If making beer is both science and art, Kurt and Rob Widmer are equal parts chemists and creators.

Lately, however, the Portland brewery brothers have been political campaigners, too.

Months after they worked at the Legislature to kill an increase in the state beer tax, the Widmers are weighing in on the quality of Portland’s water.

This week, Portland City Council will consider a resolution that, if approved, would direct the Water Bureau to proceed with plans for a new water filtration plant in the Bull Run watershed, where Portland gets almost all of its drinking water.

The $385 million proposal aims to keep Portland in compliance with rules from the U.S. Environmental Protection Agency designed to protect drinking water from cryptosporidium, an illness-causing microorganism that has not been found recently in Portland’s water supply. In terms of scale and expense, the proposal dwarfs Portland’s recent foray into stadium building.

But far fewer people are aware of filtration’s consequences, which include higher water rates estimated to jump 18 percent a year for five years.

The Widmer brothers certainly are aware of it.

Last week, the Widmers caught the attention of David Shaff, the city’s Water Bureau director, after they wrote Commissioner Dan Saltzman and members of Oregon’s congressional delegation to oppose the plan for a water filtration plant.

They called the federal rules “completely unnecessary,” and urged elected leaders to push the EPA for a variance.

“The same pristine water supply which you are presently discussing filtering, chemically treating and ultimately altering in ways we don’t completely understand is the very foundation and soul of the great beer that has placed Portland on the map as one of the outstanding brewing cities in the world,” the Widmers wrote July 20.

After many months of study, Shaff says he was “surprised” by the Widmers’ concern. He says a new filtration plant could actually improve the Widmers’ beer-making process, “because the water will be a more consistent quality.”

One of the Widmer brothers’ chief concerns is money. Their 25-year-old company, which employs 160 Portlanders and has Anheuser-Busch as a minority investor, will use 40 million gallons of Bull Run water this year, at a cost of about $100,000.

That’s a drop in the keg of their $86 million-a-year company, even amid a recession that has kept business “flat,” in the words of Kurt Widmer.

But changing the water could cost the company far more in indirect expenses, if those changes alter their beer’s taste and require costlier solutions, which is the brothers’ biggest worry. And so far no one has been able to guarantee the new water treatment for removing potential microorganisms won’t irreparably harm their product, which is 95 percent water. Fixing it could cost “millions,” the Widmers say.

The Water Bureau says nearly all large U.S. cities now filter their water in the way Portland proposes. San Francisco, New York, Boston, Seattle and Tacoma do not, but are also taking action to remain in compliance.

The Widmers weren’t willing to point to another beer that has been harmed by filtered water.

The problem, they say, is that they won’t know how their product might change until it’s too late to reverse it. And at that point, the brewery will be forced to address the problems on the fly. In 24 hours, the company might brew up to 70,000 gallons of beer. “We can’t stop and say, ‘Time out, we need to figure this out,’” Rob Widmer says.

An alternative exists that the Widmers support as a last resort. Instead of filtering the water, the city could use UV technology to kill cryptosporidium, as San Francisco, New York, Boston and Seattle plan to do under EPA guidelines.

That option would cost Portland less than $100 million, but the Water Bureau says filtration is superior because it will allow Portland to get more water from Bull Run. Commissioner Amanda Fritz, however, supports UV.

“It’s not just less expensive, it’s actually a better treatment option,” Fritz says.

The Widmers need two other commissioners to side with Fritz if they are to prevail in this years-long battle. Saltzman has interest in the UV process but supported filtration as the Water Bureau commissioner in 2002. Mayor Sam Adams and Commissioner Nick Fish have not said how they will vote.

Commissioner Randy Leonard, who oversees the Water Bureau, says a filtration plant is the best option.

A few of Portland’s other high-volume water users are coming to the Widmers’ defense, even if the change won’t affect their beverages’ taste.

“The Widmer brothers have it right,” says Steve McCarthy, founder of Clear Creek Distillery. “Water is a very important component of all these products, and to the extent that you have water that has some character to it, you have a better product.”

News intern Aaron Mendelson contributed to this report.