A large amount (25–60%) of degraded organics is converted directly to CO 2 during anaerobic digestion (AD) process, which substantially lowers the energy (methane, CH 4 ) yield. In this study, endogenous CO 2 fixation by H 2 from in-situ iron corrosion was explored to enhancing the CH 4 yield. The results demonstrated that a substantial enhancement (up to 61%) in the CH 4 yield could be achieved with both nano-scale zero-valent iron (NZVI) and waste iron scraps (WIS) being the added iron. Additionally, the added iron could also achieve effective phosphorus removal from the AD supernatant.