Wastewater Treatment Plant

Domestic Wastewater

Wastewater Treatment: Why It Matters

Clean water keeps people, communities, and our planet healthy, yet many of us take clean water for granted. Do you know where the water goes once it goes down the drain or down the toilet?

If you live in a rural area, your water may go into a residential septic system. But if you live in an urban area, your wastewater likely goes to a wastewater (sewer) treatment plant.

Thanks to wastewater treatment, the dirty wastewater that leaves our homes, offices, schools and buildings is treated before it is released back into the environment. This helps keep everybody healthy and protects the health of our lakes, rivers, and marine waters.

All across the state, people are working to protect and restore Washington’s waters. One of the more constant—and least known and understood—is the wastewater treatment process and the people working behind the scenes to provide you and your family with clean, safe water.

Here is why it matters: Properly treated wastewater is our first line of defense to protect public health and clean water. Untreated sewage spreads a number of deadly diseases, including cholera and typhoid. These diseases are now largely absent in developed nations thanks to modern water and wastewater treatment –but they still plague developing countries that lack modern sanitation

There are more than 300 wastewater treatment facilities in Washington State.

The basic function of the wastewater treatment plant (WTTP) is to speed up the natural processes by which water purifies itself. Before wastewater treatment systems were refined, people relied on the natural treatment process in streams and lakes to perform basic wastewater treatment. In the 19th century, scientists discovered the connection between disease and untreated sewage. Since that time, municipal governments have protected their domestic water supplies and treated sewage discharges, often through disinfection.

Currently, the most common form of pollution control in the United States consists of a system of sewers and wastewater treatment plants. The sewers collect municipal wastewater from homes, businesses, and industries and deliver it to facilities for treatment before it is discharged to water bodies or land, or recycled for other uses.

The Department of Ecology oversees domestic wastewater treatment in some of the following ways:

  • We certify wastewater treatment plant operators to ensure they are qualified to operate the wastewater treatment facilities across Washington State.

  • We issue permits to wastewater treatment plants that allows them to discharge treated wastewater. The permits are called National Pollution Discharge Elimination System (NPDES) permits.

  • We provide technical support to wastewater treatment plants to ensure they are operating at the highest standards and protecting Washington waters.

  • We oversee the state’s reclaimed (water recycling) program.

  • We set water quality standards that set pollution discharge limits for industries. The standards protect Washington waters.

  • We conduct studies and provide ongoing evaluation of wastewater treatment plants for compliance with water quality standards.

Some of the key challenges faced by wastewater treatment  professionals today:

  • Many of the wastewater treatment and collection facilities are now old and worn, and require further improvement, repair or replacement to maintain their useful life.
  • The character and quantity of contaminants presenting problems today are far more complex than those that presented challenges in the past.
  • Population growth is taxing many existing wastewater treatment systems and creating a need for new plants.
  • Farm runoff and increasing urbanization provide additional sources of pollution not controlled by wastewater treatment.
  • Much of our new development is served by decentralized systems (for example septic systems) as population migrates further from metropolitan areas.


State recognizes top treatment plants

Technical and Economic Evaluation of Nutrient Removal Technologies



Find WTTP permits in our database, PARIS


Learn what you can do @ Washington Waters - Ours to Protect


Combined Sewer Overflows



Criteria for Sewage Works Design
(Orange Book)


Learn more about a career as a professional wastewater treatment plant operator by visiting the
operator certification page.