Oyster shell waste as potential co-substrate for enhancing methanogenesis of starch wastewater at low inoculation ratio

•Utilization of oyster shells to produce CH4 has never been reported.•Co-fermentation generated CH4 yield close to the theoretical maximum.•Oyster shells accelerated and intensified methanogenesis steps.•Oyster shells contributed 17% as substrate for the total CH4 production.•Enriched acetotrophic and methylotrophic methanogens accounted for high CH4 yield.