Study On The Optimization Of Constructed Wetland For Advanced Treatment Of Rural Domestic Sewage Tailwater

Henan Province is a major agricultural province with a large number of villages,wide distribution and large differences.There is a long way to go in sewage treatment.The rural domestic sewage treatment rate is planned to reach 45%by 2025,and a large area of rural black and odorous water bodies will be basically eliminated.Nowadays,rural sewage treatment in Henan Province has made some progress,but due to large fluctuations in water quality and limited maintenance and operation levels,the sewage treatment efficiency is not high,the effect of nitrogen and phosphorus removal is poor,and the effluent quality is seriously not up to standard.Aiming at the characteristics of rural domestic sewage and the existing problems of constructed wetlands（such as weak reaeration capacity,poor or unstable treatment efficiency,and pore clogging due to prolonged operation time）,a horizontal subsurface flow（SFCW）,wavy subsurface flow（W-SFCW）,and tidal flow（TFCW）constructed wetland system using aerated concrete,volcanic rock,and gravel as a combined matrix was constructed.The effects of influent hydraulic load,influent C/N ratio and influent water quality fluctuation on the removal of pollutants were investigated,and the constructed wetlands with economic and practical,strong impact load resistance and certain feasibility were selected.Taking corn cob and wheat straw,two common agricultural wastes in Henan Province,as the research object,suitable solid carbon sources are screened out,and solid carbon source leach solution is prepared according to the operation characteristics of tidal flow as the additional carbon source of wetland,which provides an economic scheme for efficient nitrogen removal of constructed wetlands in rural areas.The main research conclusions are as follows:（1）With the increase of hydraulic retention time,the concentrations of COD,NH₄⁺-N,TN,and TP in the effluent of W-SFCW and SFCW showed a downward trend.Compared to SFCW,W-SFCW has improved its ability to remove NH₄⁺-N,TN,and TP,but the extent was limited.The average removal rates of NH₄⁺-N of both wetlands were relatively low,only40.86%and 34.22%,respectively.Pollutants in the W-SFCW system decreased to different degrees with the increase of the path,and the main removal sites of each pollutant were in the front and middle of the system.（2）Through experiments,it is determined that the operating cycle of the tidal flow constructed wetland was 36 hours for submerged operation and 6 hours for idle emptying.Increasing the influent C/N ratio can enhanced the removal rate of COD,TN and TP of the two constructed wetlands to different degrees,but has no obvious effect on the removal rate of NH₄⁺-N.By changing the nitrogen composition of the influent,W-SFCW has a good removal effects on NH₄⁺-N and TN in wastewater with high proportion of nitrate nitrogen at a low carbon ratio,but has a poor treatment effect on wastewater with a high proportion of ammonia nitrogen;TFCW has a good and stable treatment effect on wastewater containing high ammonia nitrogen or with large fluctuations in ammonia nitrogen,but its TN removal ability was weak.（3）Through the analysis of microbial community structure and functional genes in W-SFCW and TFCW,the detected microbial species belong to 11 categories.Proteobacteria,Actinobacteria and Bacteroidetes were the main dominant groups in the two wetland systems.The abundance of Patescibacteria,Bacteroidetes,Actinobacteria and Chloroflexi varied significantly.（4）The carbon release rate of corn cob,alkali treated corn cob,heat treated corn cob,wheat straw,alkali treated wheat straw and heat treated wheat straw was faster in the early stage,but decreased significantly in the middle and late stage.All of them could fit the second-order kinetic equation well.The respective c_m values at 120 h were 192.30,344.83,384.62,386.10,507.61,and 588.24 mg/（g·L）,respectively.The carbon release of the three wheat straw materials is relatively high,which will lead to the addition of carbon source is not easy to control,and the TN released is easy to cause secondary pollution risk.Combined with the experiment of biodegradability and denitrification ability of the leach solution,the heat treated corn cob leach solution is selected as the external carbon source.（5）The average concentration of COD in tidal flow wetland effluent with added carbon source is 28.80mg/L,and the average removal rate is 76.14%,which is 9.72%lower than that when glucose is used as carbon source,which met the grade A standard.The average concentration of NH₄⁺-N in effluent was 5.13 mg/L,and the average removal rate was 66.43%,which was higher than the removal rate of NH₄⁺-N using glucose as carbon source,and the effluent concentration could reach grade A standard in most days.The average concentration of TN in effluent was 11.66 mg/L,and the average removal rate was 53.55%.Under the condition of increasing C/N ratio,the removal rate of TN was only 3.25%higher than that of using glucose as carbon source,which met the grade A standard.After 41 days of cooling,the average removal rates of COD,NH₄⁺-N and TN in the tidal flow wetland decreased by 3.68%,5.22%and 10.23%,respectively.According to the characteristics of rural domestic sewage,this paper uses constructed wetland technology to conduct deep purification treatment of simulated rural domestic sewage,explores the optimal operating parameters,provides improvement measures,and finally provides certain theoretical reference and data support for the application and promotion of constructed wetland in rural areas.