Struvite precipitation under changing ionic conditions in synthetic wastewater: Experiment and modeling

Abstract

A computational model was developed and applied to investigate struvite precipitation under different pH levels and ionic concentrations. Ionic species, including ammonium (NH4-), hydrogen phosphate (HPO42-), magnesium (Mg2+), calcium (Ca2+), and struvite, were incorporated into the proposed model. The unknown kinetic coefficients of struvite were identified from the experimental data. In this study, the kinetics of struvite precipitation, which combined the ionic reactions (NH4+/NH3 and HPO42-/H2PO4) on the basis of pseudo-equilibrium conditions was parameterized. The experiments for model verification were conducted at a constant initial Mg/P ratio with changing ionic concentrations at pH levels of 8.7 and 9.7. The batch experiments showed high struvite precipitation (>90% for 300 mg-N L-1 and 100 mg-P L-1). The presence of Ca2+ (Ca2+/Mg2+ >0.5) in the reactors interfered with the formation and growth of struvite. The decrease in the pH level with the struvite precipitation verified the simulation data. The model also confirmed the optimal ionic conditions in order to maximize the struvite precipitation (300 mg-P L-1, and N/P molar ratio >7). The model responding to the ionic conditions provided good prediction of the decrease in the pH levels and the positive role of the nitrogen levels for struvite precipitation. High nitrogen concentrations provided high P removal due to pH buffering and crystal purity.