Phosphorus recovery from municipal wastewater via a two-step process of ozonation and crystallization: process development, optimization and upscaling

Abstract

A novel process consisting of ozonation and crystallization for the recovery of calcium phosphates with improved purity from digester supernatant was tested in a 2.5 L ozonation column and 1 L stirred tank crystallization reactor. The kinetics of natural organic matter (NOM) degradation has been studied and the optimal dose of ozone was determined to be 1.25 mg O₃ per mg of total organic carbon (TOC). Ozonation at pH 2.8 was the most effective in terms of P-recovery rate (86.4%) and product crystallinity improvement, due to the simultaneous stripping of CO₂. Ozonation at pH 8.1 did not have a significant effect on the P-recovery rate but resulted in the best improvement of product purity. A significant effect of NOM partial degradation on product purity and visual characteristics has been proven. With less than 15% of TOC reduction during the ozonation, the organic carbon content in final solid products decreased by approx. 50%. Thereafter, the process was preliminary scaled up to a 10 L set-up with a fluidized bed reactor. For the large set-up, the optimal dose of ozone was 2.2 mg O₃ per mg TOC. During crystallization in the fluidized bed reactor, the dosing of chemicals through the center of the reactor resulted in better control over nucleation and delayed flocculation of the NOM present in the supernatant. It leads to an increase of the P-recovery rate (73–75%) compared to the dosing from the bottom of the reactor (61–67%). In all experiments, low crystallinity hydroxyapatite (HAp)/amorphous calcium phosphate (ACP) were obtained.