Study On Advanced Nitrogen Removal Of Two Types Of Electron Donor Enhanced Floating Treatment Wetlands

When low carbon/nitrogen ratio water enter into the surface water,it will cause water eutrophication,endanger human health.To overcome the shortage of floating treatment wetlands in treating nitrate,two organic and inorganic electron donors were selected to improve the denitrification performance of floating treatment wetland.Heterotrophic enhanced floating treatment wetland?HEFTW?,sulfur autotrophic enhanced floating treatment wetland?AEFTW?,enhanced floating treatment wetland?EFTW?,floating treatment wetland?FTW? and control floating wetland?CON? were constructed.In the nearly two-years operation,the nitrogen removal performances of different systems were compared.The effects of hydraulic retention time?HRT?,temperature?T?,carbon/nitrogen ratio?C/N? and sulfur/nitrogen ratio?S/N? on nitrogen transformation and denitrification performance were investigated,kinetic models of nitrogen removal were established,the dynamic variation of plant uptake on nitrogen removal were studied,and the variation rules of greenhouse gas emission in five systems were clarified.The detailed changes of microbial community composition in the FTW systems were analyzed,and the mechanism of microbial nitrogen removal was analyzed.Finally,the optimum operation parameters were confirmed.This can provide an important theoretical support for the application of electron donor enhanced floating treatment wetlands for the advanced nitrogen removal in low C/N water treatment.The study of nitrogen removal performance of EFTW and FTW showed that the nitrogen removal performance of EFTW and FTW were similar with TN removal efficiency?TNRE? of 4.99%-68.5% and 4.70%-67.8%,TN loading removal rate?TNRR? of 0.11-5.37 and 0.10-4.82 g m^-2 d^-1.When the water temperature was 25-30?,the denitrification capacity of EFTW and FTW significantly increased in the second year,and the highest NO₃^--N RR reached the peak value at HRT=1d.When HRT was 3 d,T had little effect on the NH₄⁺-N removal performance,however,had significant effect on the TN and NO₃^--N removal.Nitrogen removal of EFTW,FTW conformed to the first-order kinetic model.The?value were high and the k₂₀ of TN and NO₃^--N significantly increased in the second year.Greenhouse gas emissions of EFTW and FTW were approximately the same.The mean N₂O flux in the systems was 2.85-9.60 mg m^-2 d^-1,and the mean CH₄ emission was 1.72-8.8 mg m^-2 d^-1.The mean CO₂emission was-5160-2394 mg m^-2 d^-1.Despite macrophyte promoted the emission of N₂O and CH₄,the CO₂ fixation was increased,and EFTW and FTW generally were"carbon sink"in annual.The nitrogen removal performance of electron donor enhanced floating treatment wetlands showed that:The TNREs of HEFTW and AEFTW significantly higher than those in EFTW and FTW?68.1%-85.3% and 56.1%-90.8%respectively?,TN effluent meets surface water class V standard in summer.Mixotrophic denitrification evidently occurred in AEFTW,NH₄⁺- N became the key factor to limit the improvement of nitrogen removal performance of HEFTW.Electron donors promoted the plant uptake of N,S,and C elements.Sulfur-electron donors significantly increased the TN loading removed by plant uptake and delayed the release of nitrogen.The electron donor decreased the flux of N₂O,the carbon electron donor significantly increased CH₄ flux,and the AEFTW had the advantage of reducing GWP in the hot seasons.Combined with the results of microbial high-throughput sequencing,the mechanisms of biological nitrogen removal in HEFTW and AEFTW were revealed.The electron donors significantly changed the microbial community structure,and Dechloromonas and Thiobacillus became the most dominant genus in HEFTW and AEFTW,respectively.Mixotrophic denitrification mechanisms formed in both HEFTW and AEFTW.Carbon electron donors promoted the proliferation of denitrifying bacteria and sulfate-reducing bacteria.Sulfate-reducing bacteria used organic electron donors to reduce sulfate to S⁰ or S^2-.Sulfur autotrophic denitrifying bacteria then used S⁰ or S^2- as the electron donors for autotrophic denitrification.sulfur electron donor not only enhanced the plant uptake of nitrogen and autotrophic denitification,flourishedr plants in AEFTW could secrete more organic carbon to promote the heterotrophic denitrification.In addition,the nitrifiying bacteria were not inhibited in AEFTW.The operation parameters of HEFTW and AFTW were further investigated and nitrogen removal predition models were established.The optimum C/N and HRT of HEFTW at<10?,15-20? and 25-30? were 2.5,1.7,1.5 and 3,2,0.5,respectively.The optimum S/N and HRT of AEFTW at the corresponding temperature were 1,0.8,0.6-0.7 and 3,3,1,respectively.The most significant parameters in the TNRE and TNRR prediction models for HEFTW and AEFTW were temperature and HRT,while the most significant parameters for the nitrogen removal prediction models in EFTW and FTW were all temperature.