Sustainability trade-offs in sludge-to-energy systems: a process-resolved assessment of thermal hydrolysis pre-treatment

Thermal hydrolysis pre-treatment (THP) is increasingly integrated into sludge-to-energy systems to enhance energy recovery, yet its environmental performance remains uncertain because most life cycle assessments (LCAs) treat these systems as aggregated units, limiting the representation of process-level interactions and environmental trade-offs. This study evaluates THP at 60, 80, and 120 °C in a sludge-to-energy system by propagating experimentally derived process changes through an integrated framework that combines process modelling, LCA, and techno-economic analysis. THP at 120 °C improved system performance, increasing energy output by 43 % after accounting for thermal and electrical demands, heat recovery, and losses, while reducing operational costs by 31 %. However, these gains varied across environmental categories. Environmental assessment showed reductions across non-climate impact categories due to increased energy substitution and reduced chemical and transport requirements, while global warming impacts were influenced by methane leakage and thermal demand. Nitrogen-related emissions gained importance at higher THP temperatures, particularly for stratospheric ozone depletion. Sensitivity analysis showed that controlling fugitive methane emissions can reduce global warming impacts by up to 82 %, whereas heat integration had a smaller influence. Impact categories were governed by different mechanisms, including methane leakage, thermal demand, nitrogen removal, chemical consumption, and energy substitution. Results were highly sensitive to background dataset selection, with consequential heat datasets altering the magnitude and direction of several impact categories. These findings demonstrate that evaluating THP solely through global warming can overlook system-level trade-offs, highlighting the value of integrated modelling and multi-category assessment for greenhouse-gas mitigation and sustainable sludge management.