Recovery of nitrogen from saponification wastewater by struvite precipitation

摘要

In general, saponification wastewater produced from the separation process of rare-earthnelements contains high ammonium concentration. In this study, a series of experiments werenconducted to investigate the parameters to enhance the struvite precipitation potential fornammonium removal from the wastewater having an ammonium concentration of 4,100mg/L.nExperimental results showed that increasing the dose and grain size of pre-formed struvite,nwhich was added as the seeding material in struvite reaction, could increase ammonium removal.nThe removal efficiency increased 7.6% when the dose of pre-formed struvite with crystal grainnsize range of 0.098–0.150mm increased from 0 g/L to 60 g/L. Additionally, struvite precipitationnwas tested with the intermittent addition of magnesium and phosphate to utilize the struvitencrystals formed during the reaction process as the seeding material for the subsequent reaction.nThe results revealed that intermittently adding magnesium 7 times effectively enhancednammonium removal by around 8%, which was equivalent to that of using pre-formed struvitenas the seeding material. Furthermore, the chemical composition of the struvite recovered withnintermittent addition of magnesium was characterized, showing the struvite could be used asnfertilizer. An economic evaluation indicated that intermittent addition of magnesium 7 times cannsave 13.4% cost for recovering per kg NH+ncompared to that of bulk addition.