This paper investigates the residual nutrient load from a combined sewer system along the shore of a deep Italian lake, to provide data that can help explaining the contribution of combined sewers to lakes eutrophication. To this purpose, a stochastic methodology is applied to the analysis of the combined sewer overflows and pollutants, that provides reliable estimates, with related uncertainties, of the storage capacity to be supplied in order to mitigate the impact on the quality of receiving waters. The sewer is modeled by SWMM and two terminal combined sewer weirs are modeled in detail and instrumented for calibration purposes. A year-long campaign was accomplished, allowing to study the occurrence and relevance of the first flush. The calibrated model of the sewer network was used to extrapolate the results to a 10-years period, thus providing a statistically reliable estimate of the residual load delivered from this watershed to the lake. A conservative evaluation shows that up to 22% of phosphorus and up to 41% of nitrogen delivered to the sewer are still discharged into the environment. However, the results also show that in this case a relatively small capture volume would obtain a target pollution reduction, thus providing insights on the feasibility of this structural practice in similar watersheds along the shore of endangered lakes.
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