January 30, 2013

The City of Portland has completed the last legal requirement in its 20-year program to control combined sewer overflows, officially bringing the $1.4-billion program to a close. Bureau of Environmental Services Director Dean Marriott briefed the Portland City Council today on the program’s successful completion.

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. Portland submitted the final compliance report in December 2012.

Before the CSO program began, Portland’s combined sewers overflowed an average of 50 times a year, spilling about six billion gallons of combined sewage into the Columbia Slough and Willamette River every year. The CSO program reduced overflows to the slough by 99% and to the river by 94%. Today, combined sewers overflow to the river no more than four times per winter and once every three summers.

As little as one-tenth of an inch of rain once caused a CSO event. Now it takes a winter storm of at least 1.2 inches in 24 hours and a summer storm of 1.4 inches in 24 hours to cause a CSO. The new system is flexible enough that most of the time not even that amount of rain will fill the Willamette River tunnels.

Portland’s average rainfall is 37 inches, but in the 12 months after CSO construction ended, 53 inches of rain fell in the city. Between December 2, 2011 and November 30, 2012, seven rain storms were large enough to cause CSOs, but only four did. City wastewater operators successfully used the tunnel system to control three of the large storms without overflows.

For more information contact Linc Mann, 503-823-5328.