Enhancing methane production in anaerobic digestion through hydrogen assisted pathways-A state-of-the-art review

Anaerobic digestion has been widely accepted for energy and resource recovery from biomass residues. However, the produced biogas from the process mainly composed of methane and carbon dioxide is lower in calorific content, which is a major drawback for its direct application as an energy fuel. Therefore, different biogas upgradation systems based on physical, chemical, and biological processes have been applied to either remove carbon dioxide and other gaseous constituents from the biogas or utilize carbon dioxide into methane. This review discusses the possible hydrogen-assisted pathways for converting carbon dioxide into methane in the presence of hydrogen and improving its proportion in the biogas composition during anaerobic digestion through in-situ biogas upgradation. Additionally, a co-production of hydrogen and methane in two-stage anaerobic digestion has been proposed for methane enrichment. Technical challenges, stabilization of process parameters, innovative modification and microbial pathways have been explored and discussed. The findings and prospects from this article could be an interesting state-of-the-art for optimizing process parameters during hydrogenassisted pathways and its mainstream application on existing digestion systems.