**4. Enrichment of hydrogenotrophic methanogens**

A specially designed electrochemical bioreactor is composed of the combined anode (Fig 4) and NR-graphite cathode for enrichment of the hydrogenotrophic methanogens as shown in Fig 6. Oxygen-free and carbonate-saturated wastewater was supplied continuously from a wastewater reservoir as shown in Fig 7. The electrochemical bioreactor was operated with the electricity generated from the solar panel. The wastewater obtained from sewage treatment plant was used without sterilization, to which 50 mM of sodium bicarbonate was added. The contaminated oxygen was consumed by bacteria growing intrinsically in the wastewater reservoir. Hydrogenotrophic methanogens grow with the free energy and reducing power generated by the coupling redox reaction of carbon dioxide and hydrogen (Ferguson and Mah, 1983; Na et al., 2007; Zeikus and Wolfe, 1972). Hydrogen generated from the electrolysis of water can't function to maintain the proper oxidation reduction

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 195

produce actively carbon dioxide and methane, respectively, using the electrochemical reducing power generated from the solar panel (Cheng et al., 2011). Practically, the methane production and carbon dioxide consumption were significantly increased in the

Fig. 7. Schematic structure of an electrochemical bioreactor for continuous culture of hydrogenotrophic methanogens. The wastewater saturated with carbon dioxide is supplied continuously to the electrochemical bioreactor and headspace of wastewater reservoir was

Bacteriological conversion of carbon dioxide to methane using the electrochemical reducing power may be a technique for fixation of carbon dioxide without combustion of fossil fuel; however, may not be a way for long term storage of carbon. Cell structures of bacteria are composed of peptidoglycan, phospholipid, proteins, nucleic acids, and carbohydrates that are biochemically stable polymers (Caldwell, 1995). Bacterial cells themselves can be the carbon storage by freezing or drying without the specific engineering process. Hydrogenotrophic methanogens may not be proper carbon storage because they consume the reducing power and free energy ineffectively to maintain the lower oxidation-reduction potential than -300 mV, grow more slowly than other autotrophic bacteria, and produce the

Facultative anaerobic mixotrophs, on the other hand, not only grow heterotrophically in the condition with organic carbons but also grow autotrophically in the condition with electron donors and carbon dioxide (Johson, 1998; Morikawa and Imanaka, 1993). The metabolism and physiological function of the facultative anaerobic mixotrophs are not influenced by oxygen. These are useful character of the facultative anaerobic mixotrophs to cultivate with

refilled continuously with pure carbon dioxide without oxygen contamination.

electrochemical bioreactor as shown in Fig 8.

unstable metabolite (methane).

potential for methanogenic bacteria in the electrochemical bioreactor owing to the microsolubility. The micro-pore formed by the fiber matrices of NR-graphite cathode may be proper micro-environment for the growth of hydrogenotrophic methanogens because hydrogen generated from NR-graphite cathode may be captured in the micro-pores and the lower oxidation-reduction potential than -300 mV (vs. NHE) may be maintained by the electrochemical reducing power.

Fig. 6. Schematic structure of an electrochemical bioreactor, in which the anode compartment was replaced with the combined anode composed of cellulose acetate film, porous ceramic membrane and porous carbon plate. Water is electrolyzed in the porous carbon plate and separated into proton, electron, and oxygen.

Methyl compounds, hydrogen, low molecular weight fatty acids, hydrogen, and carbon dioxide are produced by various fermentation bacteria in the anaerobic digestive sludge. The methanogens grow syntrophically in the bioreactor cultivating anaerobic digestive sludge, which is composed of various organic compounds and anaerobic bacterial community (Stams et al., 2009; Katsuyam et al., 2009). When the anaerobic digestive sludge was applied to the electrochemical bioreactor (Fig 6 and 7), the hydrogenotrophic methanogens that are *Methanobacterium* sp*.*, *Methanolinea* sp., and *Methnoculleus* sp*.* were enriched predominantly (Jeon et al., 2009B). The predominated hydrogenotrophic methanogens consumed and

potential for methanogenic bacteria in the electrochemical bioreactor owing to the microsolubility. The micro-pore formed by the fiber matrices of NR-graphite cathode may be proper micro-environment for the growth of hydrogenotrophic methanogens because hydrogen generated from NR-graphite cathode may be captured in the micro-pores and the lower oxidation-reduction potential than -300 mV (vs. NHE) may be maintained by the

