Status of CSO Control Projects

This page presents an overview of King County’s completed, current, and planned CSO control projects. Projects began in the late 1970s. Many early projects involved sewer separation, flow diversion, and new tunnels. Most current and future projects involve construction of storage tanks and treatment facilities.

• Completed CSO Control Projects and Associated Projects
• New and Recent CSO Projects
• Future CSO Control Projects

Source: Section 1.3 from 2006-2007 Annual Report.

Completed CSO Control Projects and Associated Projects

The following tables summarize CSO control projects and other projects associated with CSO controls that have been completed to date.

Completed CSO Control Projects

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Completed Associated Projects

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New and Recent CSO Projects

• Source: Section 1.3.2 from 2006-2007 Annual Report

  Puget Sound Beach Projects
• Ballard Siphon Replacement
• Densmore Stormwater System Improvements
• Denny Way/Lake Union CSO Control Project
• Henderson/Norfolk CSO Control Project

Puget Sound Beach Projects

Four projects are currently in predesign. These four projects are referred to as the Puget Sound Beach Projects: South Magnolia, North Beach, Barton Street, and Murray Avenue. In January 2007, King County hired Carollo Engineers for the planning and predesign phase of the project. Through May 2007, CSO alternatives to be evaluated for each basin include storing flows, conveying and treating flows, removing stormwater from the combined sewer system, treating CSO at the point of discharge to Puget Sound, and a combination of the alternatives.

Initial alternative screening criteria were developed and will be further refined with community feedback. Community involvement meetings were held in each of the four project basins. Public comments are being tracked and will be used to involve stakeholders in future community meetings.

During the coming year, technical memorandums will be drafted to document planning confirmation and alternative screening criteria. Flow monitoring in the local Seattle sewer system will be conducted in each of the four basins to assess whether removing stormwater from these sewers is a viable option for CSO control.

Ballard Siphon Replacement

The Ballard Siphon, built in 1935, consists of two woodstave siphon barrels that rest on the bottom of the Washington Ship Canal. The siphon carries flows collected from Seattle's north end near Carkeek Park and from the Ballard area across the Ship Canal. From there, the flows are conveyed to the West Point Treatment Plant.

In November 2005, King County conducted a sonar inspection of the Ballard Siphon. The inspection showed spots of abnormalities in the integrity of the pipe. Since sonar inspections are a new technology, it is unclear how long the abnormalities had been present and how high the risk of failure. Subsequent analyses and inspections indicated that the anomalies were not threatening, and the concern for imminent failure was significantly reduced.

Replacing the siphon is continuing forward as a high priority project in order to maintain siphon integrity and function and because the project will yield CSO control benefits. The completed project will eliminate CSO events at the Ballard Regulator. It will also reduce overflows at 11th Avenue, likely reducing the scope of a future control project at this site. The Ballard siphon project includes two major components: (1) slip-lining the existing woodstave siphon barrels to extend their useful life, and (2) tunneling an 84-inch-diameter pipe below the canal. Final design is scheduled for completion in third-quarter 2008; construction is scheduled to begin in first quarter 2009.

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Densmore Stormwater System Improvements

The Densmore stormwater system was built to reduce CSOs at the University Regulator Station. It collects stormwater from the Haller Lake area and Green Lake drainage, as well as outflow from Green Lake, which had previously entered the combined sewer system. The Densmore system runs from Green Lake to Lake Union. A pump station located to the north of Lake Union discharges to Lake Union just west of the I-5 bridge. In the event of pump failure, high-level weirs allow stormwater to discharge to the combined sewer to prevent damage to the pump station or to Green Lake park facilities.

The Densmore stormwater system began running in 1994, but its operation has not resulted in the expected CSO reduction, mainly because of hydraulic, mechanical, and electrical problems. All three of the pumps in the pump station have been worked on since startup of the station. However, all three pumps could not be used until stormwater system hydraulic improvements were made. This work, including construction of a large vault designed to reduce the surge in the line near the outfall, was completed in October 2007. The system will be evaluated over the next several wet seasons.

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Denny Way/Lake Union CSO Control Project

The Denny Way/Lake Union CSO Control project consisted of the construction of several CSO facilities to store and treat CSOs from the County’s Dexter Regulator and City of Seattle CSOs around Lake Union, and to control the County's largest CSO at Denny Regulator on Elliott Bay. Construction was completed in May 2005. The Mercer/Elliott West CSO Annual Report (Appendix C) summarizes the performance of the facility over this last wet season.

The project consisted of three major elements: the East Portal, which captures flow from a number of sewer lines in the South Lake Union area; the 14-foot-diameter Mercer Street Storage and Treatment Tunnel; and the Elliott West CSO Treatment Facility located on Elliott Bay. Two new CSO outfalls were built in Elliott Bay—one outfall to replace the outfall structure at the Denny Way Regulator and another outfall for the Elliott West CSO Treatment Facility. The Mercer Tunnel provides storage for up to 7.2 MG and primary clarification for all flows entering the tunnel. The Elliott West Treatment Facility was designed to provide final treatment— screening, disinfection, and dechlorination—to settled flows that exceed the capacity of the tunnel. Such treatment is expected to occur about 14–20 times per year.

During the first two years of operating these CSO facilities, King County faced several challenges, which is typical for such large and complex systems. The seasonal and intermittent operation of these facilities prolongs the commissioning period. During the 2006–2007 season, the Mercer Tunnel operated for 28 CSO events and final treatment at Elliott West with discharge occurred for 13 events. This does not include the steady stream of non-CSO flows from City of Seattle east Lake Union sewers that were overflowing into the tunnel.

