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

Abstract

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. HIGHLIGHTS Microorganisms preferentially transformed aqueous-phase NO3−, and no indicators of resin-phase NO3− transformation were observed.; Aqueous- and resin-phase ion concentrations are driven toward an equilibrium state that is dependent on ion selectivities.; The balance between adsorption equilibria and biotransformations is relevant to the implementation of a continuous-flow BIEX reactor.;

Keywords

• adsorption
• biex
• biotransformation