Applicability of pretreatment using subcritical water liquefaction (subCWL) for supercritical water gasification (SCWG) of sewage sludge was investigated. Effects of pretreatment conditions on liquefaction and viscosity were studied; the results revealed that treatment at 250 degrees C for 30 mm was optimum in fluidizing the sewage sludge used herein. The pretreatment tended to enhance H-2 and CH4 yields in the subsequent SCWG while suppressing CO2 production. A continuous SCWG system assembled with a subCWL pretreatment process was designed. From energy balance and cost analysis on SCWG-installed municipal wastewater treatment plant (WWTP) with conventional activated sludge process, the system was capable of producing surplus energy by utilizing exhaust heat. Cost analyses indicated that the capital cost and treatment fees were cost competitive compared to conventional treatments. (C) 2013 Elsevier B.V. All rights reserved.