Project Overview
The Salmon Creek Wastewater Treatment Plant (SCTP) receives and treats roughly 8–10 million gallons of wastewater per day from approximately 120,000 Clark County homes and businesses. The Phase 5 Expansion program is a planned investment which builds on four previous expansions over the past 40 years. The Phase 5 Expansion projects will continue through 2024. The Phase 5 Expansion projects will ensure that the treatment plant can continue to meet the region’s long-term wastewater treatment needs and continue safeguarding public health, environmental quality and our community’s economic future. Once complete, the Phase 5 Expansion projects will allow for future decommissioning of Ridgefield’s aging wastewater treatment plant and outfall into Lake River (after other projects are also completed within the local collections systems).
Phase 5 Expansion projects include: Columbia River Outfall, Columbia River Effluent Pipeline, Secondary Treatment Process Improvements, and Odor Control and Existing Facilities Improvements (completed in Fall 2021).
Interactive Map
Use the interactive map below to learn more about each project and the potential impacts related to construction.
Columbia River Outfall and Effluent Pipeline
The Salmon Creek Wastewater Treatment Plant (SCTP) treats wastewater from approximately 120,000 Clark County residents. Treated water, or effluent, is conveyed through a 30-inch pipeline built in 1974 and discharged into the Columbia River via a submerged outfall diffuser. The Columbia River Outfall and Effluent Pipeline project will plan, design and build a larger effluent pipeline and an improved outfall diffuser.
The new 48″ diameter pipeline and new outfall diffuser are needed to replace the existing aging effluent pipeline and outfall, and for future increased plant capacity to support planned growth in the community. The new outfall diffuser will improve mixing and dilution of treated water discharged into the Columbia River. The project will also reduce ongoing maintenance at the discharge location and allow future decommissioning of Ridgefield’s aging wastewater treatment plant.
Project Design and Pipeline Route
The first task of this project was to determine the preliminary design and preferred route for the new conveyance pipeline. This step in the process has been in discussion for over a decade and is now complete. All necessary permits and easements have also been obtained. The route of the replacement pipeline is parallel to the existing effluent pipeline. The route will cross private property, the Burlington Northern Santa Fe Railway, existing utility lines, Salmon Creek, Lake River, and two road rights of way.
Improved Outfall Diffuser for Water Quality
As part of our goal of fostering a healthy environment, an improved river discharge assembly, called an outfall diffuser, will be built at the end of the new effluent transmission pipeline. The outfall diffuser will improve Columbia River water quality by better diluting and mixing discharged treated water. Since the Department of Ecology (Ecology) monitors and regulates the health of the Columbia River, we’ve worked closely with Ecology to determine the best location and design for the outfall (discharge location). The mixing zone will meet stringent standards set by Ecology to ensure clean water for swimming, boating, fishing, and aquatic resources.
An improved outfall diffuser will ensure that discharge of treated water is mixed and diluted efficiently and will meet water quality standards.
Project Documents
See project documents on the Documents & Links page.
Schedule
- Learn more in the project fact sheet (PDF)
Secondary Treatment Process Improvements
As part of the Salmon Creek Treatment Plant Phase 5 projects, the Alliance will expand the biological treatment process to increase the plant’s treatment capacity from approximately 15 million gallons per day to 17.5 million gallons per day within the existing facility footprint. This will be accomplished by constructing a new Aeration Basin and Secondary Clarifier in the middle portion of the plant and an older structure will need to be demolished to make space for these new facilities. In addition to these new structures, improvements to several mechanical pumping and air supply systems will be made inside existing buildings.