A large hurdle to effective operations was the substantial amount of dry-weather flows that entered the Mercer Storage and Treatment Tunnel. These flows were reducing the tunnel’s storage capacity for CSO flows by 1–2 MG, causing pump damage, and complicating treatment compliance. Investigation by King County determined that extensive sedimentation in city sewers upstream of the tunnel were causing base flows to back up and overflow into the tunnel.

The City conducted an extensive pipeline cleaning in spring 2006, removing 80 tons of sediment, mainly sand and gravel, from the sewer lines. This cleaning resolved the dry-weather flow problem until about November 2006 when portable monitors placed at the weir indicated that the pipelines were refilling. A second cleaning during spring 2007 removed another 16 tons of sediment. The second cleaning reduced the volume of dry-weather flow to the tunnel but did not resolve it. The City and the County have agreed to make temporary weir modifications to prevent these flows from entering the tunnel. Design will begin under the current emergency contract. The schedule for modifications will depend on the alternative chosen and the period of dry weather required for the construction. In parallel, the City will perform a source investigation of its east Lake Union system. If the sedimentation can be corrected, the weir will be reopened.

The large storms of November 2–15 and December 9–15, 2006, severely challenged the system. The intense rain—up to 1.45 inches on November 6 and 1.8 inches on December 14— demonstrated that the facilities were not hydraulically capable of managing the resulting flows. Flooding at Elliott West and effluent overflows in Myrtle Edwards Park near the Denny Regulator and the outfalls highlighted the need for improvements.

A consultant was hired under an emergency waiver to assist the County in identifying solutions. The duckbill valve was removed from the outfall in March 2007. The beneficial impacts of this removal will be assessed after wet-weather flows resume.

On September 6, 2007, Ecology issued Notice of Violation (NOV) 5059, citing monitoring, disinfection, and dechlorination failures. King County’s response, submitted October 5, 2007, details the corrective actions completed and under way to bring the facility into compliance. The NOV and King County's response are attached as Appendix C2.

The Dexter CSO was also to be controlled under the Denny/Lake Union CSO Control Project. In the first two wet seasons, the facility continued to have frequent overflows but of much lower volume than before the facilities went online. Investigation identified needed programming changes to the gate controls. A new programmable logic controller was installed and the program changes were made in August 2007. The success of this correction will be assessed during the 2007–2008 wet season.

Although the Denny/Lake Union facilities have not yet achieved complete CSO control, they have made substantial inroads into controlling CSOs at the Denny and Dexter locations:

• Untreated overflows at the Denny regulator have decreased from a baseline of 32 to 6 events (both seasons):
  • Of the 513.57 MG of CSO that would have discharged into Lake Union and Elliott Bay in 2005–2006, 38.4 percent received secondary treatment, 61.4 percent received primary treatment and some disinfection, and 0.2 percent was discharged untreated. This represents a 99.2 percent reduction from pre-project levels.
  • Of the 764.53 MG of CSO that would have discharged into Lake Union and Elliott Bay in 2006–2007, 28.5 percent received secondary treatment, 64 percent received primary treatment and some disinfection, and 7.5 percent was discharged untreated. This represents a 92.5 percent reduction from pre-project levels.
• Untreated overflows at the Dexter regulator have remained close to the baseline of 15 events per year, at 16 events in 2005–2006 and 17 events in 2006–2007; the volume of untreated CSO discharges has decreased from 24 MG to 6.21 MG in 2005–2006 and 8.77 MG in 2006–2007. This is a 74 percent (2005–2006) and 63.5 percent (2006–2007) reduction from pre-project levels.

King County is committed to completing the refinements to these facilities to achieve full control as quickly as possible.

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Henderson/Norfolk CSO Control Project

The Henderson/Norfolk CSO control project was implemented to control the Henderson and Martin Luther King (MLK) CSOs into Lake Washington and the Norfolk CSO into the Duwamish River. King County upgraded the Henderson Pump Station and constructed a large storage and treatment tunnel between Henderson Street and Norfolk Street in the Rainier Valley. The facilities were designed to chlorinate and dechlorinate flows that exceed the capacity of the storage and treatment tunnel and to discharge treated flows at the Norfolk CSO in the Duwamish Waterway. This discharge is expected to occur approximately two to four times per year. Base flows, settled solids, and stored flows from the tunnel are conveyed to the South plant at Renton or to the West Point plant, depending on capacity in the Elliot Bay Interceptor, for secondary treatment.

The Henderson/Norfolk project was also completed in May 2005 but did not operate during its first season. Following adjustments in the influent gate control programming, the Henderson Tunnel began filling and treating CSO in 2006. During the 2006–2007 season, 61.5 percent of the CSO managed by this system received secondary treatment at the South plant and 38.5 percent received primary treatment and disinfection in the tunnel. One treated discharge from the Henderson tunnel to the Norfolk outfall was designated as an “untreated” event for purposes of calculating solids limit performance, otherwise, no untreated discharges occurred at the three system outfalls (Henderson, MLK, and Norfolk).

For additional information on the performance of the Henderson/Norfolk CSO Control Facilities, see Section 2.3.5 and Appendix D.

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For questions about the CSO Program, contact Karen Huber.

Department of Natural Resources
Wastewater Treatment Division
201 South Jackson, Suite 512
Seattle, WA 98104
Phone: 206-684-1246
Fax:206-684-1741

Updated: Nov. 9, 2007