Study On The Performance And Mechanism Of Iron-Carbon Micro-Electrolysis Constructed Wetlands For Purification Of Low C/N Ratio Wastewater With The Addition Of Electron Donors

Secondary effluent from wastewater treatment plants is often used for ecological water replenishment to achieve the purpose of water reuse,but the residual nitrogen and phosphorus need to be further removed.Constructed wetland has the advantages of low construction price,simple operation and maintenance,and good treatment performance of low-polluting water.It is an ecological engineering technology suitable for advanced treatment of secondary effluent.However,the traditional constructed wetland substrate is not conducive to the accumulation of denitrifying bacteria to play their roles in a low C/N ratio environment,resulting in poor treatment performance of secondary effluent.Therefore,it is necessary to improve the substrate of constructed wetland to enhance the removal efficiency of pollutants in the secondary effluent,especially nitrogen and phosphorus,so that the constructed wetland technology can be widely used in the efficient treatment of secondary effluent and upgrading of wastewater treatment plants.In this study,simulated secondary effluent of wastewater treatment plant was used as the treatment object,and four C/N ratio gradients were set,namely C/N=5,3.5,2,0.5.Using iron（iron scrap）-carbon（biochar）as a new substrate of constructed wetland,sole biochar substrate as a control,and introducing walnut shell（organic）or pyrite （inorganic）on the basis of iron-carbon wetlands,to compare the pollutants removal efficiency of each group of constructed wetlands.And to evaluate the changes in the degree of corrosion of iron scraps substrate in different wetlands with the experiment prolonged.High-throughput sequencing technology was used to analyze and compare the microbial diversity,community composition and functional microbial abundance in different wetlands at C/N=5 and 0.5.It also investigated and compared the fluxes of greenhouse gases in each wetland at C/N=5 and 0.5.The main research results are as follows:1.The removal performance of pollutants in constructed wetlands under different C/N ratios is different to a significant extent,especially for NO₃^--N.When C/N=5 and3.5,the nitrogen removal performance of each system was good（TN removal>90%）.When the C/N ratio decreased to 2 and 0.5,nitrogen removal efficiency in iron-carbon wetland was significantly higher than that of control（p<0.05）.After 80 days of operation,the concentration of P in the effluent of the control system increased significantly,while the iron-carbon wetland could still effectively remove P.The addition of walnut shells strengthened the nitrogen and phosphorus removal efficiency of the iron-carbon wetland,while the addition of pyrite slightly reduced the nitrogen removal efficiency of the iron-carbon wetland.In a typical treatment cycle,the changes of NO₃^--N concentrations and fitting results of removal rates showed that the removal rates of NO₃^--N were always the highest in the iron-carbon wetland added with walnut shells,while that were the lowest in the control system.2.Iron scraps were corroded to different degrees in different wetland systems.The content of Fe in the iron scraps decreased after use,while the content of O element increased.The addition of electron donors,especially in the wetland system added with walnut shells,the Fe content of iron scraps decreased the least,indicating that the addition of electron donors delayed the corrosion of iron scraps.The analysis of the morphological characteristics showed that there were particles of different sizes on the surface of the used iron scraps,and the aggregation features were significant different in each system.The voltage measurement results of the galvanic battery device constructed with iron scraps as the negative electrode showed that the voltage generated by the used iron scraps was less than that of the fresh iron scraps,which indirectly indicated that the iron scraps were corroded to a certain extent after use.3.When C/N=5,the number of OTU,richness index（Chao）and diversity index（Shannon）of the microbial community in wetlands substrates were higher than when C/N=0.5.The addition of iron scraps reduced the abundance and diversity of microorganisms in the wetland system.When C/N ratio decreased from 5 to 0.5,the abundance of autotrophic denitrifying microorganisms increased in the wetland system added with iron scraps.The dominant autotrophic denitrifying microorganisms in wetland systems added with iron scraps at the C/N=0.5 stage were Dechloromonas,Ferritrophicum,Thiobacillus and Sulfuritalea,showing that these autotrophic denitrifying bacteria play an important role in wastewater nitrogen removal when the carbon source is insufficient.4.When C/N=5,the greenhouse gas emissions in constructed wetlands were significantly higher than when C/N=0.5,and the characteristics of greenhouse gas emission in each wetland system were different.When C/N=5,the addition of iron scraps and electron donors can inhibit the release of CO₂ and N₂O,thereby reducing the global warming potential（GWP）value(158.03-202.73 mg m^-2 h^-1)in wetlands systems.When C/N=0.5,compared with the control system,the CH₄ and N₂O emission fluxes of iron-carbon wetland were higher,resulting in higher GWP value.The emissions of CO₂ and CH₄(26.69 mg m^-2 h^-1,0.18 mg m^-2 h^-1)in the iron-carbon wetland system added with walnut shells increased most significantly,leading to the GWP value(39.64 mg m^-2 h^-1)the greatest at this stage.