Columbia River Outfall and Effluent Pipeline Project

This project builds on decades of planning to ensure we can meet the region’s long-term wastewater treatment needs and continue safeguarding public health, environmental quality and our community’s economic future. The project will upgrade the transmission pipeline system from the Salmon Creek Wastewater Treatment Plant to the Columbia River and improve water quality in the Columbia River.

Project Documents

Engineering Report Appendices:

Archaeological Investigations Northwest, Inc. (AINW), Cultural Resource Survey for the Columbia River Outfall and Effluent Pipeline Project, Clark County, Washington, Report No. 3957, 2017 (subject to RCW 42.56.300)

Project Benefits

The project will:

  • Ensure continued reliable service at a stable, affordable rate by planning for the long term and avoiding the increased cost of isolated improvements, while coordinating with other planned projects in the area.
  • Support planned growth within the community.
  • Ensure adequate mixing and dilution of treated wastewater discharged into the Columbia River.
  • Manage shoreline stability at the discharge point.
  • Enable future decommissioning of Ridgefield’s aging wastewater treatment plant and outfall into Lake River (after additional projects are in place).

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Contact Us

For more information contact Dale Lough, Alliance Capital Program Manager, at (360) 9993-8856 or

Project information can be provided in other formats or languages upon request.

Project Overview

The Salmon Creek Wastewater Treatment Plant treats wastewater from approximately 120,000 Clark County residents. All wastewater received at the plant is treated to a high standard. Clean water, or effluent, is conveyed and discharged into the Columbia River via a 30-inch pipeline and submerged diffuser that were built in 1974. The Columbia River Outfall and Effluent Pipeline project will plan and build a larger transmission pipeline and improved discharge system.

Columbia River Outfall and Effluent Pipeline Project Map

The project will build a pipeline between the Salmon Creek Wastewater Treatment Plant and the Columbia River, as well as an improved discharge assembly at the river.

The new conveyance pipeline will be 48 inches in diameter, large enough to ensure continued reliable service at a stable, affordable rate for the long-term, avoiding the increased cost of isolated improvements.

The pipeline and diffuser are needed for future increased plant capacity to support planned growth in the community, as well as to ensure adequate 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, while coordinating with other planned projects in the area.

The information in this web page is also available as a fact sheet (PDF).

Design and Preferred Pipeline Route

The current design calls for a 48-inch pipeline with a preferred route that parallels the existing effluent pipeline. However, planning the route for the new conveyance pipeline is complicated because it must cross private property, the Burlington Northern Santa Fe Railway, existing utility lines, Salmon Creek, Lake River, and one state owned right-of-way. A variety of permits will need to be obtained before construction can begin. We are negotiating with property owners for easements. The community will be kept informed as the design process moves forward.

Diffuser Diagram

An improved diffuser will ensure that discharge of treated water is mixed and diluted efficiently and will meet water quality standards.

Improved Discharge Mechanism for Water Quality

As part of our goal of fostering a healthy environment, an improved river discharge assembly, called a diffuser, will be built at the end of the new effluent transmission pipeline. The diffuser and pipe will improve Columbia River water quality by better diluting and mixing discharges of 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.

Aligning Planning Considerations with Community Values

Special care is being taken to make sure that construction and operation of the new effluent pipeline and outfall will be environmentally sound, cost-effective, fiscally responsible, and address neighbor and ratepayer concerns. As we work through the local, state, and federal permitting processes, we are collaborating with a variety of stakeholders to ensure responsible conservation and stewardship of the environmental and social resources that we treasure. Any landscape disturbances made during construction will be fully restored and we will work to minimize disruptions to commercial and recreational interests. Once built, the new pipeline will require minimal energy and maintenance to operate. The existing pipeline will be retained to facilitate continuous treatment plant operation during the transition and allow for routine maintenance.


Every two years, the Alliance updates and adopts a comprehensive Capital Plan that provides for infrastructure needs and outlines the funding requirements for each project. Planning and design costs for the Columbia River Outfall and Effluent Pipeline Project are covered in the Capital Plan.

In total, the project is anticipated to cost approximately $25 million. Those costs are allocated to the two funding members of the Alliance partnership – the City of Battle Ground and the Clark Regional Wastewater District. The City and the District have included the project costs in long range financial planning efforts to ensure affordability of rates and charges.

Diffuser Diagram

Reliable, expandable sewer service is essential to our economy and the health of our communities and environment.

Planning for the Future

It’s important to plan for the future. Our personal health, the community’s economic health, and the health of our rivers and environment depend in large part on treating the wastewater we all generate so that it can be returned safely to the environment. As our community grows, we continue to make investments to ensure our system adequately meets our needs.

The Columbia River Outfall and Effluent Pipeline Project is part of the Phase 5 expansion program, a planned future investment which will build on four previous expansions over the past 40 years. The Phase 5 expansion will take place over several years and will be performed as two separate and independent projects.

n addition to this project, Phase 5 also includes the Salmon Creek Treatment Plant Improvements Project, which will add additional treatment facilities within the existing plant footprint to increase the plant’s capacity from the current 15 million gallons per day to approximately 17.5 million gallons per day. As part of this project, an odor containment and treatment system will be built to serve the Preliminary and Primary Treatment Facilities at the plant. You can read more about the Salmon Creek Treatment Plant Improvements Project at

Staying ahead of the reliability and resiliency needs is vitally important. By proactively planning now, the Alliance ensures that ample wastewater treatment facilities will be in place to continue to provide reliable service at an affordable rate well into our future.

Design Work is Underway, Though Construction is Years Off

We are completing information and data gathering to verify pipeline location and design. This includes surveying, geotechnical exploration, environmental analysis, and determining permit and easement needs. As planning and design progress, cost estimates will also be updated.

There is a great deal that needs to be completed before construction can begin, including detailed design and engineering, acquisition of the necessary permits and easements, and selecting a construction contractor.


Outfall Project Schedule

Stay Up to Date

Throughout the project, interested community members can follow progress and provide feedback through the Alliance website,