Aggressive flushing for discolouration event mitigation in water distribution networks

Abstract

Discolouration is one of the biggest causes of customer complaint associated with potable water. The flushing of systems has been widely identified as an appropriate pro-active means of removing material from distribution networks and hence controlling discolouration. Such flushing involves creating aggressive hydraulic forces within the network such that the materials that contribute to discolouration are mobilised and removed. Understanding of the causes and mechanisms leading to discolouration are poor. Previous work has been presented on the characterisation of material and particles collected at hydrants during flushing trials in the UK. From this data it was hypothesised that the materials causing discolouration originated from cohesive layers on pipe walls, and that once disturbed the material is maintained as a permanent suspension even under the most quiescent of networks conditions. The work presented in this paper attempts to validate the hypothesis that the discolouration materials originate from cohesive layers, and investigate the nature and variability of such layers within live distribution systems. The study involved the aggressive flushing of a long discrete length of cast iron pipe with known discolouration problems. The results showed a progressive generation of material over the length of the pipe, confirming that the material originated from a uniformly distributed cohesive source. This was followed by a sequence of flushing operations for the systematic cleaning of a complex network area, encompassing a mixture of pipe materials and ages. All measured turbidity traces showed exponential decay with time. Such exponential decay may be predicted by a model based on a change in layer strength with degree of erosion. Hydraulic forces appear to be a key factor governing the availability and mobilisation of material. Iron is the dominant material mobilised from all the pipes. There is no direct trend between the amount or the composition of the material mobilised from the different pipes.

Operational management of trunk main discolouration risk

“…This indicates that particulate turbidity is typically the cause of discolouration as reported by customers, Vreeburg and Boxall (2007). Flushing trials also report iron and manganese as dominant within discoloured water samples with a strong correlation between their concentration and the turbidity of the water, Gauthier et al (2001), Mc Coy andOlsen (1986) and Boxall et al (2003).…”

“…Discolouration as reported by customers is either caused by the presence of fine particles in the water, measured as turbidity, or by true colour (Gauthier et al 1996;Marshall 2001;Polychronopolous et al 2003;Ryan et al 2003). However, flushing experiments carried out by Boxall et al (2003), Polychronopolous et al (2003), Prince et al (2003) and Seth et al (2003) found very little true colour, but significant turbidity. This indicates that particulate turbidity is typically the cause of discolouration as reported by customers, Vreeburg and Boxall (2007).…”

“…Extensive work by the University of Sheffield, (Boxall et al 2001;Boxall et al 2003; discolouration events and carried out extensive flushing experiments to characterise discolouration particles and mobilisation processes. This has led to the development and verification of a Prediction of Discolouration in Distribution Systems (PODDS) model (Boxall et al 2001).…”

Microbial Growth in Drinking-Water Supplies: Problems, Causes, Control and Research Needs

“…The reasoning behind this is partly the theoretical approach based on Shields' and Stokes' equations, though this leads to lower velocities than 1.5 m/s for example (Boxall et al 2003). For the other part, the value of 1.5 m/s is based on practical experience that these velocities are significantly above the normal daily maximum velocities.…”

Establishment of a Practical Approach for Characterizing the Source of Particulates in Water Distribution Systems

“…However, recent studies have shown that other sources of particulate matter, aside from corrosion, can play a role in water discoloration [4][5][6][7]. Additionally, it has been reported that the accumulation of particulate matter is related to biological activity, and 1% to 12% of the organic matter may consist of bacterial biomass, thus making deposits an important factor to consider when evaluating the hygienic safety of drinking water [4].…”