Experimental Study On Of Nitrous Oxide Emissions Reduction By Sequencing Constructed Wetland Treating Wastewater

Global warming is one of the biggest challenges for human beings in the21stcentury. In the near future, the climate would exceed the range that human and naturecould possibly endure if proper mitigation measures are absent in most countries, andthere would be a high price to pay in social, environmental and economic area. Wastewater treatment is one of the energy-intensive businesses and0.75kWh electricity isneeded for per ton wastewater. Up to2011,Centralized wastewater treatment capacityhad reached1.36×108ton per day in China, and the energy consumed in the processequaled to9.18×104ton CO2per day. Decentralized wastewater treatment methods arecharacterized by small-scale and localized, which can control pollution from source andreduce energy. Constructed wetland, an important method of decentralized treatment, iswidely applied to wastewater treatment and surface water environment modification.However, the emission of CH4and N2O from constructed wetland, which still needs tobe further studied, can never be overestimated.In this study, a Sequencing Constructed Wetland which is especiallysuitable formountain city is developed. Water drops to next level of wetland could bring moreoxygen to the substrate, and in the discharge phase water flows out from the substrate,the oxygen would come into more easily. In this way, nitration is strengthened anddenitrification is suppressed. The effect of hydraulic retention time, influentconcentration and temperature on the wastewater treatment and N2O emission werestudied; the emission pattern of the Sequencing Constructed Wetland was monitored;the nitrogen mass balance of the constructed wetland was analyzed; optimizationmeasure to reduce N2O emission from the Sequencing Constructed Wetland wasproposed. Main research contents and conclusions are as following:1) The study on the effect of load on the treatment efficiency and N2O emissionflux of Sequencing Constructed Wetland. The removal efficiency was increased byprolonging HRT within a certain range. The optimum HRT was4h; further prolongingHRT could not reduce the pollutants more significantly. When influent carried mediumconcentration of pollutants, the removal rate of BOD5, NH3-N was63.16%and57.14%, respectively. The BOD5in the effluent was closed to the concentrationrequirement for urban landscaping in the Resue of Recycling Water for Urban WaterQuality Standard for Urban Miscellaneous water consumption(GB/T18920-2002);NH3-N could satisfy the requirement of construction site and urban landscaping. When influent carried low concentration of pollutants, the removal rate of BOD5, NH3-N was53.38%and44.44%, respectively. The effluent could satisfy the requirement of toiletflushing, road and automobile cleaning, construction site and urban landscapingaccording to the Standard. The HRT of each constructed wetland was1h,2h,4h and6h,and the according N2O emission flux was65.16μg m-2h-1,179.94μg m-2h-1,172.33μgm-2h-1and320.46μg m-2h-1. There was a negative correlation between dissolved oxygenand N2O emission flux. The difference of N2O emission flux under different influentconcentration was not significant.2) The study on the effect of temperature on the treatment efficiency and N2Oemission flux of Sequencing Constructed Wetland. When the temperature was25-35oC,the removal rate of BOD5, NH3-N was66.67%and62.26%, respectively. The effluentcould be applied to construction site and urban landscaping. An obvious variation ofemission could be observed in summer and autumn; however, such variation was not soobvious in winter. The maximum emission flux in summer, autumn and winter wasobserved at12:00,16:00and16:00, respectively. The minimum emission flux was allobserved around midnight. The seasonal variation of emission was also significant.Highest emission flux occurred in September and lowest average emission occurred inJanuary.3) Study on nitrogen mass balance in the Sequencing Constructed Wetland. TheN2O emission flux in different season could be estimated quickly using JN2O=K·TNinput,emission coefficient was3.03×10-4in summer,5.67×10-5in spring and autumn,2.73×10-5in winter. The emission quantity by using Sequencing Constructed Wetlandand wastewater treatment plant treating the domestic wastewater from a residentialdistrict in Chongqing was estimated, respectively; N2Oproduction by different treatmentmethod was compared, which proved Sequencing Constructed Wetland has a greatermitigation potency.The result in the study of N2O mitigation in the Sequencing Constructed Wetlandtreating wastewater could serve as theoretical basis for the control of greenhouse gasemission reduction of constructed wetland wastewater treatment system. This study,which deepened the foundation of microscopic mechanism of greenhouse gas from thesystem analyzed, had the important practical significance.