Increasing system capacity -- building storage tanks, storage tunnels, pump stations, regulator stations, and pipelines

Another method of decreasing CSOs is by building combined sewer storage tanks or tunnels. As mentioned earlier, during heavy storms, the additional water in the combined system may cause wastewater pipelines to exceed their capacity and overflow. To reduce CSOs from entering area water bodies, King County has built storage tanks and tunnels. These storage tanks and tunnels hold combined sewage until the rainstorm subsides. As soon as the wastewater pipelines have more room to accept the stored combined sewage, it is released into the wastewater pipelines.

The North Creek storage facility, completed in 2004, is designed to temporarily contain potential wastewater overflows, particularly during peak storms, to help eliminate overflows into Lake Washington near Kenmore.

The pipelines then convey the combined sewage to a CSO treatment plant or the West Point Treatment Plant for treatment and then discharge into Puget Sound. For some projects, the storage tank or tunnel may also provide primary treatment. The Henderson/MLK Way/Norfolk CSO Control Project and the Denny Way/Lake Union CSO Control Project provide both storage and treatment.

Pump stations are structures that move wastewater uphill (against gravity). Pump stations push wastewater through pipelines to a treatment plant where it is treated and then discharged to Puget Sound. King County has more than 40 pump stations throughout its service area.

Regulator stations are structures that control the flow of wastewater from a trunk sewer to an interceptor sewer. Regulators can be used to restrict or halt flow, thus causing combined sewage to be stored in the collection system until the main interceptor can handle it or discharged to the receiving water as a CSO. King County has 19 regulator stations.

Pipelines are also built to convey the combined sewage to CSO Treatment Plants or the West Point Treatment Plant in Seattle and the South Treatment Plant in Renton.