The role of endogenous and exogenous hydrogen in the microbiology of biogas production systems.

: Anaerobic digestion is an effective process for the treatment of organic solid waste and wastewater and the production of biogas, which is a clean energy source. The carbon dioxide in the biogas can be converted into methane using hydrogen generated from water electrolysis through an approach referred to as power-to-gas. Recently, hydrogen has been added to digesters as an in-situ or ex-situ biogas upgrade to reduce the levels of carbon dioxide. Biogas production systems consist of microbial complexes with highly organized microorganisms in different niches, which can either produce or consume hydrogen. However, the produced endogenous hydrogen should be constantly consumed to maintain a low hydrogen partial pressure. This review addresses the biochemical processes of anaerobic digestion and hydrogen-related microorganisms, including fermentative acid-producing bacteria, syntrophic organic acid degrading bacteria, syntrophic acetate-oxidizing bacteria, homoacetogens, hydrogenotrophic methanogens, and newly reported hydrogen-dependent methylotrophic methanogens. This study also investigates (1) the role of endogenous hydrogen as an intermediate metabolite and of interspecies electron transfer in anaerobic digestion, (2) effects of exogenous hydrogen addition on microbial community structure and metabolic processes, and (3) recent developments regarding in-situ and ex-situ biogas upgrading systems via hydrogen addition.