Current understanding on the fate of contaminants during hydrothermal treatment of sewage sludge

Sewage sludges (SS) are by-products of the wastewater treatment process and are considered critical source of contaminants as they contain a diverse range of microbial, organic, and inorganic pollutants that are concerning to public health and the environment. Hydrothermal processes are particularly suitable for treating SS; however, their viability for the effective degradation and potential destruction of persistent contaminants, such as heavy metals, microbial pathogens, microplastics, per- and polyfluoroalkyl substances, pharmaceuticals, and personal care products, among others in SS is still under rapid investigations. This article reviews the source, transformation, and fate of prominent contaminants in SS during hydrothermal treatment (HT). Most contaminants in SS are to a certain extent degraded or transformed into other products under typical HT at subcritical conditions. Transformation pathways can be complex due to the diverse physicochemical and biochemical properties, including thermal stability and hydrophobicity. Critical findings were summarised with conclusions and perspectives for future works provided.