Recovery of nitrogen from saponification wastewater by struvite precipitation

In general, saponification wastewater produced from the separation process of rare-earth elements contains high ammonium concentration. In this study, a series of experiments were conducted to investigate the parameters to enhance the struvite precipitation potential for ammonium removal from the wastewater having an ammonium concentration of 4,100 mg/L. Experimental results showed that increasing the dose and grain size of pre-formed struvite, which was added as the seeding material in struvite reaction, could increase ammonium removal. The removal efficiency increased 7.6% when the dose of pre-formed struvite with crystal grain size range of 0.098–0.150 mm increased from 0 g/L to 60 g/L. Additionally, struvite precipitation was tested with the intermittent addition of magnesium and phosphate to utilize the struvite crystals formed during the reaction process as the seeding material for the subsequent reaction. The results revealed that intermittently adding magnesium 7 times effectively enhanced ammonium removal by around 8%, which was equivalent to that of using pre-formed struvite as the seeding material. Furthermore, the chemical composition of the struvite recovered with intermittent addition of magnesium was characterized, showing the struvite could be used as fertilizer. An economic evaluation indicated that intermittent addition of magnesium 7 times can save 13.4% cost for recovering per kg NH₄⁺ compared to that of bulk addition.