Publication
The disposal of sewage sludge poses significant challenges due to storage difficulties and the presence of pollutants and pathogens. Conventional treatment methods, such as incineration and anaerobic digestion (AD), are compared to advanced technologies like hydrothermal gasification (HTG) with syngas upgrading for methane production. Various indicatorsi.e. environmental impacts, exergy efficiency, capital expenditures, and operational expensesare assessed to evaluate these pathways. The scope of the life cycle assessment (LCA) accounts for the waste recovery, the treatement infrastructure, and material and energy flows. It additionally account for the substitution of valorized products. For instance, the methane produced is assumed to replace fossil methane with a substitution rate ranging from 100% (entire replacement of fossil methane) to 0%, considering either no substitution or future decarbonized energy systems. As a result, HTG achieves an exergy efficiency as low as 10.2 %. Yet, carbon management strategies, such as co-electrolysis (co-SOEC), can improve the exergy efficiency up to a value of 62.2 %. The most favourable routes in terms of GHG emissions are those mineralizing CO 2 from sludge gasification (− 1,100 kg CO 2-eq/FU) or maximizing sludge-to-methane conversion (e.g., HTG with co-SOEC, − 790 kg CO 2-eq/FU). However, this benefit reverses under a 0 % substitution scenario (+770 kg CO 2-eq/FU). In contrast, AD-based routes with lower energy demand show impacts between − 328 and + 70 kg CO 2-eq/FU, also being more competitive in terms of costs. Beyond GHG emissions, trade-offs emerge across other impact categories, notably water scarcity, ecosystem quality, and fossil and nuclear energy use, particularly for routes involving CO 2 mineralization and co-SOEC due to their high energy demand.