Comparison between the performances of two combined sewer overflow devices in the reduction of water volume and pollutant discharges

Rain waters have revealed to be the source of relevant pollution in urban catchments, being the vehicle of both conventional pollutants and toxic substances. The growing attention towards environmental problems has consequently determined the necessity of adopting proper combined sewer overflow (CSO) devices able to reduce the discharge of sewer pollutant into receiving waters during rain events. In this paper, one of these devices – the high crested side-weirs with bottom orifice – is analysed. In particular a comparison, in terms of reduction of discharged water volumes and pollutants, between the performances of the high crested CSO device and the more common low crested side-weir is presented. For this purpose, numerical simulations have been run using experimental data concerning flow rates and pollutant concentration measured during different rain events in the urban catchment of Cascina Scala, Italy. A model based on the solution of the fully-dynamic De Saint Venant equations for the water flow and of the advection-dispersion equation for the propagation of pollutants inside the sewer has been implemented and applied. The simulations show the better performances of the analysed high crested CSO device compared to the low crested side-weir