Research On The Effect Of LID Facilities On Rainwater Runoff Pollution Reduction In Campus Area

In recent years,with the acceleration of urbanization and the increase of urban population,significant changes have taken place in the underlying surface of the city,which has led to a series of problems such as reduced rainwater infiltration,increased non-point source pollution,and shortage of water resources.The traditional rainwater drainage system focuses on the rapid collection and discharge of rainwater,neglecting the storage,purification,and recycling of rainwater,which makes the stormwater runoff reach peak flow in a relatively short time,resulting in frequent urban waterlogging.Therefore,the concept of Low Impact Development came into being,and its essence is to build a sustainable stormwater management system.This study takes rainwater runoff of a university in Chongqing as the research object,investigates the characteristics of rainwater runoff water quality on the three underlying surfaces of campus roads,roofs,and green areas,analyzes the impact of rainfall intensity,rainfall duration,and rainfall on runoff water quality.State probability distribution map to count the average concentration of pollutants（EMC）,analyze the correlation between pollutants,etc.;and build a green roof and planting ditch model,carry out a small pilot study on the treatment of rainwater in LID facilities.The main conclusions are as follows:（1）The variation rules of rainfall runoff pollutants on different underlying surfaces on the campus are different.Generally,as the rainfall duration increases,the concentration of rainwater runoff pollutants gradually declines from the initial high concentration,and the decreasing trend is steep first and then slow.The concentration decay time mainly occurs in the first 20 minutes,and the pollutant concentration gradually stabilizes after 20 minutes.Correlation analysis shows that there is a significant linear correlation between SS and COD;however,in most rainfall events,the correlation between SS and TN,NH₄⁺-N,NO₃^--N,TP is weak.（2）Among the 3 types of underlays,the largest average value of each pollutant EMC（Event Mean Concentration）is the roof.It is:7.956～19.479mg/L,3.342～4.688mg/L,2.684～6.218mg/L,0.182～0.291mg/L,117～297mg/L,138～270mg/L,the old roof,the roughness is relatively Large,easy to accumulate and adsorb pollutants;and in the case of low rainfall intensity,long periods of early dry days and low production flow,the pollutants are not diluted by rainwater,making the pollution more serious.Except for TP,the pollution indexes of roofs,roads and green areas have not met the TN,NH₄⁺-N,COD,and SS EMC values of surface water classⅤ.Rainfall intensity,dry period before rain,and underlying surface type all have an impact on surface rainfall runoff pollutants,of which rainfall intensity and underlying surface type have a greater impact on runoff pollutants,followed by the dry period before rain;（3）Through LID device research,it is found that grass p lanting ditches and green roofs have a certain effect on the total runoff control of rainwater runoff and the removal rate of pollutants.Setting a certain submergence depth for planting ditches can improve the removal effect of pollutants.And correspondingly prolonging the contact time between the rainwater runoff and the device can also significantly improve the water quality of the rainwater runoff.The analysis of the planting ditches shows that the total runoff control rate in the non-submerged area of planting ditches is 31.20%～40.87%,and the average removal rates of TN,NH₄⁺-N,NO₃^--N,TP,COD and SS are41.33%,25.30%,47.23%,51.56%,54.59%and 45.63%;set 15cm submerged area,the total runoff control rate is 39.51%～56.08%,except NH₄⁺-N,the removal rate of TN,TP,SS is 60%About 50%of the sponge requirements are exceeded.In addition,under the observation that the hydraulic retention time is 0 days,1 day,2 days,...,7 days,it is found that when the retention time is 2 days,TN,NH₄⁺-N,NO₃^--N,TP,SS can reach 80%The analysis of the green roof installation shows that the total green roof runoff control rate is 31.197%～34.69%;the removal effect of SS and TP is good,the highest can reach 57%and 68%,respectively,and the removal e ffect of ammonia nitrogen is weak,At around 35%.（4）SIMCA-P numerical simulation shows that the height of the submerged area is an important factor affecting the total runoff of grass planting ditch,TN,TP reduction rate,hydraulic load is an important factor affecting the COD reduction rate of planting ditch;the outflow time is affecting the green roof The important factor of the runoff reduction rate,the influent concentration is an important factor affecting the TN reduction rate of the green roof,the influent hydraulic load is an important factor affecting the COD reduction rate of the green roof,and the impact of the dry period before the rain and the height of the uneven drainage plate on the device uncertain.By analyzing the impact intensity o f external factors on the total runoff control rate and pollutant reduction rate,it is possible to effectively grasp which influencing factors have a significant impact on the treatment of stormwater runoff by the LID facility,thereby improving the total runoff control rate and pollution The removal effect of objects can achieve the effect of reducing urban rainwater runoff.