of greenhouse gas and the recovery of energy can be realized in a short time. So in this study, we tried to recover CH4 using autotrophic methanogens or hydrogenotrophic methanogens, converting CO2 and H2 to CH4. 4H2 þ CO2 ! CH4 þ 4H2 O The metabolism described above was achieved through the hollow fiber membrane
reactor (Hf–MBfR) and anaerobic sludge. This type of reactor has mainly been used for nitrification and denitrification [3–6], for removal of organic materials in the wastewater [7], and biodegradation of toluene [8]. This reactor was supplied electron donor and acceptor such as oxygen and hydrogen through the inner membrane of hollow fiber. Supplied gases were used by biofilm formed on the surface of the hollow fiber membrane. Gases are controlled by the pressure of inner membrane and supplied without bubbles [5]. Shin et al. reported that autotrophic denitrifier was supplied hydrogen, electron donor through the lumen of hollow fiber membrane of Hf–MBfR. And under the condition they were obtained high efficiency of denitrification rate. According to this result, we showed the possibility for application of biological conversion using Hf–MBfR [5]. In 1980s, Jee et al. reported about biomethanation of H2 and CO2 by Methanobacterium thermoauto-trophicum in membrane and ceramic bioreactors and a fixed-bed reactor [9,10]. In 1993, there was H2 using the acclimated-methanogen in batch culture [11]. The purpose of this study is to convert CO2 to CH4 with hydrogenotrophic methanogens using CO2 and H2 through the Hf–MBfR and to establish the operating conditions of this reactor.
2. Materials and methods 2.1. Hf–MBfR system Fig. 1 shows the schematic diagram Chemicore Hf–MBfR used in this study. The total volume of this reactor is 330 mL, and the working volume 195 mL. The reactor’s hollow fiber membrane was made from polysulfone, and 1000 membranes were packed in the reactor. Two reactors, CRS1 and CRS2, used in this study were operated at acidic (pH 5) and neutral (pH 7) condition, respectively. The reactors were maintained in anaerobic condition by
D.-H. Ju et al. / Desalination 234 (2008) 409–415 Table 1 Mineral medium composition of the reactor
Recycle line Gas meter Temp. (°C) Effluent Biofilm
pH
ORP Heating circulator
Gas Hollow fiber membrane
Influent Water Feed
411
H2:CO2 (4:1) gas supply
Composition
Concentration (mg L1)
MgCl2 6H2O CaCl2 2H2O ZnCl2 Na2Mo 2H2O MnCl2 4H2O CuCl2 2H2O CoCl2 6H2O KCl FeCl2 2H2O EDTA NaCl (NH4Cl)2HPO4
16.05 1.20 5.91 1.29 13.19 2.61 0.3 1.00 5.23 9.75 200 200
Fig. 1. Schematic diagram of hollow fiber membrane biofilm reactor (Hf–MBfR) system.
2.2. Gas analysis purging mixed gases (H2:CO2 ¼ 4:1) through the hollow fiber membrane, and the inner reactor temperature was mixed gases were used as substrate for biological conversion of carbon dioxide to methane. Supply pressure of mixed gases were 0.2–0.3 psi. Sieved anaerobic digest sludge which was sampled at a wastewater treatment plant was inoculated at each reactor as the level of 20% of the working volume. The mineral medium composition of the reactor is shown in Table 1. The medium of CRS1 reactor was adjusted to pH 5 by using phosphate buffer, and that of CRS2 was adjusted to pH 7 through sodium bicarbonate buffer. The inner reactor was mixed through up-flow by using a recycling pump. ORP, temperature and pH were continuously observed by ORP probe, thermometer and pH meter. A wet gas-meter (Model W-NK-0.5, Shinagawa, Japan) was used to measure the volume of produced gases. Gases taken from the sampling port installed at the gas effluent line were analyzed by GC-TCD. VFAs analysis was performed with pretreated samples by GC-FID.
Gas chromatograph (Agilent Technology, 6890N) with Porapak Q packed column (80–100 mesh 6 ft 1/8"S stainless steel) and TCD was used to analyze the quality of the gas produced from the reactor. The GC-TCD was operated at 100 C injection temperature, 200 C detector temperature and Ar carrier gas (25 mL min1). We obtained the standard curves on each gas (H2, CO2, CH4) for quantitative analysis.
