Abstract
Discharges of untreated wastewater from combined sewer overflows (CSOs) can affect hydraulic stress and have significant environmental impacts on receiving water bodies. Common flow rate and water level sensors for monitoring of CSO events are expensive in terms of investment costs, installation, operation and maintenance. This paper presents a novel surrogate method to detect CSO events by using two low-cost temperature sensors. The novelty is the experimental setup for installation of temperature sensors in CSO structures and an algorithm developed to automatically calculate the duration of CSO events considering the response time of the system. The occurrence and duration of CSO events is computed based on the convergence of the two temperature signals. The method was tested under field conditions in a CSO structure, and the results were compared to the information gathered from a parallel installed flow sensor. The application of two temperature sensors installed inside a CSO structure was proven to be robust and accurate for the automatic detection of the occurrence and duration of CSO events. Within the 7-month test phase, 100% of the 20 CSO events could be detected without false detections. The accuracy of detecting the start and end of the CSO events was 2 min in comparison to the flow sensor.
Highlights
Discharges of untreated wastewater from combined sewer overflows (CSOs) can lead to hydraulic stress, oxygen depletion or a temporary increase of pollutant concentrations in receiving waters (Ellis and Hvitved-Jacobsen 1996)
Calibration results of the algorithm for the used type of temperature sensor An initial calibration procedure was conducted to determine the optimum value of the CSO detection parameter ΔTCSO for the used type of temperature sensor
CSO detection by the reference method was defined by a threshold value of a 5 L/s flow rate, which leads to a detection of 20 CSO events in total during the observation period by using a defined CSO gap parameter tGAP of 10 min
Summary
Discharges of untreated wastewater from combined sewer overflows (CSOs) can lead to hydraulic stress, oxygen depletion or a temporary increase of pollutant concentrations in receiving waters (Ellis and Hvitved-Jacobsen 1996). The directive does not define standards at European level, but since the implementation of the Water Framework Directive (EC 2000), the characterisation of CSO events is of great importance. In countries such as Belgium, Denmark, 209 Page 2 of 18 parts of Germany and the Netherlands, the overflow frequency and partly overflow duration are used as design criteria for CSO structures (Dirckx et al 2011). The collected information can be used at operational level for function control, quality assessment, prior determination for CSO maintenance and improvement of the sewer system while implementing control strategies (Alferes et al 2013; Benedetti et al 2013). The information is useful to assess, improve and maintain combined sewer systems (e.g. Montserrat et al 2015) as well as calibrating hydraulic urban drainage models (Duchesne et al 2001; Montserrat et al 2017)
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