Nutrients removal and recovery by crystallization of magnesium ammonium phosphate from synthetic swine wastewater

With synthetic swine wastewater, lab-scale batch experiments were carried out to investigate the effects of pH value, magnesium dosage, calcium and carbonate concentrations on magnesium ammonium phosphate (MAP) crystallization. The morphology of the precipitates obtained was observed by using scanning electron microscopy with energy dispersive X-ray analysis, and the composition of the precipitates was analyzed with X-ray diffraction. The results show that the optimum pH value for MAP crystallization is in the range of 9.5-10.5; the phosphorus removal efficiency increases with the increase of Mg/P molar ratio; the optimum Mg/P molar ratio is 1.4, and excessive Mg dosage does not show significant effect on the efficiency improvement of MAP crystallization; the co-existing of Ca in solution disturbs the morphology and purity of the MAP product, and amorphous calcium phosphate will form when Ca increases to a certain concentration; CO3(2-) can affect the P removal efficiency, but does not obviously affect the morphology and purity of MAP.