Behaviour of particle mobilization and reattachment under flushing conditions in PVC pipes using a full-scale laboratory system

Abstract

Excessive accumulation of particulate material and biofilms on the inner walls of drinking water pipes increases the risk of water discoloration events, known to be the major cause of customer complaints worldwide. As a result, water utilities use pipe flushing operations to mobilize material deposits from ‘dirty sections’ of their pipe networks. Nevertheless, the development of preventative strategies is still limited by the lack of knowledge about the material accumulation process and the behaviour of resuspended particles during flushing. The goal of this paper is to investigate the behaviour of insoluble iron oxide particles during controlled accumulation and flushing processes in PVC drinking water pipes. A set of four experiments was completed where water with a known concentration of iron oxide particles was introduced into a full-scale pipe loop laboratory system under steady flow conditions producing the accumulation of particles along half the pipe length. The system was then flushed using two sequential velocities (0.7 and 1.2 m s⁻¹) and the direction of flush was changed between each independent flushing stage. During the flushing operations, it was found that a small number of mobilized particles can reattach to downstream sections pipes, and resist mobilization to elevated wall shear stresses of 1.2 Pa. Furthermore, even after successive flushes in one direction, a subsequent flush of equal velocity in the opposite direction was able to mobilize new particles from the pipe wall surface. These findings revealed a new mechanism of particle resistance to mobilization that is independent of the WSS. These results may assist water utilities in improving flushing strategies for DWDSs and managing accumulated material in their networks.