Quantifying the hydraulic conditions that govern discolouration material behaviour in drinking water distribution systems

Abstract

Recurring consumer complaints of discoloured water indicate a key management challenge for drinking water distribution systems (DWDS). This research examines the causes by investigating and quantifying the impacts of hydraulic conditions on the transition of material between the bulk water and infrastructure surfaces. It is the first to acknowledge and differentiate between particulate accumulation and mobilisation due to the processes of sedimentation and cohesive layers and provide evidenced based operation and maintenance guidelines to improve discolouration management. With the range of physical, chemical and biological interactions within DWDS the specific processes enabling particles to remain on pipe surfaces are not investigated or quantified here with the focus on delivering evidence based practical support. Results from 8 independent week long trials incorporating 18 hydraulic profiles in a full sized and extensively monitored experimental pipe-loop with flow control precision of 0.002 m s⁻¹ and using discolouration material collected from multiple operational networks are presented. Reynolds number is proposed to best describe the limit of sedimentation as a measure of the turbulence forces that are balanced by the self-weight forces. Shear stress, that quantifies surface forces, is proposed to best describe mobilisation. For practical guidelines and uptake these can be converted to velocities. Sedimentation was found to dominate up to a Reynolds number of 15 100 (0.25 m s⁻¹, 0.21 N m⁻²); above this 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 m⁻² (0.5 m s⁻¹, Re 30 300), representing an idealised flushing value as forces above this showed negligible benefit on their removal. No effective upper flushing value was found in these trials for effective full mobilisation of cohesive layers.