Sediment accumulation in drinking water trunk mains

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A large proportion of the sediment found in drinking water networks originates from the treated water. Resuspension of this accumulated sediment is the main cause of discolouration and associated complaints. In distribution networks, accumulation of sediment depends in part on the velocity of the water. Networks which experience a daily peak in velocities in the order of 0.20 to 0.25 m/s are proven to be self cleaning. These velocities are relatively rare in conventional distribution networks, but occur more frequently in trunk mains, suggesting self cleaning of transport networks. To verify the hypothesis of self cleaning trunk mains an experiment was set up with two trunk mains connecting two different treatment plants to distribution networks. They are monitored for sediment build-up in relation to the treatment characteristics and hydraulic conditions. The trunk mains have a diameter of 300 (AC) and 315 mm (PVC) and an average length of 4500 m. The water quality at the treatment plant is characterised through continuous monitoring of turbidity; the sediment build-up is measured through flushing with water at velocities between 1.43 to 1.79 m/s and taking sufficient samples. The pattern of sediment build up is reproducible over the length of the pipe. In a third verification experiment the same pattern was found. The most important explanatory factor in this case is the average turbidity of the treated water. The ratio between the average values of the turbidity of the treated water is similar to the ratio between the amounts of sediment found in the pipes.

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Reducing discolouration events in aging water supply networks is a major challenge facing water utilities worldwide as they are required to comply with increasingly stringent water quality regulations in a proactive and cost-effective manner. Emerging control methods for dynamic reconfiguration of network topologies could reduce the risk of discolouration by regulating hydrodynamic conditions to facilitate the implementation of self-cleaning networks. This review paper provides a holistic assessment of mechanisms and pathways which govern discolouration processes, associated models and the impact of system hydraulics so that control rules can be formulated. The possible role of biofilms in conditioning the wall-bound cohesive layers is identified and methods for their effective removal are discussed. The information presented identifies key variables and research gaps to facilitate the development of near real-time hydraulic and network topology control for the long-term and proactive management of discolouration risk.
Various particle transport mechanisms play a role in the build-up of discoloration potential in drinking water distribution networks. In order to enhance our understanding of and ability to predict this build-up, it is essential to recognize and understand their role. Gravitational settling with drag has primarily been considered in this context. However, since flow in water distribution pipes is nearly always in the turbulent regime, turbulent processes should be considered also. In addition to these, single particle effects and forces may affect radial particle transport. In this work, we present an application of a previously published turbulent particle deposition theory to conditions relevant for drinking water distribution systems. We predict quantitatively under which conditions turbophoresis, including the virtual mass effect, the Saffman lift force, and the Magnus force may contribute significantly to sediment transport in radial direction and compare these results to experimental observations. The contribution of turbophoresis is mostly limited to large particles (>50 μm) in transport mains, and not expected to play a major role in distribution mains. The Saffman lift force may enhance this process to some degree. The Magnus force is not expected to play any significant role in drinking water distribution systems.