Research On Influencing Factors And Mechanism Of Aerated Constructed Wetland On Denitrification Of Low-polluted Water

Rapid urbanization and economic growth have made problems such as water ecological imbalance and water environmental pollution more and more prominent.Some water bodies that meet the discharge standards after pollution source engineering treatment have relatively low concentrations of various pollutants and are low-polluted water.However,it still threatens the lake’s water body and needs to be taken seriously.Nitrogen is the main pollutant in low-polluted water.It is considered to be the inducement of eutrophication and eco-toxicity in China’s water bodies,which has seriously affected the quality of water environment and urban water landscape ecosystems.Therefore,effective denitrification of low-polluted water is imminent.Constructed wetland（CW）has the advantages of low infrastructure and operating costs,low technical requirements,strong impact load resistance,strong nutrient removal ability,stable effluent quality,etc.,so that it has been continuously valued and promoted in the purification of low-polluted water..However,traditional constructed wetlands mainly rely on natural reoxygenation and oxygen secretion by plant roots,and the poor oxygen environment of wetlands severely limits the application of constructed wetlands in sewage treatment.In order to improve the oxygen supply capacity of the wetland,this research introduces the aeration device into the traditional vertical flow constructed wetland.First,by studying the influence of hydraulic retention time（HRT）,aeration rate,carbon-nitrogen ratio（C/N）and temperature on the denitrification effect of wetland,the best operating conditions of the wetland system are sought.Secondly,in order to further explore the difference in the denitrification effect of wetlands under different C/N conditions,the nitrification and denitrification intensity of the upper,middle and lower substrates in the constructed wetland system were tested,so as to explore the nitrification and reaction along the process of wetland denitrification.The change characteristics of nitrification,and finally analyze the difference between the denitrification mechanism and denitrification effect of the constructed wetland from the perspective of microorganisms,and the results are partially quantified.Provide theoretical basis and scientific guidance for the construction and operation of a more efficient aeration-type CW low-pollution water treatment system.The main research conclusions obtained are as follows:Aeration can effectively increase the level of dissolved oxygen in the wetland.The dissolved oxygen concentration during aeration can reach 2.7～6.1mg/L.When the aeration is stopped,the dissolved oxygen concentration will quickly drop below 0.5mg/L,which is generally built in the wetland.Alternating aerobic and anoxic environments promote nitrification and denitrification respectively.1)Through continuous monitoring of the constructed wetland for 24 hours, it was found that the wetland environment was in an anaerobic state before aeration,and the dissolved oxygen（DO）content was below 0.5 mg/L.After aeration,the DO concentration of the effluent water began to increase with the increase of the aeration rate.When the aeration rate>0.4L/min,the DO content of the effluent water leveled off,and the DO concentration of the effluent water was basically maintained at5.7～6.1mg/L.The introduction of aeration can significantly increase the DO level in the wetland,and continuously create an alternate good anoxic environment in the wetland,which can respectively promote nitrification and denitrification.2)By changing the aeration intensity and hydraulic residence time（HRT） during aeration,study the aeration rate at 0.2L/min,0.3L/min,0.4L/min,0.6L/min,0.8L/min;hydraulic When the residence time is 1d,2d,3d,the nitrogen removal situation of the vertical flow constructed wetland,the results show that the best aeration rate is 0.2L/min,and the best hydraulic retention time is 1d.Under these conditions,the wetland’s removal rate of TN,NH₄⁺-N and NO₃^--N in low-polluted water can reach 90.15%,98.47%and 86.22%,respectively,achieving simultaneous and efficient removal of TN,NH₄⁺-N and NO₃^--N.From the perspective of pollutant removal rate and economic benefits,this condition is the best operating plan for the system.In addition,with the increase of aeration and the extension of HRT,the removal of TN and NO₃^--N by wetlands showed the same trend,that is,under the same aeration conditions,the removal rate of TN and NO₃^--N increased with the extension of HRT.Under the same conditions of HRT,the removal rate of TN and NO₃^--N decreases with the increase of aeration volume.The removal rate of NH₄⁺-N is always above 98%.Correlation analysis shows that the removal rate of TN and NO₃^--N has a very significant positive correlation with HRT and a very significant negative correlation with aeration volume.The NH₄⁺-N removal rate has no significant correlation with HRT and aeration volume.It can be seen that in the aerated constructed wetland,HRT and aeration volume have a significant effect on the removal of TN and NO₃^--N.3)Under the best operating plan,by changing the carbon to nitrogen ratio （C/N）,the nitrogen removal situation of the vertical flow constructed wetland at 0,1,2.5,5,and 10 C/N is studied.With the increase of C/N,the wetland denitrification rate gradually increases.Correlation analysis shows that C/N has a very significant positive correlation with TN,NO3-N and organic matter removal rate;it has no significant correlation with NH₄⁺-N removal rate.When C/N is 5,the treatment effect is the best.At this time,the removal rate of organic matter,TN,NO₃^--N and NH₄⁺-N can reach 96.72%,68.49%,50.48%and 98.88%respectively;the removal rate of NO₃^--N is increased from the negative removal of zero carbon source to 50.48%,the removal rate of organic matter and TN were increased by 5.64%and 33.43%respectively compared with zero carbon source.The removal rate of NH₄⁺-N did not change with the increase of C/N,and was always above 98%.It shows that increasing C/N plays an important role in improving the removal rate of NO₃^--N,TN and organic matter.4)Under the best working conditions（the best aeration rate,HRT,C/N） conditions,when the temperature rises from 8℃～12℃to 30℃～34℃,the wetland denitrification rate gradually increases.Correlation analysis shows that temperature has a very significant positive correlation with the removal rate of TN and NO₃^--N.Compared with low temperature,its removal rate at high temperature is increased by20.55%and 39.85%,respectively;it has no difference with the removal rate of NH₄⁺-N and organic matter.Significant correlation,the removal rate of both remains above 95%.5)By comparing the nitrification and denitrification rates of microorganisms in different layers of the constructed wetland,the changes of nitrification and denitrification along the bed under different C/N conditions were discussed,and the effect of denitrification in the constructed wetland was analyzed from the perspective of microorganisms.difference.The results show that the denitrification rate of constructed wetlands is significantly higher than the nitrification rate.With the increase of C/N,the denitrification rate gradually increases,from29.23mg·kg^-1·h^-1to 161.74 mg·kg^-1·h^-1,the difference in nitrification rate is not obvious,basically maintained at 1.45mg·kg^-1·h^-1～1.74mg·kg^-1·h^-1.Under different C/N conditions,the nitrification rate of different layers of the bed body increased slightly along the way,and the nitrification rate of the wet underground layer and middle layer was higher than that of the upper layer;the denitrification rate of the middle layer of the bed body was lower than that of the upper and lower layers,thus denitrification The function is mainly carried out in the upper and lower layers of the system,but the middle layer can also carry out strong denitrification,and the system can fully play the role of denitrification and denitrification along the way.