A view of Mercer Island from above with Lake Washington and Mt. Rainier

Construction Ahead for Mercer Island and Bellevue

Continuing our commitment to clean water and our communities

Water matters to all of us

Our relationship to water is part of what makes this region special. King County Wastewater Treatment Division is a partner in this relationship.

Water is what keeps us connected and is one of the area's greatest resources.

We all play a part in protecting this resource for future generations and preserving the beauty and health of Lake Washington and Puget Sound. 

A view of Seattle and the East Side from the air, with Lake Washington in between
A view of Seattle and the East Side from the air, with Lake Washington in between

Regional wastewater treatment was born when our water quality was suffering.

Up until the 1950s, wastewater flowed into Lake Washington and Puget Sound and many rivers and smaller lakes without enough treatment, fouling water and making a mess of local beaches.

1930s sewer project at Alki beach to extend sewer and stormwater pipes to Puget Sound
1930s sewer project at Alki beach to extend sewer and stormwater pipes to Puget Sound
1950s image of children looking at Lake Washington but unable to swim due to pollution

Lake Washington in the 1950s

A voter initiative resulted in the creation of King County Metro in the late 1950’s to address pollution concerns in Lake Washington. 

Since Metro's founding, wastewater treatment has transformed Lake Washington into one of the cleanest urban lakes in the world. 

Children looking out over a clean Puget Sound

Puget Sound today

Aerial of I-90 bridge and Lake Washington
Aerial of I-90 bridge and Lake Washington

King County Wastewater Treatment Division manages a big system that needs regular inspection, maintenance, and upgrades to keep sewage flowing.

Our system system serves 17 cities, 17 sewer districts and 1.7 million people. We operate 5 treatment plants, 42 pump stations and more than 350 miles of pipes.

Upgrades occur throughout the system as they are needed.

In your neighborhood, the existing regional sewer system was installed in the 1970’s. The pipes from your homes and businesses bring wastewater south to Renton's South Treatment Plant, built in 1965.  

The pipes are now aging and reaching capacity. They must be upgraded to provide service into the future.

What are we building in your neighborhood?

The  North Mercer Island and Enatai Sewer Upgrade Project  includes a new sewer pipe that will run across Mercer Island, underneath Lake Washington and over to the Sweyolocken Pump Station near Mercer Slough.

The project also includes upgrades to the North Mercer Pump Station and the City of Mercer Island’s Lift Station 11. 


We are working for you

Upgrading the sewer system in your area is a big project that's been years in the making and will involve years of construction. Some of the work will be in heavily used public spaces, and some will be in residential areas. The end product will provide great public benefit, but the work to get there will involve inconveniences to residents and users of public spaces.

Designing a highly complex new sewer line in a dense urban neighborhood takes time to get right. While the engineers were designing the pipes and equipment, our team was talking with neighbors and park and trail users to understand your values and concerns. 

Project schedule from 2014 through 2025, when construction ends

Project timeline - click to enlarge

From left: sharing information at a public meeting; Bike Everywhere Day on the I-90 Trail; neighborhood street walk; in the community; at the Mercer Island Summer Celebration; in Enatai Beach Park

After seven years, we are wrapping up the design phase and preparing for construction.


Get to know the work area

We expect construction to get underway toward the end of this year. You can take a virtual tour of the work area using the map below. Here's how to take the tour:

  • Click on any pin on the map
  • Once you are in the map, click on another pin or scroll to move through the different work areas

North Mercer Pump Station - upgrade of operations

78th Ave SE - sewer pipe under street

SE 24th St - sewer pipe under street

North Mercer Way - traffic re-routes

Luther Burbank Park - equipment staging

I-90 trail - sewer pipe construction

90th PL SE - pipe transition in cul-de-sac

96th Avenue SE - pipe modification

SE 36th St - pipe modification

Lift Station 11 - equipment upgrades

97th Ave SE - pipe modification and upgrade

SE 35th PL to Mercer Island boat launch - sewer pipe construction

Mercer Island boat launch - pipe construction and equipment staging

East Channel - pipe in the lake bed and floating pipe

Enatai Beach Park - multiple construction activities

Enatai hillside - pipe deep underground

Enatai shore - pipe rehabilitation

Mercer Slough - pipe rehabilitation

Sweyolocken pump station and boat launch area - new pipes and equipment storage

North Mercer Pump Station - upgrade of operations

What's happening here:

New pumps will increase the station's capacity from 9 million gallons per day (mgd) to 10.5 mgd.

