A Study On The Effect Of Nitrogen And Phosphorus Removal By Iron-Carbon Constructed Wetland Treating Rural Domestic Tail Wastewater

During the 13th Five-Year Plan period,our country’s urbanization process continues to advance,the economy grows rapidly,and the quality of rural life also continues to improve.However,with the economic developing,the construction of rural domestic sewage collection and treatment facilities fail to keep up with the pace of development.A large amount of sewage is directly discharged into natural water bodies without treatment,and the rural water environment is facing severe challenges.The existing urban sewage treatment process has disadvantages such as large investment,high operating cost and complicated management,and cannot be widely used in rural areas.Therefore,it is imperative to find a process that meets the characteristics of rural areas,has a low construction cost and is easy to maintain to improve the quality of rural domestic sewage tail water.Aiming at the problem of excessively high nitrogen and phosphorus concentrations in the tail water of rural domestic sewage,this paper used iron-carbon fillers and activated carbon as additives（5%）on the basis of traditional constructed wetlands with zeolite as the main material,designed and built three kinds of vertical subsurface flow constructed wetlands.The effects of three types of wetland systems including iron-carbon constructed wetland(W_IC)on the treatment of rural domestic sewage tail water under different HRT,iron-carbon mass ratio,influent C/N ratio,and temperature were investigated.And the community structure and functional genes for nitrogen and phosphorus removal were also determined and analyzed.The main results achieved are as follows:1.Compared the operating effects of the three systems at different hydraulic retention times at an ambient temperature of 25℃-35℃.When HRT is 1d and surface load is 0.2081m³/m²·d,the removal rates of COD,NH₄⁺-N,NO₃^--N,TN,and TP of W_ICwere 83.2%,87.3%,81.9%,87.8%,83.7%,respectively.The effluent water quality reaches the surfaceⅤwater standard.Compared with the control group,the removal efficiency of W_IC’s NO₃^--N,TN,and TP increased by 19.7%-52.9%,6.4%-21.8%,44.6%-52.5%,respectively.The pollutant concentration and physical and chemical parameters at different points were measured.The results showed that the iron-carbon filler significantly increased the removal rate of the lower layer of the system（at 40cm）.Compared with the control group,the removal rate of NH₄⁺-N,NO₃^--N,TN,TP of W_ICincreases by 4.9-11.1%,53.2-58.3%,26.5-41.8%,and25.5-28.6%respectively.At the same time,the consumption of DO in the lower layer of the system increased,the p H rose,and the ORP potential was lower than 100mv,which was more reducing.2.Compared and analyzed the influence of iron-carbon mass ratio,influent C/N ratio,temperature on the operation effect of different wetland systems under a typical cycle（HRT=1d）.Different iron-carbon ratios affected the treatment effect of iron-carbon fillers.When the iron-carbon mass ratio was 5:1,W_IChad the highest removal rate of NO₃^--N,TN,and TP,which were 81.9%,83.9%,and 87.8%,respectively.Next,the C/N ratio of influent water,one of the factors limiting nitrogen and phosphorus removal,was studied.The concentrate of COD in the three systems remained stable,and the removal rates of NH₄⁺-N,NO₃^--N,TN,and TP increased significantly.W_ICand W_Fwre more affected by carbon source than W_C;when the influent C/N ratio was less than or equal to 3.52,there was no significant change in the ratio of each wetland system TN.When the influent C/N ratio was greater or equal to 4.70,the NH₄⁺-N ratio in the effluent of W_ICrose to 87.6%,and the ratio of NO₃^--N decreased by 12.0%,which was the opposite of the control group.Finally,one of the factors limiting microbial activity,temperature,was studied,and the operating effects of three types of constructed wetlands in summer and winter were compared.When the ambient temperature was lower than 15℃,the removal rate of NH₄⁺-N,NO₃^--N,TN,TP of the three systems decreased by 3.1%-32.9%,44.6%-96.0%,19.0%-59.3%,2.7%-20.7%,respectively.The NO₃^--N in W_Fand W_Cshowed negative growth.The removal rates of NH₄⁺-N,NO₃^--N,TN,and TP in the W_ICwere 84.2%,35.2%,64.8%,and 90.5%,respectively.3.In order to explore the influence of the matrix on the colonization and distribution of functional microorganisms in the system,high-throughput sequencing was performed on bacteria in different matrix layers of the three systems.In the Alpha diversity analysis,the OUT number,observed＿species,shannon index,and simpson index of the W_Cand W_ICsubstratum were higher than those in W_F.The addition of activated carbon and iron-carbon materials could increase the abundance and diversity of soil bacterial communities in the system.In the Beta diversity analysis,the Weighted Unifrac heat map and PCA analysis showed that the bacterial communities in different systems and different substrate layers in the same system were significantly different.The results of the annotation analysis showed that the addition of activated carbon and iron-carbon materials reduced the abundance of Enterobacteriales;The addition of iron-carbon materials further enriched the denitrifying bacteria（Rhodocyclales,Nitrosomonadales,Burkholderiales）and electron transfer related bacteria（Methanobacteriales,Geobacter,Desulfuromonadales）in the system,and a more efficient denitrification method existed in the system.4.In order to further clarify the nitrogen and phosphorus removal capabilities of different systems,real-time fluorescent quantitative PCR was performed on some functional genes involved in nitrogen and phosphorus removal,and the abundance of different functional genes was determined.The detection results showed that the number of AOB-amo A in W_ICwas one order of magnitude higher than that of the control group,but the chemical reaction of the iron-carbon filler caused an increase in the NH₄⁺-N concentration in the effluent..The number of functional denitrification genes nir S,nir K,nos Z,and the anammox gene hzs A in the substratum of W_ICwere 1 to 3 orders of magnitude higher than that of W_Fand W_C,respectively.Iron-carbon filler could promote the enrichment of functional genes for denitrification.The test results of the PPK gene also showed that the iron-carbon filler also promoted the enrichment of phosphorus accumulating bacteria（PAOs）.The number of PPK genes in the W_ICiron-carbon layer was 4.67 times and 3.01 times that of the W_Czeolite layer and W_Factivated carbon layer,respectively.There were multiple ways to strengthen phosphorus removal with the coordination of chemistry and biology.The research results show that the iron-carbon constructed wetland has a good ability to remove nitrogen and phosphorus,which can provide a new idea for improving the quality of rural domestic sewage tail water,and also provide a certain reference for scientific research and engineering practice in related fields.