Prediction of Struvite Formation to Recover Phosphorus from Human Urine Using an Equilibrium Model

Abstract

Phosphorus recovery from wastewater has been a growing concern. Since human urine is one of the main sources of phosphorus in the environment, several studies have been undertaken to recover phosphorus from human urine diverted from feces. Phosphorus in human urine can be recovered as a precipitate of MAP (i.e., struvite, MgNH₄PO₄·6H₂O) by adding magnesium salt under alkaline conditions. However, little is known about the chemical reaction of MAP formation in human urine, which is essential for the practical application of the reaction. In this study, we established an equilibrium model to understand such a chemical phenomenon and to predict MAP formation in human urine. In addition, MAP precipitates were formed experimentally from synthesized and diverted human urine for the validation of the model. As a result, there was a considerable validity of the model of MAP formation. Using this model, we predicted MAP formation in diverted human urine at different pHs and amounts of magnesium salt added. We concluded that the addition of at least 10 mM MgCl₂ is necessary in MAP formation in urine in a urine-diversion toilet and in achieving a PO₄-P concentration of 0.1 mg·l^-1 in the supernatant. In such a case, the optimal pH is 9.7.