Effects Of Constructed Wetlands On Pollutants Removal Under Added Carbon Source And Reoxygenation

Constructed wetlands are widely used because of their simple processes,low investment costs,and low operating costs.They are particularly favored in terms of the depth of tail water from sewage plants,but constructed wetlands are not ideal for low C/N wastewater treatment.In order to improve the treatment effect of constructed wetlands on low C/N sewage,from the aspects of carbon source and dissolved oxygen control,an experimental study on the effects of additional plant carbon sources and aeration to promote horizontal flow constructed wetlands to remove pollutants was carried out.Reed straws and cattails,which are typical wetland plants,were selected as supplementary carbon sources,and these two carbon sources were treated with acid and alkali respectively to analyze their surface structural characteristics,functional group composition,and plants’carbon release and denitrification capabilities.By comparing the two carbon sources,cattail is preferred as the external carbon source for experiments.From the perspective of controlling dissolved oxygen,intermittent aeration in the wetland was conducted for 4 hours,8 hours,and 12 hours to investigate the effect of wetland on pollutant removal under aeration conditions,and to analyze the difference in the effect of wetland on pollutant removal under different aeration conditions.The experiment mainly analyzed the removal effect of wetland on nitrogen,phosphorus and COD,and tested the change of dissolved organic matter（DOM）in the wetland.The research results are as follows:1.The surface structure of the untreated plant is complete,and the plant is destroyed after acid or alkali treatment,and a large area of voids appears,which is conducive to the release of cellulose and hemicellulose from the plant.The large surface area is conducive to the adsorption of nitrogen,phosphorus,and other pollutants in water.At the same time,the EDS test showed that the carbon content of cattail was more than that of other substances.The overall absorbance of each functional group of cattail is stronger than that of reed.Different pretreatment methods have a greater influence on the absorbance of each functional group of cattail,while the reed absorbance of each functional group of reed has less change under different treatment methods.Cattail and reed straws have approximately the same peak positions,and they contain the same functional group types,but the number of different functional groups in cattails is greater than that of reeds.In addition,the alkali-heat cattail released more carbon source and had higher denitrification rate,which was suitable as an additional carbon source for this experiment.2.For low C/N sewage,the TN,NO₃^--N,and NH₄⁺-N effluent concentrations of the carbon source group wetlands added with alkali-heat cattail in the second stage were lower than those of the blank group without any carbon source,and the pollutants of the carbon source group were effluent.The concentration showed a trend of decreasing first and then increasing.The concentration of each pollutant in the blank group gradually increased.After adding the newly treated alkali-heat cattail in the third stage,the removal rates of TN and NH₄⁺-N in the two groups of wetlands were lower than those in the second stage.The removal rate of the carbon source group was still higher than that of the blank group.The effluent concentration of NO₂^--N in both stages was greater than that in the influent,and the carbon source group was higher than the blank group.Excitation-emission matrix spectroscopy（EEMs）test showed that the humic acid fluorescence intensity at the back of the carbon source group was higher than that of the blank group.3.When intermittent aeration was performed for 4 hours,the removal rate of TN by the aerated and cattail group was 54.2%,and the removal rate of NH₄⁺-N was the highest,which was 45.93%.Under different aeration time,the removal rate of NO₃^--N in blank group,aerated group and aerated and cattail group were all better,all above97%.This shows that the denitrification process in the wetland is better.The effluent concentration of NO₂^--N was maintained below 0.16 mg/L in several groups of wetlands.The best effect of intermittent aeration for 4h is 14.32%.Adding cattail will increase the concentration of TP in the wetland effluent.The aeration group had the best removal effect of COD,both of which could reach more than 95%.Among them,intermittent aeration for 8h was the best,which was 97.17%.Excitation-emission matrix spectroscopy（EEMs）test showed that three kinds of more obvious peaks appeared,namely:A peak,P peak and B peak.Among them,the P and B peaks of the blank group and the aerated and cattail group were more obvious.The A peak in the aeration group is more obvious.Under different aeration times,the maximum fluorescence peaks of humic acid,tyrosine and protein in the aerated group were lower than those in the blank group and the aerated and cattail group.