Combined effect of thermal hydrolysis process and low-temperature pyrolysis on the classification and bioavailability of phosphorus in sewage sludge.

Existing phosphorus (P) resources are becoming increasingly scarce, so it is necessary to recover P from potential sources. This paper is based on thermal hydrolysis process (THP) at 140-180 °C, coupled with low-temperature pyrolysis at 300 °C, to study its effect on the recovery and conversion of P from sewage sludge. Most significant change was observed in apatite P, which increased from 3.43 ± 0.48 mg/g in raw sludge to 30.17 ± 1.17 mg/g in biochar (BTHP-180-4-300) during optimal process (THP condition: 180 °C, 4 h; pyrolysis condition: 300 °C). Reactions between phosphates and metal ions became more complete during this combined process. Unstable forms of P were converted into more stable forms, with transformations from Al-P and Fe-P toward Ca-P compounds like Ca3(PO4)2, Ca3Mg3(PO4)4, Ca2P2O7, and Ca(H2PO4)2, making P less degradable and more suitable as slow-release fertilizers. Additionally, P characteristics of actual THP in a sewage treatment plant were similar to those of laboratory THP.