Pyrolysis of Hydrothermal Sewage Sludge and Food Waste Digestate for Heavy Metals Stabilization and Ecological Risk Reduction

The application of municipal sewage sludge is often limited by concerns over heavy metal (HM) safety. This study explored the reduction of HM content in hydrothermal sewage sludge (HTS) through co-pyrolysis with food waste digestate (FD), aiming to lower ecological risks in the produced biochar. Results indicated that FD addition effectively lowered HM concentrations in biochar, mainly via dilution effect. Moreover, increased pyrolysis temperatures and FD addition promoted the stabilization of Cr, Ni, Cu, Zn, As, Cd, and Pb. Notably, a 50% FD mix significantly increased the proportion of HMs in the residual fraction of Ni (75.66%), Cu (71.66%), Zn (98.13%), and Cd (58.14%) compared to solo pyrolysis at 700 °C. Consequently, the potential ecological risk index significantly dropped from 47.86 to 26.29. Biochar created under optimal conditions (700 °C with a 50% FD ratio) showcased improved application prospects due to reduced bioavailability, thus diminishing HM-related ecological dangers.