ObjectiveTo control combined sewer overflow (CSO) pollution, this study explores a design method for storage tank volume in CSO pollution control.MethodsTaking the ecological park area of Dongguan City as the research object, based on the storm water management model (SWMM), the drainage channels and related catching-up zones were generalized to construct the regional CSO hydraulic model, and the influence of the designed volume of the storage tank on the pollution control effect of the CSO under typical hydrological year conditions, annual total overflow volume, annual total overflow frequency and the annual total overflow COD load as the indices to evaluate the pollution characteristics of the CSO.ResultsIncreasing the volume of the storage tank could enhance the control rate of overflow water volume. However, due to the diminishing marginal benefit, there exised an optimal point. Before this point, increasing the volume of the storage tank could significantly improve the control effect of overflow pollution. After the optimal point, further increasing the volume of the storage tank had limited effect on improving the control effect of overflow pollution. In this study case, the optimal volumes of the storage tanks corresponding to the four drainage channels were 8 000, 10 000, 15 000, and 1 000 m³, respectively. The control rates of the overflow pollutant load and overflow water volume in the corresponding study area were 62.4% and 51.8% respectively. Compared with the working condition without storage tanks, the number of overflows in the following year was reduced by 43.ConclusionThere is an initial scouring effect in the pollution of CSO. When the volume of the storage tank is the same, the control rate of the storage tank for the pollutant load of the overflow is higher than that for the overflow water volume. Adopting the mathematical model method that can reflect the real confluence process is conducive to seeking a technical and economic balance point between the pollution control efficiency of the storage tank and the construction scale. The design method proposed in the research is a beneficial supplement to the current technical specifications' design guidelines for the volume of CSO pollution control and storage tanks.