Red Water and Discoloration in a WDS; A Numerical Simulation

Abstract

Discoloration is a major source of consumer complaints associated with a water distribution system (WDS). Depending on the pipe material, different colors can be attributed to varying concentrations and chemical oxidation states of iron or other metals. Overall, the discoloration materials are apparently entrained from particles at the pipe wall during the passage of water. The major material sources for discoloration materials are organic and inorganic materials entering a WDS at the source, materials from treatment processes, processes within a WDS such as corrosion and erosion, biological growth and chemical reactions, external contamination occurring during operations and repairs, intrusion and back flow. Traditionally, two mechanisms – hydraulic entrainment and iron uptake – have been used to described discoloration events. The hydraulic-entrainment mechanism is the physical motion of loose deposits of particles at the pipe wall into the bulk flow due to abrupt changes in the flow condition. In iron-uptake mechanism, some particles of ferrous hydroxide (produced by corrosion reactions) are oxidized to ferric hydroxide, which then can cause red bulk events in the bulk flow. Considering the hydraulic-entrainment a key main mechanism initiating discoloration event, this paper seeks in at least a preliminary way to simulate and approximate red water events in a water a WDS. Abrupt changes in flows can create aggressive shear forces, sometimes causing the accumulated particles at the pipe wall to be resuspended and be carried with the bulk flow. In this study, one- and twodimensional (1-D and 2-D) transient simulation models are proposed, by which the turbidity of water is modeled with a transport equation. The transport equation, which is then coupled with the appropriate continuity and momentum equations for flow, considers advection and diffusion as the main mass transport mechanisms. Defining a self-cleaning threshold limit for wall shear stress, Boxall’s approach is used here to model the release of the discoloration material from the pipe wall into the bulk flow. To overcome the complexity in the determination of the turbulent shear stresses and to reduce the computational burden, a five-region model is used to model the turbulence. In this model, molecular viscosity is responsible for damping turbulence in the region near the pipe wall (viscous sublayer) while eddy viscosity is the main damping-mechanism in the core region (nearer the centerline of the pipe). The turbulent diffusivity coefficient is accurately approximated from the eddy-diffusivity concept and the dimensionless turbulent Schmidt number; whereas the 1-D counterparts are determined using steady state Darcy-Weisbach formula (for wall shear stress) and Taylor’s formula (for diffusion coefficient). An implicit/explicit finite difference method; namely an Alternating Direction Implicit (ADI) approach, is superposed with a fixed grid method of characteristic (MOC) to numerically integrate the governing equations for unknown flow parameters including velocity, pressure head, and water turbidity. Finally, the results of both 1-D and 2-D models for a straightforward reservoir-pipe-valve case study are compared with available field data reported in the literature by other researchers. The overall agreement of the two sets of results supports the basic validity of the proposed 1-D and 2-D models.