Effects of pH conditions on the biological conversion of carbon dioxide to methane in a hollow-fiber membrane biofilm reactor (Hf–MBfR)

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 dayÀ1 on average from 20 to 58 days at neutral pH and 135 mL dayÀ1 on average from 36 to 43 days, 247 mL dayÀ1 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.

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 high conversion rate (90%) from the initial period. Hydrogenotrophic ethanogenesis 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.