A comparative assessment of sewage sludge valorization via anaerobic digestion and supercritical water gasification: A techno-economic case study in Norway

This study compared the performance of supercritical water gasification (SCWG) to conventional anaerobic digestion (AD) for treating sewage sludge from a wastewater treatment plant (WWTP). Based on plant data from a WWTP in Norway, kinetic-based process models have been developed in Aspen Plus® to investigate the energy and economic feasibility of AD and SCWG for sewage sludge (SS) treatment. The findings showed that low ambient temperatures significantly reduced the energy efficiency of the AD, while their effect on the SCWG process was negligible. At the average ambient temperature of 7 °C and SS concentration of 7 wt%, AD and SCWG achieved thermal energy self-sufficiency (TES) of 0.76 and 0.63, respectively. Under these conditions, the AD and SCWG had energy conversion efficiencies (η) of 25.8% and 46.6%, respectively. However, increasing the SS content in the SCWG to 15 wt% improved the TES to 1.22 and η to 73.2%, making it a more attractive alternative to the AD. At 7 wt% SS concentration, the AD and SCWG processes required an investment cost of $ 4.08 million and $ 6.83 million, respectively. Under the study assumptions, neither AD nor SCWG proved profitable, revealing negative net present values (NPV) and operating costs of $0.96 million and $0.92 million, respectively. • Conversion of sewage sludge into bioenergy via SCWG and AD is evaluated. • SCWG modeled using kinetic data on supercritical water reactions. • The SCWG and AD processes have negative NPVs at the set economic assumptions. • The total annualized costs of SCWG and AD processes are comparable. • The SCWG with 15 wt% sludge concentration was a viable alternative to the AD.