Combined Sewer Overflows (CSOs)
Feb. 16, 2017 - A CSO advisory for the Willamette River went into effect for 48 hours, following an overflow that began around noon.
Portland’s combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed a 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. The improvements eliminate 99% of CSOs from the slough and 94% from the river.
During extremely heavy rain, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During very heavy rain storms, some combined sewage can overflow. That is what the system is designed to do and that is the way it operates.
A combined sewer overflow is about 80% stormwater and 20% sanitary sewage. Before the city completed the CSO control program, combined sewers overflowed an average of 50 times a year. Today, the combined system overflows to the Willamette River an average of four times per winter and once every three summers.