Centralized versus conventional anaerobic digestion of sewage sludge: An environmental and economic assessment

High-solid anaerobic digestion (HSAD), in which dewatered sewage sludge from several small wastewater treatment plants is treated centrally, is an alternative to conventional low-solid anaerobic digestion (LSAD) for sludge stabilization and energy recovery from biogas. This work develops a modelling framework to compare centralized HSAD and decentralized LSAD through a coupled life cycle assessment (LCA) and economic analysis approach, supporting case-specific decision-making for sludge management under defined local boundary conditions. Four different sub-scenarios were modeled for both HSAD (S1.1-S1.4) and LSAD (S2.1-S2.4), forecasting different biogas uses, alternative sludge transportation routes, digestate valorization pathways, and ammonia extraction. The sub-scenarios were compared with the respective baselines (S1.0 for HSAD and S2.0 for LSAD), corresponding to full-scale Italian plant configurations. Under the investigated full-scale conditions, HSAD showed lower environmental impacts than conventional LSAD when transport distances were limited, due to higher credits generated by biogas and the avoided mineral fertilizer use in agriculture. However, input sludge/digestate transportation is a dominant driver of both environmental and economic impacts. Moreover, biogas upgrading to biomethane is environmentally preferable to electricity generation, but economically unfavourable at current market conditions. Overall, centralized HSAD can represent a favorable solution under regional-scale planning when logistics, digestate valorization, and gate-fee conditions are suitable, whereas optimized LSAD with dynamic sludge thickening can achieve comparable performance. Although the specific results are related to an Italian case study, this work provides a useful tool for decision-making among alternative sludge management solutions when local boundary conditions (sludge characteristics, transportation distances, HSAD/LSAD plant availability) are specified.