How much have CSOs been reduced ?

Controlling CSOs has been a priority for King County and the City of Seattle since the 1960s, when 20 billion to 30 billion gallons of untreated CSOs were discharged into area water bodies. Although CSOs still happen during heavy rainfall, the volume of discharge is far less. Because of control efforts during the past 30 years, King County CSO volume has been reduced significantly from a 1980-83 baseline of 2.3 billion gallons to about 1.5 billion gallons per year on average.

King County is committed to protecting water quality and human health. Under its Regional Wastewater Services Plan, King County has developed CSO control projects to bring the remaining uncontrolled CSOs to the Washington state requirement of one untreated discharge per year. These projects will be completed by 2030.

What are CSO control costs?

This graph shows the amount of CSO reduction from the many control projects King County has completed.

When CSO control is complete, it will have cost about $600 million. CSO control is expensive for several reasons:

• Large storms generate large volumes of CSO quickly. This requires large facilities to treat and/or contain the large volumes.
• Large facilities require more land, additional contractor work, and materials.
• Most of the projects are in heavily urbanized areas, and construction must be carried out in ways that minimize disruption and fit into the character of the community.