Treating Portland’s water supply for a microscopic pathogen, cryptosporidium, will increase the average rate customers pay for tap water between $2.22 and $10.38 in coming years, according to the Portland Water Bureau.

The Water Bureau is considering two options: an ultraviolet treatment facility or a filtration plant.

Bureau staff presented estimates to the Portland Utility Board Tuesday and looked at the effect on rates between 2019 and 2034.

Water rates are forecast to go up gradually at first — adding between 42 cents and 82 cents to monthly bills in 2019 — but cost increases would steepen in future years.  The increases are projected to peak at between $3.01 and $18.41, and then gradually fall.

The Water Bureau cautioned that the rate increase projections are an estimate and are likely to change.

The need to hike the rates comes after 14 water samples taken from the city’s drinking water intake in the Bull Run watershed tested positive for low concentrations of cyptosporidum this winter.

The Oregon Health Authority ordered the city to adopt new technology to kill the parasite, meaning the city will need hundreds of millions of dollars for a treatment facility.

The Water Bureau currently treats its drinking water with a combination of chlorine and ammonia to kill other pathogens, but cryptosporidium can survive that process.

Some species of cryptosporidium, a single-celled protozoan parasite, cause severe diarrhea in people. It can also cause serious illness in children or people with compromised immune systems. Other species of cryptosporidium are not known to be infectious in humans.

Public health officials said the parasites in Portland’s water likely came from wildlife droppings, and no illnesses were directly linked to the cryptosporidium.

Regardless, the detection of the parasite has triggered federal regulations that require the city to treat for it.

The cost to the city’s ratepayers will depend on what technology the Portland City Council selects to kill the parasite.

Cryptosporidium parvum

Cryptosporidium parvum

Michael Wunderli/Flickr

The Water Bureau is considering two options: an ultraviolet treatment facility or a filtration plant.

It estimates that an ultraviolet light treatment plant will cost roughly $105 million. It would increase monthly bills by an average of $2.22 a month between 2019 and 2034.

A filtration system will cost between $350 million and $500 million. The Water Bureau used the $500 million estimate to create its rate projections, and calculated the filtration plant would increase rates for the average customer by $10.38 a month over the same time period.

The Portland City Council is scheduled to vote on a treatment option at a public hearing Aug. 2.

Aging infrastructure, new federal water treatment requirements and reductions in water use have driven up water service rates in most American cities over the past decade.

But Portland in particular has faced steep costs to comply with the Environmental Protection Agency’s stricter regulation of crytposporidium in drinking water systems.

That’s because a 2006 regulation targeted unfiltered drinking water systems and water systems with open reservoirs — and Portland had both.