Wet Well Cleaning & Restoration
Project Details/Neighborhood:
This project will take place at all seventeen sewer pump station sites around the perimeter of Mercer Island.
Background
Mercer Island’s sewer system includes a series of pump stations that convey wastewater through the lakeline to King County’s South Treatment Plant. Most are concrete wet well/dry well designs, though four are, or will be, submersible. Aside from Pump Stations 4 (built in 2010), 14 (rebuilt in 2014), 11 (under reconstruction), and 20 (in design), the pump station system has seen minimal upgrades since the 1960s.
Although the City has performed routine maintenance, the wet wells haven’t been cleaned since 2018, and the structures are showing signs of deterioration. A 2024 assessment of five pump stations found poor concrete conditions, including corrosion and exposed rebar. Given their critical role and risk of failure, the City plans to reinstate annual cleaning and immediately begin structural restoration as needed.
Description
This project includes three key tasks:
- Cleaning: Wet wells will be cleaned using a vactor truck to remove accumulated debris and residue from the sides and bottom. The process will include high-pressure washing to prepare the surface for assessment.
- Condition Assessment: Fifteen wet wells will be evaluated after cleaning. The assessment will include observations, photographs, documentation of defects, and a prioritized ranking from most to least deteriorated.
- Restoration Design and Construction: Based on the assessment findings, the consultant will prepare a bid package for prioritized repairs. Restoration activities may include:
- Pressure grouting
- Structural concrete repair
- Spray-applied liners
- Lid and riser replacement
- Ladder removal
- Vent installation with in-line carbon filters for odor control
This work will help ensure long-term performance and reliability of the City’s pump station infrastructure.
Project Timeline
- Cleaning: June – July 2025
- Assessment: August – September 2025
- Design: October 2025 – January 2026
- Construction: April 2026 – November 2026
Estimated Budget
$750,000
Contact
Christopher Marks, Utilities Engineer, 206-677-1027; chris.marks@mercerisland.go
Status Updates
July 17, 2025
Ventilation Power Cleaning wraps up the project this week with wet well cleaning at Stations 16, 18, 20, 21, 22, and 23, with Station 25 scheduled for completion by Friday. The most significant challenge during the work was site access for the 40-foot, 30-ton Vactor truck. At several locations, crews had to join multiple 100-foot lengths of flexible vacuum hose to reach from the nearest available staging area to the wet well. The Island’s steep and rugged shoreline terrain not only reduced the trucks’ suction efficiency but also made the work especially strenuous, labor-intensive, and time-consuming. While VPC’s contract work wraps up this week, RH2 Engineering’s efforts are just beginning. Their team will now review inspection footage, identify station deficiencies, and work with the City to develop a rehabilitation design package for construction in 2026.
June 16, 2025
The City has contracted with Ventilation Power Cleaning to perform pressure jetting and debris removal at each of the sewer pump station wet well vaults. This work is tentatively scheduled to take place between June 30 and July 25. Cleaning is expected to take 1 day at each site and will be performed using a vactor truck. While most sites are accessible by land, a barge service will be used to access pump stations 10, 13, 17, 19, and 24. Representatives from the City and RH2 Engineering will be on site during each cleaning to oversee the work and perform visual inspections of the wet wells once cleaning is complete.
July 11, 2025
This project is underway. Over the past two weeks, Ventilation Power Cleaning (VPC) has completed wet well cleaning at sewer pump stations 1, 4, 14, and 15, as well as stations 10, 13, 17, 19, and 24. This week’s stations were unique in that they required use of the Seahorse, a 142-foot, 65-ton, crane-equipped work barge operated by Northern Marine Salvage. The work barge allowed VPC to reach otherwise inaccessible sites. Occurring simultaneously with the cleaning, staff from RH2 Engineering used a telescoping camera to inspect the interior structure at each station. The results of the inspections will be used by the City to prioritize station improvements later this year.
residential_overlays_2025_map.pdf (384 KB)