Development of Stable Mixed Microbiota for High Yield Power to Methane Conversion

The performance of a mixed microbial community was tested in lab-scale power-to-methane reactors at 55 °C. The main aim was to uncover the responses of the community to starvation and stoichiometric H 2 /CO 2 supply as the sole substrate. Fed-batch reactors were inoculated with the fermentation effluent of a thermophilic biogas plant. Various volumes of pure H 2 /CO 2 gas mixtures were injected into the headspace daily and the process parameters were followed. Gas volumes and composition were measured by gas-chromatography, the headspace was replaced with N 2 prior to the daily H 2 /CO 2 injection. Total DNA samples, collected at the beginning and end (day 71), were analyzed by metagenome sequencing. Low levels of H 2 triggered immediate CH 4 evolution utilizing CO 2 /HCO 3 − dissolved in the fermentation effluent. Biomethanation continued when H 2 /CO 2 was supplied. On the contrary, biomethane formation was inhibited at higher initial H 2 doses and concomitant acetate formation indicated homoacetogenesis. Biomethane production started upon daily delivery of stoichiometric H 2 /CO 2 . The fed-batch operational mode allowed high H 2 injection and consumption rates albeit intermittent operation conditions. Methane was enriched up to 95% CH 4 content and the H 2 consumption rate attained a remarkable 1000 mL·L −1 ·d −1 . The microbial community spontaneously selected the genus Methanothermobacter in the enriched cultures.