Study On The Regularity Of Migration Of Stormwater Runoff Pollutants In Bioretention Cell

Urban stormwater runoff usually carries a certain amount of pollutants,and direct discharge to receiving water bodies can cause pollution of the water environment.Practices have proved that a well-designed bioretention cell can effectively control the stormwater runoff pollutions.In this thesis,columnar and static adsorption experiments in the bioretention cell were combined with the established mathematical model to study the migration and transformation of main pollutants(COD,ammonia nitrogen,nitrate nitrogen,and phosphorus)of stormwater runoff in the bioretention cell.Secondly,using nitrate-nitrogen as the main pollutant to study,the Hydrus-1D software was used to establish a migration model of runoff pollutants in the bioretention cell during the rain,to simulate the operational situation of the bioretention cell under different scenarios.Finally,the sensitivity of the hydraulic parameters and the solute transport parameters were analyzed.The runoff pollutants in this study included COD,ammonia nitrogen,nitrate nitrogen,and total phosphorus.In the experiment,mix sand and soil with a ratio of sand to soil of 4:6,which were selected as the filler layer of the bioretention cell.Static migration experiments and breakthrough experiments were used to study the regularity of migration of stormwater runoff pollutants in the bioretention cell.The results showed that K_d(the adsorption constants of the linear isothermal equations)of COD,ammonia nitrogen,nitrate nitrogen and total phosphorus were 3.11L/kg,2.11L/kg,0.04L/kg,and 1.64L/kg respectively.It shows that the bioretention cell filler has strong adsorption capacity for COD,ammonia nitrogen,and phosphorus,while the filler has almost no adsorption for nitrate nitrogen.The COD breakthrough curve conformed to the general rule of the breakthrough curve in the first 20 hours.After 20 hours,the concentration of water released remained at 150 mg/L,and the reduction rate of COD was 65%under the microorganisms,reflecting the microbial degradation play the dominant role in COD reduction in the bioretention cell.The breakthrough time of ammonia nitrogen,nitrate nitrogen,and phosphorus in the bioretention cell were 8d,1d,and 7d,respectively.Because the soil particles had a stronger adsorption capacity for ammonia nitrogen and phosphorus and extended their breakthrough time in the bioretention cell.The reason for the shorter breakthrough time of nitrate nitrogen is that nitrate-nitrogen itself has a negative charge and mutually exclusive with soil particles,so the filler cannot effectively adsorb and retain it.The denitrification intensity increases with time.In order to further study the transport of pollutants in the bioretention cell,stormwater runoff pollutants transport model parameters in the bioretention cell were fitted by using RETC software and CXTFIT software,and the Hydrus-1D model of breakthrough experiments was established to verifiy the reliability of the parameters.The results show that in addition to COD the simulation results of the model have good consistency with the measured results and the R~2are all greater than 0.8,indicating that the model parameters are credible.The low degree of R~2of the COD is due to the microbial activity caused a large error between the simulated and measured results.The model of runoff pollutants in the bioretention cell during the rainfall period was established by Hydrus-1D software.The nitrate nitrogen was used as the main pollutant to study the influence of water content,rainfall intensity,and the thickness of the filling layer on the control effect of the bioretention cell on stormwater runoff volume and water quality.The results showed that when the initial water content of the bioretention cell increased from 15%to 30%,the reduction rate of rainwater runoff and pollution load was changed from 33.78%and60.52%to 17.97%and 71.57%,respectively.With the increase in the initial moisture content.the higher the initial moisture content becomes,the higher the pollution load reduction rate is,the lower stormwater runoff reduction rate is.When the thickness of the filling layer is increased from 50cm to 90cm,the reduction rate of rainwater runoff and pollution load is changed from30.51%and 58.90%to 52.76%and 92.13%.respectively.The bioretention cell have improved the effects of rainwater runoff reduction and pollutant load reduction.When the rainfall intensity increases from 2.19cm/h to 2.79cm/h,the rate of rainwater runoff and pollution load reduction became 32.30%and 62.78%to 28.50%and 53.41%,respectively.By establishing the Hydrus-1D model to study the regularity of the migration of pollutants in the bioretention cell,it is possible to analyze and predict the treatment effect of the bioretention cell on the runoff quality and quantity of water under different working conditions.Through the sensitivity analysis of soil hydraulic parameters,solute diffusion coefficient,longitudinal dispersion,and solute partition coefficient,the results showed that:regression equation established by K_s,?,n,?_r,?_s these five soil hydraulic parameters and cumulative flow of water changes.The slopes of the regression equations are 1.108,-1.6216,-6.4017,0.5765,-4.4189,respectively.The slopes of the regression equation for the solute diffusion coefficient,longitudinal dispersion,solute partition coefficient,and pollutant removal rate were-0.0023,-0.0145,and 1.2009,respectively.The soil hydraulic characteristic parameters and the solute partition coefficient are the sensitive parameters of the model.The determination of the sensitivity parameters reduces the amount of work for model tuning and parameter calibration.