Experimental Study On Nitrous Oxide Emissions Reduction By Aerobic Sequencing-Anoxic Continuous Flow Constructed Wetland

As a harmful greenhouse gas, Nitrous oxide(N2O) is the main reason to cause globe warming, ozone depletion and acid rain. N2 O has a globe warming potential(GWP) that is 130~296 times more than carbon dioxide in the 100-year time frame. A large number of studies have shown that constructed wetlands(CWs) wastewater treatment system are considered important sources of N2 O. N2 O is mainly produced in the process of nitrification and denitrification. Having the advantage of lost cost, easy operation and maintenance, CWs are widely used for wastewater treatment. Therefore, it is significant that the production and emission of N2 O in CWs are controlled.In this study, an aerobic sequencing-anoxic continuous constructed wetland was used to reduce N2 O emissions in the process of nitrification and denitrification. Water drops in series and the operation of sequencing could increase the concentration of DO in the substrate in the process of nitrification. As a result, the nitrification process could be strengthened. The operation of continuous flow could reduce the concentration of DO in the substrate in the process of denitrification. So, the denitrification reaction was completely finished and the emissions of N2 O could be reduced. To find the optimal hydraulic retention time, we could study the effect of hydraulic retention time on wastewater treatment and N2 O emissions. Under the optimal HRT, we could study the effect of influent concentration and temperature on wastewater treatment and N2 O emissions. The nitrogen mass balance model was established to analyze the law of transport and transformation of nitrogen in aerobic sequencing-anoxic continuous constructed wetland and measure the reduction potential of N2 O emissions. The content and conclusions are as following:① Study on the effect of hydraulic retention time on wastewater treatment and N2 O emissions indicate that, the treatment efficiency could be increased with the lengthening of HRT within a certain range. When the HRT of each aerobic sequencing CW was 3h or 4h and anoxic continuous CW was 3h, the concentration of COD, NH3-N, TN, TP in effluent could reach the B standard in the first rating of the GB18918-2002. When the HRT of each aerobic sequencing CW was 1h, 2h, 3h or 4h, and anoxic continuous CW was 1h or 3h, the average N2 O emission flux was 70.2μg m-2h-1, 66.5μg m-2h-1, 103.7μg m-2h-1, 100.1μg m-2h-1, 139.7μg m-2h-1, 135.1μg m-2h-1, 238.5μg m-2h-1,234.4μg m-2h-1 respectively. The emissions of N2 O could increase with the lengthening of HRT in aerobic sequencing CW and the emissions of N2 O could decrease in anoxic continuous CW. The optimal HRT was all 3h in aerobic sequencing CW and anoxic continuous CW.② Study on the effect of influent concentration on wastewater treatment and N2 O emissions indicate that, when the influent concentration was high, medium or low, the removal of COD was 77%, 69%, 57% respectively. The concentration of COD in effluent could reach the B standard in the first rating of the GB18918-2002. When the influent concentration was high, the concentration of NH3-N, TN in effluent was 10mg/L, 21.71mg/L and reach the B standard in the first rating of the GB18918-2002 approximately. When the influent concentration was medium or low, the concentration of NH3-N, TN in effluent could reach the B standard in the first rating of the GB18918-2002. When the influent concentration was high, medium or low, the average N2 O emission flux in aerobic sequencing-anoxic continuous CW was 105.6μg m-2h-1, 132.6μg m-2h-1, 142.4μg m-2h-1 respectively and the average N2 O emission flux in anoxic continuous CW was 105.6μg m-2h-1, 132.6μg m-2h-1, 142.4μg m-2h-1 respectively. The emissions of N2 O could increase with the increase of influent concentration on the whole. However, the emissions of N2 O could decrease in anoxic continuous CW with the increase of influent concentration. The difference of N2 O emission flux under different influent concentration was not significant.③ Study on the effect of temperature on wastewater treatment and N2 O emissions indicate that, when the temperature was 25~35℃ or 12~25℃ under the optimal HRT, the concentration of COD, NH3-N, TN in effluent could reach the B standard in the first rating of the GB18918-2002. When the temperature was under 12℃, the concentration of NH3-N and TN could reach the B standard in the first rating of the GB18918-2002 approximately. The N2 O fluxes in CW presented obvious seasonal and diurnal variation. The highest N2 O fluxes was in October and the lowest N2 O fluxes was in January both in aerobic sequencing CW and anoxic continuous CW. The maximum N2 O fluxes during one day were appeared at noon and the minimum N2 O fluxes during one day were appeared at 24:00.④ Study on nitrogen mass balance and measurement the reduction potential of N2 O emissions indicate that, the N2 O emission accounted for nitrogen inflow in summer, autumn, winter and spring was 8.95×10-4、8.95×10-4、2.27×10-4、6.66×10-4 respectively. By comparing the emissions of N2 O produced in aerobic sequencing-anoxic continuous CW and wastewater treatment plant, it demonstrate aerobic sequencing-anoxic continuous CW has a greater potency of reducing N2 O emissions.An aerobic sequencing-anoxic continuous constructed wetland was used to reduce N2 O emissions in the process of nitrification and denitrification. The study results above would provide theoretical basis for reduction of greenhouse gas in the process of nitrogen and phosphorous removal in CWs, which had the important practical significance.