The effects of voltage on the methane production rate (k) of various types of anaerobic sludge biomasses are quantitatively demonstrated. Voltage is supplied to the anaerobic digestion (AD) sludges independently and quickly improves the methane production rates and specific methanogenic activities (SMAs), regardless of bio-electrochemical activation of the sludge. This supports previous findings that bio-electrochemical AD (BEAD) shows higher stabilization and methane production rates than those of AD. However, in reactors fed with H-2/CO2, SMA and k are not significantly increased by the voltage supply, indicating that direct electron transfer for hydrogenotrophic methanogenesis is not a major methane formation pathway. The voltage supply contributed to indirect electron transfer for H-2 production. Therefore, indirect electron transfer via H-2 is a significant factor in hydrogenotrophic methanogenesis. These findings indicate changes in the general pathways of AD achieved using a voltage supply and provide a better understanding of previous BEAD studies. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.