A new building will house a generator that will keep the station running during power outages.

Upgraded odor control with new technology.

Site improvements:

A new retaining wall to protect slopes.

A new screening wall to buffer neighbors' views.

Low maintenance, drought-tolerant native plant landscaping.

Duration: 3 years

Learn more in our  detailed fact sheet 

78th Ave SE - sewer pipe under street

What's happening here:

New sewer pipe underneath residential street, installed with open cut trenching to lay the pipe.

The trenches will be approximately 8 feet wide.

Learn more about open cut construction below.

The street will be paved after construction.

Duration: approximately 8 months

Learn more in our  detailed fact sheet 

SE 24th St - sewer pipe under street

What's happening here:

New sewer pipe underneath residential street, installed with open cut trenching to lay the pipe.

The trenches will be approximately 8 feet wide.

Learn more about open cut construction below.

The street will be paved after construction.

Duration: approximately 8 months

Learn more in our  detailed fact sheet 

North Mercer Way - traffic re-routes

What's happening here:

New sewer pipe under 500' stretch of North Mercer Way, crossing Island Crest Way to the I-90 Trail.

Pipe will be installed with open cut trenching to lay the pipe.

The street will be paved after construction.

This stretch of North Mercer Way will remain closed 24/7 during construction. Detours will be in place for that time.

Duration: approximately 2 months

Luther Burbank Park - equipment staging

What's happening here:

Luther Burbank's south parking lot will be used for equipment storage and preparation.

Duration: October 1 - May 15 2021-2024

I-90 trail - sewer pipe construction

Description of work:

New sewer pipe underneath I-90 trail will be installed with "open cut" trenching to lay the pipe.

Segments of the trail will be closed at different times.

The work zone will be up to 30 feet wide.

The trail will be widened and paved after construction.

New trees and native landscaping will be planted.

Duration: approximately 1 year, in segments.

Learn more in our  detailed fact sheet 

90th PL SE - pipe transition in cul-de-sac

What's happening here:

New sewer pipe underneath residential street, installed with open cut trenching to lay the pipe.

New underground structures will be built in the cul-de-sac to transition between two types of pipe.

Underground equipment includes a discharge structure where the two pipes meet and an odor control facility.

Above ground equipment supports the underground structures.

Above ground equipment - which will be just past the guard rail at the top of the ravine - includes an electrical cabinet, air vent, and a stack to release clean air.

Duration: approximately 8 months

Learn more in our  detailed fact sheet 

96th Avenue SE - pipe modification

What's happening here:

An existing pipe that runs underneath I-90 will be modified to move sewage more efficiently.

Equipment will be stored nearby to support construction.

Crews will excavate around the existing maintenance holes near the intersection of 96th Ave SE and SE 35th St.

There will be a sewer bypass to carry sewage during construction.

Duration: approximately 4 months

SE 36th St - pipe modification

What's happening here:

An existing pipe that runs underneath I-90 will be modified to move sewage more efficiently.

Equipment will be stored nearby to support construction.

Crews will excavate around the existing maintenance holes on SE 36th St in front of Mercer Island City Hall.

There will be a sewer bypass to carry sewage during construction.

Duration: approximately 4 months

Lift Station 11 - equipment upgrades

What's happening here:

Improvements to the City's Lift Station 11 to align with new location for North Mercer Interceptor pipe.

Park will be closed for duration of construction.

