Effect of the stirring speed on the struvite formation using the centrate from a WWTP

The formation of struvite (MgNH4PO4·6H2O) for nutrient recovery in wastewater treatment plants has been widely investigated; however, little attention has been paid to the effect of stirring speeds on the resulting particle size, which could affect its agronomic value as a slow-release fertilizer. In this study, struvite formation from the centrate of sewage digestate was performed under six stirring speeds (0, 100, 200, 300, 400, 500 rpm). The resulting struvite crystals were characterised using X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy. The average particle size of struvite crystals increased from 55 µm at 0 rpm to 127 µm at 100 rpm and 128 µm at 200 rpm. Further increments in stirring speeds resulted in smaller crystal sizes. These results indicated that the largest particle size can be obtained at stirring speeds ranging from 100 to 200 rpm, equivalent to a velocity gradient between 79 and 188 s-1, as there was no statistically significant difference between mean values (t-test, p<0.05). The optimum stirring speed range reported herein can be used to set operational conditions for struvite crystallisation with the benefit of producing large crystals and reducing energy consumption in stirring tanks.