In December 2011, the City of Portland completed its 20-year CSO Control Program. The work reduced CSOs to the Columbia Slough by more than 99% and to the Willamette River by 94%. Instead of an average of 50 CSOs to the river each year, there are now an average of four overflows to the Willamette each winter and one every third summer during only very heavy rain events. The city has met all of its required CSO program milestones on time.
How We Got Here
Older Portland neighborhoods have a sewer system that mixes untreated sewage and stormwater runoff in a single pipe. During very heavy rain storms, runoff from buildings, streets, and other hard surfaces can fill these combined sewers to capacity and cause them to overflow.
Controlling CSOs is an important part of Portland’s efforts to improve Willamette River water quality. CSO solutions include projects to keep stormwater runoff from entering sewers and building new facilities to carry sewage and stormwater to the Columbia Boulevard Wastewater Treatment Plant.
Portland started CSO construction with a set of Cornerstone Projects that keep stormwater out of the combined sewer system. Projects included installing street sumps and sedimentation manholes, building separate sewers for stormwater in some neighborhoods, encouraging homeowners in targeted neighborhoods to disconnect downspouts from the sewer system, and removing underground streams from the combined sewers.
Big Pipe Projects
Environmental Services completed Columbia Slough CSO projects in 2000. They included the Columbia Slough Big Pipe, a 3.5-mile long, six and 12-foot diameter pipeline to collect combined sewage and transport it to the Columbia Boulevard Wastewater Treatment Plant. Environmental Services expanded plant capacity to accommodate the extra flow. The slough projects controlled 13 CSO outfalls and reduced CSO volume to the slough by 99%.
Environmental Services completed the West Side Big Pipe and a CSO pump station in 2006 to control 16 CSO outfalls on the west side of the Willamette River. The 3.5-mile, 14-foot diameter tunnel carries combined sewage to the Swan Island CSO Pump Station, which pumps the sewage to the Columbia Boulevard Wastewater Treatment Plant.
Also in 2006, Environmental Services began construction of the East Side Big Pipe, a six-mile long, 22-foot diameter tunnel to collect combined sewage from the east side of the Willamette. The city completed tunneling in October 2010 and completed connecting combined sewers to the east side tunnel in September 2011 to control the 19 remaining Willamette River CSO outfalls.
1993 - Work begins on projects to divert stormwater runoff from the combined sewer system. The sump installation, sewer separation, stream diversion and residential downspout disconnection projects are known collectively as the Cornerstone Projects.
2000 - Environmental Services completes construction of the Columbia Slough Big Pipe and other slough projects to reduce CSOs to the slough by more than 99%.
2006 - The 14-foot diameter West Side Big Pipe and Swan Island Pump Station are completed, and work starts on East Side CSO projects.
2007 - The Downspout Disconnection Program disconnects downspout number 50,000, removing more than one billion gallons of stormwater annually from the combined sewer system.
2010 - Tunneling is completed on the East Side Big Pipe Project.
2011 - Work is complete on all CSO Program construciton.
Environmental Services promotes other innovative projects to manage stormwater onsite instead of piping it into sewers or streams. Projects include ecoroofs, green streets, rain gardens, swales and stormwater planters.