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Marine Outfall

Learn about Brightwater construction activities.

Marine Outfall

The marine outfall will discharge effluent (treated wastewater) from the Brightwater treatment plant into Puget Sound. An outfall has two components: a long solid pipe that extends from the shoreline deep into the water, and a section called a diffuser at the end, where the treated wastewater is released from ports into the marine environment.

The outfall will connect to underground pipe near the Point Wells Portal and extend offshore for approximately 5,200 feet (nearshore and offshore portions). Part of the portal site will serve as a staging area for outfall construction. The main part of the outfall pipe will be constructed off site.

The outfall will originate near the portal located at Point Wells (Portal 19) and extend offshore for approximately 5,200 feet (nearshore and offshore portions). The portal site will serve as a staging area for outfall construction, as well as portal and conveyance construction.

Outfall Details

Graphic of typical outfall section through the nearshore and offshore environs

The Brightwater marine outfall will be sited and designed to:

• Discharge effluent deep in the Puget Sound where strong currents will result in rapid dispersion
• protect, marine life, human health, and the health impacts to area residents and commercial enterprises
• protect the outfall pipelines from potential seismic events and submarine landslides
• maintain acceptable hydraulic performance
• minimize maintenance

Diffuser Design

The twin diffusers will be 250 feet long and will be installed at a depth of about -600 feet MLLW. Sixty-two diffuser ports will be available for effluent release.

Although some maintenance will be required, the marine outfall and diffuser will on-site staff for operations. Design features used to minimize maintenance requirements will include self-cleaning end ports and diffuser port risers.

Pipeline Size and Materials

A single 84-inch steel pipe will extend from the underground sampling facility at Point Wells to a point in the nearshore where the pipe branches. Two 63-inch outfall pipes will convey flows from the Brightwater treatment plant to the marine environment. These pipes will be constructed of high-density polyethylene, or HDPE pipe. Protections will be in place to reduce corrosion of steel pipe and metal fittings during operation.

Outfall Construction

The outfall will pass through several slope gradations. First, the pipeline will pass through relatively flat intertidal zone where marine life is most dense. Beyond this shelf, the seafloor slope increases (up to 35 percent) before reaching the main channel area of Puget Sound approximately 5,000 feet offshore. This is where the diffuser will be located, at a depth of about 600 feet

Onshore and nearshore

The 84-inch marine outfall connector pipe, which extends from the underground sampling facility to the marine outfall, will be installed on the beach using a remotely-controlled tunneling machine.

The marine outfall will be laid in a trench on the beach, and in the nearshore zone to -80 feet mean low low water, or MLLW. Shoring will be used to support trench walls and minimize the trench size, limiting environmental impacts on the sensitive nearshore habitat.

The marine outfall contractor will fence off beach areas used for construction to protect public safety.

Offshore Construction Methods

The outfall pipelines, including the diffuser section will be laid directly on the seafloor using a method called controlled submergence. The pipelines will be constructed offsite and towed to Point Wells by tug boats. Entrapped air provides buoyancy for this phase. Seawater is carefully added beginning at the nearshore end to submerge each pipeline, causing it to settle to seafloor. The entire process is expected to take 2-3 days for each pipeline.

Construction Duration

Construction on the marine outfall is expected to start in 2008 and be completed within two years.

Nearshore Restoration

To prepare for nearshore restoration, eelgrass plants will be harvested in mid-June 2008 for culture. Scientists will conduct nearshore monitoring of marine plants and animals tol provide a picture of conditions prior to construction. Once construction is complete, eelgrass will be replanted and the area will be monitored for post-construction recovery.

Contact us if you need more information about Brightwater.

King County Brightwater Project

22505 State Route 9
Woodinville, WA 98072-6010
E-mail: Brightwater
Phone: 206-684-6799 or
TTY Relay: 711

Updated: January 15, 2008