Research On Pollution Characteristics And Reduction Rate Of Initial Rainwater Runoff In Sponge City Construction

With the rapid promotion and development of the concept of sponge city construction in our country,Chongqing has become one of the first sponge city construction pilot cities.In 2015,Bishan Green Island New Area of Chongqing became one of the first sponge city construction pilot projects in Chongqing.Pollution control of initial rainwater runoff is one of the important goals of sponge city construction.The study of pollution characteristics and reduction rate of initial rainwater runoff in sponge city will contribute to the control of rainwater runoff pollution in sponge city construction.In this study,the initial rainwater runoff of residential area（RA）,municipal road（MA）,commercial area（CA）,square area（SA）and public service area（PSA）was selected as the research object in Bishan Green Island New Area.Solid suspended solids（SS）,chemical oxygen demand（COD）,five-day biochemical oxygen demand（BOD₅）,total nitrogen（TN）,total phosphorus（TP）and iron content（Fe）were taken as pollution indicators.The temporal variation of pollutants,the correlation between pollutants,the biodegradability of pollutants,the first flush effect,the even mean concentration of pollutants（EMC）,the pollution load of pollutants,the pollution factors of runoff were studied.The reduction rate of pollutants after low impact development（LID）was investigated by SWMM model.The main conclusions are as follows:1.The results of the instantaneous concentration distribution and variation with time of pollutants show that for MA,the concentrations of pollutants varied widely,and the average concentrations are significantly higher than those of the other functional areas.With the prolongation of rainfall time,the pollutant concentration fluctuates to some extent,but the overall trend is attenuation,and the decline trend is steep before and slow after.The attenuation of pollutants mainly occurs in about 10²0 minutes and the pollutant concentration basically tends to be stable in about 50⁶0 minutes.2.The results of correlation between SS and other pollutants and biodegradability of pollutants show that SS has significantly correlation with COD and BOD₅,and poor correlation with TP and Fe.The BOD₅/COD ratio of each functional area is much less than 0.3,which indicates that the biodegradability of the initial rainwater in Bishan District is poor,and it is not suitable for biochemical treatment.3.The analysis results of the first flush effect based on M（v）curve and b parameter show that the first flush effect occurs in different functional areas in Bishan District.The initial flush effect of different pollutants is Fe>TN>SS>BOD₅>COD>TP for RA;Fe>SS>COD>BOD₅>TN>TP for MA;SS>COD>Fe>TN>BOD₅>TP for CA;Fe>SS>TP>COD>TN>BOD₅ for SA;TP>BOD₅>COD>Fe>TN>SS for PSA.By comparing the average values of b parameters of pollutants in different functional areas,it can be concluded that the first flush effect of CA is the strongest and that of PSA is the weakest.Initial runoff interception rates for RA,MA,CA,SA and PSA suitability are 30%,30%,40%,40%and 40%,respectively.4.The analysis results of the EMC and annual pollution load show that the average EMC values of SS,COD,BOD₅,TN,TP and Fe for RA are 66.56,56.70,4.15,2.52,0.09 and 0.12 mg/L,respectively;For MA,are 101.03,60.45,4.86,3.80,0.11 and 0.05 mg/L,respectively;For CA,are 101.03,60.45,4.86,3.80,0.11 and 0.05 mg/L,respectively;For SA,are 106.99,55.93,4.57,3.87,0.07 and 0.13mg/L,respectively;For PSA,are 117.60,68.10,5.51,3.94,0.13 and 0.28 mg/L,respectively.The annual pollution load of SS,COD,BOD₅,TN,TP and Fe are17.58,14.98,1.10,0.67,0.02 and 0.03 t/km² for RA,respectively;238.39,76.14,6.99,4.09,0.16 and 0.24 t/km² for MA,respectively;46.04,27.48,2.21,1.73,0.05 and 0.02 t/km² for CA,respectively;23.18,12.12,0.99,0.84,0.02 and 0.03 t/km²for SA,respectively;20.88,12.09,0.98,0.70,0.02 and 0.05 t/km² for PSA,respectively.MA are the main contributors to the non-point source pollution in Bishan Green Island New Area.5.The results of influence factors of runoff pollution show that the average concentrations of MA are significantly higher than those of the other functional areas.The annual pollution load is strongly correlated with rainfall and mean rainfall intensity,but poorly correlated with dry days before rain.6.The results of the reduction rate based on SWMM model show that the total emission of pollutants increases with the increase of the repetition period before and after LID modification,but the increase value decreases,the maximum emission occurs when the repetitionperiod is 10 years;The reduction rate of each pollutant decreases gradually with the increase of the repetition period,but the reduction rate of pollutants all reaches 50%.