Water Treatment

The Green River water supply gets treated with four chemicals: chlorine, fluoride, caustic soda and ozone. Those treatments improve your water’s taste and smell.


Chlorine is our primary disinfection treatment. While it does an excellent job of killing the microorganisms that may be harmful to you, chlorine also reacts with natural organic material commonly found in surface water sources like lakes, rivers and streams. This reaction forms compounds called “disinfection byproducts.” We must meet drinking water standards for two groups of disinfection byproducts compounds. Byproduct levels found in water depend primarily on:

•The amount of natural organic material in the water
•The amount of chlorine used to treat water
•The amount of time it takes water to reach the customer

Byproduct levels vary throughout the year. Byproducts often increase during the warmest months when the water supply has its highest levels of natural organic material and chemical reactions happen faster. We work to minimize byproduct levels and have adjusted portions of our system operations.


Some water supplies contain naturally occurring fluoride. Tacoma Water’s sources do not contain a significant amount of natural fluoride. Therefore, after a vote of the people in 1988, we began to add fluoride to the Green River water supply in 1994 to help improve dental health. The amount of fluoride we add is based upon guidance from the Department of Health and Human Services (HHS) and Washington State Department of Health regulations.

In 2011, Tacoma Water reduced its dose of fluoride in the water supply from 1.0 mg/L (milligrams per liter) to 0.7 mg/L. The change was based on updated recommendations from HHS; it is well within the range determined by the U.S. Environmental Protection Agency to be safe.

The federal recommendation is based on the latest science and reflects a more conservative approach at balancing the benefits fluoride provides in protecting teeth against the risk of discoloring them.

Customers who prefer unfluoridated water can obtain unfluoridated well water daily between 8 a.m. and 6 p.m. at the well behind the former Southeast Mutual Water Company building at 1614 99th St. E. (You will need to use your own containers.) UPDATE – September 2020: This well is now back in service.

Ozone & Caustic Soda

We use ozone to address occasional objectionable taste and odor issues in treated Green River. Ozone also provides additional disinfection benefits, further ensuring water is safe to drink. The chemical lasts only a few minutes after being injected into the water. Ozone is not present in the water supply when it exits the treatment site.

We treat our Green River water supply with caustic soda to help minimize the amount of lead and copper found in homes. Adding caustic soda to water raises its pH (a measurement of acidity), which helps make water less corrosive on plumbing. That reduces the amount of lead and copper that can dissolve into drinking water.