Faced with sprawling growth in the greater Seattle area, King County water officials have capitalized on the population’s environmental awareness to push through a $1.7-billion wastewater treatment system that could become the country’s showcase for advanced secondary treatment. Taking advantage of technological advances that lower energy needs and produce an effluent clean enough to be sold for nonpotable use, the county’s division of wastewater control is building a 36-million-gallon-per-day plant, the largest in the U.S. to use membrane bioreactors, borrowing from systems developed to treat drinking water.
The Brightwater plant, located 11 miles north of Seattle in Bothell, will handle wastewater flows from King County’s northern reaches and part of neighboring Snohomish County. Flows will be delivered to the plant through a 13-mile conveyance tunnel now being bored under three tunneling contracts through soft ground of mostly glacial till. The 16-ft-dia. tunnel will do triple duty. It is being fitted with a force main to handle raw wastewater flows, boosted on its way to the plant with a 170-mgd pump station. Other pipes will deliver reclaimed water that will be sold for irrigation or industrial use, and treated effluent that will be discharged through a new outfall to 400-ft depths in Puget Sound. Sections of the tunnel are also designed to handle excess stormwater flow. By 2040 the plant, designed by CH2M Hill Cos.’s Bellevue, Wash., office, will be expanded to provide average daily treatment capacity of 54 mgd.