High-pressure thermophilic electromethanogenic system producing methane at 5 MPa, 55°C

Toward applications of bio-electrochemical systems in industrial processes and extreme environments, electromethanogenesis under high-pressure conditions was examined. Stainless-steel single-chamber reactors specifically designed to examine bio-electrochemical reactions under pressurized conditions were inoculated with thermophilic microorganisms originated from an oilfield formation water. The reactors were incubated at 5 MPa, 55°C in fed-batch operational mode with an applied voltage of 0.7 V. In the first few fed-batch cycles, hydrogen was mainly produced. After the third cycle, however, the reactors produced only methane simultaneously with current generation. The methane-production rate of the reactors showed an applied-voltage dependence and increased from 34.9 to 168.4 mmol m^-2 day^-1 with an increase in the applied voltage from 0.4 to 0.9 V. The efficiency of capturing electrons in the produced methane on average exceeded 70% with the applied voltage of 0.4 V or higher. Cyclic voltammetry further confirmed abilities of the bioelectrodes to catalyze electrochemical reactions at 5 MPa. Performance of the electromethanogenesis system was not altered under lower pressure conditions (1.2 and 2.5 MPa). An exoelectrogenic bacterium affiliated with the genus Thermincola and a methanogen belonging to the genus Methanothermobacter were detected as the dominant species in the bioanode and biocathode microbiotas, respectively. Thus, our results indicated that electromethanogenesis systems could be developed and operated under highly-pressurized conditions, suggesting that applications of the bio-electrochemical system in high-pressure environments (including high-temperature subsurface reservoirs) can be technically feasible.