Effect of solution composition on struvite crystallization

BACKGROUND: Struvite precipitation has been applied to several types of wastewater for removal and recovery of nutrients. Engineering criteria for these applications, particularly crystallization, have yet to be developed. In the present study, effects of solution composition on the induction period and crystallization rate of struvite were experimentally investigated. Experiments were conducted as batch tests and on synthetically prepared samples. High foreign ion concentrations up to 160 mmol L⁻¹ were used for the evaluation. RESULTS: Effect of ions such as sodium, chloride, sulfate, and acetate that frequently accompany struvite crystallization was evaluated with varying composition of solutions. The induction time decreased with increasing supersaturation. The increase in sodium concentration from 14.25 to 130.52 mmol L⁻¹ resulted in about a four-fold retardation in induction time. Increasing sulfate concentration to 40.5 mmol L⁻¹ caused a significant retardation reaching an induction time of 30 min. Sulfate ions exhibited a dependence on chloride concentration. Effect of acetic acid was marginal up to 10 mmol L⁻¹. Crystallization kinetics was represented by modified first-order kinetics. The rate constants varied between 0.0053 and 0.1622 min⁻¹. The lowest rate constants were calculated for compositions containing high concentrations of sodium and sulfate ions. CONCLUSION: Results of the study indicated that the existence of common ions in wastewaters may give rise to great variations in the induction period and crystallization rate of struvite. The crystallization process was satisfactorily modelled; and the rate constant exhibited a wide range depending on the wastewater composition. The results indicated the need for further studies considering the complex matrix of wastewaters of concern.