Portland's Combined Sewer System

Portland's first sewers, constructed in the late 19th century, combined sewage and stormwater in the same pipes and carried the wastewater directly to the Willamette River and Columbia Slough without treatment. In 1952, new interceptor pipes began collecting sewage from the combined sewers for conveyance to Portland's first sewage treatment plant.

The new system improved water quality. But because stormwater runoff during wet weather filled the combined sewers to capacity, some of the sewage and stormwater mixture still overflowed to the river and slough. New streets, rooftops and other hard surfaces increased as Portland grew, generating more stormwater runoff and increasing CSO volume.

Estimated annual CSO volume reached 10 billion gallons by the 1970s. A series of sewer improvement projects reduced volume to about six billion gallons a year by 1990.

The City of Portland and the Oregon Department of Environmental Quality (DEQ) agreed on a CSO control plan in 1991, and an amended plan in 1994. That plan, the Amended Stipulation and Final Order (ASFO), required Portland to control CSOs to the Columbia Slough by 2000 and to the Willamette River by 2011. Projects completed to date have reduced CSO volume by nearly two thirds, and have improved Columbia Slough and Willamette River water quality.