What is wastewater?

Wastewater is the water that leaves industries, businesses, farms, and homes. This includes water from sources like sinks, showers, toilets, pulp mills, and manufacturing companies. Different contaminants and pollutants enter wastewater depending on how and where water is used.  Wastewater must be treated to remove these pollutants before it can be released back into the water environment.

What is our role?

We ensure wastewater is clean enough to be released into ground and surface waters through regulating discharges from municipal and industrial treatment plants. We do this through issuing wastewater discharge permits.

The permits require facilities and municipalities to:

  • Treat their wastewater.

  • Control the discharge quality and quantity.

  • Have a certified operator in charge of municipal plants.

Wastewater discharge permits are required if the facility or municipality disposes of wastewater into surface or groundwater called waters of the state. This includes rivers, lakes, streams, bays, and aquifers.

Some industrial facilities do not discharge to a water of the state. Rather, the industry discharges wastewater to a municipal sanitary sewer that takes sewage to a treatment plant for pollutant removal prior to disposal. This still requires a discharge permit.

What is wastewater treatment?

All wastewater must receive treatment to protect human health and aquatic life. Wastewater is treated at regulated facilities called Wastewater Treatment Plants (WWTP). There are more than 600 WWTPs in Washington. Each plant’s discharge must meet our water quality standards. All of the WWTP operators in charge of running the municipal plants must also be state certified.

Plant size and treatment methods vary depending on the climate, wastewater source, population served and/or industry size. Treatment types include physical, chemical, and biological processes:

  • Physical treatment removes solids from the raw (untreated) wastewater with processes like screening or settling.

  • Chemical treatment involves the addition of chemicals that change the water chemistry to help increase settling rates or remove specific pollutants.

  • Biological treatment uses special microorganisms to consume organics.

Treatment plants use different combinations of these three treatment process categories. It all depends on the wastewater source and level of treatment required by the discharge permit.

Outstanding WWTP awards

There are more than 300 municipal plants, and some have met every single standard and deadline for the whole year. We recognize these plants through an award program. Some of the plants have received an award since we started handing them out — 23 years! Operators proudly display the awarded plaques on their walls demonstrating their excellence and continued commitment to keeping Washington waters clean.