The identification of variable shear strength of particles deposited on drinking water PVC pipes after the passage of a suspended particle plume in a full-scale laboratory system

Abstract

Understanding the adhesion properties of sediments that accumulate on the wall of drinking water pipes is an important step in the development of mitigation strategies to prevent the formation of deposits and protect drinking water. Research based on flushing of operational pipe mains that mobilized particulate sediments from isolated pipe sections has shown that fine iron oxide particles are a recurrent major component of these deposits. In addition, it has been established that adhesion forces proportional to the flow wall shear stress (WSS) develop between the pipe wall and accumulated particles, which prevents the washing-off of particles during common conditioning flows and provokes a rapid resuspension during high-flow events that cause water discolouration. Discolouration models have also showed that sediments have a variable shear strength, and, therefore, a fraction of material may resist a first increase in WSS but then be mobilized after a second increase in WSS. To explain the variable shear strength of layers, researchers have hypothesized that sediments accumulate as cohesive layers, which might be explained by the growth of biofilm among particulate material. Although current models have successfully explained sediment mobilization during flushing, the prediction of material accumulation and its shear strength is more challenging due to the lack of a comprehensive understanding about the accumulation process. The aim of this paper is to examine how particulate iron oxide that are rapidly deposited on PVC pipes develop variable shear strength under common hydraulic conditions found in drinking water distribution networks. A set of experiments were performed in a full-scale laboratory facility, where selected iron oxide particles were controlled and used to amend the feed water at the entrance of a 200 m pipe loop during a short period of time to create a suspended sediment plume with constant concentration. Experiments were realized at three different concentrations and three different velocities. In each set, three sequential plumes were used to accumulate particles on the pipe walls, followed by three sequential flushing steps used to mobilize the particulate material. The deposition of iron oxides in the PVC pipes were assessed indirectly through suspended sediment concentration (SSC) and turbidity data. Results showed that iron oxide particles predominantly accumulated in the first sections of the pipe wall. Most sediments were found to have weak shear strength and were easily mobilized with the first flushing step. However, the mobilized load from the second and third flushing steps revealed a consistent mobilization of sediment with higher shear strengths. These shear strengths were higher in the experiments with a higher inoculation concentration, and they were lower in experiments performed with a higher conditioning fluid velocity. The results suggest that variable shear strength can develop without biofilm. Additional long-term experiments are still required to evaluate the evolution of sediment shear strength which possibly can increase with time.