Hydrothermal carbonisation of waste-activated sludge: Possible route to improve sludge management in municipal wastewater treatment facilities

This study investigated the integration of hydrothermal carbonisation (HTC) and anaerobic digestion (AD) of waste-activated sludge (WAS) for sustainable sludge management. HTC were performed at 180 °C, 210 °C, and 260 °C for 30, 60, and 120 min. Hydrochar and aqueous hydrothermal liquor (AHL) were tested for biochemical methane potential. Increasing HTC severity reduced solids and carbon content, altered H/C and O/C ratios and lowered hydrochar yield while increased its stability. Under the mildest condition (180 °C for 30 min), AHL achieved the highest methane yield (387 NmL CH₄ g^-1 VS), 33 % higher than raw WAS, while hydrochar produced lower yields. Heavy metals were mainly retained in hydrochar with most within regulatory limits except cadmium, leaving AHL relatively clean. Although total biogas production output decreased due to carbon partitioning into hydrochar, AHL required only 86 % of WAS digestion capacity. HTC improves methane yield per unit of AHL, enables better use of existing plant capacity, and generates value-added byproducts.