We optimized and tested a postbioprocessing step with a single-culture archaeon to upgrade biogas (i.e., increase methane content) from anaerobic digesters via conversion of CO(2) into CH(4) by feeding H(2) gas. We optimized a culture of the thermophilic methanogen Methanothermobacter thermautotrophicus using: (1) a synthetic H(2)/CO(2) mixture; (2) the same mixture with pressurization; (3) a synthetic biogas with different CH(4) 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 ~70% at a rate such that CO(2) 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 ~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.