Modeling the crystallization of magnesium ammonium phosphate for phosphorus recovery

The crystallization of magnesium ammonium phosphate (MAP) is one of the main processes for recovering P and N from wastewater. Chemically defined solution systems were designed; the saturation indices (SIs) of the solution systems with respect to MAP were derived by using a geochemical aqueous model Program, PHREEQC 2.11; the effects of the solution conditions were evaluated using thermodynamic theories. The concentrations of P and Mg in the tested solutions were 10–600 mg l−1 and 24–720 mg l−1, respectively, the molar ratios of N/P and pH values of the solutions varied in the ranges of 1–40 and 6.0–12.0, respectively. The temperature of all the tests was set at 25 °C. The test results show that the SI value of MAP is the logarithmic functions of the concentrations of P, ammonium–N and Mg, and increases with the increase of the concentration of each element. The SI value of MAP is a polynomial function of pH value of the solution, and the optimum pH value for the crystallization of MAP is 9.0 but increases slightly with the increase of the N/P. Moreover, the SI value of MAP is a power law function of the ionic strength of solutions but decreases with its increase. The adjustment of the Mg concentration and the control of solution pH are two effective methods for the control of the crystallization of MAP. The results obtained from the research can be used to guide the design and control of MAP crystallization process for the removal and recovery of P.