The effect of combination of mechanical and chemical pretreatment of municipal waste activated sludge (WAS) prior to anaerobic digestion was studied using a laboratory scale system with an objective to decrease volatile sulfur compounds in biogas and digested sludge. Mechanical pretreatment was conducted using depressurization of WAS through a valve from a batch pretreatment reactor pressurized at 75 +/- 1 psi, while combined pretreatments were conducted using six different dosages of hydrogen peroxide (H2O2) and ferrous chloride (FeCl2) along with mechanical pretreatment. About 37-46% removal of H2S in biogas occurred for different combined pretreatment conditions. Sludge solubilization achieved due to the mechanical pretreatment increased total cumulative methane production by 8-10% after 30 days during the biochemical methane potential (BMP) test. The pretreatment also improved dewaterability in terms of time to filter (TTF), and decreased methyl mercaptan generation potential of the digested sludge. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.