This paper aims to investigate the alternative solid fuel production from paper sludge employing hydrothermal
treatment (HTT) in a lab-scale facility for implementation of the pilot-scale plant. The paper sludge was subjected to the HTT
under subcritical hydrothermal conditions. In the lab-scale experiment, the temperature conditions were 180 °C, 200 °C, 220 °C,
and 240 °C, respectively, while it was 197 °C in the pilot plant as the optimum condition. The holding time was 30 min in both
cases. The hydrothermally produced solid fuel was evaluated for the fuel property, the water removal performance, and the mass
distribution. Furthermore, the energy balance of the process was studied. The higher heating value of the HT pretreated paper
sludge was slightly improved. In addition, the produced solid fuel had comparable H/C and O/C atomic ratios with that of coal,
indicating the presence of carbonization during the HTT process. Using the mechanical dewatering, only 4.1% of moisture in the
raw paper sludge can be removed while the solid fuel production from paper sludge by HTT at 200 °C showed 19.5% moisture
reduction. According to the energy balance of the pilot plant, the recovered energy was significantly higher than the energy input,
showing the feasibility of employing the HTT to produce alternative solid fuel from paper sludge.