Influence Of Plant On Nitrogen And Fluoride Removal Performance Of Constructed Wetlands

In recent decades,the booming development of industry and the improvement of people’s living standards were usually accompanied by large-scale pollution discharge.Although the sewage treatment plant could purify wastewater to a certain extent,the reduction of some pollutants was limited(such as fluoride(F-)and nitrate nitrogen(NO3--N)).and it could still cause serious environmental problems in basin as effluent was discharged into the nature water.As a kind of advanced wastewater treatment technology,constructed wetlands(CWs)had the advantages of low operating cost and high environmental benefits in improving wastewater treatment efficiency.As an important part of the CWs,plant could affect the pollutant removal efficiency through direct absorb and root activities.However,most of the existing studies only focused on the effect of plant root functions on nitrogen(N)removal,while ignoring the potential impact of plant endogenous carbon coupled with external influent carbon sources on microbial N removal pathways.Especially,the mechanisms of plant on N removal in low carbon-to-nitrogen ratio(C/N)wastewater remained still unclear.In addition,the mechanism of the effect of plants root action on the removal of pollutants based on substrate adsorption(such as F-)was still unclear.Therefore,it was necessary to explore the contribution and mechanism of wetland plant on pollutant removal more comprehensively and deeply.Aiming at the unclear removal effects of plants on low C/N ratio wastewater and pollutants dominated by substrate adsorption,this study explored the influence mechanism of plant endogenous carbon coupled with influent external carbon sources on microbial N removal under different low C/N conditions.Taking F-dominated by substrate adsorption as a typical pollutant,the influence mechanism of plant presence on F-removal and transformation was studied in detail.Finally,the comprehensive action mechanism of wetland plants affecting N and F-removal transformation in CWs were proposed based on the above studies.This study mainly included the following contents:ⅰ)The effect of plants on N removal in CWs under different low C/N ratios,including N removal pathway of CWs under different low C/N conditions and the influence of the presence of wetland plants on N removal and greenhouse gas emission.On this basis,the regulation mechanism of plant root action on N removal-related functional genes were explored with the methods of high-throughput sequencing and real-time quantitative PCR technologies;ⅱ)F" removal and plant effects in CWs with different substrates.Taking F-as a typical pollutant adsorbed mainly by substrate,the removal effect and environmental fate of high adsorption performance substrate(calcium-based substrate)on Fwere clarified.Based on the effects of plants on the microenvironment of CWs and N and P pollutants,the mechanism of plant presence on F-migration and transformation was discussed;ⅲ)Finally,the influences of plants on the microenvironment,microbial community structure and microbial activity of CWs were revealed.The comprehensive effects of wetland plants on N and F-removal in CWs were proposed,which could provide new insights for the design and optimal management of C Ws.The main conclusions are as follows:(1)With the increase of influent C/N ratio,the N removal effect of CWs was enhanced continuously.The presence of plant enhanced the removal of NO3--N and TN,the difference of N removal performance under the influence of plants led to different levels of greenhouse gases emissions.Under different C/N ratios,plant decreased the emission fluxes of N2O and CH4,but increased the activity of bacteria and CH4 oxidation process resulted in more CO2 emission with the increase of C/N ratios.(2)The influence of the C/N ratios in influent on microorganisms associated with N removal was higher than that of plants,and N removal pathways was changed.The contribution of denitrification to N removal gradually increased with increasing C/N.and the contribution of partial denitrification-anammox(PDN/AMX)gradually decreased.Plants increased the relative abundance of microorganisms involved in nitrification and denitrification,but decreased the relative abundance of microorganisms associated with anammox.The process of PDN/AMX contributed more than 70.4%of N removal under the C/N ratio of 0～1.5,while complete denitrification dominated N removal at C/N of 3.The relative contribution of PDN/AMX to N removal was lower in CWs than in the control group,due to the adverse effect of radial oxygen loss on anammox.However,the N removal efficiency in CWs was higher than that in the control group,which was related to plant uptake of N and root exudates to enhance denitrification.(3)Calcium-based CWs could improve the removal of F-by co-precipitation and promoting plant absorption in a short time.As a typical representative of calcium-based substrate,limestone had rough and porous surface which could directly absorb F-,and the released Ca2+ could co-precipitate with F-to form calcium fluoride phosphate(Ca5(PO4)3F)and calcium fluoride(CaF2)to remove F-.In addition,limestone could provide trace elements for plant to promote photosynthetic activity,and enhanced the direct absorption of F-by plants.However,the direct contribution of plant absorption to the removal of F-was relatively limited,accounting for only 1.16%of the TN input.The accumulation of F-was different in different parts of the plant(root>stem>leaf).(4)The presence of plants indirectly affected the migration and transformation of F-by changing CWs microenvironment.The pH value of CWs decreased by 0.57 ± 0.07 with plant presence,and dissolve oxygen(DO)concentration decreased by 0.11 ± 0.03 mg/L.The relative abundance of fluorine-tolerant bacteria decreased from 5.87%to 1.14%alter plants removal.A highly significant negative correlation between F-effluent concentration and pH value(r=0.67.p<0.05)was observed.The decreased pH value induced by rhizosphere acidification affected the morphology of F-in the substrate,resulting in the release of F-which had been adsorbed by the substrate from the substrate by desorption or fluorine precipitation dissolution.(5)In terms of CWs microenvironment and pollutant removal,both unplanted group and plant-cleared group showed the same performance.Compared with the planted group,unplanted group and plant-cleared group had no significant effect on NH4+-N removal(p≥0.05),the removal efficiency still remained above 95%.The removal efficiency of NO3--N and TN were decreased by 42%compared with the planted group,and there was no significant difference between the unplanted group and plant-cleared group.(6)The presence of plants did not significantly change the microbial community structure(p≥0.05),but significantly enhanced the abundance of functional microorganisms.The absolute abundance of 16S rRNA and functional genes related to N metabolism in planted group was significantly higher than that in plant-cleared group and unplanted group(p<0.05).At the same time,plants improved the relative abundance of fluorine-tolerant bacteria,especially in calcium-based CWs.Although the microbial community structure of the plant-cleared group was significantly different from that of the planted group(p<0.05).the microbial diversity difference between the plant-cleared group and the planted group was less than that between the plant-cleared group and the unplanted group.Plant clearance significantly decreased alpha diversity in both calcium-base group and control group(p<0.05),and plant presence had a more significant effect on alpha diversity than substrate type(p<0.05).