Phosphorus (P) demand in agriculture has increased sharply with the global population rising exponentially, but the P ore reserves will become depleted over the long term as a non-renewable resource. Thus, P recovery from waste streams is considered to be a feasible solution to assure long-term P supply. 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).  

The Ecology Pollution Group in Wuhan Botanical Garden investigated P recovery and quality of crystals obtained from synthetic supernatant with varying Ca/Mg ratio through batch experiments.  

The results demonstrated that solution nitrogen and P concentration exhibited a similar trend with the increasing Ca/Mg ratio from 0 to 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 (XRD) 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. The competition and distribution of Ca-P and Mg-P crystals were largely correlated with Ca/Mg ratio. When the Ca/Mg ratio was above 1/2, the XRD analysis supported the dissolution of struvite characteristic peaks, and the solution was considered to be inappropriate for obtaining wanted struvite crystals with high purity. 

This study was supported by Funding Project of Sino-Africa Joint Research Center, Chinese Academy of Sciences, and the work was published in Chemical Engineering Journal with the title of "Impact of calcium on struvite crystallization in the wastewater and its competition with magnesium".  

The struvite purity decreased and the shape of crystals evolved from thin and long needle to shorter and thicker rod with the increasing Ca/Mg ratio (Image by WBG)