Struvite crystallization, a chemical precipitation method for the recovery of nutrients, has been extensively studied. However, there is a lack of knowledge about the effect of different interacting factors on the purity of struvite. In this study, a central composite design using response surface methodology, scanning electron microscopy-energy dispersive X-ray spectrometry, and X-ray diffraction was employed to investigate the effects of pH, the Mg/P ratio and Ca2+ interactions on struvite purity. These characterization results showed that pure struvite was produced when the pH was lower than 8.5. When the pH increased from 8.5 to 10.0, the purity of struvite decreased, and a large amount of calcium carbonate appeared in the precipitate. A lower magnesium salt dosage (with an Mg/P molar ratio lower than 1.8:1) failed to compete with calcium for phosphate ions, which resulted in the formation of impurities in struvite. In addition, an excessive Mg dosage was found to not improve the purity. In consideration of the purity of the obtained struvite and the cost of the chemical reagents, the operation parameters were optimized. Results showed that the best conditions for the crystallization of pure struvite were pH 8.5 and an Mg/P ratio of 1.8:1. Furthermore, the recovery efficiencies of phosphate, ammonia nitrogen, and calcium were 88.9%, 9.6%, and 0.9%, respectively.
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