Enhancing the recovery of calcium phosphates from wastewater treatment systems through hybrid process of oxidation and crystallization

Abstract

A novel two-stage hybrid process of oxidation and crystallization has been developed and optimized for recovery of high purity calcium phosphates (CaP) from digester supernatant. Oxidation using H2O2 with or without ultrasonication (US) treatment has been proved effective on degrading the natural organic matter (NOM) in digester supernatant, which exhibits a strong inhibiting effect on the crystallization of CaP. It has been discovered that the optimal pH for the oxidation process is pH 7.8. Application of US intensified the oxidation process in the first 2 h, but this effect was diminishing after 24 h. Increasing Ca/P ratio from 2 to 3 in the crystallization step led to decreased P-recovery yield, which has been attributed to the decreased operating pH at the higher Ca/P ratio. Hydroxyapatite of poor crystallinity was obtained from non-seeded crystallization experiments with the highest P-recovery rate of 93%. Seeding with 0.1 g of Dicalcium Phosphate Dehydrate was not sufficient to suppress the nucleation of Hydroxyapatite, and a mixture of Hydroxyapatite and Dicalcium Phosphate Dehydrate has been recovered from the seeded crystallization processes.