Transient Mobilisation of Pipe-Wall Adhered Material in Drinking Water Distribution Systems

Discolouration, an aesthetic indicator of drinking water quality, affects approximately 6.7 million customers annually in the UK and is perceived to mask other water quality failures. Existing management techniques cannot explain all of these discolouration failures. Therefore, understanding the processes and forces that lead to discolouration is crucial. Material associated with discolouration is mobilised from the pipe wall when its adherence strength is exceeded by imposed hydraulic forces. Transient events generate significant dynamic forces, yet, there is currently little conclusive evidence exploring their influence on mobilisation of material. This study aims to determine, for the first time, if transient forces can mobilise of material adhered to the pipe-wall, which cannot be mobilised by steady state flows at the same initial or final conditions. An innovative, rigorous laboratory experiment was designed to test this aim. Replicated adhered material was created using magnetic particles inside the pipe and an electromagnet external to the pipe, so that controlled current through the electromagnet quantified adherence force experienced by the magnetic particles. Hydraulic steady state and transient tests, for a range of flow rate and pressure conditions, were conducted to determine the current at which mobilisation occurred. A key contribution of this research was the confirmation that valve closing and valve opening transients cause mobilisation of adhered material, where steady state cannot. This is substantial finding, particularly for valve closing transients as the steady state force reduces during the valve movement. Mobilisation must be due to the dynamic forces generated by the transient. An observationally driven analysis led to development of a function to capture the magnitude of the hydraulic force generated during transients. The one dimensional function was termed the ‘Peak Dynamic Force’ and begins to quantify transient induced forces that lead to mobilisation of pipe-wall adhered material. The work presented within this thesis is unique in that it consistently isolated transient forces and quantified their mobilisation ability. This dynamic ability has theoretical and practical implications, and could ultimately lead to the development of effective management strategies for improving drinking water quality.