Recovery of ammonia nitrogen and magnesium as struvite from wastewaters in coal-fired power plant

This paper describes a cost-effective method for recovering ammonia nitrogen (NH₄ ⁺-N) from regeneration wastewater by using magnesium recovered from flue-gas desulfurization (FGD) wastewater in the same coal-fired power plant. Mg²⁺ was efficiently removed from FGD wastewater at pH of 11, and the settling performance of Mg (OH)₂ was evidently improved by adding anionic polyacrylamide and crystal seed. For NH₄ ⁺-N removal optimization by response surface methodology, the results indicated that the molar ratio of phosphorus to nitrogen (P/N) was the more dominant parameter, and the optimum conditions were predicted at Mg/N of 1.1 and P/N of 0.7. At the optimum conditions, the predicted data of NH₄ ⁺-N removal efficiency (NRE) and residue phosphorus were 71.41% and 21.90 mg/L, and actual experimental data were 72.12% and 19.60 mg/L. The inhibitory effect of NaCl concentration on NRE matches the modified Monod model with half inhibition concentration of 89.01 g/L. Morphologic analysis confirmed that salinity inhibited the formation and growth of struvite crystals.