Hydrothermal Carbonization Treatment as a Pathway for Energy Utilization of Municipal Sludge and Agricultural Residues Through Co-Gasification

Municipal sewage sludge (S.S.) and abundant olive-tree pruning on Lesvos Island present both a disposal challenge and an untapped energy resource. This study proposes and evaluates on a preliminary level an integrated system that utilizes both sewage sludge and pruning. The integrated system converts sewage sludge into Hydrochar (HC) via Hydrothermal Carbonization (HTC), removes the aqueous phase using passive solar distillation, and co-gasifies the dried HC with olive pruning in an autothermal downdraft gasifier. HTC experiments on anaerobically digested sludge produced HC with higher heating values exceeding 20 MJ kg−1 while reducing the chemical oxygen demand of the process liquor. Gasification modelling, using the MAGSY equilibrium model, demonstrated that replacing up to 50% of lignocellulosic biomass with HC increased hydrogen content and the Lower Heating Value (LHV) of syngas. Mass and energy balances suggest that the system could provide approximately 590 kW of continuous power, contributing around 4720 MWh to the island’s annual electricity generation. These results indicate that combining HTC, solar distillation, and co-gasification offers a viable pathway to close waste loops, reduce landfill needs, and deliver renewable energy. Future work will focus on Aspen Plus design and optimization, along with a life-cycle assessment in order to assess the environmental benefits.