2.3. Volatile fatty acids analysis To analyze the VFAs produced from the reactor, Gas chromatograph (Agilent Technology, 6890N) with HP-INOWAX capillary column (30 m 0.25 mm 0.25 mm) and FID was used with pretreated samples. The operating conditions of GC-FID were 250 C injection temperature, 50–170 C (10 C : min1) oven temperature, 250 C detector temperature and He carrier gas (1 mL min1). Samples for VFAs analysis were taken from the recycle line of each reactor. These samples were centrifuged at 14,000 rpm for 4 min, and then 1 mL of supernatant was mixed with 0.5 mL of 10% phosphoric acid.
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3. Results and discussion
100
3.1. CRS1 Hf–MBfR performance CRS1 reactor was operated at pH 4.2–5.5, and the supply pressure of mixed gases were 0.2–0.3 psi as shown Fig. 2. Temperature of the inner reactor was 37 C on the average, and the ORP was about 430 mV. This ORP value indicated that the reactor maintained the anaerobic condition well for the operating periods. The liquid in the reactor was continuously mixed by up-flow recycling at 84 mL min1 (Fig. 3). As a result of these conditions, CRS1 reactor started to produce methane after 2 days of inoculation of anaerobic digest sludge. After 7 days, the methane ratio of effluent gases reached 80–90%, 10
3.5
8
2.5 2.0
6
1.5 1.0
pH
4
0.5 2
0.0 0
10
20 30 40 50 Operation time (days)
60
70
Fig. 2. Variations of pressure at hollow fiber innermembrane and pH at CRS1 Hf–MBfR; ~, pressure; , pH.
40
0
0
10
20
30 40 50 Operation time (days)
60
Fig. 4. Rates of produced gases at CRS1 Hf–MBfR; CH4; , H2; , CO2.
70
,
and this ratio held the line for 65 days (Fig. 4). The volume of methane produced from 10 to 20 days was much more than that of the other periods. For this period, acetic acid was produced as shown in the VFAs analysis (Fig. 5). This acetic acid was probably produced by acetogens while the reactor operated at pH 5.5–6.8. The acetic acid produced under this condition was thought to have been used by acetoclastic methanogen, converting acetic acid to methane. So in this period, it means that acetoclastic methanogen coworked with the hydrogenotrophic methanogen, which converts CO2 and H2 to CH4. Acetogens must have been inhibited because the pH of the reactor was kept at the acidic condition after 20 days. Thus the acetic acid was not produced anymore from 37 to 65 days. Although the volume of the produced methane decreased a
40
0
35
−200
30
−400
25
−600 70
0
10
20 30 40 50 Operation time (days)
60
Recycle speed & ORP (mV)
200
45
Temp. (˚C)
60
20
Fig. 3. Variations of ORP, temperature and recycle , recycle speed; ^, speed at CRS1 Hf–MBfR; temperature; !, ORP.
600 Volume of gases (mL)
Pressure (psi)
3.0
Rates of gases (%)
80
500 400 300 200 100 0 0
10
20
30 40 50 Operation time (days)
60
70
Fig. 5. Volume of produced gases at CRS1 Hf–MBfR; , CH4; !, H2; c, CO2.
2000 1500 1000
200
40
0
35 −200 30
500 0
−400 0
10
20
30 40 50 Operation time (days)
60
70
Fig. 6. Variation of acetic acid at CRS1 Hf–MBfR.
little after 20 days, it gradually increased with time. And at this period, the rate of the methane was also 80–90%, and its volume reached 220 mL day1 on average (Fig. 6). This fact indirectly indicates that the microbial metabolism of the reactor changed the acetoclastic and hydrogenotrophic methanogenesis into hydrogenotrophic methanogenesis under acidic condition. 3.2. CRS2 Hf–MBf R performance CRS2 reactor was operated at neutral condition, pH 6.5–7.5, and the supply pressure of mixed gases was 0.2–0.3 psi as shown in Fig. 7. Temperature of the reactor was at 37 C on average, and anaerobic condition was consistently maintained for the operating periods as ORP value was about 390 mV. The working liquid of the reactor was continuously mixed by up-flow 3.5
25
0
20
40 Operation time (days)
60
Fig. 8. Variations of ORP, temperature and recycle , recycle speed; ^, speed at CRS2 Hf–MBfR; temperature; !, ORP.
