Valorization of Agricultural Biomass by Microbial Fermentation for Sustainable Biohythane Production

Agricultural biomass, comprising animal manure, food processing residues, lignocellulosic agricultural by-products, and other agro-industrial wastes, is generated in large quantities worldwide, particularly in developing countries. Although these residues pose significant environmental disposal challenges, they represent abundant renewable carbon resources that can be valorized into biofuels, contributing to sustainable waste management and circular bioeconomy initiatives. Biohythane, a gaseous fuel consisting of hydrogen and methane, is primarily produced through two-stage anaerobic digestion, in which dark fermentation generates hydrogen-rich intermediates that are subsequently converted into methane during methanogenesis. The separation of these stages enables independent optimization of hydrogen and methane production, resulting in improved substrate conversion efficiency and higher energy recovery than conventional single-stage anaerobic digestion. In addition, the presence of hydrogen enhances combustion characteristics while reducing greenhouse gas and nitrogen oxide emissions. This review critically evaluates recent advances in biohythane production from agricultural biomass through a structured assessment of peer-reviewed literature retrieved from major scientific databases. The selected studies were synthesized to examine biomass feedstocks, pretreatment technologies, microbial communities, metabolic pathways, reactor configurations, and process optimization strategies influencing biohythane production. The review further discusses the advantages and limitations of different agricultural residues, identifies current technological and economic challenges, and highlights emerging research opportunities to improve process efficiency and facilitate the sustainable commercialization of biohythane as a low-carbon renewable energy source.