Installation of larger pumps to move sewage uphill to new County pipe.

Upgraded electrical system to handle increased pumping needs.

Equipment storage in park.

Temporary pumps and piping on the surface while work in underway.

Above ground electrical cabinets and generator.

Restoration of park after construction.

Duration: approximately one year

Learn more in our  detailed fact sheet 

97th Ave SE - pipe modification and upgrade

What's happening here:

Existing pipe from Fruitland Landing to SE 34th St will be relined to extend its life.

Construction pits to access existing pipe at Fruitland Landing and intersection of 97th Ave SE and SE 34th St.

Temporary pipe on east side of street between Fruitland Landing and SE 34th St while existing pipe is relined. Temporary ramps will allow driveway access.

New pipe between SE 34th St and the I-90 trial constructed with open cut trenching.

Street will be paved after construction.

Duration: approximately 6 months

Learn more in our  detailed fact sheet 

SE 35th PL to Mercer Island boat launch - sewer pipe construction

What's happening here:

New sewer pipe underneath residential street, installed with open cut trenching to lay the pipe.

New underground structures will be built in the cul-de-sac to transition between two types of pipe.

Underground equipment includes a discharge structure where the two pipes meet and an odor control facility.

Above ground equipment supports the underground structures.

Above ground equipment includes an electrical cabinet, air vent, and a stack to release clean air.

Restoration includes tree planting and new pavement over work area.

Duration: approximately 8 months

Mercer Island boat launch - pipe construction and equipment staging

What's happening here:

The back half of the parking lot will be used for equipment storage and material preparation.

New pipe extending from SE 35th PL installed using open cut technology under the road and trail at the north end of entrance area

Installation of maintenance vault adjacent to new pipe along frontage road

Boat launch will remain accessible

Shoreline and road restoration when work is complete

Duration: up to two years

In-water work will occur during permitted fish window

Learn more in our  detailed fact sheet 

East Channel - pipe in the lake bed and floating pipe

What's happening here:

New pipe laid across the lakebed to north of bridge.

Pipe laid using cut and cover technology from barges in the lake.

Crews will maintain a navigable channel for boat traffic.

Work will be done during permitted fish windows to avoid environmental impacts.

The bottom of the lake and the east and west shorelines will be restored after construction.

In addition to installing new pipe under the lake, the pipe that will be installed under the Enatai hillside will be floated along the shoreline north of Enatai Beach Park prior to installation.

Duration: approximately 6 months

approximately 1 week for floating pipe

Learn more in our  detailed fact sheet 

Enatai Beach Park - multiple construction activities

What's happening here:

Construction equipment storage and preparation underneath I-90 bridge piers.

Pipe for multiple work segments will be assembled at the park.

New sewer pipe installed with open cut trenching across the swim beach to connect the new pipeline to the existing pipeline.

New underground structures will be built under the I-90 bridge to transition between different pipes.

Underground equipment includes a discharge structure where the two pipes meet and an odor control facility.

Above ground equipment supports the underground structures.

Above ground equipment - which will be located underneath the I-90 bridge - includes an electrical cabinet, air vent, and a stack to release clean air.

Temporary underground pump station to redirect flow to the new Enatai Siphon while downstream Enatai Interceptor is being rehabilitated.

Restoration of work site including new permanent access road underneath the I-90 bridge.

Duration: 4 years of intermittent work.

Learn more in our  detailed fact sheet .

Enatai hillside - pipe deep underground

What's happening here:

New pipe will be installed up to 160 feet under the Enatai hillside between Enatai Beach Park and Sweyolocken boat launch.

Drill pits at Sweyolocken and Enatai Beach Park.

An underground hole will be drilled between the two pits using horizontal directional drilling technology.

Crews lay a wire on the surface and use computers to guide the drill underground.

Surfaces above the new pipeline will not be disturbed.

Duration: approximately 6 months

Learn more in our  detailed fact sheet .

