Investigation of N and P removal from aqueous solution by magnesium ammonium phosphate (MAP) precipitation

Precipitation of struvite or magnesium ammonium phosphate (MgNH4PO4.6H(2)O, MAP) in wastewater treatment plants where nitrogen (N) and phosphorus (P) treatment is carried out causes very important operational problems. Also, since MAP has commercial value, its control and recovery is important. Therefore, batch experiments were conducted to examine the effects of pH, Magnesium (Mg): N and N:P molar ratio, different Mg2+ sources and the presence of calcium (Ca2+) on MAP crystallization. Experimental results showed that at a constant Mg:N:P=1:1:1 ratio, the optimum pH was 9, and the N and P removal efficiencies at this pH were 79.4% and 88.4% respectively. These removal efficiencies for N and P increased to 87.6% and 99.1%, respectively, by increasing the Mg:N ratio to 1.25:1. By increasing the N:P ratio to 1:1.15, the N removal efficiency increased to 96.4%, but the P removal efficiency decreased to 96.1%. The most effective source of Mg2+ was found to be MgCl2.6H(2)O, with N and P removal efficiencies of over 96%. It was found that the N removal efficiency was significantly reduced in the presence of Ca2+ in the solution. It was determined that the N removal efficiency, which was >96% in the absence of Ca2+, decreased to 68.6% at 1000 mg/L Ca2+ concentration. In the XRD analysis performed on the sediment, it was determined that the formation was a MAP crystal, and in the addition of Ca2+, an amorphous structure rich in Ca2+ was formed instead of MAP.