The City of Portland, Ore., has completed its 20-year program to control combined sewer overflows (CSOs) to the Willamette River and Columbia Slough. An order from the Oregon Department of Environmental Quality (DEQ), originally issued in 1991 and amended in 1994, required the city to control CSOs by Dec. 1, 2011.
Completing the CSO program reduces CSO events from an average of 50 per year to an average of four each winter and one every third summer during very heavy rain storms. The program reduced annual CSO volume to the Columbia Slough by 99 percent and to the Willamette River by 94 percent.
Bureau of Environmental Services Director Dean Marriott and Willamette River CSO Tunnel Program Manager Paul Gribbon briefed the Portland City Council on Nov. 30 on successfully completing the program before the DEQ-mandated deadline. During the briefing Environmental Services also premiered a new CSO Program video narrated by “60 Minutes” correspondent Lesley Stahl. The video is available at www.portlandonline.com/bes/csovideo.
Portland’s combined sewers carry stormwater runoff and sanitary sewage in the same pipes. When the city began planning its CSO control program in 1990, rain frequently filled the combined sewer system to capacity and combined sewers overflowed to the Columbia Slough and Willamette River.
Work started on the first CSO projects in 1994. These Cornerstone Projects were designed to remove as much stormwater runoff as possible from combined sewers, which allowed engineers to design smaller, less expensive facilities to collect and convey combined sewage. Today the stream diversion, sewer separation, sump installation and downspout disconnection projects remove an average of 2.2 billion gallons of sewage per year from the combined sewer system. Downspout disconnections alone remove an average of 1.2 billion gallons from system annually.
Construction of larger projects began in 1996 with the Columbia Slough Big Pipe, a 3.5-mile long conduit ranging in size from 6 to 12 ft in diameter. The city completed the pipeline and projects to expand capacity at the city’s sewage treatment plant in 2000 to reduce CSOs to the Columbia Slough by more than 99 percent.
Construction began on West Side CSO projects in 2002. They included the 3.5-mile long, 14-ft diameter West Side Big Pipe, the Swan Island CSO Pump Station, the Peninsular Force Main and the Southwest Parallel Interceptor. The city activated the West Side Big Pipe in 2006 to control CSO outfalls on the west side of the Willamette River.
Also in 2006, the city broke ground on the East Side CSO Big Pipe Project, the largest public works project in Portland history. The city completed tunneling the 6-mile long, 22-ft diameter tunnel in October 2010 and activated the tunnel this fall.
This fall the city also completed work on the Balch Consolidation Conduit and the Sellwood CSO Pump Station to complete CSO construction.
Over 20 years, the CSO program has cost Portland sewer ratepayers an estimated $1.4 billion. That estimate includes the costs of more than 300 CSO construction projects since 1994.