Hydraulic Challenges of Designing for a High-Capacity and High-Head Pump Station

King County is developing a new wastewater treatment plant to service a growing area of King and Snohomish Counties, Washington. To convey flows to the Brightwater treatment plant, the influent pump station (IPS) and a series of new tunnels and interceptors are being constructed. The plant IPS is located approximately 2.5 miles upstream and connected to the treatment plant via a tunnel containing the influent force main and other pipelines.The new 170-million gallon per day (mgd), 90-foot deep IPS has six centrifugal, two-stage (total dynamic head or TDH of 285 feet) raw sewage pumping systems that utilize two different sized pumps: two 20-mgd pumps, three 45-mgd pumps, and one standby 45-mgd pump.ENSR International conducted a study of the influent structure, wet well, and rock catcher using a scale physical hydraulic model. The objectives of the study were to: evaluate the performances of the wet well design, influent structure layout, and rock catcher over a range of flow conditions. The results of the modeling were used to modify the design. The design changes included revisions to the operating depth, wet well trench design, self-cleaning operating conditions, wet well inlet conditions, approach channel in the influent structure, and rock box design.