AWWA ACE65154

The objective of this project is to improve reservoir mixing while minimizing construction inside two reservoirs. The reservoirs, which are owned and operated by EPCOR Water Services Inc. in Edmonton, Alberta, Canada are arranged in parallel between the filters and the distribution pump station at the E.L. Smith Water Treatment Plant. Flow through the reservoirs is by gravity. Computational fluid dynamics (CFD) analysis has been used to identify stagnant regions and to develop a strategy to eliminate them. The CFD software used in this analysis is Fluent. Mixing effectiveness can be evaluated through examination of steady state velocity contours, but a more quantitative assessment has been completed using Fluent's multi-species modelling capabilities. Time dependent solutions produced concentration contour plots that can be used individually, or compiled into an animation sequence, to observe the mixing process. Also measurements of influent concentration at the effluent can be used to compare reservoir performance over the full range of anticipated flow rates. These results could also be used to compare with tracer study results. Due to differences in their configuration, two different approaches to improve mixing have been proposed, but not yet implemented. In one case, the reservoir is square with its influent and effluent located at adjacent corners and there is a partial internal wall along the reservoir centerline prevented flow short-circuiting. Installation of nine short wall sections between columns is sufficient to promote cross-channel mixing and eliminate regions of flow stagnation for all flow conditions. In the second case, the reservoir is very long and narrow with the influent and effluent located at adjacent corners, leading to flow short-circuiting. Since the influent has very little kinetic energy, the ultimate solution involved constructing an internal wall with small openings to promote plug flow along the reservoir length and another to effectively relocate the effluent. Flow stagnation is eliminated, but there is greater uncertainty with the proposed solution because of the low velocities. CFD is an excellent prototyping tool to cost effectively examine various possibilities to improve reservoir mixing. The results can often be used to guide the development of innovative and cost-effective solutions, but sometimes uncertain results indicate that a conservative design approach is appropriate. Includes 4 references, tables, figures.