Enhancing hydrochar production and mitigating heavy metal risks in sewage sludge: The role of ultrasonic pretreatment in hydrothermal carbonization

Hydrothermal carbonization (HTC) is a potentially valuable technology for sewage sludge treatment and resource utilization. This study proposes a combined process of ultrasonic pretreatment and HTC to improve the yield and properties of hydrochar derived from sewage sludge (SS). SS was ultrasonically pretreated at different sound energy densities (0.6 – 1.4 W/mL) and pretreatment times (5 – 30 min), and then HTC conducted for 4 h in an autoclave. Ultrasonic pretreatment markedly enhanced the apparent structure and particle size of the feedstock, thereby improving the hydrothermal reactivity of SS. After ultrasonic pretreatment at 1.4 W/mL for 15 min, the yield of hydrochar derived from SS was 11.25% higher than that obtained after direct HTC. The hydrochar yield was also correlated with the ultrasonic pretreatment parameters. Ultrasonic pretreatment increased the specific surface area and number of oxygen-containing functional groups of the hydrochar and increased the energy density and energy yield by at most 7.63% and 9.56%, respectively. The impact of ultrasonic pretreatment on the heavy-metal pollution risk of hydrochar was evaluated in terms of the geological acumination index. Pretreatment with appropriate ultrasonic parameters decreased the pollution risks of Zn, Cu, Cr, and Pb. This study provides a valuable perspective on combined feedstock pretreatment and HTC, increasing the product value of HTC of sewage sludge.