recycling as it was done at the CRS1 reactor (Fig. 8). As described earlier, the operating conditions of the CRS1 and CRS2 reactors were almost the same except the pH condition. Under these conditions, CRS2 reactor started to produce methane gas and to decrease mixed gases from 4 days after anaerobic sludge inoculation. The rate of methane among the effluent gases reached 60%, and it increased to 80% with time (Fig. 9). The volume of produced methane was 770 mL day1 on average, and acetic acid was continuously produced from 3000 to 7000 ppm for the operating period (Figs. 10 and 11). It is thought that acetic acid producing bacteria, acetogens, were not inhibited at CRS2 reactor because of the neutral pH and acetic acid produced by acetogen was used by acetoclastic methanogen. CRS2 reactor was
10 100
3.0
9
2.5 8
1.5
7
1.0 6
0.5
Rates of gases (%)
80
2.0
pH
Pressure (psi)
45
2500 Temp. (ºC)
Conc. of VFAs (ppm)
3000
413
Recycle speed & ORP (mV)
D.-H. Ju et al. / Desalination 234 (2008) 409–415
60 40 20
0.0
0
20
40 Operation time (days)
60
5
Fig. 7. Variations of pressure at hollow fiber innermembrane and pH at CRS2 Hf–MBfR; ~, pressure; , pH.
0
0
20
40 Operation time (days)
60
Fig. 9. Rates of produced gases at CRS2 Hf-MBfR; CH4; , H2; , CO2.
,
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D.-H. Ju et al. / Desalination 234 (2008) 409–415
1800 Volume of gases (mL)
1600 1400 1200 1000 800 600 400 200 0
0
20
40 Operation time (days)
60
Fig. 10. Volume of produced gases at CRS2 Hf–MBfR; , CH4; !, H2; c, CO2.
Conc. of VFAs (ppm)
8000
high conversion rate (90%) from the initial period. Hydrogenotrophic methanogenesis seems to have occurred in the CRS1 reactor as acetic acid was not produced. CRS2 reactor in the neutral condition became slowly stable and gradually increased the conversion rate from 60% to 85%. We think that hydrogenotrophic and acetoclastic methanogenesis coexisted in CRS2 reactor because acetic acid was continuously produced for the operating period. In the further study, we will investigate the effect of high supply pressure of mixed gases and the performance of scale-up pilot plant.
6000
Acknowledgement 4000
This study was supported by Korea Energy Management Corporation.
2000
0
0
20
40 Operation time (days)
60
Fig. 11. Variation of acetic acid at CRS2 Hf–MBfR.
3.5 times greater than CRS1 reactor in terms of the volume of the methane produced. These findings led us to conclude that at CRS2 reactor, aectoclastic and hydrogenotrophic methanogen produced methane together as in the initial period of CRS1 reactor. Since perchlorate reduction is sensitive to the hydrogen pressure [12], it may be necessary to check the effect on the high pressure of the mixed gases at this reactor.
4. Conclusion The purpose of this experiment was to convert CO2 to CH4 with hydrogenotrophic methanogens using CO2 and H2 through the Hf–MBfR. CRS1 and CRS2 reactors were operated at acidic and neutral pH condition. As a result of operation in this condition, CRS1 reactor in the acidic condition became quickly stable and showed a
References [1]
[2] [3]
[4]
[5]
[6]
W. Fricke and M. Wallasch, Atmospheric CO2 records from sites in the UBA air sampling network. In: T.A. Boden, D.P. Kaiser, R.J. Sepanski and F.W. Stoss (Eds.), Trends 93: a compendium of data on global change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN, USA, 1993, pp. 135–144. In our hands, Earth Summit 1992, Asahi Hhinbun, 139. K. Brindle and T. Stephenson, Nitrification in a bubbleless oxygen mass transfer membrane bioreactor, Water Sci. Technol., 34 (9) (1996) 261–267. K.C. Lee and B.E. Rittmann, Applying a novel autohydrogenotrophic hollow-fiber membrane biofilm reactor for denitrification of drinking water, Water Res., 36 (8) (2002) 2040–2052. J.H. Shin, B.I. Sang, Y.C. Chung and Y.K. Choung, The removal of nitrogen using an autotrophic hybrid hollow-fiber membrane biofilm reactor, Desalination, 183 (1–3) (2005) 447–454. A. Terada, S. Kaku, S. Matsumoto and S. Tsuneda, Rapid autohydrogenotrophic denitrification by a
