The effects mechanism of CO₃^2- and Ca² on phosphorus removal of synthetic anaerobic digestion of swine wastewater by crystallization of magnesium ammonium phosphate

Abstract   Using synthetic anaerobic digestion of swine wastewater, bench-scale experiments were carried out. The influence mechanism of carbonate (CO₃^2-), calcium (Ca²⁺) as well as the coexistence of CO₃^2- and Ca²⁺ on magnesium ammonium phosphate (MAP) precipitation reaction was analyzed by the modified Visual MINTEQ 3.0. The composition of the obtained precipitate was analyzed and characterized by X-ray Diffraction (XRD). The results showed that higher pH value and initial n(Mg):n(N):n(P) molar ratio were beneficial to MAP precipitation. However, with pH value≥9.5 and the initial n(Mg):n(P) molar ratio≥1.4:1, these two factors had negligible effects on the improvement of phosphate removal rate. Precipitated P is a binomial function of the solution pH value. At pH 9.5, the presence of CO₃²⁻ alone marginally reduced phosphorus (P) removal efficiency, while exhibiting no discernible impact on the crystalline structure or purity of MAP product. The single existence of Ca²⁺ increased P removal efficiency. The concentrations of Ca²⁺ obviously affected the components of the precipitate and the purity of MAP. At the condition that n(Mg): n(Ca)=2:1, MAP became the main component of the precipitate, with the purity of MAP be about 66.7%. Amorphous calcium phosphate(ACP) was produced with the concentration of Ca²⁺ increased. The effects of coexisting CO₃^2- and Ca²⁺ on the precipitation process was consistent with the single existence of Ca²⁺. CaCO₃ precipitation was absent at Mg:CO₃:Ca molar ratios of 2:1:1 and 1:1:1. At a 1:2:2 ratio, CaCO₃ may have formed, but its concentration was below the XRD detection limit, precluding definitive identification.