AWWA WQTC57011

AWWA WQTC57011 Assessing Improvements to Sedimentation Basins Using Computational Fluid Dynamics Modeling
Conference Proceeding by American Water Works Association, 11/01/2002

Seckinger, Sarah E.; Herrick, Jason D.; Gadoury, Paul J.; Sheldon, Robert H.; Hannoun, Imad A.

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The Providence Water Supply Board's Water Purification Plant in Scituate, Rhode Island provides water to more than half the population of the state of Rhode Island. Clarification is provided by two very large uncovered sedimentation basins. The plant's North and South Sedimentation Basins, with a combined capacity of 161 million gallons, provide several treatment and supply functions. They are used to remove suspended particulate matter, to achieve adequate chlorine contact time during winter months to comply with the Surface Water Treatment Rule (SWTR) CT requirements for Giardia inactivation, and for flow balancing and meeting variable water demand. MWH conducted an evaluation of the existing sedimentation treatment process to determine whether future capital improvements should include concrete repair of existing facilities or more extensive upgrades or modifications to achieve other treatment benefits. The existing sedimentation basins were found to provide sufficient sedimentation prior to filtration and result in stable and excellent finished water quality. Thus, improvements to the basins focused on reducing short-circuiting and minimizing recirculation zones particularly because the South Sedimentation Basin is chlorinated in the winter months to meet SWTR CT requirements for Giardia disinfection. Flow Science developed a three-dimensional computer model of the basins using Computational Fluid Dynamic (CFD) modeling. The model considered the intricate basin geometries and predicted the detailed flow and velocity distribution in the basins. The modeling results were used to evaluate both the existing configuration and flow conditions as well as prospective design changes that would: reduce short-circuiting by rerouting the flow into portions of the basins that were previously dead water zones; and, create a more uniform flow pattern to improve settling performance. Recommended improvements include installing additional baffles to the sedimentation basins to modify the existing flow path and achieve more uniform flow velocities. Includes 2 references, tables, figures.