Effect Of Hematile And Biochar On The Treatment Of Sewage And The Control Of N₂O And CH₄ In Horizontal Subsurface Flow Constructed Wetland

As a global environmental problem,climate warming has played a significant impact on human life,ecosystems and economic development.The major cause of climate warming is the increasing of the concentration of greenhouse gases in the atmosphere.In recent years,comparing with tradition centralized sewage treatment facilities,constructed wetlands?CWs?are becoming a reasonable options for treating wastewater and have been widely used in rural and other non-network of pipes areas,because of their strong removal of phosphorus,less operation and maintenance requirements,lower cost,lack of reliance on energy inputs,and good water purifying and landscape effects.However,because of the low dissolved oxygen during the running time for the traditional subsurface constructed wetlands,the removal of nitrogen is not effective.At the same time,a large amount of greenhouse gases will be produced during the removal of pollutants.In this study,four groups included blank-constructed wetland,hematite-constructed wetland,biochar-constructed wetlands and hematite-biochar-constructed wetland?referred to as Tiesheng-constructed wetland?were constructed by adding hematite and biochar in these systems,in order to study the influence of emission reduction substances on the effect of wastewater treatment,the effect of nitrogen removal in the constructed wetland environment,the transformation process and flow direction of carbon and nitrogen,and the structure of the key microbial community to reveal the mechanism of the addition of emission reduction substances to the reduction of CH4 and N2O emissions.According to the operating conditions of wetlands and the characteristics of influent water quality,the construction method of biochar wetlands is optimized,and strategies for reducing greenhouse gas emissions are proposed to provide theoretical basis for greenhouse gas emission reduction and scientific evaluation of constructed wetlands in China.The main results are as follows:1.Effect of different emission reduction substances on the removal of pollutants in constructed wetlands?1?The average effluent COD concentration of each group was below 50mg·L^-1,and the average COD removal rates were all above 90%.There was no significant difference in the removal effect of hematite-constructed wetland and blank-constructed wetland.Biochar-constructed wetland and Tiesheng-constructed wetland significantly reduced the COD effluent concentrations of constructed wetlands?P?0.05?.?2?The average effluent DO concentration of constructed wetlands were below0.3 mg·L-1,which were in anoxic state.There was no significant difference in the effluent DO between hematite-constructed wetland and blank-constructed wetland,but the DO concentrations of biochar-constructed wetland and Tiesheng-constructed wetland were both signifecantly different comparing with blank-constructed wetland?P?0.05?,indicating that the addition of biochar significantly increased the DO of effluent in the constructed wetland system,but each group of constructed wetlands was still in anoxic state.The pH of biochar-constructed wetland and Tiesheng-constructed wetland were significantly lower than that of blank-constructed wetlands?P?0.05?,but there was no significant difference between hematite-constructed wetlands and biochar-constructed wetlands,indicating that the addition of biochar was significant lower the pH of the constructed wetland system.?3?The NH₄⁺-N effluent concentrations of biochar-constructed wetland and Tiesheng-constructed wetland were significantly lower than that of blank-constructed wetlands and hematite-constructed wetlands?P?0.05?,but for hematite-constructed wetland and blank-constructed wetland there was no significant difference in NH₄⁺-N effluent concentration,indicating that the addition of biochar reduced the NH₄⁺-N effluent concentration and improved the NH₄⁺-N efficiency of wastewater treatment in constructed wetlands.For TN,the removal effects of the four groups of constructed wetlands were similar to NH₄⁺-N.The TP removal rates were above 50%in the four groups,and the TP removal rates of biochar-constructed wetland and Tiesheng-constructed wetland were significantly higher than that of blank-constructed wetland and hematite-constructed wetland?P?0.05?,indicating that the addition of biochar was beneficial to improve the removal effect of TP in sewage.2.Effect of different emission reduction substances on CH4 and N2O emissions from constructed wetlandsThe average CH₄ emission fluxes of the four groups of constructed wetlands during the experimental period were 35.30±12.18,23.66±5.08,9.83±9.74,and9.26±5.18 mg·m^-2h^-1.Compared with the blank-constructed wetland,the CH₄ fluxes of hematite-constructed wetland,biochar-constructed wetland,and tiesheng-constructed wetland ware decreased by 32.97%,72.15%,and 73.77%,respectively.It indicated that hematite,biochar,and hematite+biochar all had a CH4emission reduction effect in the