A critical review on the use of non-conventional materials for phosphorus removal and recovery in municipal WRRFs.

This review critically examines the use of non-conventional materials, primarily industrial by-products, for phosphorus removal and recovery in WRRFs, with particular attention to their potential implementation within existing treatment processes. Materials assessed include alum sludge, ferric sludge, steel slag, fly ash, red mud, bittern, and magnesium-rich by-products. Their performance is evaluated in terms of removal mechanisms, efficiency across a wide range of operating conditions, operational integration into full-scale WRRFs, potential agronomic value of the resulting products, and regulatory barriers. Evidence shows that many of these materials achieve high phosphorus removal efficiencies (>80%) under diverse conditions, while also offering opportunities for valorization within circular economy frameworks. Nonetheless, gaps remain in large-scale validation, techno-economic assessments, and regulatory harmonization. The review highlights potential pathways for integrating these non-conventional materials into sustainable phosphorus management strategies that can help to reduce reliance on phosphate rock, mitigate eutrophication, and foster the transformation of WWTPs into WRRFs.