Struvite crystallization under a marine/brackish aquaculture condition

The results in this study show that struvite was formed in the digester at pH 7.7 due to the magnesium naturally present and the released ammonia and phosphate, resulting in low phosphate concentration in the digester. Apparently the digester already provided proper conditions for struvite formation. Under the brackish condition, the estimated thermodynamic solubility product and enthalpy change of struvite formation were 10 13.06 and 25.7 kJ mol 1, respectively. The average crystal size under marine/brackish condition decreased with pH, but increased with temperature. X-ray diffraction measurements indicate struvite (NH4MgPO4 6H2O) and dittmarite (NH4MgPO4 H2O) were predominant phosphorus species produced in filtrates of the digester. However, struvite and newberyite (HMgPO4 3H2O) were the predominant species precipitated from synthetic brackish waters after dosing MgCl2. It is pronounced that (waste)water characteristics played also an important role on the nature of phosphate precipitates. Under high NH4 + condition, phosphorus precipitates containing ammonia were dominant, compared to other amorphous phosphates.