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membrane biofilm reactor equipped with a fibrous support around a gas-permeable membrane, Biochem. Eng. J., 31 (1) (2006) 84–91. M. Pankhania, K. Brindle and T. Stephenson, Membrane aeration bioreactors for wastewater treatment: completely mixed and plug-flow operation, Chem. Eng. J., 73 (2) (1999) 131–136. M. Parvatiyar, G. Govind, R. Bishop and F. Dolloff, Biodegradation of toluene in a membrane biofilter, J. Membr. Sci., 119 (1–2) (1996) 17–24. H.S. Jee, T. Yano, N. Nishio and S. Nagai, Biomethanation of H2 and CO2 by Methanobacterium thermoautotrophicum in membrane and ceramic bioreactors, J. Ferment. Technol., 65 (4) (1987) 413–418. H.S. Jee, T. Yano, N. Nishio and S. Nagai, Continuous CH4 production from H2 and CO2 by
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methanobacterium thermo-autotrophicum in a fixed-bed reactor, J. Ferment. Technol., 66 (2) (1988) 235–238. [11] J.H. Shin, B.I. Sang, Y.C. Chung and Y.K. Choung, The removal of nitrogen using an autotrophic hybrid hollow-fiber membrane biofilm reactor, Desalination, 183 (1–3) (2005) 447–454. [12] Z.Y. Zhang and T. Maekawa, Kinetic study on fermentation from CO2 and H2 using the acclimated-methanogen in batch culture, Biomass Bioenergy, 4 (6) (1993) 439–446. [13] K.C. Lee, B.E. Rittmann, R. Nerenberg, T.E. Gillogly, G.E. Lehman, S.S. Adham and I. Najm, Hydrogen-based hollow-fiber membrane biofilm reactor (MBf R) for removing oxidized contaminants, Water Sci. Technol. Water Supply, 4 (1) (2004) 127–133.
Effects of pH conditions on the biological conversion of carbon dioxide to methane in a hollow-fiber membrane biofilm reactor (Hf–MBfR) Dong-Hun Jua, Jeong-Hoon Shina, Hong-Kyun Leeb, Sung-Ho Kongb, Jong-Il Kimc, Byoung-In Sanga* a
Center for Environmental Technology Research, Korea Institute of Science and Technology, Seoul, Korea Tel. þ82(2)9586751; Fax þ82(2)9586858; email: [email protected] b Department of Chemical Engineering, Hanyang University, Seoul, Korea c Hansol EME Co. Ltd., Sungnam, Korea Received 14 August 2007; accepted revised 28 September 2007
Abstract Carbon dioxide is one of the global warming gases. Utilization of a sustainable energy is one of the effective technologies for the mitigation of CO2 accumulation in the atmosphere. If renewable energy can be used for methane synthesis, H2 is converted to methane by reducing CO2. We investigated the conversion of CO2 to CH4 using a novel hollow-fiber membrane biofilm reactor. We have converted CO2 to CH4 with autotrophic methanogens using CO2 and H2. All the gases were diffused into water through the membrane without bubbles. We have successfully operated the Hf–MBfR for stable methane production from CO2 and H2 under continuous operations for 60–70 days at acidic and neutral pH. The methane ratio of the gas produced depended on the pH condition and reached about 60% at neutral pH and 80–90% at acidic pH. The produced methane contents were 751 mL day1 on average from 20 to 58 days at neutral pH and 135 mL day1 on average from 36 to 43 days, 247 mL day1 on average from 44 to 70 days at acidic pH. At neutral pH, during the operating periods, acetic acid was continuously produced to 4000–7000 mg/L so that produced methane was considered to have been produced by hydrogenotrophic methanogens and acetoclastic methanogens. At acidic pH, during the initial operating periods, pH was maintained to 5.9–6.6 so that acetic acid was produced by acetogens. After adjusting to less than pH 5.5, however, it was decreased. At the same time, the methane contents produced were considered to have increased by acetoclastic methanogens. Keywords: Carbon dioxide; Hydrogen; Methane; Hydrogenotrophic methanogen; Hf–MBfR
*Corresponding author. Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.111
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D.-H. Ju et al. / Desalination 234 (2008) 409–415
