Methane yield enhancement of mesophilic and thermophilic anaerobic co-digestion of algal biomass and food waste using algal biochar: Semi-continuous operation and microbial community analysis

[Display omitted] •Algal biochar enhanced co-digestion of algal biomass and food waste for biomethane.•75% algal biomass + 25% food waste mixture co-digestion showed a synergistic effect.•Average methane yields enhanced by 12–54% due to 15 g/L of algal biochar amendment.•Proteiniphilum enriched in mesophilic reactor like Defluviitoga in thermophilic one.•Biochar shifted methanogens towards a balance of two flexible methanogenic pathways. The impact of algal biochar addition on mesophilic and thermophilic anaerobic co-digestion of algal biomass and food waste was investigated with a focus on semi-continuous operations and functional microbial communities. Under batch co-digestion, the highest co-digestion synergy was observed for a mixture of 25% food waste and 75% algal biomass. During semi-continuous co-digestion of 25% food waste-75% algal biomass mixture, biochar amended digesters exhibited a 12–54% increase in average methane yield (275.8–394.6 mL/gVS) compared to the controls. Elevated temperature induced narrow distributions of volatile fatty acids (VFAs) by inhibiting the production of branched VFAs. Genus Proteiniphilum was selectively enriched by 3.2 folds in mesophilic digesters with biochar amendment while genus Defluviitoga was selectively enriched in thermophilic digesters due to elevated temperature. Methanogenic communities were significantly different in mesophilic and thermophilic digesters. Biochar amendment contributed to shifts in the predominant methanogens leading to a more balanced state of two methanogenic pathways.