Impact of calcium on struvite crystallization in the wastewater and its competition with magnesium

Phosphorus (P) recovery in the form of struvite might be impacted by the calcium (Ca) in the wastewater due to its competition reaction with magnesium (Mg). This study designed synthetic supernatant with varying Ca/Mg ratio, and focused on P recovery and quality of crystals obtained through batch experiments. The results demonstrated that an obvious change in solution pH was exhibited under the Ca/Mg ratio of 1/3 and 3/2, while little difference under the Ca/Mg ratio of 1/2 and 1/1, compared to the condition of no Ca containing. Solution nitrogen (N) and P concentration exhibited a similar trend with the increasing Ca/Mg ratio from 0 to 3/2. The remaining N and P concentration both became higher under the Ca/Mg ratio of 1/3 and 1/2 accompanied with lower nutrients removal rate, while the opposite phenomenon was observed under the Ca/Mg ratio of 1/1 and 3/2. The shape of crystals formed evolved from thin and long needle to shorter and thicker rod with the increasing Ca/Mg ratio from 0 to 3/2. When the Ca/Mg ratio was 1/2 or above, the X-ray diffraction analysis and scanning electron-energy dispersive spectrometry microscopy supported the apparent disappearance of struvite characteristic peaks and the appearance of a smaller amorphous Ca-P substance covered. Meanwhile, the increasing supersaturation index of Ca-P compounds corresponds to a decrease of that for struvite with an increasing Ca/Mg ratio, leading to an inhibition effect on the struvite crystallization. Thus, the wastewater with the Ca/Mg ratio above 1/2 was thought to be no longer suitable for P recovery due to low proportion of struvite.