Phosphorus species transformation and recovery without apatite in FeCl3-assisted sewage sludge hydrothermal treatment

Abstract Phosphorus recovery from sewage sludge is necessary to promote the sustainable recovery of agricultural fertilizer products and prevent the negative environmental problems. In this study, an innovation procedure of FeCl3-assisted sewage sludge hydrothermal carbonization (HTC) treatment for selective elution of apatite phosphorus (AP) has been developed to recover phosphorus as K-struvite. AP of above 88% was eluted from the sludge solid phase at pH of 2.5, and the dissolved phosphate from AP was mainly fixed by FeCl3. The transformation of phosphorus fractions under different HTC temperature (200–260 ℃) and time (0.5–4 h) with or without FeCl3 were subsequently investigated. Phosphorus-31 nuclear magnetic resonance (31P NMR) results showed that prolonging HTC time and increasing HTC temperature could disintegrate organic phosphorus (OP), while higher HTC temperature was effective for decomposition of pyrophosphate (Pyro-P). The content of orthophosphate (Ortho-P) in hydrochar accounted for above 95% of total phosphorus (TP) under HTC temperature of 200 ℃ for 4 h with FeCl3. The phosphate desorption behavior of hydrochar with FeCl3 treated was well described by Pseudo-second-order kinetic model, while the hydrochar without FeCl3 treated followed Pseudo-second-order and Elovich kinetic models. About 96.1% of phosphate was extracted from the hydrochar under pH of 12.0 and contact time of 120 min. Finally, about 92.6% of TP in sewage sludge was recovered as K-struvite to alleviate the depletion of phosphorus resource.