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 H₂ and CO₂ environments. In this review, recent developments and applications of CH₄ enrichment in microbial methanation processes are systematically reviewed. During biological methanation, either H₂ can be injected directly inside the anaerobic digester to enrich CH₄ by a consortium of mixed microbial species or H₂ can be injected into a separate bioreactor, where CO₂ contained in biogas is coupled with H₂ 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 H₂ injection are thoroughly discussed. Finally, the summary of existing demonstration, pilot plants and commercial CH₄ enrichment plants based on microbial biomethanation are critically reviewed.