Assessing the Environmental Impact of Combined Sewer Overflows Through a Parametric Study

Design and management of combined sewer overflows (CSOs) have been, so far, mainly based only on complying a fixed dilution rate of wastewater in stormwater during rain events. This poses serious environmental issues, since the definition of the acceptable dilution does not con-sider the characteristics of the upstream urban catchment, nor the climatic features, nor those of the receiving water body. Overflows are usually designed for activation when the mixed dis-charge reaches about five times the mean wastewater discharge (though it may vary, depending on countries regulations), the latter being the mean dry weather wastewater discharge. Accord-ingly, recent regulations started enforcing limits also on the frequency of overflows. Overflow activation frequency and discharged volumes of pollutants may depend on the upstream catchment features as well as on the precipitation regime. The great variability of these factors could make the impact on the receiving water body of similarly designed overflows to be quite different. In this study, the behaviour of a CSO placed at the outlet of urban catchments with same size, but different characteristics, has been simulated with SWMM. The considered hydrological parameters were catchment imperviousness, width and slope, Manning coefficient and depres-sion storage. Served population characteristics affecting the combined sewer hydraulic regime were studied by changing the population density and the mean wastewater discharge per capita. After defining realistic ranges for each parameter, the time series of discharged overflows have been calculated for all the combinations of the variable catchment parameters, corresponding to 20 years long precipitation series from a single rain gauge. The obtained results, although pre-liminary, indicate that CSOs impact on the receiving water body strongly depends on the char-acteristics of the upstream urban catchment. Therefore, such characteristics should be considered in CSO design and management.