Hydrothermal carbonization of digested sewage sludge: The fate of heavy metals, PAHs, PCBs, dioxins and pesticides

Hydrothermal carbonization (HTC) is emerging as a promising technology for the management of sewage sludge. The fate of phytosanitary products, Polycyclic aromatic hydrocarbons (PAHs) and PCBs (Polychlorinated biphenyls) after HTC, as well as the formation of dioxins and furans, is still unclear. Moreover, only little information is available on the distribution of heavy metals and major nutrients between the hydrochars and the process water. Here, we aim to contribute to fill these gaps. HTC of sewage sludge from six different wastewater treatment plants has been carried out at 220 °C for 85 min. Feedstock, hydrochars and spent liquor have been then characterized and discussed. HTC is here proven to be a suitable technology for the immobilization of both heavy hydrocarbons and heavy metals, with the exception arsenic, which was also found in the spent liquor at a significant proportion (∼15–∼50%). DDD, DDT, DDE were detected in all sludge samples and their content was reduced by nearly one order of magnitude after the process. HTC is here proven to not be responsible at an appreciable extent of PCBs enrichment of the processed solids. Moreover, the sum of PCDDs and PCDFs in hydrochars never exceeded 20 ng kg−1 s.s. The results obtained encourage further developing of HTC, with the aim to improve the sustainability of sewage sludge management. Additional studies on the environmental impact of hydrochar when used as alternative fuel, as well as soil amendment, could lead to the overcoming of the issues which still hinder these applications.