Multi-Method Fractionation of Phosphorus Forms in Dewatered Sludge: Implications for Phosphorus Release and Recovery

Determining the distribution of phosphorus forms in dewatered sludge is crucial, as it directly impacts the choice and effectiveness of recovery strategies. Analyses using the Standards, Measurements and Testing (SMT) method, soluble/insoluble fractionation and extracellular polymeric substance (EPS)/EPS-residue fractionation revealed that sludge characteristics strongly influenced phosphorus speciation and content. Inorganic phosphorus (IP) was the dominant form in dewatered sludge, primarily regulated by the levels of Al, Fe, and Ca, while dissolved orthophosphate (ortho-P) constituted only 1% of the total phosphorus (TP). Notably, phosphorus exhibited a distinct distribution pattern between EPS and EPS residues: EPS comprised only 2.09% of TP, 74.19% of which was organic phosphorus (OP), whereas EPS residues contained 93.26% of TP, with a much lower OP proportion (15.04%). These results indicate that most phosphorus in dewatered sludge is retained in insoluble and residue-bound forms. Therefore, pretreatment strategies that promote the conversion of solid-phase phosphorus into dissolved ortho-P are necessary for improving phosphorus recovery from dewatered sludge, supporting waste valorization and more sustainable sludge management.