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Overview of the existing and new system

The existing King County system is shown in purple; the CSO outfall is shown in red; the new gravity sewer pipeline is shown in yellow.

Overview of the existing and new wastewater conveyance system in South Magnolia

System operation

View how the system works when a storm event occurs (ranging from "one cloud" representing a light rain to "four clouds" representing a very large or sustained storm events).

Under dry weather conditions, the combined sewer system sends flows directly to the Interbay Pump Station via the existing South Magnolia Trunk for conveyance to the West Point Treatment Plant. The new project elements (new diversion structure, conveyance pipeline and underground storage tank) are bypassed.

dry weather conditions

Under moderate flow conditions (one rain cloud), the combined sewer system continues to send flows directly to the Interbay Pump Station via the existing South Magnolia Trunk for conveyance to the West Point Treatment Plant. The new project elements are bypassed.

moderate flow conditions (one rain cloud)

During large storms (two rain clouds), the combined sewer system continues to send flows to the Interbay Pump Station via the existing South Magnolia Trunk. In addition, excess flows travel from the new diversion structure through the new conveyance pipeline to the Interbay Pump Station, bypassing the underground storage tank.

large storms (two rain clouds)

During larger storm events (three rain clouds), flows up to 7 million gallons per day (MGD) continue to the Interbay Pump Station via the existing South Magnolia Trunk and the new conveyance pipeline. Flows that exceed the capacity of the existing South Magnolia Trunk back up into the underground storage tank.

larger storm events (three rain clouds)

During very large or sustained storm events (four rain clouds) that exceed both conveyance and storage capacity, gates at the diversion structure and tank automatically close preventing additional flows from travelling through the new conveyance pipeline and reaching the storage tank. Excess flows will be discharged through the existing CSO outfall via the existing diversion structure.

very large or sustained storm events (four rain clouds)

Once the storm is passed and water levels return to normal, the flows stored in the tank are sent to the Interbay Pump Station and on to the West Point Treatment Plant.

When the CSO underground storage tank is in service, people will see one or two crew members on site about once a week for inspection and maintenance. After a storage event, more crew members may be on site to empty and flush the facility. Every three to five years, one to two days of major maintenance will be required. Advance notice will be provided for maintenance activities that cause disruptions.