Assessing struvite precipitation in a pilot-scale fluidized bed crystallizer

The recovery of phosphates from biological wastewater treatment plants, through struvite crystallization, minimizes operational downtime and offers the potential for cost-effective recovery. The pilot-scale, fluidized bed reactor developed at the University of British Columbia (UBC) was found to be effective in recovering phosphate in the form of nearly pure struvite product, from an anaerobic digester centrate. The desired degree of phosphate removal was achieved by maintaining operating pH 8.0-8.2, and recycle ratio 5-9, to control the supersaturation conditions inside the reactor. The performance of the system was found to be optimal when the in-reactor supersaturation ratio was 2-6. In-reactor magnesium to phosphate molar ratio was found to be an important parameter to maintain system performance. In-reactor ammonium to phosphate molar ratio was also found to maintain a good correlation with phosphate removal. The influence of organic ligands on the struvite precipitation was investigated for a small molecular weight organic ligand, acetate, using a chemical equilibrium model PHREEQC. An acetate concentration below about 100 mg l(-1) was not found to affect the precipitation potential of struvite. Calcium and carbonate ion did not have any noticeable effect in struvite crystallization of struvite, under the operational concentrations utilized. Since the precipitation of calcium and carbonate compounds was controlled by kinetic factors, rather than thermodynamic solubility alone, the solid precipitates harvested were pure struvite, with undetectable impurities.