Techno-economic analysis of hydrothermal carbonization of sewage sludge for phosphorus retention and carbon sequestration.

Sludge management is a challenging aspect in wastewater treatment plants (WWTPs). Hydrothermal carbonization (HTC) is a promising thermochemical process, converting wet biomass into a valuable hydrochar (HC), while improving environmental performance. This study evaluates the techno-economic feasibility of integrating HTC into a medium-size WWTP. Approximately 21,000 t yr⁻1 of sludge could be converted into ∼ 3,066 t yr⁻1 of HC, achieving a substantial reduction in transported material, while retaining valuable phosphorus. Approximately 73% of the initial sludge carbon was retained in the HC, of which an estimated 49% remained as stable carbon assuming a 67% carbon-stability factor. A techno-economic analysis was conducted considering capital expenditure, operating costs, heat integration, carbon-credit scenarios, and governmental incentives. The estimated capital investment was approximately €2.8 million, with equipment costs accounting for most of the expenditure. Heat integration based on pinch analysis enabled recovery of ∼ 414 kW from a total thermal demand of ∼ 489 kW, corresponding to a heat recovery efficiency of 86% and significantly reducing external energy requirements. The minimum selling price of HC was estimated at €246 t⁻1, with strong sensitivity to HC production. A sensitivity analysis suggests that a minimum carbon-credit, discount rate and project budget subsidy of €60 t⁻1 CO2-eq, 7% and 40%, respectively, are together needed to reduce the dynamic payback-period to below 10 years. Overall, the results demonstrate that HTC can serve as a viable waste-to-resource strategy for WWTPs, supporting circular economy through phosphorus and carbon retention, and reduced management costs.