The City of Portland has completed the last legal requirement in its 20-year program to control combined sewer overflows, officially closing the books on the biggest public works project in its history.
City of Portland Bureau of Environmental Services Director Dean Marriott briefed City Council members Wednesday on the successful completion of the Comined Sewer Overlfow system (CSO), also known as the "Big Pipe" project.
Before the $1.4 billion-dollar CSO program came online last year, Portland’s combined sewers overflowed up to 100 times a year, spilling about six billion gallons of combined sewage into the Columbia Slough and Willamette River every year. Today, combined sewers overflow to the river no more than four times per winter and once every three summers.
Portland’s average rainfall is 37 inches, but in the 12 months after CSO construction ended, 53 inches of rain fell in the city. During that time, Marrott said seven rain storms were large enough to cause CSOs, but only four did.
Environmental Services began its CSO control program in 1991. The city finished CSO construction and activated the Willamette River CSO tunnel system before its December 1, 2011 deadline. Portland’s legal agreement with the Oregon Department of Environmental Quality (DEQ) required a final report twelve months after the deadline that demonstrates that CSO controls work and meet DEQ requirements.