Improving combined sewer overflow and treatment plant performances by Real Time Control operations

In the last years, urban stormwater discharges have been recognised as one of the main causes of the environmental problems of receiving water bodies. During rain events, in fact, large amounts of water and pollutants are directly discharged through combined sewer overflow (CSO) devices into the receiving waters without any treatment. In addition, storm flow peaks and pollutant shock loads can arrive at the treatment plant (TP) reducing its efficiency or causing malfunctionings. Recently, environmental problems have induced researchers to focus on the qualitative impacts of the CSO discharges. In particular, more efficient CSO devices, such as the high crested side-weir with bottom orifice and many kinds of vortex separator devices, have been proposed by different authors. These devices can perform a relevant reduction of the discharged pollutant loads, besides the usual flow partition function. A further significant improvement of the performances of CSO devices can be obtained by increasing as much as possible the storage capacity of the sewer system. For this objective, the real time control (RTC) has been proven to be a cost-effective solution since it is based on the activation of existing unused capacities and consequently allows for reducing the need of new storage infrastructures. Schilling reports significant monetary savings achieved by the implementation of RTC for several case studies. Benefits obtained with the implementation of RTC have been shown by different authors by comparing the performances of various urban drainage systems in RTC scenarios and in uncontrolled system scenarios. One common RTC technique is to activate the sewer in-line storage capacity by placing moveable regulators, such as sluice gates, weirs and orifices, into the main trunk sewers. These regulators, if adequately controlled, can reduce the CSO discharges in terms of both water volumes and pollutant masses and can perform also a better regulation of the flows conveyed to the TP. In this paper, the improvement of the performances of a CSO device deriving from the adoption of RTC techniques is analysed in terms of reduction of discharged water volumes and pollutants. In particular, a simple quality-oriented CSO device, the high crested side-weir with bottom orifice, has been considered. This device is made up of a high crested side weir and of a downstream fixed gate with bottom orifice and allows for storing some in-line volume behind the gate until the water level rises up over the weir crest. The storing process leads to reduce the discharge of first flush waters and of pollutants into the receiving water body and to increase the water volumes conveyed to the treatment plant. Benefits obtained with RTC techniques have been evaluated considering moveable sluice gate regulators placed upstream of the CSO device. The control of the gates has been performed adopting a basic local strategy aiming to the activation of the maximum in-line storage. In particular, standard proportional (P) control units have been considered for the regulation of the gate movements. The analysis has been carried out setting up and applying a numerical model to the experimental data of two Italian urban catchments, considering both quantitative and qualitative measurements recorded during several rain events. The implemented model is based on the fully-dynamic De Saint Venant equations for the water flow and on the advection-dispersion equation for the analysis of the pollutants inside the sewer system.