PORTLAND, Ore. (Portland Tribune) — Portland Water Bureau officials will ask the City Council to consider spending between $105 million and $500 million to treat a potentially deadly parasite in the Bull Run watershed on Tuesday.

“We will explain how we got here and go over the options,” says Gabriel Solmer, the bureau’s deputy director.

The work session was scheduled after the Oregon Health Authority announced it will revoke the city’s variance from federal rules requiring treatment for cryptosporidium on Sept. 22. Portland owns and operates the only large municipal water system in the country that does not currently treat for the parasite.

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Bureau officials will testify that the city could build a plant that treats crypto — as it is commonly called — with ultraviolet light for $105 million. Such a plant would not treat or filter any other contaminant out of water in the Bull Run reservoir, the city’s primary source of water that is also sold to suburban customers.

A filtration plant could be built for between $300 million and $500 million. In addition to removing crypto from the water, such a plant could also filter out contaminants that might be prohibited by the federal government in the future. Although none are currently under discussion, regulations have changed many times in previous years.

The vast majority of U.S. cities have built filtration plants to comply with the requirement.

Such a plant could also filter sediments out of the reservoir, such as dirt and mud that enters the reservoir because of landslides triggered by an earthquake. It could also filter ash deposited by a catastrophic wildfire in the watershed. Although fires in the watershed are usually small and infrequent, a catastrophic one is thought to occur once every 350 years. It is not known when the last one occurred.

The City Council fought complying with the rule adopted by the U.S. Environmental Protection Agency for many years, because Bull Run water has historically been so clean. It is currently only treated with chlorine and ammonia. The EPA granted a variance in March 2012, provided the city regularly test for crypto and report its findings. None was found after that — until earlier this year, after heavy winter storms may have washed it into the reservoir.

Once the city realized it could no long comply with the variance, it notified the health authority, which enforces the variance for the EPA. The health authority announced the revocation on May 11. It has given the city until Aug. 11 to submit a plan with measurable mileposts for complying with the rule by treating Bull Run water for crypto.

Planning for a UV plant is much farther along that planning for a filtration plant. When the council decided to pursue the variance, it also directed the bureau to draw up plans for a UV plant. They are approximately 30 percent complete, so construction could be completed in five years. Planning for a filtration plant would have to start at scratch and construction could take 10 to 12 years, the Water Bureau says.

There are no estimates yet on the impact of either project on potential water rate increases.

The City Council is not expected to make a decision on Tuesday. Water Bureau officials hope it will give them enough direction to draft a response to the Oregon Health Authority. If not, the officials will draft a resolution summarizing the discussion for the council to consider in the near future.

Crypto is transmitted through animal feces. It can cause cryptosporidiosis, a respiratory and gastrointestinal illness, which killed 104 people and sickened thousands of others in 1993 in Milwaukee, Wisconsin. That outbreak prompted the EPA to adopt its treatment rule.