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Proposed Alternatives for the Carnation Wastewater Treatment Facility.
Also available as PDF
(529 KB).
City-owned and Weckwerth Treatment Plant Sites.
Also available as PDF
(522 KB).
Conceptual River Discharge.
Also available as PDF
(111 KB).
Conceptual Wetland Discharge.
Also available as PDF
(2.0 MB).
Conceptual Upland Discharge.
Also available as PDF
(127 KB).
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Treatment plant alternatives
Two site alternatives were selected for the treatment plant:
- City-owned site: a nine-acre parcel west of the business district and state Highway 203 on city-owned property at the end of Entwistle Street (formerly known as the Schefer property).
- Weckwerth site: a five-acre parcel owned by the concrete forming company immediately east of the Carnation Fire Station and Highway 203.
Pipeline alternatives
Six alternative pipeline routes were identified, one route for each possible combination of treatment plant site and discharge location. The pipelines range from 1.5 to 3 miles long and would be 8 to 10 inches in diameter. Most pipeline routes would be in the public right-of-way. The pipelines would be buried 3 to 8 feet underground and built by trenching.
Discharge facilities alternatives
King County and the City of Carnation were evaluating three discharge alternatives.
1. River Discharge Alternative: Discharge to the Snoqualmie River may be the most cost-effective option. However, it would have to meet or exceed stringent water quality standards and Endangered Species Act requirements. Water quality modeling will be used to determine necessary levels of treatment to meet critical water quality standards in the Snoqualmie River.
Highly treated water from the plant would be discharged to the Snoqualmie River at the Carnation Farm Road Bridge about a mile north of Carnation. An 8 to 10-inch pipeline would lie on the river bottom and extend about 15 feet into the river.
2. Wetland Discharge Alternative: highly treated water from the treatment plant would be used to create and enhance wetlands in the Stillwater Wildlife Area, about two miles north of Carnation. This alternative would enhance habitat for native plants, fish and wildlife.
Two options were considered within this alternative. Under the Basic Option, several wetlands would be created or enhanced. Wetland creation or enhancement could consist of introducing highly treated water to existing low areas in former agricultural fields or existing wetlands, with native plantings. The Expanded Option would add large woody debris clusters on one or more streams to keep water in the existing wetlands for a longer period of time and create pools for fish and wildlife habitat.
3. Upland Discharge Alternative: highly treated water from the treatment plant would be discharged to constructed basins, where it would filter through the ground. The upland discharge site would occupy up to 10 acres within the 240-acre upland discharge study area southeast of Carnation. Up to eight half-acre infiltration basins would be built and used sequentially.
Water reuse
In the summer time, some of the treated water may be able to be used for water reuse. Reclaimed water is wastewater that has been treated to such a high level that it can be used safely for non-drinking water purposes like irrigation. Potential irrigation uses in the area, such as a park or agricultural land, will be evaluated.
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