Study On Pollution Characteristics Of Initial Rainwater And Its Intercepted Storage At Hefei Economic And Technological Development Area

The initial rainwater is formed during the early stages of rainfall.During the rainfall,the rain leaches the exhaust gas and dust in the atmosphere.On the underlying surface,runoff flushes roads,roofs and green areas,causing initial rainwater to contain large amounts of pollutants,which seriously affects the aquatic environment.This paper studied the pollution characteristics of rainwater runoff and the interception percentage of initial rainwater in Hefei Economic and Technological Development Area.By the way,a Stormwater Flood Management Model?SWMM?was established on the basis of measured rainfall events to simulate the water quality and quantity of rainfall at different recurrence interval and determine the volume of the storage tank.Firstly,the characteristics of water quality and quantity of rainwater runoff were analyzed,and the initial interception flow rate for Hefei was determined.The correlation analysis of pollutant concentration was carried out in the meantime.And actual measurements of water quantity and quality was detected there times.By analyzing the concentration characteristics of initial rainwater,the concentrations of COD_Cr and SS were affected by rainfall intensity,when the rainfall intensity turned greater,the scour effect would get stronger and the concentration would get higher.The concentration of TN,TP,and NH₃-N was related to the ADWP.The larger the ADWP got,the more pollutants accumulated on the underlying surface and the higher the concentration there would be.The EMC value of SS was 9.43-69.38 times of the first-level A emission standard in Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant?GB 18918—2002?.The EMC values of COD_Cr,TN,NH₃-N and TP were 2.00-6.44,2.98-8.25,1.47-5.15 and 2.13-3.18 times higher than that of grade V in Environmental Quality Standards for Surface Water?GB3838—2002?,respectively.On the other hand,the results of MFFn curve analysis showed that there was an initial flushing effect in the rainwater in the catchment area,but the initial flushing effect was not obvious and the proportion of the initial rainwater interception should be increased.According to the M?V?curve,the runoff volume of rainwater could be determined in Hefei City by 40%-45%of the initial rainfall cutoff.The software SPSS was used to analyze the correlations between the contaminant concentrations,and the results showed that TN and NH₃-N,COD_Cr and SS were very significantly correlated,and the regression was the best if the relationship was a cubic function.Secondly,the SWMM model was constructed by using the measured rainwater volume and water quality data in the study area.Based on general principles,the study area was generalized into 75 subcatchments,76 nodes,76 rainwater pipelines and 1storm outlet.The parameters in the SWMM model were rated using the flow and water quality data for the rain events on May 14,2017.The flow and water quality data of the rainfall event on June 5,2017 were used to verify the parameters,which proved the reliability of SWMM model at this area.Finally,in order to calculate the volume of the storage tank,the rainfall process line was established based on different recurrence interval in the study area.With the increase of rainfall duration,the rainfall intensity at different recurrence interval gradually increased and then decreased.By the way,the greater the recurrence interval,the greater the rainfall.At P=0.5 a,the concentration curves over time of COD_Cr,SS,TN,NH₃-N,and TP all increased at first and then decreased.When P=1 a,3 a,and 5 a,overflow occurred at the nodes,and the concentration curves over time of COD_Cr,SS,TN,NH₃-N,and TP firstly increased and then decreased,and then increased again and then gradually decreased.According to the Code for Design of Outdoor Wastewater Engineering?GB 50014-2006??2016 Edition?,the maximum volume of the storage tank was 32400 m³.Considering the environmental benefits and economic benefits,a return period of 3 years was selected as the storage tank design standard,and 45%of the stormwater runoff volume was selected as the cutoff volume of the initial rainwater,and the determined volume of the storage tank was 53000 m³.