Batch tests were operated to investigate the bioelectrochemical enhancement of methane production from the high-solid anaerobic digestion of waste sludge in the microbial electrolysis cells (MEC) with iron-graphite electrode. Compared with the control tests, methane production in the MEC with iron-graphite electrode increased by 22.4% and VSS removal rate increased by 11% at an applied voltage of 0.3 V. However the methane production decreased and hydrogen was cathodically produced when increasing the voltage to 0.6 V. At the higher voltage, the excessive utilization of H+ in the cathode led to the alkaline pH to inhibit the methanogenesis. The applied voltages of 0.3 V could also enhance the removal of suspended and volatile suspended solids. The input of energy at 0.3 V could be neglected compared to the incremental energy generated from the methane. Denaturing gradient gel electrophoresis analysis revealed that the operation at 0.3 V had a bioaugmentation effect for both archaea and bacteria in the high-solid anaerobic digestion of waste sludge, which might be useful for enhancing VFA formation and methane production. (C) 2014 Elsevier B.V. All rights reserved.