Study On The Effect Of Rainwater Purification By The Bioretention System Of Different Substrates

With the rapid development of urban economy,the problems such as the increasing of water under impervious surface,flooding and runoff pollution are becoming more and more serious.As a key part of Low Impact development(LID)technology,bioretention plays an important role in reducing runoff and controlling pollutants.In this paper,the substrates of bioretention system were screened,and the screened substrates were used in bioretention system.The law of the stagnation of the rainfall runoff by the bioretention and the difference purification of pollutants between the different rainfall scenarios,submerged outflow height and the drying period were studied.the following conclusions:(1)The isothermal adsorption,adsorption kinetics analysis and equation fitting were carried out on the construction waste,volcanic rock,river sand,gravel and other substrates.It was found that 4 substrates have less adsorption on ammonia nitrogen and construction waste showed good adsorbability to phosphate,saturated adsorption capacity reached 2.83(mg/g),adsorption rate was 0.014(mg/(g·h)),which was obviously better than the other 3 kinds of substrates.(2)The ability of reducing the total runoff volume and controlling the peak flow rate of the bioretention unit increased with the decrease of the particle size.And the smaller particle size filled bioretention unit has better effect on pollutant removal.(3)With the increase of rainfall recurrence period,the detention effect of bioretention unit and the reduction of runoff peak volume are decreasing.Different rainfall(rain,heavy rain and rainstorm)have an effect on the removal of runoff pollutants.With the increase of rainfall intensity,the removal of phosphate has little effect and the removal of ammonia,total nitrogen and COD are decreasing.(4)The increase of submerged outflow height can increase the entire hydraulic retention time.The hydraulic efficiency of three submerged outflow heights(0mm,300 mm,600mm)is 0.431,0.845,1.019 respectively.The former is closer to the hydraulic model of the push flow,and the latter is closer to the hydraulic model of the mixed flow.With the rise of the submerged flow height,the removal rate of ammonia nitrogen in the bioretention unit decreased,the removal efficiency of nitrate nitrogen first increased and then decreased,and the average removal of COD had an obvious decreasing trend.In general,the height of the submerged flow was 150 mm,which was better for the removal of pollutants.(5)With the extension of the drying period,the bioretention unit has little effect on the removal of ammonia nitrogen and phosphate,but it has a great influence on the removal efficiency of COD and nitrate nitrogen.There are obvious spillovers phenomena in nitrate nitrogen.After several days of humid recovery,the bioretention unit can achieve a higher pollutant removal effect.(6)Surface layer microbial activity is higher than the bottom in the bioretention unit,and the biomass in the bioretention unit of larger particle size is larger than that of small particle size.