A Life Cycle Assessment of Hydrothermal Carbonization and Anaerobic Digestion of Municipal Organic Waste and Sewage Sludge for Biomass-Based Carbon Removal and Storage

Achieving net zero targets requires both major emission reductions and durable carbon dioxide removal. This study uses life cycle assessment (LCA) to evaluate the sustainability of a Biomass Carbon Removal and Storage (BiCRS) technology: the conversion of wet waste through hydrothermal carbonization (HTC) and subsequent long-term hydrochar storage. Two waste streams, the organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS), and two treatment options, direct HTC and a cascaded use combining anaerobic digestion and HTC, were assessed in 13 scenarios, comparing environmental impacts, carbon removal efficiency, and key trade-offs. The results show a trade-off between energy generation and carbon storage: in cascaded configurations, biogas replacing fossil fuels can improve environmental performance. Carbon removal efficiency varied greatly, from 0 to 86%. Cascaded pathways with sewage sludge emitted significantly more carbon dioxide than they removed. Only configurations with favorable feedstock properties, moderate energy demand, low-carbon energy supply, and short transport distances achieved robust net-negative emissions. OFMSW performed better than sewage sludge due to higher dry matter and carbon contents. The findings suggest that HTC of wet waste biomass can provide sustainable carbon removal under specific, optimized conditions, with feedstock choice and energy sourcing as key levers for maximizing net environmental benefit.