The purpose of this study was to increase the efficiency of anaerobic digestion of waste activated sludge (WAS). Either thermochemical or biological hydrolysis was used as a pretreatment and the effects of both were investigated and compared. Two different three-stage digestion systems showed improved performance, although thermochemical hydrolysis showed better results than biological hydrolysis in a bench-scale operation. After anaerobic digestion with thermochemical pretreatment, the total chemical oxygen demand (tCOD) reduction, volatile solid (VS) reduction, methane yield and methane biogas content were 88.9%, 77.5%, 0.52 m(3)/kg VS and 79.5%, respectively. These results should help in determining the best hydrolysis pretreatment process for anaerobic digestion and in improving the design and operation of the large-scale treatment of WAS by anaerobic digestion with hydrolysis systems.