Effects Of Spatiotemporal Aeration On Nitrogen Removal In Vertical Flow Constructed Wetlands

Water shortage and water ecological pollution have become global problems in recent years.Constructed wetlands have become an effective solution for sewage treatment due to their high efficiency,low cost and easy maintenance,which has attracted more and more attention.However,because of the lack of oxygen and other problems,the nitrogen cycle in traditional constructed wetlands is restricted,which directly affects the water purification capacity of constructed wetlands.Intermittent aeration,as an enhanced oxygen supply method,can effectively solve the problem of oxygen deficiency in constructed wetlands.At the same time,it is widely used in the enhanced denitrification process of constructed wetlands because of its economy and ease of operation.This study is focused on exploring the effects of intermittent and spatial aeration on pollutant removal rate,so as to determine the optimal aeration mode and provide theoretical guidance for the decontamination practice of constructed wetlands.Thus,this study set up three types of vertical flow constructed wetlands(VFCWs),including Total Intermittent Aerated VFCWs(TIA VFCWs),Partial Constant Aerated VFCWs(TIA VFCWs)and None Aerated VFC Ws(NA VFCWs).The data of pH,DO,COD and nitrogen removal in three systems under different aeration conditions were continuously monitored in 9 cycles to determine the optimal water treatment mode.Subsequently,the Partial Intermittent Aerated VFCWs(PIA VFCWs)were established.In order to optimize nitrogen removal efficiency in constructed wetlands,the nitrogen purification effects of the upper and down chamber of PIA VFCWs,TIA VFCWs and NA VFCWs under two different pollution loads were investigated.Furthermore,the mechanism of nitrogen cycle in constructed wetlands was explored at the molecular level.The gene abundance of microbial nitrogen metabolism in three constructed wetlands was analyzed by QPCR technology to show the performance on pollutant removal.The main research results are as follows:(1)The average removal rates of COD by PCA,TIA and NA were 95.4%,90.9%and 49.8%under the same influent concentration,respectively.PCA and TIA showed higher removal rates of NH4+-N,which were 90.9%,88.8%,respectively.NA performed poor on NH4+-N removal,only 32.8%.For NOx--N,the removal rate of NOx--N by NA was the highest,reaching 96.6%.PCA followed by 59.4%,TIA was the lowest,only 31.3%.The removal rates of total nitrogen(TN)in the three systems were:TIA>PCA>NA,79.0%,70.4%and 54.0%respectively.Therefore,compared with non-aeration,aeration can significantly enhance the removal of total nitrogen.At the same time,the removal rate of ammonia nitrogen in partial constant aerated chamber is lower than that in intermittent aerated ones,because of the accumulation of nitrate nitrogen.Therefore,this experiment will establish a composite aeration system(i.e.,the upper chamber is intermittent aeration,the down chamber is non-aeration)to reduce the excessive consumption of COD by constant aeration,improve the nitrate nitrogen removal rate,and further optimize the nitrogen removal effect of constructed wetlands.(2)Under two types of pollutant concentration(influent1 and influent2),the removal rates of COD by PI A and TIA were 91.8%and 91.9%,73.0%and 89.5%,respectively,the removal rates of NH4+-N were51.9%and 70.7%,56.1%and 96.1%respectively,the removal rates of NO,--N were97.5%and 13.7%,88.5%and-63.5%respectively.The removal rates of TN were 63.0%and 50.4%,61.5%and 36.8%,respectively.The results showed that the deficiency of COD in PIA was alleviated.In addition,the removal rate of COD in PIA and TIA was similar,but the removal rate of NOx--N in PIA was significantly higher than that of TIA,especially in the low concentration pollutants.This indicates that the periodic aerobic-anaerobic conversion of TIA is not conducive to denitrification,and a large amount of COD in the influent is consumed by aerobic microorganisms.PIA can separate the denitrification step into a stable anaerobic space and improve the removal rate of NOx--N.At the same time,the difference of total nitrogen removal rate between the two influent concentrations was 4.9%for PIA and 16.1%for TIA.The PIA VFCWs have the optimal total nitrogen removal ability and are more stable in TN removal under different pollutant concentration.(3)qRT-PCR was used to detect the microbial genes in PIA,TIA and NA VFCWs.The results showed that nitrification gene amoA and nxrA in PIA-U had high activity with a gene abundance of 106?108copies/g.Furthermore,the denitrification genes narG,nirK,nosZ and nirS in PIA-D also had high activity with a gene abundance of 108?109copies/g.The denitrification genes narG,nirK,nosZ and nirS in TIA were inhibited,while four types of genes in NA had a high abundance.This further indicated that the periodic aerobic-anaerobic cycle inhibited the activity of denitrification microorganisms in the system,while PIA achieved a relatively balanced state of nitrification and denitrification by reserving a stable anaerobic chamber,in order to improve the removal capacity of pollutants in wetlands.Compared with the abundance of genes under different influent loading,low pollution load inhibited nitrification and denitrification genes,especially on nxrA,narG and nosZ,but Anammox gene showed higher abundance under low concentration of pollutants.The results showed that nitrification-denitrification was easily affected by the concentration of pollutants than that of anaerobic ammonia oxidation,and denitrification restriction might provide favorable conditions for anaerobic ammonia oxidation.