Thermodynamic modeling of ferric phosphate precipitation for phosphorus removal and recovery from wastewater

Phosphorus removal and recovery by ferric phosphate (FePO4·2H2O) precipitation has been considered as an effective technology. In the present study, we examined chemical precipitation thermodynamic modeling of the PHREEQC program for phosphorus removal and recovery from wastewater. The objective of this research was to employ thermodynamic modeling to evaluate the effect of solution factors on FePO4·2H2O precipitation. In order to provide comparison, with the evaluation of thermodynamic modeling, the case study of phosphate removal from anaerobic supernatant was studied. The results indicated that the saturation-index (SI) of FePO4·2H2O followed a polynomial function of pH, and the solution pH influenced the ion activities of ferric iron salts and phosphate. The SI of FePO4·2H2O increased with a logarithmic function of Fe3+:PO43− molar ratio (Fe/P) and initial PO43− concentration, respectively. Furthermore, the SI of FePO4·2H2O decreased with a logarithmic function of alkalinity and ionic strength, respectively. With an increase in temperature, the SI at pH 6.0 and 9.0 decreased with a linear function, and the SI at pH 4.0 followed a polynomial function. For the case study of phosphate removal from anaerobic supernatant, the phosphate removal trend at different pH and Fe/P was closer to the predictions of thermodynamic modeling. The results indicated that the thermodynamic modeling of FePO4·2H2O precipitation could be utilized to predict the technology parameters for phosphorus removal and recovery.