Removal of nitrate from groundwater using ZVI treatment system combined with continuous CO2 gas bubbling

A zero-valent iron (ZVI) treatment system combined with pH adjustment, aeration, sedimentation and microfiltration processes was developed for the robust removal of nitrate from groundwater. Powdered ZVI dosed at 10 g L-1 removed nitrate by 166 mg L-1, of which 80% were converted to ammonium ion for a given contact time of 125 h. By diffusing gaseous carbon dioxide (CO2) into the ZVI reactor, pH was desirably controlled in the range of 5.5-6.5, and concurrently < 0.5 mg L-1 dissolved oxygen was consistently maintained during the longer-term of dissolved oxygen ZVI treatment system. The combination of ZVI treatment and CO2 gas bubbling reached up to 35% removal of nitrate and passivation on the surface of ZVI was prevented by diffusing CO2 gas. Soluble Fe (II) concentration of mixed liquor in the ZVI reactor ranged between 0.2 and 7.8 mg L-1 and Fe (II) dissolved in the ZVI-treated water was almost completely removed by subsequent aeration. The overall result shows that the novel ZVI treatment system is significantly promising to remove nitrate in groundwater and therefore to meet the regulatory limit for drinking water.