New wastewater treatment filters will help provide drought-proof source of reclaimed water

King County has selected a vendor for an innovative process that treats wastewater to such a high level it can be used safely as a drought-proof water source for irrigation. Using membrane technology to filter wastewater, the process will be installed at the future Brightwater and Carnation wastewater treatment plants.

King County's Wastewater Treatment Division will buy the membranes from the Zenon Environmental Corp. of Ontario, Canada. Wastewater treatment plants in California, Colorado, Michigan, Georgia, Florida, Ontario and other locations around the world use Zenon membranes. Jurisdictions in other states and countries also use Zenon membranes to produce drinking water.

"We selected Zenon's membrane bioreactor system for our new plant because of its performance in providing consistently higher water quality," said King County Executive Ron Sims. "As we are planning to discharge the treated wastewater into Puget Sound, protecting our marine life as well as our communities was a primary concern. We are confident that Zenon's technology will help us to preserve our surrounding environment."

The Brightwater and Carnation systems will convert a standard biological wastewater treatment system into a membrane bioreactor (MBR) system by immersing the membranes in wastewater. The MBR system will suck wastewater through hollow fibers with microscopic pores small enough to filter out particulate matter and even individual bacteria.

"Besides protecting public health and the environment, the MBR treatment process will produce reclaimed water for beneficial uses," said Christie True, manager of the wastewater capital improvement program. "And the MBR technology will need less space, simplify odor control and produce better water quality than traditional wastewater treatment."

When completed in 2010, the Brightwater plant will initially treat 36 million gallons of wastewater a day to serve the growing population in south Snohomish County and north King County. The Carnation plant, requested by the City of Carnation, will initially treat about 400,000 gallons a day.

The MBR system will produce Class A reclaimed water, which meets strict standards of the state departments of Ecology and Health for use in nondrinking purposes. Those purposes include landscape and agricultural irrigation, heating and cooling, and industrial processing as well as safe discharges into freshwater.

"The wastewater will be seven to 10 times cleaner than typical secondary treated wastewater," True said. "Secondary treatment already meets tough environmental requirements for discharges into Puget Sound. By reducing the discharge of pollutants even more with the MBR process, we'll further improve water quality and protect the marine environment."

For example, 36 million gallons of the treated wastewater discharged from the Brightwater MBR system will contain only the same amount of microscopic impurities as 5 million gallons of treated wastewater from a typical secondary treatment plant. Brightwater is being designed to hold up to 220 of the large membrane units, called cassettes. Zenon says Brightwater will have the largest MBR system in the world.

Five cassettes at the Carnation plant will produce highly treated water that's discharged into either the Snoqualmie River or a wetlands area on the river. If funding for the wetlands discharge becomes available, the reclaimed water would be used to enhance wetland habitat.

The MBR system will replace the large round settling tanks used in conventional plants. Because the MBR system makes more efficient use of space, the plants will be easier to screen from public view. When covered, the MBR process also can capture and eliminate odors.

The cost for the Brightwater membranes, excluding tanks and support equipment, will be about $23.7 million. The price for the Carnation system will be $898,000.

King County's Wastewater Treatment Division protects public health and water quality by serving 17 cities, 17 sewer agencies and more than 1.4 million residents in King, Snohomish and Pierce counties. Formerly known as Metro, the regional utility has been preventing water pollution for 40 years.