Fig. 6. Schematic structure of an electrochemical bioreactor, in which the anode

carbon plate and separated into proton, electron, and oxygen.

compartment was replaced with the combined anode composed of cellulose acetate film, porous ceramic membrane and porous carbon plate. Water is electrolyzed in the porous

Methyl compounds, hydrogen, low molecular weight fatty acids, hydrogen, and carbon dioxide are produced by various fermentation bacteria in the anaerobic digestive sludge. The methanogens grow syntrophically in the bioreactor cultivating anaerobic digestive sludge, which is composed of various organic compounds and anaerobic bacterial community (Stams et al., 2009; Katsuyam et al., 2009). When the anaerobic digestive sludge was applied to the electrochemical bioreactor (Fig 6 and 7), the hydrogenotrophic methanogens that are *Methanobacterium* sp*.*, *Methanolinea* sp., and *Methnoculleus* sp*.* were enriched predominantly (Jeon et al., 2009B). The predominated hydrogenotrophic methanogens consumed and

electrochemical reducing power.

produce actively carbon dioxide and methane, respectively, using the electrochemical reducing power generated from the solar panel (Cheng et al., 2011). Practically, the methane production and carbon dioxide consumption were significantly increased in the electrochemical bioreactor as shown in Fig 8.

Fig. 7. Schematic structure of an electrochemical bioreactor for continuous culture of hydrogenotrophic methanogens. The wastewater saturated with carbon dioxide is supplied continuously to the electrochemical bioreactor and headspace of wastewater reservoir was refilled continuously with pure carbon dioxide without oxygen contamination.

Bacteriological conversion of carbon dioxide to methane using the electrochemical reducing power may be a technique for fixation of carbon dioxide without combustion of fossil fuel; however, may not be a way for long term storage of carbon. Cell structures of bacteria are composed of peptidoglycan, phospholipid, proteins, nucleic acids, and carbohydrates that are biochemically stable polymers (Caldwell, 1995). Bacterial cells themselves can be the carbon storage by freezing or drying without the specific engineering process. Hydrogenotrophic methanogens may not be proper carbon storage because they consume the reducing power and free energy ineffectively to maintain the lower oxidation-reduction potential than -300 mV, grow more slowly than other autotrophic bacteria, and produce the unstable metabolite (methane).

Facultative anaerobic mixotrophs, on the other hand, not only grow heterotrophically in the condition with organic carbons but also grow autotrophically in the condition with electron donors and carbon dioxide (Johson, 1998; Morikawa and Imanaka, 1993). The metabolism and physiological function of the facultative anaerobic mixotrophs are not influenced by oxygen. These are useful character of the facultative anaerobic mixotrophs to cultivate with

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 197

transfer and supply to any electronic device. The electrochemical bioreactor is also the simplest electronic device to convert electric energy to biochemical reducing power. The wastewater and exhausted gas can be used directly without purification or separation as the nutrient source for bacterial metabolism. Experimentally, the electricity generated from the 25 cm2 of the solar panel is very enough for operation of the 10 L of electrochemical

Fig. 9. Schematic diagram of the cylinder-type electrochemical bioreactor equipped with a built-in anode compartment for the cultivation of CO2-fixing bacteria. The glass filter septum equipped at the bottom end of the anode compartment functions as redox separator between anode and cathode compartment and micropore for transfer of catholyte to anode

During enrichment of the carbon dioxide-fixing bacteria using the cylinder-type electrochemical bioreactor, bacterial community was changed significantly as show in Fig 10. Some of bacteria community was increased or enriched as shown in the box A and C but decreased or died out as shown in the box B and D. These phenomena are a clue that the

bioreactor.

compartment.

wastewater containing other reduced organic and inorganic compounds and exhaust containing carbon dioxide, and the electrochemical reducing power as the electron donor (Skirnisdottir et al., 2001).

Fig. 8. Carbon dioxide consumption () and methane production () by anaerobic digestive sludge cultivated in conventional bioreactor (reactors A and B) and electrochemical bioreactor (reactors C and D). Duplicate reactors were operated to enhance the comparability between the conventional bioreactor and the electrochemical bioreactor.
