Implementation of Flushing Tanks in Combined Sewer Networks to Comply with Optimal Self-Cleansing Properties

Accumulation of in-sewer sediments causes problems such as loss of hydraulic capacity of sewers and pollution consequences when re-suspended by storm water flow. Thus, the issue of designing sewer systems to be self-cleansing becomes important. This is however not always promising due to e.g. local topographical features. The use of flushing devices that generate controlled flush waves could be a proper solution. The effect of such devices as external sources to help remove the settled particles from sewer pipes has been investigated by many researchers (Bertrand-Krajewski et al, 2005; Bouteligier et al, 2006; Campisano et al, 2004; Dettmar et al, 2002). In the paper, an evaluation on the proper implementation of flushing devices as instruments for eroding deposited sediments from a combined sewer network is presented. The research focuses on the hydraulic characteristics of the flushing tank (released flow rate as a function of time). The methodology consists of utilizing the InfoWorks CS model (Wallingford Software, United Kingdom) in order to calculate the resulting shear stresses as a function of the pipe diameter and slope to evaluate eroding capabilities of the generated flush waves into an existing combined sewer network. Emphasis is given to simulation-based assessment of spatially distributed shear stresses throughout the sewer network and the specific effects on in-sewer sediment transport. Thus, implementation of these flushing devices in different locations of a sewer network is studied with regard to subsequent modifications of hydrodynamic components of the flow throughout the network (shear stress, flow discharge, flow velocity). Of particular concern is the proper location of multiple flushing devices all over the studied 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 in-sewer water depth modifications leading to sewer surcharging or flooding. In fact, there are various influencing parameters engaged in 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) could not be satisfied in all parts of the considered sewer network and even some undesired effects emerged. However, the capability of such devices to produce effective forces for removal of the settled particles in combined sewer networks is well accepted.