Hydrothermal Coupled Pyrolytic Treatment of Sewage Sludge and Food Waste Digestate for Heavy Metal Immobilization and Biochar Properties Enhancement

Hydrothermal combined pyrolytic treatment is a promising technology for improving the dewaterability of sewage sludge (SS) and food waste digestate (FD) with additional heavy metal (HM) risk reduction. The effect of cohydrothermal coupled pyrolysis with different temperatures (300, 500, and 700 °C) at different mixture proportions (1:0, 3:1, 1:1, 1:3, and 0:1) on the migration and transformation of HMs was investigated. In addition, the effects on the properties of biochar and the assessment of the risk of HMs in biochar were studied. The results showed that the addition of FD increased the residual fractions of Cr, Zn, and Cd during the hydrothermal process of SS. Followingly, Cu and As were better immobilized in biochar from cohydrothermal combined pyrolytic treatment than SS treatment alone, with the low potential ecological risk (indices: 9.13–11.62) of biochar at 700 °C. In addition, cohydrothermal combined pyrolytic treatment (1:1, w/w) at 700 °C can significantly improve the aromaticity and specific surface area of SS biochar. This work reveals that the codisposal of SS and FD has superior application potential in HM immobilization and biochar property improvement.