Removal of Low Concentration Nitrous Oxide from Gas Streams Using a Self-Sustaining Biofiltration System with Gravitationally-Fed Wastewater as the Nutrient Solution

Abstract

N2O is one of the major greenhouse gas with a global warming potential 298 times that of CO2 and a potent ozone-depletion agent. Physical or chemical methods for removal of highly-concentrated sources of N2O have been put into practice for years; however, these techniques are not suitable for mitigation of low-concentration N2O emitted from wastewater treatment plant (WWTP) as the life-cycle CO2 emissions resulting from electricity and chemicals would exceed the removed greenhouse gas. In this study, we have developed a biological system for removal of N2O emitted at low concentrations (<200 ppmv) from WWTP that would require no extra energy or nutrients (chemicals). A lab-scale biofiltration system was constructed with polyurethane foams as the support for biofilm, where N2O would be consumed as electron acceptors. Wastewater collected from 1st sedimentary area of Daejeon WWTP was used both as the source of electron donor and nutrient and the inoculum for the biofilter. The biofilter was initially operated at room temperature under anoxic condition (influx of ~100-200 ppmv N2O in N2) and later under aerobic condition (influx of ~100 ppmv N2O in air). The velocity of inlet gas was varied from 37.5~150 ml/min (the empty bed residence time > 30 mins). The operation of the lab-scale biofilter with pure water and anoxic stream of N2O (negative control) confirmed no N2O was removed abiotically by physical or chemical interactions with the packing materials. In the positive controls with Pseudomonas stutzeri as the inoculum and synthetic wastewater with acetate as the nutrient medium, >94% removal efficiency was observed. With actual wastewater collected from Daejeon WWTP as the inoculum and nutrient medium, >85% removal efficiency (<18 ppmv N2O in the effluent gas) was achieved and maintained for a prolonged period (100 hours), as long as circulated wastewater was exchanged upon depletion of bioavailable carbon. Even under aerobic operation where 100 ppmv N2O in air was used as the influent gas, >50% removal efficiency was observed, suggesting that N2O still took place in anoxic niches within the biofilter. In the designed system, unlike in the laboratory setup, the biofilter would receive wastewater continuously from the 1st sedimentary tank located at higher elevation, using gravitational force and thus, would require no additional power or chemical input. We expect that incorporation of this biofiltration system into designs of WWTPs would significantly contribute to engineering efforts for mitigation of life-cycle greenhouse gas emissions from WWTPs.