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How we clean wastewater 

When you flush the toilet or wash your hands in the sink, the water—referred to as “wastewater”—that travels down your drain flows into a system of pipes stretching across the region. The pipes carry the wastewater to a treatment plant to get cleaned before it can be released into regional water bodies.

The process we use to clean wastewater at our 3 regional wastewater treatment plants (Brightwater, South, and West Point) and our 2 local treatment plants (Carnation and Vashon) involves these basic steps:

  • Primary treatment

    Solids are separated from liquids in sedimentation tanks during this first step of the wastewater treatment process.

  • Secondary or biological treatment

    Next, oxygen is added to activate the living microorganisms (such as bacteria) that eat the dissolved organic material still in the wastewater. We use what’s called an “activated sludge” treatment process, where the organisms are maintained at a very high level to accelerate the consumption process.

  • Disinfection

    The treated wastewater is then disinfected using either hypochlorite bleach or ultraviolet light to remove pathogenic microorganisms before being returned to the environment.

  • Advanced treatment

    Some of the wastewater treated at our regional treatment plants is reclaimed water, which must undergo an additional level of filtration and disinfection to meet state environmental and health Class A standards.

Get involved in clean water 

Visit our education site to access special program resources and upcoming events, sign up for a plant tour, and learn how you can help protect our water.

The graphic below provides an overview of the entire wastewater treatment process, from start (sources of wastewater) to finish (recycled & reused products).

Diagram showing wastewater sources (from homes, businesses and industries), local sewer pipes (maintained by local sewer agencies), regional sewer pipes and pumps (maintained by King County), regional wastewater treatment plants, recycled and reused products (energy recovery, bisolids recycling, recycled water) and the clean water that flows back to Puget Sound.