A Single-Culture Bioprocess of Methanothermobacter thermautotrophicus to Upgrade Digester Biogas by CO2-to-CH4 Conversion with H-2

We optimized and tested a postbioprocessing step with a single-culture archaeon to upgrade biogas (i.e., increase methane content) fromanaerobic digesters via conversion of CO2 into CH4 by feeding H-2 gas. We optimized a culture of the thermophilic methanogen Methanothermobacter thermautotrophicus using: (1) a synthetic H-2/CO2 mixture; (2) the same mixture with pressurization; (3) a synthetic biogas with different CH4 contents and H-2; and (4) an industrial, untreated biogas and H-2. A laboratory culture with a robust growth (dry weight of 6.4-7.4 g/L; OD600 of 13.6-15.4), a volumetric methane production rate of 21 L/L culture-day, and a H-2 conversion efficiency of 89% was moved to an industrial anaerobic digester facility, where it was restarted and fed untreated biogas with a methane content of similar to 70% at a rate such that CO2 was in excess of the stoichiometric requirements in relation to H-2. Over an 8-day operating period, the dry weight of the culture initially decreased slightly before stabilizing at an elevated level of similar to 8 g/L to achieve a volumetric methane production rate of 21 L/L culture-day and a H-2 conversion efficiency of 62%. While some microbial contamination of the culture was observed via microscopy, it did not affect the methane production rate of the culture.