Application of subcritical water liquefaction as pretreatment for supercritical water gasification system in domestic wastewater treatment plant

[Display omitted] ► Subcritical water liquefaction was proposed as a pretreatment. ► Effect of pretreatment condition on supercritical gasification was specified. ► Supercritical water gasification system for waste water treatment plant was designed. ► Energy balance and cost analysis were conducted for the designed system. ► Superiority of the proposed system was demonstrated. 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°C for 30min was optimum in fluidizing the sewage sludge used herein. The pretreatment tended to enhance H2 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.