Effects of biochar supported nano zero-valent iron with different carbon/iron ratios on two-phase anaerobic digestion of food waste

•BC/nZVI with carbon/iron of 3:1 was found to be optimum for this system.•VFAs concentration was increased by 31.4% in acidogenic phase with BC/nZVI added.•The addition of BC/nZVI enhanced cumulative methane production by 24.8%.•BC/nZVI promoted the abundance of Methanothrix in methanogenic phase.•BC/nZVI stimulated methanogenesis through the acetoclastic methanogenic pathway.