For officials in Portland, Ore., the Big Pipe project is no longer a dream. The recently completed $1.4-billion, 20-year combined sewer overflow (CSO) control program was implemented in response to an Oregon Dept. of Environmental Quality order to reduce significantly CSO events by December 2011. Thanks to large-diameter underground tunnels installed along the Columbia Slough and on both sides of the Willamette River, the city now meets the overflow frequency criteria set forth in 1991 by the DEQ. Annual CSO volume to the Columbia Slough and the Willamette River has dropped by 99% and 94%, respectively, according to city officials.
The city’s Big Pipe program consisted of three major phases and more than 300 individual projects to meet the Oregon DEQ water-quality standards for the Willamette River and the Columbia Slough, says Virgil Adderley, CSO program manager with the city of Portland’s Bureau of Environmental Services. “The first phase, the cornerstone projects, included measures such as stream diversions, sewer separations and downspout disconnections for removing stormwater from the combined sewers,” Adderley says. “The other two phases were implemented in parallel over a 15-year period and involved treatment-plant expansions to handle higher wet-weather flows as well as the construction of large underground tunnels and pump stations for collecting and transporting combined sewage.”