Study On The Control Of The Turbidity Of Pollutants In Road Rainwater Runoff By Prefabricated Filter Bed

With the continuous promotion of the path of urbanization,the proportion of the urban impervious surface continues to rise,and the decrease of infiltration leads to the higher flood peak flow and the aggravation of the pollution load of the non-point source pollution.Turbidity of pollutant is not only one of the main pollutants in the runoff,but also one of the key substances affecting the subsequent purification effect and operational stability of all kinds of LID facilities.In order to alleviate the urban non-point source pollution and to remove the turbidity of pollutants in the runoff,a kind of perfabricated filter bed with good interception effect and easy cleaned and long cleaned cycle is developed,and it can be used as the pretreatment facility for other LID facilities.In view of the rainwater runoff quality in Xi'an,the filter material of the prefabricated filter bed was optimized,the optimum particle size and the optimum thickness were determined,and the flow direction of the prefabricated filter bed was clearly defined.Finally,the operation period of the prefabricated filter bed was simulated and predicted.The main conclusions of this study are as follows:?1?Under the different influent loads,The purification effect order of three kinds of filter materials on turbidity removal is:plastic beads>ceramics beads>silica sand.However,the purification effect of the dissolved pollutants(COD_Cr,TP and TN)in the three kinds of filter materials is poor.The main reason is that the process of rainwater filtration is mainly by physical interception,with less biological and adsorption.Comprehensive analysis the physical properties and the differences in the removal efficiency of pollutants of the three kinds of filter materials.The plastic beads are recommended as filter materials for the prefabricated filter bed in actual engineering.?2?The discharge capacity of the three filter devices?filter particle size of 1mm,3mm and 5mm?gradually weakened with the extension operation time,and the smaller the size of the filter material,the greater the decrease of the discharge capacity.The relationship between the discharge capacity?L/h??Y?of 1mm,3mm and 5mm plastic beads and the extension operation time was Y=90.8+8.45X-1.14X²?Y=103.17+1.11X-0.29X² and Y=97.34+3.85X-0.41X² respectively.?3?The order of removal effect of three kinds of plastic filter divices?filter particle size of 1mm,3mm and 5mm?on turbidity of pollutants was as follows:3mm>5mm>1mm,and the plastic bead with the particle size of 3mm was the best to control the particle size of particulate pollutants,so the plastic filter material recommended by 3mm is used as filter material in practical engineering.Purification effect of turbidity of pollutants based on the test,the thickness of the filter layer is 60cm when the plastic bead with a particle size of 1mm is used in engineering.The thickness of the filter layer is 40⁶0cm when the plastic bead with a particle size of 3mm or 5mm is used.For areas with heavy load of rainwater runoff,plastic beads with a particle size of 1mm is not recommended as practical engineering filter material.?4?The order of purification efficiency of the three kinds of prefabricated filter beds is as follows:horizontal flow>upward flow>downward flow.In the whole operation period,the decrease percentage of voidage of the upward flow,horizontal flow and downward flow filter bed are 42.2%,38.6%and 27.7%respectively.But considering the difficulty of cleaning the filter material in practical engineering,it is recommended to use the horizontal and downward flow prefabricated filter bed.?5?The filter bed clogging time of the three prefabricated type filter beds?down flow,horizontal flow and upward flow?predicted by the Kozeny-Gaman equation is 1.12a,1.27a and 1.73a respectively.The operation cycle of the three prefabricated filter beds is more than 1 years,so the cleaning frequency is also smaller,and it is feasible in practical engineering.