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How We Got to Filtration

For a long time, Tacoma Water was one of the handful of remaining water utilities in the country not required to filter its water supply.

Nationally, filtration was and continues to be the standard for treating surface water. Because of the high quality of the Green River water, we avoided filtration for many decades. Then in the early 1990s, disaster struck 2,000 miles away.


Cause & Options

About 100 people die and 400,000 get sick after drinking water from Milwaukee's municipal water supply, which contained cryptosporidium. According to the Milwaukee-Wisconsin Journal Sentinel, it was the largest documented waterborne disease outbreak in U.S. history. 

Tacoma Water begins testing the Green River for cryptosporidium. By 1997, we test for it each month.

The Washington State Department of Health adopts an EPA regulation requiring treatment for cryptosporidium, which takes effect in 2014. Tacoma Water begins a two-year period of mandatory cryptosporidium testing.

We focus on two treatment options: filtration and ultraviolet light.

  • Filtering: would improve taste, reduce the amount of sand and silt entering the pipes, reduce the need and cost for disinfection byproducts and provide more protection from contaminants like cryptosporidium.
  • UV: would meet the EPA requirements, but does not offer the additional benefits. 

Water concludes a two-year monitoring period without detecting cryptosporidium and launches a small-scale pilot program for filtration and UV.  


Decisions & Pilot Testing

Extensive analyses of risks, costs and benefits of each treatment option are developed. Public outreach begins with goals of informing people and getting feedback on the two options.

Tacoma Water recommends the use of filtration to the Public Utility Board and the Tacoma City Council; both approve, as do Tacoma's Regional Water Supply Partners.

Decision to use General Contractor/Construction Manager approved. MWH Americas selected for plant design and permitting support. Pilot testing continues; preliminary design starts.  



Preliminary designs finalized. Hoffman Construction Company selected as General Contractor/Construction Manager. Environmental reviews are underway. Third party value engineering effort identifies numerous cost-saving ideas; reduces project cost by $10 million.

Broke ground. Project schedules for early work are accelerated to take advantage of a favorable bidding climate. Full construction is underway. Project is approximately 20% built.

Construction continues; 150 people work at the site each day. Improvements to Green River intake are added to project. Project is 60% constructed.

Intake improvements completed, along with emergency repairs to a tunnel upstream of the filtration plant. On Dec. 16, filtered water flows to Tacoma.

Plant operations are fine-tuned. Fully filter all water starting Jan. 15. Regulatory "switch" from unfiltered to filtered started May 1.