Deposition characterization and predictive modeling of phosphorus and heavy metals during sewage sludge incineration

The deposition of heavy metals during sewage sludge incineration restricts phosphorus recovery from ash. This study investigated the distribution and mineral phases of phosphorus and heavy metals in sewage sludge ash (SSA) across particle sizes and built predictive models. At 750 °C, total phosphorus increased from 52.18 to 60.23 mg/g at 0.2-0.09 mm and then dropped to 57.29 mg/g. Apatite phosphorus (AP) rose continuously, while non-apatite phosphorus (NAIP) declined. With higher temperatures, Fe-P gradually transformed into Ca-P, improving phosphorus bioavailability. Zn was strongly enriched in finer particles at 900 °C (897-1406 mg/kg), and Cu (205-235 mg/kg) showed enrichment in both coarsest and finest fractions at 800-850 °C. More crystalline HMs-NAIP appeared in coarse fractions, while HMs-AP dominated fine ones, with differences decreasing at higher temperatures. Phosphorus and heavy metals co-migrated through stable phosphate formation. Regression and 3D models revealed temperature-size-dependent peaks of TP, Zn, and Cu, guiding efficient phosphorus recovery.