1. Introduction One of the major global environmental problems facing mankind is the global warming of the atmosphere. The increasing consumption of fossil fuels since the industrial revolution (i.e., since 1750) has caused a marked increase in the global concentration of carbon dioxide, which now amounts to 360 ppm [1]. In 1990, the intergovernmental panel on climate change (IPCC) issued a statement declaring that more than 60% of CO2 emissions must be decreased to retain a level of long-life greenhouse gases [2]. Mitigation of these greenhouse gas emissions to the atmosphere can be achieved by various control technologies (capture, disposal or chemical recycling, immobilization). There are several ways of immobilizing carbon dioxide: first, production of CO, HCOO, CH3OH, CH4 through reduction of carbon dioxide by sun light; second, production of CO, HCOO and H2 by electrical reduction of water and carbon dioxide using the energy obtained from solar-powered battery; third, production of methanol, formic acid, formamide, and so on through hydrogenation of carbon dioxide by regular or irregular catalyst; and fourth, production of hydrogen and immobilization of carbon dioxide by genetically engineered photosynthetic bacteria and algae. However, in the disposal and digest process of organic waste, CO2 and CH4 are reproduced at the level of 40–50%, 50–60%, while the yield of usable methane gas as energy is relatively low. If the occurrence of CO2 is minimized (10–20%) while the yield of methane is maximized (80–90%), we may expect that the reduction of greenhouse gas and the recovery of energy can be realized in a short time. So in this study, we tried to recover CH4 using autotrophic methanogens or hydrogenotrophic methanogens, converting CO2 and H2 to CH4. 4H2 þ CO2 ! CH4 þ 4H2 O The metabolism described above was achieved through the hollow fiber membrane
reactor (Hf–MBfR) and anaerobic sludge. This type of reactor has mainly been used for nitrification and denitrification [3–6], for removal of organic materials in the wastewater [7], and biodegradation of toluene [8]. This reactor was supplied electron donor and acceptor such as oxygen and hydrogen through the inner membrane of hollow fiber. Supplied gases were used by biofilm formed on the surface of the hollow fiber membrane. Gases are controlled by the pressure of inner membrane and supplied without bubbles [5]. Shin et al. reported that autotrophic denitrifier was supplied hydrogen, electron donor through the lumen of hollow fiber membrane of Hf–MBfR. And under the condition they were obtained high efficiency of denitrification rate. According to this result, we showed the possibility for application of biological conversion using Hf–MBfR [5]. In 1980s, Jee et al. reported about biomethanation of H2 and CO2 by Methanobacterium thermoauto-trophicum in membrane and ceramic bioreactors and a fixed-bed reactor [9,10]. In 1993, there was H2 using the acclimated-methanogen in batch culture [11]. The purpose of this study is to convert CO2 to CH4 with hydrogenotrophic methanogens using CO2 and H2 through the Hf–MBfR and to establish the operating conditions of this reactor.
2. Materials and methods 2.1. Hf–MBfR system Fig. 1 shows the schematic diagram Chemicore Hf–MBfR used in this study. The total volume of this reactor is 330 mL, and the working volume 195 mL. The reactor’s hollow fiber membrane was made from polysulfone, and 1000 membranes were packed in the reactor. Two reactors, CRS1 and CRS2, used in this study were operated at acidic (pH 5) and neutral (pH 7) condition, respectively. The reactors were maintained in anaerobic condition by
D.-H. Ju et al. / Desalination 234 (2008) 409–415 Table 1 Mineral medium composition of the reactor
Recycle line Gas meter Temp. (°C) Effluent Biofilm
pH
ORP Heating circulator
Gas Hollow fiber membrane
Influent Water Feed
411
H2:CO2 (4:1) gas supply
Composition
Concentration (mg L1)
MgCl2 6H2O CaCl2 2H2O ZnCl2 Na2Mo 2H2O MnCl2 4H2O CuCl2 2H2O CoCl2 6H2O KCl FeCl2 2H2O EDTA NaCl (NH4Cl)2HPO4
16.05 1.20 5.91 1.29 13.19 2.61 0.3 1.00 5.23 9.75 200 200
Fig. 1. Schematic diagram of hollow fiber membrane biofilm reactor (Hf–MBfR) system.
