Migration and Transformation of Phosphorus during Hydrothermal Carbonization of Sewage Sludge: Focusing on the Role of pH and Calcium Additive and the Transformation Mechanism

Recovering phosphorus (P) from sewage sludge has been proposed as a promising substitute for phosphate ores, while hydrothermal carbonization (HTC) has emerged as an energy-efficient method for sewage sludge treatment with substantial merits. Migration and transformation of phosphorus in the sludge during HTC is vital for subsequent P recovery. In this study, the role of pH and calcium additive (CaCl2) in the partitioning and transformation of phosphorus species as well as the associated transformation mechanism was investigated via Standards, Measurements, and Testing Program analysis, X-ray diffraction analysis, and the aid of thermochemical equilibrium approach. The results indicated that higher feedwater pH and dosage of CaCl2 favored the enrichment of phosphorus in the hydorchars with RP > 80%, which was mainly presented as inorganic phosphorus. The obvious transformation from nonapatite inorganic phosphorus to apatite phosphorus was observed, promoting by higher feedwater pH or dosage of CaCl2. Unlike the addition of CaO, Ca3(PO4)2, and Ca2P2O7 have been calculated as the main Ca-associated phosphorus compounds at equilibrium state while less hydroxyapatite (Ca5(PO4)3OH) was reported. These findings were confirmed by XRD analyses well. The work described here can provide new insights into the phosphorus recovery from sewage sludge.