Research On The Improved Effect Of Nitrogen Removal And Characteristics Of N₂O And CO₂ Emission In Bioretention System

Bioretention system is an in-situ control method for urban stormwater runoff based on the infiltration principle.It has efficient runoff reduction capacity,water purification capacity,and pollution load reduction capability.It has been widely recognized and applied at home and abroad.In this paper,the bioretention system was used as the research object,and the effect of the bioretention system on the removal of TN,TDN,NH₄⁺-N and NO₃^--N from the rainwater runoff and N₂O,CO₂ emissions was studied by simulation experiments and semi-manual water distribution methods.Then,discussed the factors that affecting nitrogen removal efficiency and greenhouse gas emissions.The simulated results of the bioretention system on the rainfall runoff showed that the addition of biochar to the traditional bioretention system significantly improved the nitrogen removal efficiency and runoff reduction capacity.The removal efficiency increased with the increase of biomass carbon consumption.but the difference is not significant.The system’s mass reduction rate of nitrogen pollutants under medium and high pollution loads is higher than that of low pollution loads,and a nine-day dry period can make the next rainfall N reduction rate under the same rainfall pollution load conditions reach the highest value.Planting plants promotes the removal of nitrogen pollutants from bioretention systems.During the whole rainfall process,nitrogen was eluted mainly in the form of NO₃^--N,and NH₄⁺-N had the best removal effect.The submerged zone significantly improved the system’s removal rate of TN and NO₃^--N,in which the removal rate of NO₃^--N increased greatly and the removal rate of NH₄⁺-N decreased slightly.In a comprehensive comparison,the optimal addition amount of biomass carbon is 2%.The experimental results of bio-retentive system gas emissions show that the total change trend of N₂O and CO₂ emission flux is opposite after rainfall,N₂O peak appears in the initial stage after rain,and then the emission flux gradually decreases until stable.The peak of CO₂ appeared in the dry period after the rain,and the initial discharge was the lowest,and gradually increased until stable.During the peak period,Both N₂O and CO₂ fluxes increased with the increase of biochar.Biochar inhibits the emission of N₂O but significantly promotes CO₂.The ¹⁵N tracer method indicates that the main source of N₂O emissions is the denitrification process when the N₂O emission is high at the early stage after rainfall.The BC0 device has the highest emissions of N₂O and CO₂ under the inlet condition of NO₃^--N&NH₄⁺-N,The N₂O emission is highest under the condition of NO₃^--N.The setting of submerged zone significantly increased the emissions of N₂O but inhibited the CO₂ emissions in the biochar system.The impact of dry season on N₂O emissions was significantly higher than that of CO₂ emissions.Among the two greenhouse gases in this experiment,CO₂ is the main contributor to the comprehensive greenhouse effect,and the contribution of N₂O is less than 1%.The comprehensive greenhouse effect of various treatment devices increases with the increase of biochar content.The addition of biochar will promote the comprehensive greenhouse effect of greenhouse gas emissions from bioretention system in a short period of time,however,the addition of 2%biochar and submerged zone can reduce the comprehensive greenhouse effect of N₂O and CO₂.