Partial vs total hydrothermal carbonization: process design, energy analysis and economic assessment for flexible sewage sludge treatment

The paper presents a techno-economic analysis assessing two different configurations of a hydrothermal technology processing 10 kt/y of sewage sludge (23 % w solids) from municipal wastewater treatment as an alternative to their current fate mainly consisting in incineration or landfilling. The process carries out a hydrothermal carbonization of sludges (210 °C, 34.5 bar, 35 min) producing hydrochar, and a liquid stream with lower organic content. Mass and energy balance closure and preliminary sizing have been performed in Matlab® environment, estimating capital and operating plant costs to calculate a levelized costs for sludge treatment. Two process configurations have been assessed: Configuration A features a hydrothermal process treating the entire sewage sludge flowrate. In Configuration B, a split fraction of the sludge flow rate is sent to the hydrothermal process to produce the same amount of biochar for combustion as Configuration A, however the remaining sludge stream is sent to co-combustion reducing HTC wastewater recirculated to the water treatment plant. In both configurations A and B, HTC enables a reduction in primary energy consumption related to natural gas use, as well as lower fossil CO 2 emissions compared to direct incineration (−68.3 % and −35.3 % respectively). The levelized cost of sludge treatment remains below 97.40 €/ton and can decrease to 48.37 €/ton in case of no disposal costs for excess biochar. The work includes the design of a dedicated unit for off-gas treatment; moreover, both configurations have been assessed in terms of primary energy consumption and fossil CO 2 emissions.