A pressurized hydrogenotrophic denitrification reactor system for removal of nitrates at high concentrations

Abstract

A pressurized hydrogenotrophic denitrification reactor to remove nitrate from groundwater was recently presented. To enable treatment of nitrate-concentrated brines, we demonstrate here the removal of high nitrate concentrations without nitrogen gas accumulation in the reactor’s headspace over time. In order to eliminate nitrogen gas build up in the pressurized reactor’s headspace, the main unsaturated flow pressurized reactor was connected to an external degassing unit via liquid recirculation. Pressurized liquid from the main reactor was intermittently discharged to the degassing unit where dissolved N2 gas effervesced and escaped to the atmosphere. The degassed effluent water was continuously recirculated back to the main reactor to maintain steady-state nitrogen and hydrogen gas partial pressures. Denitrifying rates of up to 6 g N/L-reactor/d were achieved in the reactor system under freshwater and brine conditions. Nitrate removal was over 97 %, with nitrate effluent concentrations lower than 10 mg N/L for influent concentrations of about 400 mg N/L. The reactor system maintained steady-state nitrogen and hydrogen gas partial pressures using low external recirculation rates at low reactor total pressures (< 5 atm). At high denitrification rates, dissolved nitrogen concentrations in the reactor were super saturated (125–150%) while on the other hand, dissolved hydrogen was undersaturated (30–60%) due to intense bacteria uptake from the water. These dynamic conditions reduced the external recirculation flow requirement for degassing and resulted in much better hydrogen utilization than expected (greater than 93 %).