Modeling assessment for ammonium nitrogen recovery from wastewater by chemical precipitation

Chemical precipitation to form magnesium ammonium phosphate (MAP) is an effective technology for recovering ammonium nitrogen (NH4(+)-N). In the present research, we investigated the thermodynamic modeling of the PHREEQC program for NH4(+)-N recovery to evaluate the effect of reaction factors on MAP precipitation. The case study of NH4(+)-N recovery from coking wastewater was conducted to provide a comparison. Response surface methodology (RSM) was applied to assist in understanding the relative significance of reaction factors and the interactive effects of solution conditions. Thermodynamic modeling indicated that the saturation index (SI) of MAP followed a polynomial function of pH. The SI of MAP increased logarithmically with the Mg2+/NH4+ molar ratio (Mg/N) and the initial NH4(+)-N concentration (CN), respectively, while it decreased with an increase in Ca2+/NH4+ and CO3(2-)/NH4+ molar ratios (Ca/N and CO3(2-)/N), respectively. The trends for NH4(+)-N removal at different pH and Mg/N levels were similar to the thermodynamic modeling predictions. The RSM analysis indicated that the factors including pH, Mg/N, C(N), Ca/N, (Mg/N)x (CO3(2-)/N), (pH)2, (Mg/N)2, and (C(N))2 were significant. Response surface plots were useful for understanding the interaction effects on NH4(+)-N recovery.