Thermodynamic Assessment of Effects of Solution Conditions on Precipitation and Recovery of Phosphorus from Wastewater

Abstract

Abstract

Chemical modeling calculations and batch tests were carried out to investigate the effect of solution chemistry on formation of phosphate precipitation in synthetic flushed dairy manure wastewater for phosphorus recovery. Saturation index (SI) of different calcium phosphate precipitates in solution with a PO₄³⁻ concentration range of 0.032–9.8 mM (1–300 mg P/L), Ca/P molar ratio of 1–20, pH value of 5.0–12.0, and the CO₃²⁻ concentration of 0–100 mM was calculated separately using the geochemical aquatic modeling program, PHREEQC. Results show that the SI of calcium phosphate is the logarithmic function of Ca²⁺ and PO₄³⁻ concentration, increasing with the increase of Ca²⁺ and PO₄³⁻ concentrations. SI of calcium phosphate is the polynomial function of the solution pH value. SIs of hydroxyapatite (HAP) and tricalcium phosphate (TCP) increase with the growth of the pH value, while the SIs of octacalcium phosphate (OCP) and dicalcium phosphate dehydrate (DCPD) reach the maximum value at pH 9.0–9.5 and 7.0–7.5, respectively. The SI of calcium phosphate decreases with the growth of the CO₃²⁻ concentration following a linear function pattern. Meanwhile, the SI of calcium phosphate decreases with growth of ionic strength following a logarithmic function pattern. In the case study of phosphate removal from synthetic flushed dairy manure wastewater, the PO₄³⁻ removal trend under a different pH value and Ca/P molar ratios was closer to the predictions of thermodynamic modeling. CO₃²⁻ can affect the PO₄³⁻ removal efficiency and turn on the marked inhibiting effect on HAP growth, but does not obviously affect the structure of the precipitate.