Co-Hydrothermal Carbonization of Sewage Sludge with Wood Chip: Fuel Properties and Heavy Metal Transformation Behavior of Hydrochars

To further reduce the toxicity of heavy metals (HMs) in sewage sludge (SS), organic acid (oxalic acid, citric acid, acetic acid) washing was used before co-hydrothermal carbonization (co-HTC) with pinewood. The results showed the total concentrations of six HMs (As, Cu, Ni, Cr, Mn, and Zn) in hydrochar produced at 230 °C decreased obviously during co-HTC. More importantly, F1 + F2 fractions of Ni, Cr, Mn, and Zn were also lowered. With the pretreatment of citric acid, the hydrochar from co-HTC exhibited the lowest potential ecological risk among all simples. Compared with the hydrochar produced from SS, the coalification degree, higher heating values (5.61–6.19 MJ/kg), and combustion behavior of the hydrochar derived from co-HTC were upgraded and improved. These findings offered a viable technology that could lower the bioavailability of HMs and realize the energy value of SS.