Hydrothermal-alkaline conversion of sewage sludge: Focus on optimization of process parameters and characterization of humic acid

Abstract Sewage sludge (SS) dewatering is a key step in sludge disposal, which plays an important role in reducing sludge volume, facilitating transportation and subsequent treatment. In recent years, many studies have focused on sludge hydrothermal dewatering, while resource recovery during this process is less studied, especially humic substances. In this paper, a facile and low-cost hydrothermal-alkaline treatment for SS was proposed, which can be used for sludge dewatering and humic acid (HA) recycling at the same time. Response surface methodology (RSM) was used to determine the optimal conditions, and a mathematical model was established to accurately predict the change of sludge water content and the extraction rate of HA. Under the optimal conditions of 170 °C/42 min/0.05 (for hydrothermal temperature, hydrothermal time and mass ratio of KOH to wet sludge, respectively), the water content decreased to 46.7% and the extraction rate of HA (with a purity of 96.2%) was 89.1%. The improvement of the dewatering performance effectively facilitates the subsequent disposal of the sludge. The extraction of HA not only solves the problem that the sludge separation liquid needs further treatment, but also has potential economic value in agriculture, biological medicine, environment and other fields.