A novel sequential incineration-melting process for phosphorus fertilizer from sewage sludge

The phosphorus (P) recovery from sewage sludge (SS) for fertilizer production faces a trade-off among technical, economic, and environmental aspects. To address the low P availability and involvement of hazardous chemicals, a stepwise incineration-melting process using SS supplemented with CaO and MgO additives was developed in this study to produce calcium magnesium phosphate fertilizer (FCMP) at reduced energy consumption. The optimal additive ratio as Ca_7.25Mg_4.57 was determined, yielding total P content of 9.41 % and P bioavailability of 100 % in FCMP. The applicability of an NBO/T formula in the SS system was examined, with a recommended range of 2.3-2.5. The co-incineration of SS with additives significantly prevented P loss during the combustion stage, achieving over 95 % P fixation in the ash. Model compound experiments indicated that additives facilitated orthophosphates formation. Material characterization and molecular dynamics simulations revealed that CaO and MgO depolymerized the glass network, with MgO exhibiting a stronger depolymerizing effect. Additives can facilitate the depolymerization of Al-containing networks, while Fe-containing networks maintain tighter aggregation, due to the conversion of Fe²⁺ (network modifier) to Fe³⁺ (network forming). This study offers new insights into P recovery from SS for fertilizer production with emphases on glass network depolymerization mechanism and process verification.