Field Studies of Discoloration in Water Distribution Systems: Model Verification and Practical Implications

Discoloration in water distribution systems has been studied in partnership with a number of U.K. water companies by measuring the turbidity response to changes in hydraulic conditions induced by systematic flushing. The resulting data was used to verify a predictive empirical model and hence the underlying assumptions made in its development. Model simulations, made using previously established parameters defined solely by pipe diameter and pipe material, are presented alongside measured data to demonstrate this verification. The primary cause of discoloration observed is the mobilization of material from cohesive layers bonded to pipe walls. These layers demonstrate a profile of increasing shear strength with increasing degree of discoloration. Differences are demonstrated in the layer and ultimate shear strength characteristics of the discoloration layers formed in iron and plastic pipes, with a modeled shear stress of $1.2\text{N}/{\text{m}}^2$ shown to exhaust material layers in plastic pipes. Based on the observed data it is theorized that accumulation of material to the pipe walls is primarily dependent on two mechanisms; ubiquitous background concentrations in the bulk water, and if present corrosion by-products from iron pipes and fittings. A consequence of this is that all pipes within a water distribution system are susceptible to the development of material layers. In the formulation of operation and maintenance strategies it is suggested that iron and plastic pipes should be treated differently to obtain optimum operational effectiveness and minimize discoloration risk.