Hydrothermal treatment of sewage sludge in sub- and supercritical water: Fundamentals, mechanisms, and sustainable valorization

Hydrothermal treatment (HT) technologies offer a promising route for the sustainable and efficient management of sewage sludge by enabling harmless disposal, volume reduction, energy recovery, and resource valorization in subcritical and supercritical water conditions. This review systematically summarizes the principles, operating conditions, and product characteristics of key HT pathways, including thermal hydrolysis, hydrothermal carbonization, wet oxidation, hydrothermal liquefaction, supercritical water gasification, and supercritical water oxidation. Several aspects are comprehensively discussed, including the degradation mechanisms of organic matter, the transformation behaviors of pollutants such as heavy metals and microplastics, as well as the formation pathways of value-added products. Comparative analysis highlights the advantages, limitations, and application potential of each technology. Finally, key challenges including cross-component reaction networks, emerging contaminant fates, catalyst deactivation, and nutrient partitioning are identified. Future research directions are proposed to support the advancement and large-scale implementation of HT technologies for sewage sludge treatment.