Alleviating Na+ effect on phosphate and potassium recovery from synthetic urine by K-struvite crystallization using different magnesium sources.

Human urine is characterized by high concentrations of nitrogen (N), phosphorus (P) and potassium (K), of which the P and K can be recovered as K-struvite crystals. This study first investigated the formation of Na-struvite because of the high Na+ present in the urine. From the results, the optimal pH for the Na-struvite crystallization was observed to be 12, and the rise in the Na+ concentration distinctly favored the Na-struvite formation. As magnesium needed to be added to induce the K-struvite crystallization, several magnesium sources including MgCl2, Mg sacrificial electrode and Mg(OH)2 were applied to recover P and K from synthetic urine. The findings indicated that when MgCl2 was used as the magnesium source, the K removal could be slightly enhanced by prolonging the reaction time, which would correspondingly decrease the Na concentration in the precipitates; besides, the intermittent addition of MgCl2 could noticeably improve the removal efficiency of K by 6%, but simultaneously raise the Na content in the precipitates recovered. With respect to the use of the Mg sacrificial electrode, the recovery efficiencies of the P and K from synthetic urine were close to those with the use of MgCl2. However, when Mg(OH)2 was used as the magnesium source, the recovery efficiencies of P and K achieved only roughly 50%, which was much lower than those noted when MgCl2 and the Mg sacrificial electrode were employed. A comprehensive analysis revealed that the MgCl2 was the best magnesium source for the K-struvite crystallization, followed by the Mg sacrificial electrode and Mg(OH)2.