An overview of microbial biogas enrichment

Biogas upgrading technologies have received widespread attention recently and are researched extensively. Microbial biogas upgrading (biomethanation) relies on the microbial performance in enriched H-2 and CO2 environments. In this review, recent developments and applications of CH4 enrichment in microbial methanation processes are systematically reviewed. During biological methanation, either H-2 can be injected directly inside the anaerobic digester to enrich CH4 by a consortium of mixed microbial species or H-2 can be injected into a separate bioreactor, where CO2 contained in biogas is coupled with H-2 and converted to CH4, or a combination hereof. The available microbial technologies based on hydrogen-mediated CH4 enrichment, in particular ex-situ, in-situ and bioelectrochemical, are compared and discussed. Moreover, gas-liquid mass transfer limitations, and dynamics of bacteria-archaea interactions shift after H-2 injection are thoroughly discussed. Finally, the summary of existing demonstration, pilot plants and commercial CH4 enrichment plants based on microbial biomethanation are critically reviewed.