Research On The Efficiency Of Plant Carbon Source Enhanced Wetland-MFC Coupling System On Tail Water Nitrogen Removal

In-depth treatment of tail water needs urgent attention and improvement,because the amount of sewage treated in China is increasing day by day,and the upgrading and upgrading of urban sewage treatment plants are also ongoing.At present,constructed wetland is widely used for deep nitrogen removal in tailwater.The coupling of MFC（Microbial Fuel Cell）system and constructed wetland system can effectively improve the nitrogen removal efficiency.However,there are still problems such as insufficient carbon source of influent.The pretreated leaves of Iris pseudacorus were added to the cathode and anode regions of the MFC-CW system as plant carbon sources respectively,which were labeled as MFC-CW-C and MFC-CW-A,a CW system was constructed as the control.HDPE and gravel were used as the matrix in all three systems.The pollutant removal effect and electricity generation performance of each system at low C/N influent were studied,the influence of adding position and adding period on the system’s pollution removal efficiency and electricity generation was discussed,and the impact of plant carbon source on the system’s anti-impact load capacity was studied,and microbial analysis was conducted to explore the nitrogen removal mechanism in the system.The main research results are as follows:（1）After planting Iris pseudacorus,the TN removal efficiency of CW and MFC-CW system increased from 65.53%and 77.52%to 70.20%and 83.32%,respectively.After alkali treatment,there were a large number of flake protrudes on the leaf surface,which was much rougher than the surface of other plant carbon source materials.Alkali treatment destroyed the cellulose structure in the leaves,which was more exposed than that of untreated leaves.In the static release experiment,the alkali treatment had the best carbon release performance and the highest cumulative carbon source release amount,which reached 172.95 mg COD·g^-1,and the C/N release effect in the whole release experiment is very stable,which is more suitable for the addition of plant carbon source adding device.（2）Compared with the addition of plant carbon source to the cathode,the addition of plant carbon source to the anode region can improve the pollutant removal efficiency of the system for a longer time.After the secondary addition of plant carbon source,the pollutant removal effect of the system was further improved.At the same time,in the twice adding experiments,the removal efficiency of pollutants had decreased to the level before adding 24 days after adding,so 24 days was selected as the adding cycle of plant carbon sources.（3）Adding plant carbon source significantly increased the output voltage of MFC-CW system.The average output voltage of MFC-CW-A increased significantly from 97.15±28.02 mV to 138.39±28.26 mV and 125.14±28.47 mV during the period when the plant carbon source was added to the anode area two times.Compared with the cathode,the effect of plant carbon source on the output voltage is more significant.（4）When plant carbon sources（5 g）were added to the anode region of MFC-CW,the mass loss of the two additions was 2.37 g and 2.57 g,respectively.After alkali treatment,the proportion of cellulose in leaves increased from 26.02%to 39.15%.After the static release experiment,the proportion of cellulose decreased from 39.15%to 33.16%,and the proportion of lignin from 12.76%to 3.96%.On the 10th day after the initial addition of plant carbon sources,the main dissolved organic compounds in the anode region of the system were aromatic proteins like folic acid and tyrosine,and microbial metabolites like tryptophan.The main dissolved organic matter in the cathode region is the microbial metabolites of tryptophan.On the 20th day,there were only a small amount of tryptophan-based microbial metabolites in the effluent of the system.Fourier infrared spectrum analysis showed that the peak shape related to cellulose,hemicellulose and lignin in the leaves of Iris pseudacorus treated with alkali was significantly weakened.On the one hand,alkali treatment may have damaged the structure of some cellulose in the leaves.On the other hand,alkali treatment reduced the content of cellulose and other components in the leaves.（5）Under the HRT of 24 h,36 h and 48 h,The average TN removal rates of MFC-CW-A effluent were 73.45%,53.98%and 63.91%,respectively.The average TN removal rates of effluent from MFC-CW system were 76.94%,54.06%and 63.38%,respectively.The TN removal rate of MFC-CW-A fluctuated the least during the change of HRT,indicating that the alkali-treated Acorus calamus leaves added to the anode area could improve the shock load resistance of the MFC-CW system and enable the MFC-CW system effluent to become more steady under the change of HRT.（6）The addition of plant carbon source can promote the growth of functional microorganisms related to nitrogen removal.At the genus level,Chloroplast was the dominant bacteria in the three devices,while Saccharimonadales was the dominant bacteria in the anode region,and the relative abundance of chloroplast was significantly higher than that in the cathode region.The abundance of metabolites function genes related to denitrification in the system was higher than that related to nitrification,and the abundance of denitrifying bacteria was also significantly higher than that of nitrifying bacteria.The relative abundance of denitrifying bacteria in MFC-CW system is significantly higher than that in CW system,indicating that the added carbon source can improve the relative abundance of denitrifying bacteria in the system,and thus improve the nitrogen removal efficiency.