Development and validation of an equilibrium model for struvite formation with calcium co-precipitation

• A model was developed using numerical computation with MINTEQ and to predict the P recovery.
• The model included an estimation of struvite solubility constants with HPO42− as well as PO43−.
• A high correspondence between the modeled and experimental P-recoveries was achieved.

This study developed an equilibrium model to predict the P recovery and struvite amounts by newly incorporating two separate equilibrium constants on the struvite formation with HPO42− and PO43−, as well as free ammonium (NH4+), phosphate (PO4), magnesium (Mg2+), and calcium (Ca2+) ion species. The equilibrium struvite reaction and its solubility constant with HPO42− species was verified by deriving a reasonable correlation between solution pH and the conditional solubility products that were obtained from the equilibrium reaction. Also, based on the Visual MINTEQ software program, the potentially precipitated Ca phosphates and struvite precipitates were selected, and these compounds were utilized as target precipitants for the modeling to simulate P recovery and struvite formation under the competitive inhibition of Ca ions. The resultant simulated P recovery data were validated by experimental data with synthetic wastewater. The model data showed good agreement with the experimental results (R2>95%). The model also confirmed that the purity of struvite in the precipitate and the pH that maximizes the struvite fraction are dependent on the initial concentrations of NH4+, Mg2+, and PO4. Because only PO43−, not HPO42−, was regarded in Ca precipitation, Ca phosphate precipitation was underestimated as compared with the experimental results.