Article

Sediment accumulation in drinking water trunk mains

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Abstract

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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