Repeated use of MgNH4PO4·6H2O residues for ammonium removal by acid dipping

A novel process was developed to repeatedly utilize MgNH4PO4·6H2O (MAP) for ammonium removal from wastewater by releasing ammonium in MAP through acid dipping. In acid solutions, ammonium can be released to solution from MAP while MAP was transformed to MgHPO4, which can be used to remove ammonium again. Ammonium-N concentration in acid solutions increased with the decreasing pH and increasing temperature, up to 3.27×104 mg/l at 60°C and pH 4.8. MAP also dissolved into acid solutions when transformation to MgHPO4·N:P, indicating the ratio of MAP transformation to MAP dissolution, was maximum at pH 6.0 and increased with the temperature. N:P was up to 38.8 at 60°C and pH 6.0. Recycling experiments were done. Ammonium-N in effluents was lower than 20 mg/l in the first five cycles, which indicates the feasibility of this process. XRD spectra also validate the transformation between MAP and MgHPO4.

The application of ammonia removal through MAP formation has been limited by two problems, high chemical cost and disposal of MAP precipitates. A novel process was developed to solve the problems by recovering ammonium using acid and repeatedly using MAP residues. Ammonium could be efficiently released from MAP with a low N/P ratio at a pH ≤ 5.0 and temperature ≥40°C. The MAP was mainly transformed to MgHPO4·3H2O in acid solutions, which could be used for ammonium removal again. With a dosing ratio of MgHPO4·3H2O to NH4+–N of 2, the MAP precipitates could be at least repeated used for 5 times, and the residual NH4+–N in the treated effluents was below 20 mg/l in the first 4 cycles.