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Most of King County's wastewater flows to three regional treatment plants (BrightwaterSouth, and West Point) where the wastewater flows through a series of treatment processes that remove waste from the water and protect public health and the environment. In addition, there are two local treatment plants (Carnation, Vashon) that serve smaller communities within the King County service area. 

primary secondary disinfection advanced

Standard wastewater treatment process

Typically, wastewater treatment begins with primary treatment where solids are separated from liquids. This happens in sedimentation tanks.

The next phase is called secondary treatment. It is also called biological treatment because oxygen is added to activate the living microorganisms (such as bacteria) that eat the dissolved organic material still in the wastewater. We use an 'activated sludge' treatment process where the organisms are maintained at a very high level to accelerate the consumption process.

After secondary treatment, King County disinfects the treated wastewater and returms it to the environment.

Advanced treatment

Some of the water treated at the regional treatment plants is reclaimed. Reclaimed water must be treated beyond secondary treatment to meet Washington State Departments of Health and Ecology Class A standards.

This advanced treatment involves an additional level of filtration to remove any remaining solids, and a disinfection process.

In some cases (Brightwater, Carnation), the additional filtration occurs in the same vessels as the biological treatment.