Phosphate recovery from anaerobic digester effluents using CaMg(OH)4

Dolomite lime (DL) (CaMg(OH)₄) was used as an economical source of Mg^2 + for the removal and recovery of phosphate from an anaerobic digester effluent of a municipal wastewater treatment plant (MWWTP) wastewater. Batch precipitation results determined that phosphate was effectively reduced from 87 to less than 4 mg-P/L when the effluent water was mixed with 0.3 g/L of DL. The competitive precipitation mechanisms of different solids in the treatment system consisting of Ca^2 +–Mg^2 +–NH₄⁺–PO₄^3 − CO₃^2 − were determined by comparing model predictions with experimental results. Thermodynamic model calculations indicated that hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), Ca₄H(PO₄)₃?3H₂O, Ca₃(PO₄)₂(beta), and Ca₃(PO₄)₂(am2) were more stable than struvite (MgNH₄PO₃?6H₂O) and calcite (CaCO₃). However, X-ray diffraction (XRD) analysis determined the formation of struvite and calcite minerals in the treated effluent. Kinetic experimental results showed that most of the phosphate was removed from synthetic effluent containing NH₄⁺ within 2 hr, while only 20% of the PO₄^3 − was removed in the absence of NH₄⁺ after 24 hr of treatment. The formation of struvite in the DL-treated effluent was due to the rapid precipitation rate of the mineral. The final pH of the DL-treated effluent significantly influenced the mass ratio of struvite to calcite in the precipitates. Because more calcite was formed when the pH increased from 8.4 to 9.6, a pH range of 8.0–8.5 should be used to produce solid with high PO₄^3 − content. This study demonstrated that DL could be used for effective removal of phosphate from the effluent and that resultant precipitates contained high content of phosphate and ammonium.