The CSO Control Program --
Controlling Combined Sewer Overflows

How do we control CSOs?

Major methods of CSO control include:

• maximizing the transfer of flows to and between King County Treatment Plants using computer controls
• increasing conveyance capacity by building storage tanks, storage tunnels, pump stations, regulator stations, and pipelines
• building local CSO treatment facilities
• reducing the amount of stormwater entering the system by separating sewers in combined areas, repairing leaky side sewers, and disconnecting roof and yard drains.

New! View animation: Controlling Combined Sewer Overflows

Transferring flows

One method King County uses to decrease CSOs is transferring wastewater flows from the part of the system that lacks capacity to another part of the system that has excess capacity.

For example, in the winter during times of heavy flow when the northern part of the King County sewer system may exceed its capacity, treatment plant operators may transfer a part of the northern wastewater flow to the southern service area (where it is then routed to the South Treatment Plant for secondary treatment). This transfer of wastewater flow provides more capacity in the northern service area pipes to retain combined sewage and thus decrease CSOs.

Increasing 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 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.

King County wastewater treatment service area maps

Puget Sound Beach CSO Projects -- North Beach, South Magnolia, West Seattle

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.

Building local CSO treatment facilities

CSO treatment facilities are designed to provide primary treatment (physical settling of solids), disinfection (usually chlorination), and dechlorination of CSOs. They are built to directly serve the area where they are located. Unlike the West Point Treatment Plant, these facilities operate only during heavy rainfall. King County CSO treatment facilities include the Alki CSO Treatment Plant, the Carkeek CSO Treatment Plant, the Denny Way/Lake Union CSO Control System , and the Henderson/MLK Way/Norfolk CSO Control System. Many of King County's local CSO treatment facilities include a CSO storage tank or tunnel.

Reducing stormwater entering the system

There are several methods to reduce stormwater from entering the wastewater system. One method is sewer separation. Sewer separation takes place in combined sewer areas when additional pipelines are laid in the area. The new pipeline then conveys stormwater directly to the nearest water body and the older pipeline conveys sewage to a secondary treatment plant (such as the West Point plant). Once these new pipes are in place, the area is considered a separated system. For more information on separated and combined systems visit: Why Do We Have CSOs?

Another method of reducing stormwater into the wastewater system is by repairing leaky side sewers. This method keeps rainwater out of the wastewater system, which reduces CSOs.

In many areas, roof and yard drains have been connected into the wastewater system. Disconnecting and rerouting these drains keeps rainwater out of the wastewater system, which reduces CSOs. The CSO Program Library has more information on reducing stormwater into the wastewater system.

More on CSOs:
Introduction
Reduction and Cost
The CSO Program
Being a Good Neighbor
Library
Glossary of Terms

Department of Natural Resources and Parks
Wastewater Treatment Division
201 S. Jackson St., Suite 512
Seattle, WA 98104-3855
Phone: 206-684-1246
Fax: 206-684-2057

Updated: July 12, 2007
 

Related Information:
Seattle-King County Public Health CSO Public Notification Program

Duwamish Waterway Programs