Techno-Economic and Environmental Assessment of Hydrothermal Gasification, Incineration, and Biodigestion Routes for Biomethane Production from Sewage Sludge

Conventional sewage sludge disposal methods, such as incineration, are compared to advanced technologies like hydrothermal gasification (HTG) and anaerobic digestion (AD) with syngas upgrading for biomethane production. Various indicators — including environmental impacts, energy requirements, exergy efficiency, CAPEX, and OPEX — are assessed to evaluate these pathways. Results show that HTG achieves an exergy efficiency of 59.8% when coupled with a transcritical CO2 power cycle and a carbon conversion rate of 80%. Carbon management strategies, including CO2 mineralization and co-electrolysis, are also explored. Introducing a new reference environment where atmospheric CO2 is considered mineralized, the HTG route achieves 37.4% exergy efficiency with magnesium carbonate. Gasification with CO2 mineralization emerges as the preferred configuration, achieving net-negative emissions of -466 kgCO2-eq/ton of sludge. A solution space illustrates optimal trade-offs among metrics. While AD-based routes offer lower exergy efficiencies (≃16%), they balance costs and climate impact, presenting viable alternatives.