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Portland’s City Council must decide by early August whether to direct the Water Bureau to build a $150 million ultraviolet-light reactor or a $350 million to $500 million water filtration plant to fight microscopic parasites.

The city has been ordered by the Oregon Health Authority to treat its water for cryptosporidium after the parasites turned up in a series of tests in January, February and March.

Health authorities revoked Portland’s 10-year variance from federal drinking water rules requiring treatment for cryptosporidium in the aftermath of the tests that detected the parasite.

OHA issued the variance in 2012 and revoked it on May 19. The City Council held a work session Tuesday to discuss the way forward.

鈥淲e cut the deal that we did, and it became obvious that our water system does not deliver water of the quality of a filtration plant,鈥� said Michael Stuhr, director of the city’s Water Bureau.

City commissioners now must make a decision quickly. They must decide on a treatment option by Aug. 11 to adhere to an OHA compliance schedule. To meet that deadline, an Aug. 2 hearing has been set aside for commissioners to pass a resolution directing the Water Bureau on a course of action.

Building a UV treatment facility would be cheaper and faster. It could be built in about five years, Stuhr said. A UV treatment facility would also fit at the city’s headwaters facility at Bull Run Lake.

A filtration plant would cost much more, take 10-12 years to construct and would be built most likely at Lusted Hill, on a parcel along Carpenter Lane in east Gresham, Stuhr said. The city purchased the 90-acre site years ago with a treatment facility in mind.

Either facility could include solar panels to offset their energy use, Stuhr said.

Filtration plants are more effective than UV facilities for a wide variety of water-quality issues, Stuhr said.

鈥淭he water professionals’ answer has been filtration plants, because they are so effective and have been able to handle anything anyone’s cooked up,鈥� he said.

Portland could build a UV facility that would work for 20 to 30 years, while setting aside funds to convert the facility to a treatment plant, Stuhr said.

Cryptosporidium outbreaks cause a diarrheal disease that is particularly dangerous to those with weakened immune systems, such as AIDS patients, according to the Centers for Disease Control. During the worst such recorded outbreak in U.S. history, in Milwaukee, Wisconsin, in 1993, 69 deaths were attributed to the cryptosporidium outbreak, according to a Milwaukee Journal-Sentinel account.

Such an outbreak is a possibility in Portland as long as the water is untreated, Stuhr said.

鈥淚f crypto takes hold, it can wreak havoc with your most vulnerable populations,鈥� he said.

No outbreak was detected in Portland despite the presence of the parasites earlier this year, said Paul Lewis, a Multnomah County health officer.