Description

The North Creek Diversion Project diverts flows from the Kenmore Lake-Line and West Point Treatment Plant to the East Division Reclamation Plant. A new pump station pumps the diverted flow to the existing York Pump Station. The peak flow is 39 MGD. T/WI was the prime consultant for Contract B. Contract B involved the design of 8,350 feet of dual 30-inch diameter force mains, a drop structure, and an odor control facility. Specifications for the entire 24,000 feet of pipeline were prepared. The project also involved connecting into the existing 72-inch interceptor to the York Pump Station; and providing support for permits, community relations, and environmental compliance. Thomas/Wright, Inc. also provided construction engineering assistance to King County.

The North Creek Diversion project required the design of dual 30-inch force mains terminating in a drop structure at King County's existing York Pump Station. At the drop structure, the force main rose above ground to a level that assures that the mains will not be subject to a vacuum during pump shutdown. This resulted in a 30-foot drop from the force main outlet to the existing interceptor sewer below. Long force mains hold the raw wastewater for long periods of time in an anaerobic environment. When the wastewater is discharged in an aerobic and turbulent environment, hydrogen sulfide gases are released from the wastewater.

In the past, King County has relied on vortex drop structures to minimize turbulence and hydrogen sulfide release. Vortex structures, however, are expensive to construct and have been only marginally successful in containing hydrogen sulfide. T/WI engineers suggested that, instead of a vortex, the drop structure should be a simple structure that would allow the wastewater to free fall. This would release hydrogen sulfide in a controlled environment and provide some aeration to the wastewater before it enters the York Pump Station. The drop structure was designed to accommodate the hydrogen sulfide gas through an air-lock at the wastewater outlet and with a PVC liner on all interior concrete surfaces. An odor control facility was designed next to the drop structure to clean the gas within the drop structure before discharge to the atmosphere.