Quantifying the Hydraulic Conditions that Govern Discolouration Material Behaviour in Drinking Water Distribution Systems

Abstract

Recurring consumer complaints of discoloured water are a key management challenge for drinking water distribution systems (DWDS). This research examines and quantifies the impacts of hydraulic conditions on the transition of material between the bulk water and infrastructure surfaces. It is the first to simultaneously consider and integrate material accumulation and mobilisation due to the processes of sedimentation and or cohesive layers. The specific processes enabling particles to become trapped and remain on pipe surfaces are not investigated, the focus is on how the hydraulic conditions dictate the different processes to provide evidence for operational best practice. Results from 8 experiments incorporating 18 hydraulic profiles in a full sized, extensively monitored, laboratory based pipe-loop with flow control precision of 0.002 m/s and using discolouration material collected from multiple operational networks are presented. Reynolds number is shown to best describe the limit of sedimentation, a measure of turbulence forces that balance self-weight forces. Shear stress, that quantifies surface forces, is shown to best describe mobilisation. Sedimentation was found to dominate up to Reynolds number of 15100 (0.25 m/s, 0.21 N/m2), above these cohesive layers were dominant. This indicates discolouration risks from sedimentation, likely in the tertiary zones of a DWDS, may be mitigated if flows regularly attain these values. Material accumulated following sedimentation was shown to be effectively removed by a shear stress of 0.77 N/m2 (0.5 m/s, Re 30300), representing an optimum flushing value with forces above this showing negligible benefit in terms of sediment removal. No upper flushing value was found for cohesive layers. These evidenced based values can be incorporated into operation and maintenance policy and practice to improve the management of discolouration within drinking water distributions systems.