Mechanisms and modelling of phosphorus solid–liquid transformation during the hydrothermal processing of swine manure
Phosphorus (P) recovery from swine manure by hydrothermal processes has recently attracted considerable interest; however, research has been limited by knowledge gaps and challenges in understanding the mechanisms of soluble and insoluble P transformations and the evaluation of the effects of the reaction conditions. In this study, the transformation mechanisms were investigated and the soluble and insoluble phosphorus distributions in swine manure during the hydrothermal processes were modelled. By increasing the severity of the exogenous conditions, P transformed from insoluble to soluble, and then polymerized with the formation of orthophosphates; meanwhile, the formation of hydrochar was enhanced thereby facilitating further P reclamation. The effects of the endogenous conditions showed there may be a threshold of calcium content, which limited the combination of Ca and P. Calcium ions mainly reacted with P in the form of hydroxyapatite and octacalcium phosphate. The modelling and prediction results showed that a coalification model gives a good fit (RSP2 = 0.9205 and RIP2 = 0.8559) for changes in the concentrations of solid total P and liquid inorganic P. The prediction level of mean absolute error was good as well (MAESP = 0.74 mg g−1 and MAEIP = 0.62 mg g−1). These findings provide a range of scientific opportunities for achieving a comprehensive understanding of the basis of sustainable utilisation of P.