Columbia Slough Projects

Environmental Services completed construction of the largest of the Columbia Slough projects, the Columbia Slough Big Pipe, in October 2000. The pipeline collects the sewage and stormwater that once overflowed into the slough when it rains.

The Columbia Slough Big Pipe is a 3.5-mile long conduit that extends from Northeast 13th Avenue to the Columbia Boulevard Wastewater Treatment Plant. The reinforced concrete pipeline prevents about 300 million gallons of combined sewage from overflowing into the Columbia Slough each year.

Construction took three years. Environmental Services built the conduit in three segments.

Segment 1

Segment 1 construction started at the Columbia Boulevard Wastewater Treatment Plant and moved east to Chautauqua Boulevard. Environmental Services installed this 12-foot diameter segment using open cut trench construction.

Segment 2

The city used a tunnel boring machine to install this section of 12-foot diameter pipe from Chautauqua to Interstate Five

Segment 3

Environmental Services used open trench construction to install this six-foot diameter segment from I-5 to Northeast 13th Avenue.

Columbia Boulevard Treatment Plant Additions

The Columbia Slough CSO projects included several significant additions to the Columbia Boulevard plant to accommodate the new flows collected by the Big Pipe. 

• Environmental Services constructed an influent pump station to pump up to 105 million gallons per day of combined sewage from the Big Pipe for treatment.
• Environmental Services constructed new primary clarifiers and refurbished older primary tanks to provide significant wet weather treatment capacity at the plant for the peak flows from the Columbia Slough CSO system.
• The projects included construction of a second outfall pipe, 84 to 96-inches in diameter, to transport the treated wastewater effluent to the Columbia River.

•  A Dechlorination Facility was constructed to reduce chlorine in disinfected wet weather effluent to less than one part per million before the flow is discharged to the Columbia River.

The Columbia Slough projects reduced CSOs to the slough by 99%.