Technique And Mechanism Of Intensified Nitrogen Removal In Horizontal Subsurface Flow Constructed Wetland

The present water pollution situation is serious in China.Due to the traditional wastewater treatment technology of low nitrogen removal efficiency and plenty of farmland irrigation water directly discharged into water bodies without proper treatment,contributing to severe eutrophication in lakes and rivers,which will also lead to the decline of ecological function.Constructed wetlands（CWs）have been widely used as an ecological way for nitrogen removal owing to the advantages of small footprint and low construction and maintenance costs.Nitrogen removal mainly depends on nitrification and denitrification dominated by microorganisms in CWs.This study starts with the influencing factors of nitrification and denitrification in CWs.Building new spray aeration system and exhaust system will improve oxygen condition in CWs and then promote complete nitrification.Meanwhile,this study pays attention to the effect on nitrogen removal efficiency with different adding location of carbon source.Put forward the solution of restrained denitrification caused by insufficient carbon source.In addition,this study applies microbial immobilization technology to CWs.The slow growth and survival rate usually causes low nitrifier abundance in CWs,which significantly limit nitrification reaction.The immobilization technique may be a promising solution to enhance the microbial abundance of nitrifier for effective NH₄⁺-N removal in CWs.The prime work and results are as follows:（1）Research on intensified nitrogen removal by external carbon addition and denitrification exhaust system in horizontal subsurface flow constructed wetlands（HSSFCWs）.Four wetland plants and four disposal methods were chosen to observe the release of carbon source.The results showed that the best carbon release was Eichhornia crassipes treating with 5% dilute sulphuric acid,COD release was 368.6 mg.L^-1.The research also focused on two different adding method of carbon source to HSSFCWs,which involves adding through influent and adding through denitrification exhaust pipe,to detect the effect of nitrogen removal in HSSFCWs.Better performance was obtained in adding carbon source through denitrification exhaust pipe system.NO3--N removal efficiency reached 95.56% while TN removal efficiency was 86.70%.Additional carbon source as electron donor,participate in the redox reaction and change the system redox potential to high reducibility environment.This promotes the transition of the inorganic nitrogen form and improves nitrogen removal efficiency by complete denitrification in HSSFCWs.In addition,the influence of nitrogen removal by denitrification exhaust pipe was also investigated.According to Henry’s law,using denitrification exhaust pipe will diminish N₂ and N₂O gas partial pressure,thus reduce mole fraction of gas solubility.This is beneficial to the discharge of N₂ and N₂O gas,in favor of denitrification chemical equilibrium,and promotes NO3--N removal through complete denitrification in HSSFCWs.（2）Research on intensified nitrogen removal by novel spray aeration system in HSSFCWs.The distributing pipe and spraying nozzle which could generate atomization spray were used to improve oxygen condition in HSSFCWs.The results showed spray water distribution increased time and aera contact of influent water with air.The formation of water droplets in the air was beneficial to the rate and quantity of oxygen dissolved in water.The oxygen-enriched influent was also helpful to enhance the nitrogen removal ability of nitrification in HSSFCWs.The effluent NH₄⁺-N concentration was significantly lower in aerated HSSFCWs,proven that spray aeration system could increase DO concentration,promote nitrification and thus accelerate NH₄⁺-N removal in HSSFCWs.（3）Research on microbial community structure and spatial distribution in spray aerated HSSFCWs.The change of microbial community structure and quantity was detected under the spray aeration condition.The results showed the number of ammonifying bacterial in the upper layer was larger than that in the bottom layer.The number of ammonifying bacteria in the upper layer was 7.92×10⁶-4.29×10⁷ MPN.g^-1.The nitrate bacteria increased along the flow direction in the upper layer.The nitrate bacteria reached 6.33×10⁴ MPN.g^-1 in rear end close to the effluent zone.The denitrifying bacteria in the bottom layer increased by almost one order of magnitude than the upper layer.The denitrifying bacteria reached 3.92×10⁷ MPN.g^-1 in fore end close to the influent zone in the bottom layer.The increase of nitrite and nitrate bacteria is beneficial to complete nitrification and improves NH₄⁺-N removal.Meanwhile,the increase of denitrifying contributes to complete denitrification and guarantees high removal efficiency of NO3--N.（4）Research on optimal immobilization condition for efficient nitrification of immobilized pellets.The effect on immobilized pellets formation,strength and mass transfer performance by additives and crosslinking agent were investigated.Nitrifier immobilized on a carrier pellet containing 10% polyvinyl alcohol（PVA）,2.0% sodium alginate（SA）and 2.0% calcium chloride（CaCl₂）was benificial to COD and NH₄⁺-N degradation.The immobilized pellets were easy to made and had high mechanical strength,better mass transfer performance under this situation.（5）Research on optimal degradation condition for efficient nitrification of immobilized pellets.The variations of temperature,p H,DO and dosage was investigated to obtain the optimal degradation condition of immobilized pellets.Batch experiments were conducted to inspect characteristics of immobilized pellets for the subsequent application in CWs.The ideal reaction conditions were obtained as follows: p H,8.5;DO,4.0 mg.L^-1;temperature,30°C and PVA pellets dosage,60 g.L^-1.In the process of practical application,the temperature controls from 25 to 35°C,p H keeps from 7.0 to 8.5 and DO concentration remains about 4.0 mg.L^-1,could also maintain high NH₄⁺-N removal efficiency.（6）Research on efficient nitrogen removal by the utility of immobilized nitrifier as a viable technique in HSSFCWs.Higher NH₄⁺-N and TN removal performance were obtained in immobilized CWs than that in non-immobilized CWs under three influent COD/N ratios.65.27%% of TN was removed in immobilized CWs when influent COD/N reached 9:1.Immobilized pellets will promote nitrification processs and contribute to NH₄⁺-N removal by artificially increase the number of nitrifiers in HSSFCWs.The diffenent influent COD/N ratios will also provide sufficient carbon source for denitrification.Thus efficient nitrification and denitrification finally facilitated TN elimination.（7）The novel spray aeration system generates water droplets in the air,which is beneficial to the rate and quantity of oxygen dissolved in water.The oxygen-enriched influent is helpful to enhance NH₄⁺-N removal and promotes nitrification in HSSFCWs.The application of immobilized nitrifier promotes nitrification processs and contributes to NH₄⁺-N removal by artificially increase the number and activity of nitrifiers in HSSFCWs.In addition,using denitrification exhaust pipe diminishes N₂ and N₂O gas partial pressure,which is beneficial to the discharge of N₂ and N₂O gas,in favor of denitrification chemical equilibrium,and promotes NO3--N removal through complete denitrification in HSSFCWs.Furthermore,external plants’ carbon addition contributes to sufficient electron donor of denitrification,which benefits NO3--N removal and intensifies denitrification in HSSFCWs.