Design Of Subsurface Flow Constructed Wetland And The Simulation Research On Nitrogen Removal Performance

Constructed wetlands are widely used in nitrogen removal of wastewater treatment due to their low cost and energy consumption,easy operation,environment-friendly,and high nitrogen removal efficiency.As one of the types of constructed wetlands,subsurface flow constructed wetlands are widely used in northern China and alpine regions.In constructed wetlands system,N and organic matter can be removed through the combined action of plant absorption,substrate adsorption and microbial degradation.Therefore,substrate types,dissolved oxygen concentration,hydraulic retention time?HRT?,hydraulic load,plant specis,temperature,pH and carbon source are the main factors which affect the removal performances of constructed wetlands in wastewater treatment.In the current study,although the influencing factors of nitrogen removal have been extensively explored,the lack of efficient wetland systems and the suitable operation conditions are still the bottlenecks of constructed wetlands in wastewater treatment.In this study,a new subsurface flow constructed wetland system was designed to investigate the effects of substrate types?natural zeolite or biological ceramsite?,aeration?intermittent aeration or not?and hydraulic retention time?3 d or 6 d?on the nitrogen removal performance and microbial community of the wetland systems.The results not only provide a theoretical basis for the nitrogen removal mechanism of constructed wetland systems,but also provide a practional guide for the practical application of the new subsurface flow constructed wetland system.The main research results and conclusions are as follows:1.Compared with biological ceramsite,the wetland system packed with natural zeolite substrate has a higher nitrogen removal efficiency under without aeration operation.The average concentration of NH₄⁺-N in effluents of CW1?natural zeolite?and CW2?biological ceramsite?was 8.09 mg L^-11 and 9.41 mg L^-1,respectively.And CW1 obtained lower NO₃^–-N and TN concentrations.The DO values of two wetland systems were maintained below 1.00mg L^-1.There is a larger plant biomass in CW1,which is about 10 g higher than CW2.The transpiration rate and net photosynthetic rate of the plant in CW1 were higher than those in CW2.The enzyme activities in rhizosphere soil in CW1 were higher.Furthermore,the microbial activity was higer than that in CW2.Urease activity and nitrification potential were significantly higher than CW2.2.Aeration significantly increased the dissolved oxygen concentration of the wetland system and the maximum was 3.78 mg L^-1.The removal efficiency of NH₄⁺-N was increased from 64.82%?CW1,natural zeolite aeration?to 88.27%?CW3,natural zeolite,aeration?.The NH₄⁺-N and TN removal efficiency in CW4?biological ceramsite,aeration??biological ceramsite,aeration?,were increased by 20%than those in CW2.The removal efficiencies of NO₃^–-N and COD were increased by about 10%.The aerated wetland system obtained higher plant biomass and its enzyme activity of rhizosphere soil were significantly higher than those in the wetland system without aeration.Especially,the dehydrogenase activity was increased by almost 40%.The removal rates of HN₄⁺-N,NO₃^–-N,TN and COD on CW4were 88.78%,74.86%,71.02%and 64.68,respectively.And this is no significant change compared to CW3?The removal rates of HN₄⁺-N,NO₃^–-N,TN and COD were 88.27%,75.82%,70.30%and 62.21%,respectively?.The results showed that the effect of substrates on the removal efficiency in wetlands was not obvious after aeration,.Aeration is favorable to the growth of wetland plants and microorganisms.Morever,this operation could significantly improves the nitrogen removal performance of the wetland system.3.The removal efficiency of HN₄⁺-N,TN and COD was significantly improved when HRT was extend to 6 d in both wetlands packed with natural zeolite or biological ceramsite.However,the removal efficiency of NO₃^–-N and enzyme activity of rhizosphere soil were hardly affected.As far as HN₄⁺-N removal efficiency was concerned,the removal efficency in CW5?natural zeolite,no aeration,HRT=6 d?was increased by 10%compare with CW1?natural zeolite,no aeration,HRT=3 d?.Similarly,the removal efficiency of CW6?biological ceramsite,no aeration,HRT=6 d?was increased by 10%than that in CW2?biological ceramsite,no aeration,HRT=3 d?.The removal efficiency of TN in CW5 and CW6 was 20%higher than those in CW1 and CW2.The removal efficiencies of N and COD in CW7?natural zeolite,aeration,HRT=6 d?system was significantly higher than CW3.CW8?biological ceramsite,aeration,HRT=6 d?significantly improved the removal efficiency of COD compared with CW5.The plant biomasses of CW7 and CW8 were higher by 55 g and 21 g than those of CW3 and CW4,respectively.The results showed that the nitrogen removal performance of wetland system can be effectively improved by properly prolonged HRT after intermittent aeration treatment.4.In this study,the effects of substrate types and HRT on the richness and diversity of the bacterial community was not obvious in wetland systems.However,intermittent aeration treatment could significantly improve the microbial abundance and diversity in wetland systems.Proteobacteria was the dominant bacterium in all the wetland systems.Proteobacteria had a higher abundance in the aerated wetland systems The abundance of Alphaproteobacteria,Betaproteobacteria and Gammaproteobacteria in wetland systems were relatively higher in the aerated wetland systems with 3-day HRT The Deltaproteobacteria and Actinobacteria in the wetland systems of HRT=6 d had higher abundance.The abundance of the nitrifying functional genes amoA and nxrA in wetland systems treated with intermittent aeration was significantly higher than the unaerated wetland systems,while the absolute abundance of the denitrifying functional genes varied in different trends.The results shows that intermittent aeration treatment and proper HRT have remarkable influence on the microbial community structure in wetland systems.Moreover,intermittent aeration significantly improved the nitrification performance,and there was no significant inhibition of denitrification performance.The results indicated that the improvement of microbial functions by the proper operation condition could enhance the nitrogen removal performance in the constructed wetlands.