Thermochemical routes for phosphorus recovery from waste activated sludge: Advances, limits, and pathways to practice

Phosphorus recovery from waste activated sludge (WAS) is a sustainable solution to the ever-increasing global phosphorus challenge. Thermochemical technologies, including pyrolysis, incineration, and gasification, reduce sludge volume and retain phosphorus in the solid phase, but their performance is strongly dependent on WAS characteristics and upstream treatment processes. Existing reviews often overlook the effects of WAS's physical and chemical characteristics on recovery efficiency, pre-treatment requirements, and technology selection. This review analyses correlations between upstream factors, sludge characteristics, and thermochemical pathways, and maps phosphorus transformation from raw WAS to final products. Sludge with high organic phosphorus (OP) and polyphosphate (poly-P) (from biological phosphorus removal), and low moisture, favours pyrolysis with minimal post-treatment, whereas high-moisture sludge rich in inorganic phosphorus requires incineration or gasification with more intensive processing. The feasibility of thermochemical pathways is affected by energy recovery, product marketability, feedstock variability, and social and regulatory constraints. To facilitate scalable, sustainable, and financially feasible phosphorus recovery and advance circular nutrient management in wastewater treatment, it is imperative to incorporate customised technology selection, high-value products, energy and resource recovery, and supportive regulations.