Physical Modeling of Airflow Dynamics in the Brightwater Treatment Plant Effluent Drop Structure

King County, Washington’s 54 MGD Brightwater Regional Wastewater Conveyance System and Treatment Plant is currently in final design. The effluent from the plant will enter a 7-foot diameter pipe through the effluent drop structure, which will be located on the grounds of the proposed treatment plant. Effluent will be discharged to Puget Sound.

The design team has committed to designing a conveyance and treatment system that will not induce odor complaints. All components of the system must be designed according to this criterion.

During design, questions arose regarding the potential for air to be dragged in the headspace of the incoming pipe of the structure, how much air would be entrained in the de-aeration chamber of the structure and, subsequently, how much of it would be dragged into the downstream pipe. Because very little information is generally known about these phenomena that can be directly applied to such problems, bench scale physical modeling was determined to be the best way to answer these questions.

A bench scale physical model of the effluent drop structure was constructed by northwest hydraulic consultants (nhc) in Vancouver, British Columbia. Various hydraulic tests have been run on the model by nhc engineers to simulate the hydraulic conditions that the structure is expected to encounter over the next 40 to 50 years. The data that the testing
provided resulted in design changes to the structure that will ensure that air will not be entrained in the downstream pipe that could cause pressure problems and subsequent release to atmosphere.

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