Influence mechanisms of \bfCO_\bf3^\bf2- and Ca²⁺ on phosphorus removal of synthetic swine wastewater anaerobic digestate by magnesium ammonium phosphate crystallization

To enhance phosphorus recovery efficiency and the resource value of products from swine wastewater anaerobic digestate, this study systematically investigated the influence mechanisms of carbonate (\mathrmCO_3^2- ) and calcium ions (Ca²⁺) on phosphorus removal via magnesium ammonium phosphate (MAP) crystallization using synthetic swine wastewater anaerobic digestate. Batch precipitation experiments, coupled with Visual MINTEQ 3.0 thermodynamic modeling and X-ray diffraction analysis, were conducted to elucidate the phosphorus removal behavior and product characteristics under different pH conditions and molar ratios of n(Mg)∶n(N)∶n(P). The results indicated that a phosphorus removal rate exceeding 96% was achieved at pH 9.5 with n(Mg)∶n(P) = 1.4∶1; further increases in pH or n(Mg)∶n(P) molar ratio showed no significant improvement in removal efficiency. The presence of \mathrmCO_3^2- alone had minimal impact on phosphorus removal (>95%) and the purity of MAP crystals. In contrast, the presence of Ca²⁺ alone significantly altered the composition of the precipitates. When n(Mg²⁺)∶n(Ca²⁺) = 2∶1, the purity of MAP was approximately 66.7%, and as the Ca²⁺ concentration increased, the products gradually transitioned to being dominated by amorphous calcium phosphate. The combined influence of \mathrmCO_3^2- and Ca²⁺ was largely consistent with that of Ca²⁺ alone. Under high Ca²⁺/\mathrmCO_3^2- conditions, although a high phosphorus removal rate (≥95%) was maintained, the purity of the product decreased substantially. In practical applications, the Mg∶P molar ratio should be precisely controlled based on the ionic concentration ratios to balance phosphorus removal efficiency and the resource value of the recovered products.