Enatai shore - pipe rehabilitation

What's happening here:

Crews will perform spot repairs on existing in-water pipe along shoreline between Enatai Beach Park and the entrance to Mercer Slough.

Access to pipe will be through existing easements on docks.

Barges on the water will provide crew access to work area.

Temporary sewage diversion will be in place for affected homeowners at times.

Duration: approximately 6 months

Mercer Slough - pipe rehabilitation

What's happening here:

Crews will reline the existing Enatai Interceptor from the mouth of Mercer Slough to the Sweyolocken boat launch.

The work will take place at maintenance holes and angle points along the way, with the new lining inserted at one of these access points and pulled through the pipe to the next.

Most of the access points will be on land, but a few will be within the water of the Mercer Slough.

At the in-water locations, crews will surround the access points with temporary dams to create an enclosed area for the work.

The Sweyolocken Boat Launch, the slough, and the surrounding bike trails will generally remain open during this work, with short closures on the bike path as equipment is moved.

Duration: approximately 6 months

Learn more in our  detailed fact sheet .

Sweyolocken pump station and boat launch area - new pipes and equipment storage

What's happening here:

Drill pit and equipment for Enatai hillside pipe.

Equipment storage and preparation in the Sweyolocken boat launch parking lot.

Sewer connection between new Enatai pipe and pump station installed using open cut trenching.

Full closure of the access road from Bellevue Way SE to the Sweyolocken Boat Launch for the duration of work.

Duration: approximately 9 months

Learn more in our  detailed fact sheet .

Construction techniques

Our engineers are designing around a legacy of the past. They are constantly challenged to figure out the most effective construction techniques in dense urban neighborhoods. The system has to keep sewage moving while reducing community and environmental impacts wherever possible.

Watch the videos to get a taste of what you might experience as a neighbor, commuter, or park and trail user.

Open cut trenching involves digging down from the surface, laying pipe, and covering it back up. Watch this video to find out what you might experience as a neighbor or passerby.

This map shows Mercer island street and trail locations for construction.

Click the map to enlarge and see open cut locations

See examples of construction on other King County projects.

Trenchless construction allows our crews to install pipe deep underground. It also reduces impacts on the surface compared to open cut trenching. This video explains the trenchless technique we will use, called horizontal directional drilling, or HDD. It gives you a sense of what neighbors and passersby may experience while we are constructing this section of pipe.

Map showing location of pipe installed under the Enatai hillside using trenchless construction

The Enatai siphon will be constructed using horizontal directional drilling (HDD).

Pipe relining allows crews to repair damaged pipes without digging them up. Essentially it creates a new pipe inside the old one, extending the old pipe's life by fifty years or more.

Map showing Enatai Interceptor pipe that will be relined

The Enatai Interceptor will be repaired and partially relined from Enatai Beach to the Sweyolocken Pump Station.

Graphic that explains how old pipes are relined to extend their life

We're with you every step of the way

We know this is a big project, and King County is committed to being a good neighbor during construction. Here is how we will work with you:

  • We will provide you with advance notice of construction activities via mail, email, phone, web bulletins and project fliers
  • We will work with you one-on-one to resolves problems quickly
  • We will give you regular project website and construction updates
  • You will always be able to reach up with a 24-hour construction hotline
Map that shows construction area by the type of impact it will have on neighbors

Construction reference guide


When work is complete, there will be four miles of new pipe and two upgraded pump stations to keep sewage flowing efficiently for the next fifty years and more.

This project is part of King County's commitment to clean water and healthy habitat.

Aerial image of Mercer Island with Mt Rainier in the backgroud

                 

Keeping water clean and sewage flowing for 50 years and more


                        

Resources

Project timeline - click to enlarge

Construction reference guide

Lake Washington in the 1950s

Puget Sound today

Click the map to enlarge and see open cut locations

The Enatai siphon will be constructed using horizontal directional drilling (HDD).

The Enatai Interceptor will be repaired and partially relined from Enatai Beach to the Sweyolocken Pump Station.