Study On Removal Efficiency Of Pollutants In Stormwater And Snowmelt Runoff By Bioretention

In the process of rapid development of urbanization,the area of impervious floor mats has continued to increase,and pollution sources have also increased.Oil,nitrogen,phosphorus,heavy metals and other pollutants are attached to rooftops,roads and so on.In the process of rainfall or other forms of precipitation,pollutants are washed with rainwater runoff and thus enter the urban drainage network.Eventually,runoff without treatment or inadequate treatment directly enters rivers,lakes,and other receiving water bodies,causing massive pollution to urban water bodies.In recent years,China has vigorously developed the sponge city construction.Bioretention is one of the typical technologies in the Sponge City—Low Impact Development system,and it is very important.Therefore,it is of great significance for the design,development,and engineering application of bioretention to study the removal efficiency of runoff pollutants by bioretention.In this project,a small simulation system of bioretention is constructed by using traditional medium,including sand,gravel,sandy loam and plants.First,we examined the removal of pollutants from stormwater runoff by setting up bioretention with different inundation heights(30 cm,45 cm,and 60 cm).It was found that the height of the submerged area had a great influence on the removal of the pollutants.In order to ensure that all the pollutants can reach a certain level of removal,45 cm is a more suitable submerged height,and this submerged height is used in the later experiment.The average removal rates of COD were 66.48%,78.91% and 84.85% respectively,when the rainfall were heavy rain,medium rain and small rain.A nd the removal rate of ammonia nitrogen was about 80%.With the increase of rainfall intensity,the removal rate of nitrate nitrogen increased from-6.2% to 54.19%.The treatment effect of TN was positively correlated with rainfall intensity.As the rainfall intensity increases,the TP removal rate gradually decreases.Then,in the long-term dry and wet alternate operation of the bioretention,it was found that the length of the drought period(2 d,5 d,10 d)had no significant effect on the efficiency of the bioretention to remove COD,and the removal rate was 84%~91%.The effluent concentration of TP in the bioretention increased with the increase of drought period.The removal efficiency of ammonia nitrogen is between 83.9%~91.8%.The suitable length of drought period can promote the removal of nitrate nitrogen and TN by bioretention.Under different drought conditions,the migration and transformation rules of pollutants in the bioretention are different,indicating that drought is the main factor affecting the performance of the bioretention.Finally,it was found that the average removal rates of Cd?Zn?Cu?Pb were 100%,99.5%,98.6% and 94.4%,respectively.Heavy metals are mainly removed in the upper part of the bioretention.The heavy metal content in the surface soil of the bioretention showed an increasing trend with the increase of the number of t ests.In order to check the actual operation effect of the bioretention,the actual snowmelt runoff purification experiment was carried out.First,the snowmelt runoff pollution on two underlying surfaces of garden and road in a campus of Harbin is compared,and it was found that the snowmelt runoff pollution of the garden is lighter than that of the road snowmelt runoff.The average removal rates of Zn,Cu,Pb and Cd were 87.3%,76.9%,85.7% and 100% respectively.The average removal rates of TSS,COD,TN and ammonia nitrogen were 98.1%,60.8%,11.2% and 90.1% respectively.The removal of nitrate nitrogen is extremely unstable.The outflow concentration of TP is greater than that of inflow concentration,which indicates that the leaching of phosphorus is in the bioretention.In general,the bioretention can be successfully used to deal with snowmelt runoff on urban roads.