Carnation Wastewater Treatment Facility

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Why were the Treatment Plant Site and Discharge Alternatives Chosen?

Note: View Addendum 1 of the Final EIS for an update on the Wetland Discharge option.

Treatment Plant Site

Selected Carnation Wastewater Treatment Facility with Revised Wetlands Option (January 2007). Click image for larger view.

Note: The project team first looked at Stillwater Wildlife Area as the only alternative for a wetland enhancement. Piping the water more than 15,000 feet to Stillwater will cost much more than $1 million and finding funding to cover the pipe costs is proving difficult. During the summer 2005, King County dropped the Stillwater option and began looking at wetland options (Chinook Bend Natural Area) closer to the treatment plant to avoid the additional conveyance cost associated with the Stillwater location.

Carnation Wastewater Treatment Facility with Stillwater Wildlife Area as Wetland Enhancement Option (January 2005). Click image for larger view.

Either the city-owned site or the Weckwerth site would be a satisfactory location for the treatment plant. However, the city-owned site offers several advantages:

Cost – Building the plant at the city-owned site would have a total project cost of about $2 million less than the cost of building the plant at the Weckwerth site.

Community considerations – The city-owned site is larger and may provide for better visual screening. The site also offers several options for access, whereas access to the Weckwerth site is limited.

Discharge Alternatives

1. Upland discharge was eliminated for two main reasons:

Cost – Upland discharge is the most costly of the discharge alternatives in all cost categories. Land acquisition and capital costs would be an estimated $3 million higher than they would be for the river discharge alternative. Operation and maintenance costs would also be higher.

Environmental – The final EIS notes that the potential long-term operating impacts of upland discharge could include groundwater mounding because of low permeability of subsurface soils.

2. River discharge

The river discharge alternative is one of the most widely used discharge methods in the United States. Most if not all treatment plants on the Snoqualmie River use this discharge method. The river outfall is satisfactory in all decision factor categories.

Cost – The river discharge is the least costly of the three discharge alternatives. Capital costs for the river outfall and associated conveyance system are $1.6 million (versus $4.1 million for the wetlands enhancement and its conveyance pipe).

Environmental - The final EIS notes that all discharge alternatives would have short-term construction and long-term operating impacts that we could minimize with mitigation measures. The final EIS notes that the river discharge alternative would have greater potential to adversely impact spawning habitat for chinook salmon than the wetland discharge alternative.

3. Wetlands enhancement discharge

The wetlands alternative offers several clear community and policy advantages. However, it costs much more than the river discharge alternative.

Cost – The wetlands alternative is estimated to be $2.5 million more than the river discharge alternative. Total capital costs would be $4.1 million for the wetlands alternative, including the following main elements:

• Wetlands construction and native plantings–$1.2 million
• Conveyance pipe–$2.25 million
• Emergency storage at the treatment plant–$660,000.

The wetlands alternative does provide opportunities for grant funding and partnerships not available for the river outfall that could significantly reduce these costs.

Environmental – The final EIS notes that all discharge alternatives would have short-term construction and long-term operating impacts that we could minimize with mitigation measures. The wetland alternative would enhance wetlands and wildlife habitat in areas once disturbed or eliminated by past agricultural practices.

Community factors – The wetlands alternative would enhance an existing wetlands habitat and affect fewer people during construction and operation, compared with the other discharge alternatives.

Policy – The wetlands alternative offers opportunities for building partnerships, water reuse and habitat enhancement that are not associated with the other discharge alternatives. Wetlands enhancement would involve the state Department of Fish and Wildlife as a partner. Several nonprofit organizations have expressed interest in contributing resources to the habitat restoration project. No potential partnerships have been identified for the other discharge alternatives.

For more information

See description of selected Carnation Wastewater Treatment Facility.

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For more information, call 206-296-8361 (TTY 711) or e-mail us.

King County Wastewater Treatment Division
201 S. Jackson St., Suite 505
Seattle, WA 98104-3855

Updated: Jan. 8, 2007