Study On Discharge Characteristics And Pollution Control Of Combined Sewer Overflow Based On SWMM

With the rapid urbanization process,the problem of overflow pollution is becoming more and more serious.For the old urban area,the drainage system was mainly constructed by the intercepting combined drainage system.During the rainy season,the amount of mixed rain and sewage water was relatively large,and rainwater which exceeded the capacity of the intercepting main pipe overflows into the surrounding water body through overflow wells.This part of the mixed rainwater and sewage water was called combined sewers overflow（CSO）.CSO was the main reason for the imbalance of black and smelly water and water environment in cities.Therefore,this paper studied the emission characteristics and pollution control of CSO from the three aspects of source infiltration（lid facility）,pipeline system（intercepted ratio）,end storage（surge tank）.This paper used SWMM software to construct a surface runoff model of a certain place in Hangzhou.Aiming at the selection of the interception multiple n₀,the overflow flow and project investment cost in each year when the interception multiple was 1-5 were simulated and calculated,and the optimal interception multiple was selected by constructing an optimization model.Regarding the selection of the regulating storage volume of the regulating storage tank,the intercepted rainwater volume and pollutant load at different regulating storage depths were simulated and calculated,and the concentration of the pollutant load intercepting at the discharge outlet was used to determine the control effect of different regulating storage depths on the initial rainwater runoff pollution.In view of the selection of the optimal area proportion of LID facilities,the total runoff control rate and engineering cost under different LID facility area proportions were simulated and calculated,and the optimal area proportion was selected by constructing an optimization model.Research indicates:（1）When the interception ratio n₀=1,about 35-50%of rainwater and 26%of the pollutant load can be intercepted each year.When n₀=5,only 60-80%of the rainwater and 53%of the pollutant load can be intercepted.The main reason is that the proportion of the design flow rate increase of the intercepting main pipe was greater than the proportion of the pipe diameter increase.Through the ideal point method optimization,it was concluded that when the interception ratio was 2.63,the optimal balance between environment and economy can be achieved.（2）The retention rate of pollutants in the regulation storage tank increased slowly with the increase of the regulation and storage depth.At the maximum regulated storage depth（9mm）,the storage tank can intercept up to 66.9%of the pollutant load（2004）and at least 48.82%of the pollutant load（2007）.The main reason is that the runoff coefficient in 2007 was relatively large.As a result,the amount of water that forms runoff increases,and part of the rainwater overflows at the upstream node,which reduced the efficiency of pollution interception.Through the intercepted pollutant concentration curves at different regulation and storage depths,it was concluded that the initial rainwater runoff pollution can be basically solved when the regulation and storage depth was 6mm.（3）As the proportion of the area increased,the reduction effect of the green roof on the runoff gradually decreased,and the reduction effect of the infiltration pavement and rainwater garden on the runoff decreased almost linearly.The reduction effects of the three types of LID facilities on overflow pollutants all showed a linear reduction.Using the optimization model,it was concluded that when the proportions of green roof,permeable pavement,and rainwater garden were 14.7%,9.92%,and 7.45%,respectively,the total runoff control rate was the largest and the corresponding construction cost was the lowest.