Hydrothermal carbonisation products energy properties: The role of digested sludge type and operating conditions

Hydrothermal carbonization (HTC) is a promising alternative to conventional sludge drying, enhancing energy recovery in wastewater treatment plants (WWTPs). This study examines how temperature, residence time, and sludge collection point influence HTC product properties. Experiments were conducted at 200–250 °C for 30–120 min using digested sludge collected before filtration, after thickening, and after dewatering. Results show that sludge collection point strongly affects hydrochar's higher heating value (HHV), while temperature and residence time influence the biomethane potential (BMP) of HTC liquids. The highest HHV (16.31 MJ/kg) was obtained from dewatered sludge (19.8 % TS) at 250 °C, 75 min, while the highest BMP (506 mlCH₄/g NPOC) was observed from HTC liquids of thickened sludge (11.1 % TS) at 200 °C, 30 min. Findings highlight that sludge pre-treatment (thickening, dewatering) plays a crucial role in HTC efficiency, influencing both solid and liquid fractions. From a WWTP perspective, dewatered sludge processed under mild HTC conditions provides the best trade-off between hydrochar quality, HTC liquid valorization, and operational costs. These insights support the optimization of sludge-to-energy strategies, essential for implementing HTC in WWTPs.