Study On Electrochemical Enhancement Effect Of Vertical Flow Constructed Wetland On Nitrogen And Phosphorus Purification From Rural Domestic Sewage

Constructed wetland is a commonly used ecological treatment method in rural domestic sewage treatment.It has the advantages of environmental friendliness and low construction cost.However,it has the problems of large area,vulnerability to climate temperature and unstable treatment effect,which limits its popularization and application.The application of electrochemical technology in constructed wetland system can increase the hydraulic load of wetland system,reduce the area of wetland and weaken the influence of climatic conditions on the removal efficiency of nitrogen and phosphorus in the system.At the same time,electrochemistry can also provide electrons without additional carbon source to avoid secondary pollution.In this study,based on the vertical flow constructed wetland,ordinary electrochemical enhanced constructed wetland（E-BVFCW）and three-dimensional electrochemical enhanced constructed wetland（3DE-BVFCW）were constructed.By comparing the nitrogen and phosphorus removal effects with traditional constructed wetlands（BVFCW,B2VFCW）,the enhancement effect of electrochemistry on constructed wetland system was determined.The effects of hydraulic retention time（HRT）,current intensity and carbon-nitrogen ratio（C/N）on the removal efficiency of nitrogen and phosphorus in E-BVFCW and 3DE-BVFCW systems under different operating conditions were further discussed,so as to obtain the best experimental conditions.Combined with microbial community structure and diversity,the operation mechanism of three-dimensional electrochemical enhanced constructed wetland system was analyzed.The main conclusions are as follows:（1）The removal efficiency of nitrogen and phosphorus in rural domestic sewage by E-BVFCW and 3DE-BVFCW systems was significantly improved under ordinary electrochemical and three-dimensional electrochemical enhancement.The removal rates of total phosphorus（TP）,nitrate nitrogen（NO₃^--N）and total nitrogen（TN）in the E-BVFCW system reached 78.92±3.57%,76.97±7.58%and 58.53±9.48%,respectively,which were 14.66%,18.35%and 3.97%higher than those in the control group（BVFCW）.The removal rates of TP,NO₃^--N and TN in the 3DE-BVFCW system were 88.21±3.03%,74.80±7.23%and 57.46±8.86%,respectively,which were19.45%,9.21%and 0.99%higher than those in the control group（B2VFCW）.The electrochemical effect was not conducive to the removal of ammonia nitrogen（NH₄⁺-N）in the system.The removal rates of NH₄⁺-N in the E-BVFCW and 3DE-BVFCW systems were 63.46±5.32%and 63.08±4.14%,respectively,which were 9.56%and5.06%lower than the control groups（BVFCW and B2VFCW）.Compared with the E-BVFCW system,the 3DE-BVFCW system has similar nitrogen removal effect,but shows obvious advantages in phosphorus removal effect.（2）When the HRT was 24 h and the current intensity was 15 m A,the nitrogen and phosphorus removal effects of E-BVFCW and 3DE-BVFCW systems were the best.The removal rates of TP,NO₃^--N,NH₄⁺-N and TN in the E-BVFCW system were 80.43±0.72%,93.82±0.43%,46.45±1.41%and 65.55±2.12%,respectively.The removal rates of TP,NO₃^--N,NH₄⁺-N and TN in the 3DE-BVFCW system were 90.40±0.38%,97.81±0.46%,55.08±1.76%and 76.59±0.96%,respectively.At this time,the nitrogen and phosphorus removal effect of the 3DE-BVFCW system was better than that of the E-BVFCW system.The removal rates of TP,NO₃^--N,NH₄⁺-N and TN increased by 9.97%,3.99%,8.63%and 11.04%,respectively.（3）The influent C/N had a significant effect on the nitrogen removal efficiency of E-BVFCW and 3DE-BVFCW systems,but had little effect on the removal efficiency of TP.When the influent C/N was 3,the E-BVFCW and 3DE-BVFCW systems could achieve good denitrification effect,but with the decrease of C/N,the denitrification effect decreased significantly.When C/N decreased from 3 to 0,the TN removal rate of E-BVFCW system decreased from 65.55±2.12%to 38.30±4.00%,and the NO₃^--N removal rate decreased from 93.82±0.42%to 56.28±4.60%.The TN removal rate of the 3DE-BVFCW system decreased from 76.59±0.96%to 43.39±4.07%,and the NO₃^--N removal rate decreased from 97.81±0.46%to 45.99±6.86%.Through the strengthening effect of three-dimensional electrochemistry,the denitrification effect of3DE-BVFCW system was significantly better than that of E-BVFCW system under the condition of low carbon source or even no carbon source.（4）Through the analysis of ASV petal diagram,Alpha diversity and Beta diversity,the microbial richness of 3DE-BVFCW system was lower than that of E-BVFCW,but higher than that of control group B2VFCW;at the same time,the richness of the anode region（Chao1=558.98）in the 3DE-BVFCW system is higher than that of the cathode region（Chao1=408.01）and the intermediate electrode region（Chao1=411.74）.There were significant differences in the microbial community structure among the three systems of E-BVFCW,3DE-BVFCW and B2VFCW,but compared with the E-BVFCW system,the microbial community structure between the 3DE-BVFCW and B2VFCW systems was lower and more similar.There were also significant differences in the microbial community structure between the regions in the 3DE-BVFCW system,but compared with the cathode region,the microbial community structure in the middle electrode region was closer to the anode region.Through the analysis of microbial community composition at each level,there were a large number of denitrifying bacteria in the 3DE-BVFCW system,and the total abundance of denitrifying bacteria was higher than that of E-BVFCW and B2VFCW systems.The total abundance of denitrifying bacteria in the anode region of 3DE-BVFCW was the highest（69.00%）.The main denitrifying bacteria found in the anode area were Thauera（64.34%）,Hydrogenophaga（1.06%）,Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium（2.96%）,Pseudomonas（0.33%）,Denitratisoma（0.28%）and Gemmatimonadetes（0.03%）.There were also many denitrifying bacteria in the cathode region and the intermediate electrode region of the 3DE-BVFCW system,with a total abundance of43.33%and 48.33%,respectively,indicating that there was a denitrification process in each region of the 3DE-BVFCW system.Three-dimensional electrochemistry can enhance the denitrification effect of the constructed wetland system.Micropruina（0.27%）was also found in the middle electrode area of 3DE-BVFCW system,but it was significantly lower than that of E-BVFCW system（1.78%）,indicating that the effect of three-dimensional electrochemistry on TP removal in constructed wetland system was greater than that of phosphorus accumulating bacteria in 3DE-BVFCW system.