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

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 this paper, a facile 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 changes 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 hydrothermal-alkaline method not only realizes the efficient dehydration of the sludge, but also obtains HA from the sludge extract. The obtained HA has potential economic value in the fields of agriculture, biological medicine, environment, and the like.