2006-2007 Annual Report
King County
Combined Sewer Overflow Control Program

Executive Summary

• Background
• Report Requirements
• CSO Control Status

Background

The King County Wastewater Treatment Division (WTD) provides wholesale wastewater conveyance and treatment for flows from the City of Seattle and 33 other cities and sewer districts. Only the City of Seattle wastewater collection system contains combined sewers that collect both wastewater and stormwater. Seattle’s collection system conveys flows to County trunks and interceptors, which then convey flows to the West Point Treatment Plant located in Discovery Park. A small portion of flows from the combined sewer system is treated at the South Treatment Plant in Renton. When medium to large storms occur, flows may exceed the capacity of the collection system pipes, resulting in combined sewer overflows (CSOs) at 38 County CSO locations that discharge to Lake Washington, Lake Union, the Ship Canal, the Duwamish River, Elliott Bay, and Puget Sound. The City of Seattle is responsible for 92 CSO locations in its local sewer systems (Figure 1).

CSOs are a recognized source of water pollution that can result in temporary increases in bacterial counts and aesthetic degradation of shorelines, in long-term adverse effects on sediment quality at discharge points, and in raised public health concerns in areas where there is potential for public contact. Since the 1970s when the basic sewer system infrastructure was in place, King County has been implementing CSO control projects to improve water quality in the Seattle area.

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Report Requirements

This report is prepared and submitted to the Washington State Department of Ecology (Ecology) in accordance with the requirements established in the West Point Treatment Plant National Pollution Discharge Elimination System (NPDES) Permit and in WAC 173-245-090. The report provides:

1. An overview and status of King County’s Combined Sewer Overflow (CSO) Control Program.
2. CSO volumes and frequencies for June 1, 2006, through May 31, 2007.
3. Annual reports for the four CSO treatments facilities—Alki, Carkeek, Henderson/Norfolk, and Mercer/Elliott West.
4. Progress on the first Regional Wastewater Services Plan (RWSP) CSO projects at South Magnolia, North Beach, Barton Street, and Murray Avenue CSO locations.

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CSO Control Status

Despite near normal rainfall over the year, the unusually intense pattern of rainfall created significant challenges for the region. November and December 2006 brought several large storms and intense rain. Approximately one-third of the annual rainfall occurred during two storms occurring November 2–15 (8.67 inches) and December 9–15 (4.12 inches). 1.8 inches of rain fell on 12/14 alone. Although producing less total rainfall than the November storm, the December storm was extremely intense. Rains were accompanied by winds up to 70 miles per hour, power outages, and flooding, which severely impacted the County’s sewer system.

The total volume of untreated CSOs was down to approximately 691 million gallons (MG) during the year compared to a baseline of 2,339 MG. The total of 691 MG represents 1.7 percent of flow from the West Service Area, and is a 70.5 percent reduction in CSO volume over the 1981–1983 baseline average.

This report includes the annual reports for the Alki, Carkeek, Henderson/Norfolk, and Mercer/Elliott West CSO Treatment Facilities. Continued progress is being made in starting up the Mercer/Elliott West and Henderson/Norfolk treatment and storage facilities that came online May 2005. These are complex facilities that operate intermittently. Experience with past CSO treatment facilities has shown that intermittent operation prolongs the startup process. The infrequent opportunities to assess performance and the need to schedule construction for dry weather periods extend the time required to refine facility performance. King County is working diligently to enable these facilities to achieve their performance goals.

Ecology issued a notice of violation (NOV) September 6, 2007. This NOV requires the County to explain the causes of permit violations and describe corrective measures being taken. King County has kept Ecology informed of the challenges and corrective actions performed since operations began. A copy of the NOV and the County's response is attached as Appendix C2.

Although full control has not yet been achieved at the Denny and Dexter Regulators (controlled by Mercer/Elliott West facilities), much has been accomplished. Of the volume of CSO to be managed at the Mercer/Elliott West facilities, 28.5 percent was captured and received full secondary treatment, 64.0 percent received primary treatment and disinfection, and only 7.5 percent was discharged untreated at the Denny Regulator. The number of untreated discharges went from the baseline of 32 events per year to just 6 events. At the Dexter Regulator, the number of events was nearly unchanged from baseline, but they were smaller in volume— 58.5 percent of the baseline volume that had previously discharged untreated at Dexter received treatment at West Point in 2006–2007.

The Henderson/Norfolk project was also completed in May 2005. 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). Following adjustments in the influent gate control programming, the Henderson Tunnel began filling and treating CSO in November 2006. In 2006–2007, 61.5 percent of the CSO managed by this system received secondary treatment at the South plant at Renton, while 38.5 percent received primary treatment and disinfection in the Henderson tunnel.

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Download the report (.PDF file, 1.1MB).

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