Analysis Of Microbial Communities And N₂O Release Characteristics In Different Types Of Constructed Wetlands

Constructed wetlands are widely used in improving the urban water quality and ecologic as a sewage treatment technology with good effect,low investment and environmental friendliness.However,constructed wetlands will inevitably release greenhouse gases such as N₂O,which will cause harm to the environment,during purifying water quality.Therefore,it is significant that understanding the mechanism of N₂O release process correctly for improving the applicability of constructed wetlands.The project establishes that parallel horizontal subsurface and free surface flow constructed wetlands for purification of urban water quality,evaluates the effectiveness of various types of constructed wetlands on urban water quality purification,analyzes the release rules of N₂O in different types of constructed wetlands,correlates the water purification process and the N₂O release process in constructed wetlands;reveals the relationship between microbial community characteristics and N₂O release patterns in different types of constructed wetlands.The research results are as follows:（1）Both groups of constructed wetlands can improve the water quality effectively.Under the quality of influent water is COD:27.84±6.04 mg/L,SS:5.30±2.42 mg/L,TN:14.91±4.39 mg/L,NH₄⁺-N:4.02±1.06 mg/L,NO₃^--N:5.86±1.34 mg/L,NO₂^--N:0.43±0.14 mg/L,TP:0.70±0.18 mg/L,PO₄^3-:0.63±0.16 mg/L,Surface flow constructed wetlands have better nitrogen and phosphorus removal functions,while horizontal flow constructed wetlands have a better effect on the removal of organic matter.In addition,both groups of constructed wetlands were affected by seasons,but the laws of different pollutants affected by seasons were not the same.Both constructed wetlands have better resistance to load fluctuations for COD and SS.While for TN,NH₄⁺-N and TP,with the increase of intake load,the removal effect is more likely to show the trend of first increasing and then decreasing.（2）During the experiment,the annual average N₂O release flux of natural wetland,horizontal flow constructed wetland and surface flow constructed wetland were:0.869±0.263 mg/（m²·h）,0.044±0.014 mg/（m²·h）And 0.034±0.017 mg/（m²·h）.The N₂O release flux of natural wetland is much higher than that of constructed wetland,while the release of N₂O in horizontal flow constructed wetlands reaches 1.29 times to surface flow constructed wetlands.Both the horizontal flow constructed wetland and the surface flow constructed wetland show two states of source and sink,in which the sink of the horizontal flow constructed wetland mainly occurs in January,while the surface flow constructed wetland mainly occurs in March and April.Overall,the two groups of constructed wetlands were still mainly released N₂O.（3）The N₂O release fluxes of the two groups of constructed wetlands showed significant seasonal differences,it decreased in the order of summer>spring>autumn>winter（p<0.05）.In addition,the surface flow constructed wetland has a higher N₂O flux in winter（0.023 vs 0.016 mg/（m²·h））,while the horizontal flow constructed wetland has a higher flux in spring（0.043 vs 0.029 mg/（m²·h））.There is a significant positive correlation between the concentrations（intake TN and NH₄⁺-N）and the N₂O release flux,and the negative correlation between the NO₃^--N concentration and the N₂O release flux.In addition,the release flux of N₂O in constructed wetlands will show a tendency to increase first and then decrease when the COD concentration and C/N of the influent increase.（4）The microbial diversity of horizontal flow constructed wetlands is higher than the surface flow ones,meanwhile the microbial abundance in surface flow constructed wetlands is higher than the horizontal flow ones.Proteobacteria is dominant at both constructed wetlands,their proportions are 46.32%and 37.35%.In the surface flow constructed wetland,the Bacteroides phylum and Bacillus phylum are 0.99%and 1.66%higher than the horizontal flow constructed wetland.Meanwhile the Chloroflexi,Acidobacterium,Actinomycete and Nitrospirae spiral in the horizontal flow constructed wetland is 1.64%,5.15%,4.02%and 0.51%higher than the surface flow constructed wetland.Compared to the horizontal flow constructed wetlands,the dominant bacteria are Nitrospira,Gaiella,Sporacetigenium,Fonticella,Denitratisoma,Arthrpbacter in surface flow constructed wetlands.The dominant bacteria in horizontal flow constructed wetlands are:Anaerolinea,Bradyrhiaobium,Bryobacter,Bacillus,Thiobacillus.（5）The microbial community structure is positively correlated with its N2O release flux in the horizontal flow constructed wetland,while the microbial community is positively correlated with the removal of NO₃^--N and NH₄⁺-N in the surface flow constructed wetland.Microbial communities that affect their denitrification efficiency and N₂O release flux in horizontal flow constructed wetlands are mainly concentrated in the matrix region,but in surface flow constructed wetlands are mainly concentrated in the rhizosphere region of plants.The surface flow constructed wetland has better denitrification efficiency and N₂O inhibition effect than horizontal flow constructed wetland.The improvement of the influent C/N is close to 5,which can improve the efficiency of water purification of constructed wetland and effectively reduce the N₂O release flux of constructed wetland.Relative to the low temperature season,we should optimize the operation parameters of constructed wetland in the high temperature season.