Using selectivity to evaluate aqueous- and resin-phase denitrification during biological ion exchange

An increased fertilizer application for agricultural purposes has resulted in increased nitrate (NO₃^-) levels in surface water and groundwater around the globe, highlighting demand for a low-maintenance NO₃^- treatment technology that can be applied to nonpoint sources. Ion exchange (IEX) is an effective NO₃^- treatment technology and research has shown that bioregeneration of NO₃^- laden resins has the potential to minimize operational requirements and brine waste production that often prevents IEX application for decentralized treatment. In this work, batch denitrification experiments were conducted using solutions with low IEX selectivity capable of supporting the growth of denitrifying bacteria, while minimizing NO₃^- desorption from resins, encouraging resin-phase denitrification. Although only 15% of NO₃^- was desorbed by the low selectivity solution, this initial desorption started a cycle in which desorbed NO₃^- was biologically transformed to NO₂^-, which further desorbed NO₃^- that could be biotransformed. Denitrification experiments resulted in a 43% conversion rate of initially adsorbed NO₃^-, but biotransformations stopped at NO₂^- due to pH limitations. The balance between adsorption equilibria and biotransformation observed in this work was used to propose a continuous-flow reactor configuration where gradual NO₃^- desorption might allow for complete denitrification in the short retention times used for IEX systems.