Potential of biogas residue biochar modified by ferric chloride for the enhancement of anaerobic digestion of food waste

•Biogas residue was pyrolyzed to produce biochar, which further was modified by FeCl3.•BRB and FeCl3-BRB caused accelerated VFAs conversion, thus enhancing methane yield.•Obvious increase in Syntrophomonas and Methanosarcina with BRB and FeCl3-BRB.•BRB and FeCl3-BRB resulted in metabolically diverse methanogenic pathways.•BRB and FeCl3-BRB increased the functional gene pilA and fixA involved in DIET.