Optimizing Struvite Crystallization at High Stirring Rates

Phosphorus and ammonium can both be recovered in the presence of magnesium through struvite (MgNH4PO4·6H2O) crystallization. The present work aimed to optimize struvite crystallization at turbulent solution flow. Struvite was crystallized by magnetic stirring at different initial phosphorus concentrations between 200 and 800 mg·L−1 and high stirring rates between 100 and 700 rpm. The crystals obtained were analyzed by powder X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy. For all experiments, the only phase detected was struvite. It was shown that for an initial phosphorus concentration of 200 mg·L−1, increasing the stirring rate to 500 rpm accelerated the precipitation of struvite, improved the phosphorus removal efficiency, and obtained larger struvite crystals. A decrease in the phosphorus removal efficiency and smaller struvite crystals were obtained at higher stirring rates. This was attributed to the solution turbulence. The limiting effect of turbulence could be overcome by enhancing the initial phosphorus concentration or by lowering the stirring rate. The highest phosphorus removal efficiency (~99%) through large struvite crystals (~400 μm in size) was obtained for an initial phosphorus concentration of 800 mg·L−1 and a stirring rate of 100 rpm.