Application of the hydroxyapatite crystallization-filtration process to recover phosphorus from wastewater effluents

Phosphorus crystallization-filtration (PCF) was devised as a novel tertiary process for phosphorus removal from domestic wastewater. The results obtained showed that during the PCF process, high pH and excessive calcium dosage conditions were required to obtain effluents with total phosphorus (T-P) and suspended solid (SS) concentrations below 0.2 and 10 mg/L, respectively, within 2 h of operation. Phosphorus was precipitated during the pre-treatment step, and thereafter it crystallized on the surface of the fixed seed material in the PCF reactor. Furthermore, the addition of Ca²⁺ resulted in phosphorus removal efficiencies >95%, and pH, residual Ca²⁺, filtration depth, and linear velocity were identified as the main design and operation parameters of the PCF process. Following the pilot-scale PCF process, the average concentrations of T-P, PO₄-P, and SS in the effluent were 0.05, 0.04, and 1.1 mg/L, respectively, corresponding to removal efficiencies of 90.9, 86.5, and 79.7%, respectively. The investigation of the backwashing sludge characteristics of the PCF process using scanning electron microscopy (SEM), Fourier transform-infrared vacuum spectrometry (FT-IR), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses showed that owing to its high contents in calcite and hydrated phosphorus compounds, PCF sludge could be used as an alternative soil amendment resource.