June 6, 2017
2 p.m. EST
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How does a small, rural city deal with the problem of inadequate septic systems that threaten public health, pose environmental risks and have resulted in a building moratorium? The city of Carnation, Wash., protected these interests by partnering with the regional wastewater treatment utility, King County Wastewater Treatment Div., to build a central treatment plant to collect and treat the city’s wastewater.
Carnation constructed and now operates a vacuum collection system, and King County built and now operates a 0.4-million-gal-per-day (mgd) treatment plant that produces Class A reclaimed water. The reclaimed water from this facility provides additional water that supports enhancement of habitat at a nearby 58-acre park. The project has been operating since 2008 and has supported the city’s growth and development goals while protecting public health and preserving the environment. Highlights of the project are its advanced treatment technologies, innovative wetland discharge and “green” features.
An aerial photo of the Carnation treatment facility.
Carnation is nestled in the pristine Snoqualmie Valley next to the Snoqualmie and Tolt rivers, which provide the perfect spawning grounds for Chinook and Coho salmon. This setting necessitated a wastewater treatment facility design capable of meeting stringent water quality requirements.
The treatment process selected includes fine rotary drum screens, zone activated sludge basins, membrane bioreactor (MBR) technology and ultraviolet disinfection modules. The membranes in the MBR process provide a physical barrier to remove solids and bacteria, which is capable of filtering water to a final turbidity of less than 0.2 NTU. At full capacity, the treatment facility will produce 0.4 mgd of Class A reclaimed water, which is approved for all uses except drinking under Washington state reclamation and reuse standards.
The reclaimed water produced by Carnation’s treatment plant is piped about a mile north to Chinook Bend Natural Area ecological preserve. King County worked with Ducks Unlimited to secure grants and make the wetland enhancement discharge an environmentally and economically viable discharge opportunity.
The wetland design focused on enhancing native wetland plants, controlling reed canary grass using a water control structure to manage wetland soil moisture and increasing the physical landscape complexity of the wetland. The wetland size has been increased by approximately 4 acres. Wetland monitoring is ongoing to measure success in reducing invasive reed canary grass and re-establishing native wetland plant species.
The wetland at Chinook Bend Natural Area.
Early monitoring results indicate that trace amounts of native wetland emergent plant species have become established; they also have calculated a more than 90% survival rate for native wetland plantings. To date, no suppression of reed canary grass in response to moist soil management has been observed. Monitoring will continue for another eight years.
The wastewater treatment plant administration building incorporated rural community aesthetics and sustainable elements from the U.S. Green Building Council Leadership in Energy and Environmental Design rating system. The project used regionally available materials and recycled content materials; recycled construction waste whenever possible; selected native drought-tolerant plants for landscaping; incorporated energy-efficient equipment; designed for maximization of daylight to light indoor spaces; and conserved water by installing high-efficiency fixtures.
For more information on the project, visit www.kingcounty.gov/environment/wtd/about/system/carnation.aspx.