constructed wetland system,and the constructed wetland system with biochar addition had the most obvious suppression effect on CH4emissions.The average N2O emission fluxes of the four groups of constructed wetlands during the experimental period were 406.53±138.36,278.46±81.75,243.88±130.99,and 291.72±112.03?g·m^-2h^-1.Compared with the blank-constructed wetland,the N2O fluxes of hematite-constructed wetland,biochar-constructed wetland,and Tiesheng-constructed wetland were reduced by 31.50%,40.00%and 28.24%respectively,indicating that hematite,biochar and hematite+biochar all had a reduction effect on N2O emission in the constructed wetland systems,and the effect of biochar alone was the best.The integrated GWP caused by CH4 and N2O emissions in the four groups of constructed wetlands during each hydraulic retention time were3.43±1.18,2.30±0.50,0.97±0.95,and 0.92±0.51 g CO₂·m^-2;compared with the blank-constructed wetland,the integrated GWP of the other three groups of hematite-constructed wetland,biochar-constructed wetland and Tiesheng-constructed wetland were decreased by 32.94%,71.72%and 73.18%respectively,indicating that hematite,biochar and hematite+biochar had a reduction effect on the integrated GWP of constructed wetlands.3.Effect of different emission reduction substances on the structure and gene abundance of constructed wetland biological communities?1?On the basis of Phylum,all samples were divided into 47 categories in total.The main groups?the top 7 in proportion?of the four groups of constructed wetlands were the same,mainly including Actinobacteria,Proteobacteria,Bacteroidetes,Chloroflexi,Patescibacteria,Firmicutes,Acidobacteria,and the 7 most important communities account for more than 93%.The main dominant bacteria in the four groups of constructed wetlands were Actinomycetes and Proteobacteria.Analyzing the species composition of Proteobacteria at the level of the class,we found the addition of emission reduction substances changes the species structure of the Proteobacteria at the level of the class.Among them,hematite-constructed wetland,biochar-constructed wetland,and Tiesheng-constructed wetland reduced deltaproteobacteria accounting for the percentage of Proteobacteria;but hematite-constructed wetland,biochar-constructed wetland and Tiesheng-constructed wetland increased the percentage of Proteobacteria by Alphaproteobacteria and Gammaproteobacteria.?2?The four groups of constructed wetland systems showed significant differences in the abundance of mcrA gene in merogenetic bacteria?P?0.05?.The hematite-constructed wetland had the highest abundance of 3.16×10⁷ copies·g^-1,biochar-constructed wetland was the lowest which was 8.33×10⁶ copies·g^-1,hematite-constructed wetland significantly increased the mcrA gene abundance of methanogens?P?0.05?,biochar-constructed wetland and Tiesheng-constructed wetland significantly reduced the mcrA gene abundance of methanogens?P?0.05?.The abundance of pmoA gene in biochar-constructed wetland and Tiesheng-constructed wetland were 1.29×10⁷ copies·g^-11 and 1.11×10⁷ copies·g^-1,respectively,compared with 2.78×10⁶ copies·g^-11 in blank-constructed wetland,the two groups of constructed wetlands significantly increased the abundance of pmoA gene in methane-oxidizing bacteria?P?0.05?,while the pmoA gene abundance of hematite-constructed wetland was 3.54×10⁶ copies·g^-1,which was no significant difference from blank-constructed wetland.Compared with 1.36×10⁶ copies·g^-11 of AOB gene abundance in blank-constructed wetlands,the AOB gene abundance of biochar-constructed wetland(2.38×10⁶ copies·g^-1)and Tiesheng-constructed wetland?1.82×106 copies·g-1?were significantly increased?P?0.05?,while the hematite-constructed wetland(1.14×10⁶ copies·g^-1)was not significantly different with the blank-constructed wetland.Compared with the 5.18×10⁷ copies·g^-11 of blank-constructed wetland,the abundance of nirS gene in hematite-constructed wetland,biochar-constructed wetland and Tiesheng-constructed wetland were7.50×10⁷ copies·g^-1,3.73×10⁸ copies·g^-1,8.97×10⁷ copies·g^-11 significantly increased?P?0.05?.The nirK gene abundances of blank-constructed wetland(5.45×107copies·g^-1),hematite-constructed wetland(3.00×10⁷ copies·g^-1),biochar-constructed wetland(8.29×10⁷ copies·g^-1)and Tiesheng-constructed wetland?2.24×10⁸ copies·g-1?showed significant differences with each other?P?0.05?.Among them,the biochar-constructed wetland and the Tiesheng-constructed wetland significantly increased the nirK gene abundance?P?0.05?,while hematite-constructed wetland significantly reduced the abundance of the nirK gene?P?0.05?.The abundance of nosZ genes in biochar-constructed wetland and Tiesheng-constructed wetland were3.06×10⁷ copies·g^-11 and 1.72×10⁷ copies·g^-1,respectively,compared with 9.80×10⁶copies·g^-11 in blank-constructed wetlands,the two groups significantly increased the abundance of gene nosZ?P?0.05?,while the abundance of gene nosZ in hematite-constructed wetland was 1.19×10⁷ copies·g^-1,which was not significantly different from that of blank-constructed wetland?P?0.05?.