Assessing the environmental impact of combined sewer overflows through a parametric study

Design and management of combined sewer overflows (CSO) 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 (i.e. overflows are usually designed for activation when Q > ~5Q_mw, the latter being the mean dry weather wastewater discharge) does not consider the characteristics of the upstream urban catchment nor those of the receiving water body. Accordingly, more recent regulations started enforcing limits also on the yearly number of overflows or increasing dilution coefficient.

Overflows 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 with fixed dilution rate placed at the outlet of urban catchments with same size, but different hydrological and urbanistic characteristics, has been simulated through SWMM multi-scenario simulations. The considered hydrological parameters were catchment imperviousness, width, slope, routing Manning coefficient and depression storage for both pervious and impervious surfaces. Urbanistic characteristics of the catchment affecting the combined sewer hydraulic regime were studied by changing the density of population, the imperviousness, and the mean per capita wastewater discharge.

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 time series of a single rain gauge.

The obtained results indicate that CSOs impact on the receiving water body, strongly depends on the characteristics of the upstream urban catchment. Therefore, such characteristics should be considered in CSO design and management.