2.2. Gas analysis purging mixed gases (H2:CO2 ¼ 4:1) through the hollow fiber membrane, and the inner reactor temperature was mixed gases were used as substrate for biological conversion of carbon dioxide to methane. Supply pressure of mixed gases were 0.2–0.3 psi. Sieved anaerobic digest sludge which was sampled at a wastewater treatment plant was inoculated at each reactor as the level of 20% of the working volume. The mineral medium composition of the reactor is shown in Table 1. The medium of CRS1 reactor was adjusted to pH 5 by using phosphate buffer, and that of CRS2 was adjusted to pH 7 through sodium bicarbonate buffer. The inner reactor was mixed through up-flow by using a recycling pump. ORP, temperature and pH were continuously observed by ORP probe, thermometer and pH meter. A wet gas-meter (Model W-NK-0.5, Shinagawa, Japan) was used to measure the volume of produced gases. Gases taken from the sampling port installed at the gas effluent line were analyzed by GC-TCD. VFAs analysis was performed with pretreated samples by GC-FID.
Gas chromatograph (Agilent Technology, 6890N) with Porapak Q packed column (80–100 mesh 6 ft 1/8"S stainless steel) and TCD was used to analyze the quality of the gas produced from the reactor. The GC-TCD was operated at 100 C injection temperature, 200 C detector temperature and Ar carrier gas (25 mL min1). We obtained the standard curves on each gas (H2, CO2, CH4) for quantitative analysis.
2.3. Volatile fatty acids analysis To analyze the VFAs produced from the reactor, Gas chromatograph (Agilent Technology, 6890N) with HP-INOWAX capillary column (30 m 0.25 mm 0.25 mm) and FID was used with pretreated samples. The operating conditions of GC-FID were 250 C injection temperature, 50–170 C (10 C : min1) oven temperature, 250 C detector temperature and He carrier gas (1 mL min1). Samples for VFAs analysis were taken from the recycle line of each reactor. These samples were centrifuged at 14,000 rpm for 4 min, and then 1 mL of supernatant was mixed with 0.5 mL of 10% phosphoric acid.
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3. Results and discussion
100
3.1. CRS1 Hf–MBfR performance CRS1 reactor was operated at pH 4.2–5.5, and the supply pressure of mixed gases were 0.2–0.3 psi as shown Fig. 2. Temperature of the inner reactor was 37 C on the average, and the ORP was about 430 mV. This ORP value indicated that the reactor maintained the anaerobic condition well for the operating periods. The liquid in the reactor was continuously mixed by up-flow recycling at 84 mL min1 (Fig. 3). As a result of these conditions, CRS1 reactor started to produce methane after 2 days of inoculation of anaerobic digest sludge. After 7 days, the methane ratio of effluent gases reached 80–90%, 10
3.5
8
2.5 2.0
6
1.5 1.0
pH
4
0.5 2
0.0 0
10
20 30 40 50 Operation time (days)
60
70
Fig. 2. Variations of pressure at hollow fiber innermembrane and pH at CRS1 Hf–MBfR; ~, pressure; , pH.
40
0
0
10
20
30 40 50 Operation time (days)
60
Fig. 4. Rates of produced gases at CRS1 Hf–MBfR; CH4; , H2; , CO2.
70
,
and this ratio held the line for 65 days (Fig. 4). The volume of methane produced from 10 to 20 days was much more than that of the other periods. For this period, acetic acid was produced as shown in the VFAs analysis (Fig. 5). This acetic acid was probably produced by acetogens while the reactor operated at pH 5.5–6.8. The acetic acid produced under this condition was thought to have been used by acetoclastic methanogen, converting acetic acid to methane. So in this period, it means that acetoclastic methanogen coworked with the hydrogenotrophic methanogen, which converts CO2 and H2 to CH4. Acetogens must have been inhibited because the pH of the reactor was kept at the acidic condition after 20 days. Thus the acetic acid was not produced anymore from 37 to 65 days. Although the volume of the produced methane decreased a
40
0
35
−200
30
−400
25
−600 70
0
10
20 30 40 50 Operation time (days)
60
Recycle speed & ORP (mV)
200
45
Temp. (˚C)
60
20
Fig. 3. Variations of ORP, temperature and recycle , recycle speed; ^, speed at CRS1 Hf–MBfR; temperature; !, ORP.
600 Volume of gases (mL)
Pressure (psi)
3.0
Rates of gases (%)
80
500 400 300 200 100 0 0
10
20
30 40 50 Operation time (days)
60
70
Fig. 5. Volume of produced gases at CRS1 Hf–MBfR; , CH4; !, H2; c, CO2.
