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

Xueyu Liu , Zhonghou Xu , Jianfeng Peng , Yonghui Song , Xiaoguang Meng


Received January 12, 2016,Revised February 03, 2016, Accepted February 05, 2016, Available online March 17, 2016

Volume 28,2016,Pages 260-268

Dolomite lime (DL) (CaMg(OH)4) was used as an economical source of Mg2 + 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 Ca2 +–Mg2 +–NH4+–PO43 − CO32 − 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 PO43 − 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 PO43 − 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.

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