Nitrogen Removal Effect And Influencing Mechanism For Cattail And Reed Vertical Subsurface Flow Constructed Wetland Treating Reclaimed Water

In order to solve the problem of high nitrogen content in reclaimed water,it is planned to use vertical subsurface flow constructed wetland to carry out research on the process and mechanism of nitrogen deep purification in reclaimed water.According to the water quality characteristics of the reclaimed water of sewage treatment plants in Beijing area,based on the investigation and selection of suitable wetland plants and substrates,design and construct a vertical subsurface artificial wetland with deep purification of reclaimed water.Designed as two-level randomized block trial in a single factor.The single factor are wetland plants,the two levels are cattail and reed,and the pure substrate vertical subsurface flow wetland is set as the control,and each treatment is set with three replicates.During the experiment,the domestication period and operation period of wetland plants were set.By analyzing the temporal and spatial characteristics of different nitrogen forms in wetland water bodies,plants and substrates,at the same time,combined with 16S r RNA-bacterial high-throughput sequencing technology to analyze the bacterial community structure in vertical subsurface flow constructed wetlands.The main research conclusions are as follows:（1）The average removal rates of TN,NO₃^-,NH₄⁺-N,and NO₂-N by cattail treatment were85%,97.5%,83.7%,and-2.21%respectively;the average removal rates of TN,NO₃^-,NH₄⁺-N,and NO₂-N by reed treatment were 77.16%,93.6%,85%,-2.43%,respectively;The average removal rates of TN,NO₃^-,NH₄⁺-N,and NO₂-N by the pure matrix control were 71.59%,85%,78.67%,and-1.42%,respectively.Among them,the removal rate of TN by cattail treatment was significantly higher than that of reed treatment and pure substrate control,reaching the surface water level III standard.The removal rate of TN by the reed treatment was higher than that of the pure substrate control and reached the Class V standard of surface water.The removal rate of NO₃^-and NH₄⁺-N by cattail treatment and reed treatment was significantly higher than that of pure substrate control,and the removal rate of cattail treatment was higher than that of reed treatment.In addition,the cattail treatment,the reed treatment and the substrate control all have a higher removal rate of NH₄⁺-N,and all have reached the surface water level II standard.The synthetic wetland system’s removal rate of nitrogen in reclaimed water is as follows:cattail treatment>reed treatment>pure substrate control.（2）The results of high-throughput sequencing showed that the bacterial communities in the cattail treatment,reed treatment,and pure substrate control constructed wetland system mainly consist of five phylums:Proteobacteria,Bacteroidetes,Actinobacteria,Cyanobacteria,and Patescibacteria.Proteobacteria,Bacteroidetes,and Actinobacteria are the three most abundant phyla in the system,accounting for 87.97%of the total abundance,of which Proteobacteria（63.27%）is the main dominant phylum.Shannon,Simpson,Ace,Chao and other diversity indexes indicate that there are significant differences in bacterial community composition in the cattail treatment,reed treatment,and pure substrate control constructed wetland.Among them,the abundance and diversity of typha treatment and reed treatment were significantly higher than that of pure substrate control,and the richness and diversity of typha treatment and reed treatment were higher than that of reed treatment.In addition,the difference of bacterial community diversity in plant treatment and substrate control constructed wetland is related to the temperature,DO,NO₃^-,TN,NH₄⁺-N and other environmental and chemical factors in the system.（3）The contribution rate of microbial degradation and assimilation of nitrogen in constructed wetlands accounted for 39.69%～48.47%of nitrogen removal,and the contribution rate of substrate adsorption of nitrogen was 17.46%～23.15%.Compared with the control,the total nitrogen removal rate of cattail and reed treatment was 6.78%～8.98%higher,reaching a significant level.It shows that the wetland purification plants significantly increase the removal rate of total nitrogen through the joint action of microorganisms.In addition,nitrification mainly occurs in the overlying water area,and denitrification mainly occurs in the rhizosphere and matrix layer.The physicochemical factors that affect microbial-driven chemical denitrification are DO,ORP,and EC,and the key bacterial groups are Aeromonas,Flavobacterium,Bdellovibrio,and Acidovorax.Research on the fate pathways of nitrogen removal in constructed wetland systems shows that microbial nitrification-denitrification（39.69%～48.47%）is the main way of nitrogen removal in constructed wetlands,followed by matrix adsorption（17.46%～23.15%）.Plant-treated constructed wetland not only has a certain absorption effect on TN（5.95%～8.62%）,and the removal of TN by microorganisms in the matrix control constructed wetland system is significantly increased by 6.78%～8.98%.It shows that the constructed wetland can help to exert the effect of microorganisms to a greater extent under the action of plants,and should not be limited to the direct absorption of nitrogen by the plants themselves.