2000 1500 1000
200
40
0
35 −200 30
500 0
−400 0
10
20
30 40 50 Operation time (days)
60
70
Fig. 6. Variation of acetic acid at CRS1 Hf–MBfR.
little after 20 days, it gradually increased with time. And at this period, the rate of the methane was also 80–90%, and its volume reached 220 mL day1 on average (Fig. 6). This fact indirectly indicates that the microbial metabolism of the reactor changed the acetoclastic and hydrogenotrophic methanogenesis into hydrogenotrophic methanogenesis under acidic condition. 3.2. CRS2 Hf–MBf R performance CRS2 reactor was operated at neutral condition, pH 6.5–7.5, and the supply pressure of mixed gases was 0.2–0.3 psi as shown in Fig. 7. Temperature of the reactor was at 37 C on average, and anaerobic condition was consistently maintained for the operating periods as ORP value was about 390 mV. The working liquid of the reactor was continuously mixed by up-flow 3.5
25
0
20
40 Operation time (days)
60
Fig. 8. Variations of ORP, temperature and recycle , recycle speed; ^, speed at CRS2 Hf–MBfR; temperature; !, ORP.
recycling as it was done at the CRS1 reactor (Fig. 8). As described earlier, the operating conditions of the CRS1 and CRS2 reactors were almost the same except the pH condition. Under these conditions, CRS2 reactor started to produce methane gas and to decrease mixed gases from 4 days after anaerobic sludge inoculation. The rate of methane among the effluent gases reached 60%, and it increased to 80% with time (Fig. 9). The volume of produced methane was 770 mL day1 on average, and acetic acid was continuously produced from 3000 to 7000 ppm for the operating period (Figs. 10 and 11). It is thought that acetic acid producing bacteria, acetogens, were not inhibited at CRS2 reactor because of the neutral pH and acetic acid produced by acetogen was used by acetoclastic methanogen. CRS2 reactor was
10 100
3.0
9
2.5 8
1.5
7
1.0 6
0.5
Rates of gases (%)
80
2.0
pH
Pressure (psi)
45
2500 Temp. (ºC)
Conc. of VFAs (ppm)
3000
413
Recycle speed & ORP (mV)
D.-H. Ju et al. / Desalination 234 (2008) 409–415
60 40 20
0.0
0
20
40 Operation time (days)
60
5
Fig. 7. Variations of pressure at hollow fiber innermembrane and pH at CRS2 Hf–MBfR; ~, pressure; , pH.
0
0
20
40 Operation time (days)
60
Fig. 9. Rates of produced gases at CRS2 Hf-MBfR; CH4; , H2; , CO2.
,
414
D.-H. Ju et al. / Desalination 234 (2008) 409–415
1800 Volume of gases (mL)
1600 1400 1200 1000 800 600 400 200 0
0
20
40 Operation time (days)
60
Fig. 10. Volume of produced gases at CRS2 Hf–MBfR; , CH4; !, H2; c, CO2.
Conc. of VFAs (ppm)
8000
high conversion rate (90%) from the initial period. Hydrogenotrophic methanogenesis seems to have occurred in the CRS1 reactor as acetic acid was not produced. CRS2 reactor in the neutral condition became slowly stable and gradually increased the conversion rate from 60% to 85%. We think that hydrogenotrophic and acetoclastic methanogenesis coexisted in CRS2 reactor because acetic acid was continuously produced for the operating period. In the further study, we will investigate the effect of high supply pressure of mixed gases and the performance of scale-up pilot plant.
6000
Acknowledgement 4000
This study was supported by Korea Energy Management Corporation.
2000
0
0
20
40 Operation time (days)
60
Fig. 11. Variation of acetic acid at CRS2 Hf–MBfR.
3.5 times greater than CRS1 reactor in terms of the volume of the methane produced. These findings led us to conclude that at CRS2 reactor, aectoclastic and hydrogenotrophic methanogen produced methane together as in the initial period of CRS1 reactor. Since perchlorate reduction is sensitive to the hydrogen pressure [12], it may be necessary to check the effect on the high pressure of the mixed gases at this reactor.
4. Conclusion The purpose of this experiment was to convert CO2 to CH4 with hydrogenotrophic methanogens using CO2 and H2 through the Hf–MBfR. CRS1 and CRS2 reactors were operated at acidic and neutral pH condition. As a result of operation in this condition, CRS1 reactor in the acidic condition became quickly stable and showed a
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