Implementation of Flushing Tanks in Combined Sewer Networks to Comply with Optimal Self-Cleansing Properties
Combined sewer networks are designed to collect and transport the aqueous and solid wastes originating from domestic, industrial and catchment surfaces for treatment and disposal. One problem that arises is the sedimentation of the particles within such collected flows while in transport throughout such sewer networks. In fact, many sewer pipes in combined sewer systems experience considerable fluctuations in flows, ranging from high flow during short-term storm events to longer periods of much lower dry-weather-flows. In low flow periods sewers also experience a characteristic diurnal flow variation. In such periods when the pipe filling level is very low, minimum critical velocities might not be satisfied (Bertrand-Krajewski, 2002). Thus, deposition generally occurs during these periods and also during decelerating flows when storm runoff is receding. Although the flow of surface runoff throughout the sewer network generates considerable rates of shear stresses, this does not guarantee proper sediment transport in downstream sewer pipes due to lack of enough strength in the flow to constantly produce the required shear stresses. These possible sediment depositions can generate problems such as hydraulic overloading of the sewer pipes due to a reduction in flow capacity, and increase the risk of surcharging during storm events. This could also result in both surficial flooding and unwanted early operation of the combined sewer overflows (CSOs). Thus, the issue of designing sewer systems to be self-cleansing becomes important.
This is however not always promising, particularly in flat regions where the necessary slopes for sewer pipes to be self-cleansing are not available (especially in the most upstream parts of the network) and due to the costs of required deep excavations and pumping stations. In this regard, a proper solution could be the use of flushing tanks that can generate controlled flushes, produced by means of the collected volumes of surface runoff flows in their tanks, into the required sections of sewer systems. The effect of such devices as external sources to help remove the settled particles from sewer pipes has been investigated by many researchers.
In this chapter an evaluation of the proper implementation of flushing tanks for eroding sediments from a combined sewer network in the village Erpe-Mere in Flanders, Belgium, is presented. The results are based on the experiments and numerical simulations carried out at the Hydraulics Laboratory of the Katholieke Universiteit Leuven (K.U.Leuven). Implementation of these flushing tanks in different locations of the sewer network is studied with regard to subsequent modifications of the hydrodynamic components of the flow throughout the network (shear stress, flow discharge, flow velocity). The research takes account of the hydraulic characteristics of the flushing wave (released flow rate as a function of time) and the specific effects on in-sewer sediment transport.
The methodology consists of using version 7.5 of InfoWorks CS (Wallingford Software, UK) in order to calculate the resulting spatially distributed shear stresses as a function of the pipe diameters and slopes to evaluate eroding capabilities of the generated flush waves in the sewer network. Emphasis is given to verifying whether the simulated shear stresses throughout the network reasonably satisfy the required selfcleansing conditions, i.e. the generated shear stresses remain high enough for a small time interval.
Of particular concern is the proper location of multiple flushing devices all over the combined sewer network with respect to the potential degree of sediment removal and transport, and the possible drawbacks which could occur during such flushing events, such as modification of the flow cross section and consequent effects on sewersurcharging or flooding. In fact, there are various influencing parameters related to the proper installation of these devices such as sewer network characteristics, contributing catchment characteristics, etc. The evaluation results indicate that attaining a minimum shear stress criterion (e.g. 3 N/m2 for combined sewer networks based on the Flemish sewer design requirements) could not be satisfied in all parts of the considered sewer network and even some undesired effects emerged. Nevertheless, regarding the accomplished modeling analyses, the capability of such devices to produce effective forces for removal of the settled particles from combined sewer networks is well accounted for.
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