Study of the Effect of Linear and Parabolic Density Region in Settling Tanks for Potable Water.

Computational fluid dynamics (CFD) methods are employed to evaluate the influence of temperature variations on the flow field in an actual sedimentation tank for potable-water treatment. A series of experimental measurements of temperature variation has been conducted in that sedimentation tank. The mixed-convection problem caused by the density variations and the horizontal stream is considered at low temperatures (0-10°C) where the variation of water density is strongly nonlinear. Three cases have been examined: (1) one in the ascending, (2) one in the descending parabolic density region, and (3) one in the linear density region. The thermophysical properties of water have been incorporated into the CFD code. All simulations have been conducted for a transient, turbulent flow field. The momentum exchange between the primary and the secondary phase (particles) is considered. The results show that in the parabolic density region, only a few particles are trapped in the bottom, whereas many of them float near the surface and escape from the tank causing a low-tank efficiency. The opposite happens in the linear-density region where the tank efficiency is much better. [ABSTRACT FROM AUTHOR]