Evaluation of the role of inherent Ca²⁺ in phosphorus removal from wastewater system

The role of inherent Ca²⁺ in phosphorus removal from wastewater was evaluated by batch tests. Precipitates were characterized by an X-ray diffractometer (XRD), Fourier transform infrared spectrophotometer (FT-IR) and scanning electron microscope with an energy dispersive spectrometer (EDS) system. Effects of inherent Ca²⁺ on phosphorus removal through basic oxygen furnace slag (BOFS) were also analyzed. The results show that upon adjusting the pH to higher than 7.0, inherent Ca²⁺ can remove phosphorus from wastewater and form Ca-P precipitates. Residual phosphorus exhibited a linear decreasing trend with increasing the pH from 7.0 to 10.0 and then remained unchanged at higher pH than 10.0. EDS determined that the precipitates contained the elements Ca, P and O. FT-IR spectra demonstrated that the functional groups of precipitates involved PO₄³⁻, OH⁻ and CO₃²⁻. XRD indicated that the precipitates may consist of CaCO₃ and some Ca-P phosphates such as CaHPO₄, Ca₄H(PO₄)₃, Ca₃(PO₄)₂, and Ca₅(PO₄)₃(OH). During the removal process of phosphorus by BOFS, due to the presence of inherent Ca²⁺ in wastewater, the removal efficiency and rate of phosphorus increased by 15.5% and by a factor of about 3.0, respectively.