Sustainability of hybrid solar-assisted greenhouse drying for sewage sludge: An environmental and economic assessment

This study assesses the environmental and economic performance of hybrid solar-assisted greenhouse drying and its integration with hydrogen-assisted biological biogas upgrading at a municipal wastewater treatment plant in Italy. Life cycle assessment (LCA) and life cycle costing (LCC) compared the existing anaerobic digestion and mechanical-dewatering line (S0), hybrid solar-assisted greenhouse drying with photovoltaic (PV) and heat-pump support (S1), and the additional integration of PV-powered electrolysis and biological methanation (S2). Primary operating data for 2019, 2022 and 2023 were combined with full-scale engineering specifications for S1 and modelled assumptions for S2. Agricultural recovery was the reference sludge destination; incineration was examined as a screening sensitivity. Under agricultural recovery, S0 had the lowest environmental impacts because energy and material-substitution credits exceeded treatment burdens. Drying reduced the wet sludge mass by approximately 74%, but additional electricity and facility impacts caused S1 and S2 to shift from a net climate credit to net burdens under the reference grid mix. Relative to S0, S1 reduced net operating cost by 34.1% and equivalent annual cost (EAC) by 4.2%. Its mean incremental equity net present value (NPV) was €1.22 million (temporal SD €1.40 million), although the 2023 case was negative, and its dynamic payback was 13.0 years in the mean cash-flow case. S2 reduced operating cost by 18.1% but increased EAC by 3.1%; its mean NPV was −€1.35 million, with no payback within 20 years. The results demonstrate a site-specific environmental–economic trade-off controlled mainly by dewatering, electricity supply, sludge destination and wet-mass management costs.