By Dina Mishev

January 17, 2002 After pumping hundreds of gallons of water through backcountry filters, Ryan Jordan knows filter slime. And the Senior Research Engineer at Montana State University's Center for Biofilm Engineering knows what kind of water filters grow what kinds of slime.

Slime, also known as biofilm, is comprised of bacteria communities that can plug filters and possibly sicken users. Jordan's research shows that filters containing anti-microbial agents such as silver are less likely than other filters to grow slime.

Jordan and two former MSU students, Lee Richards and Marc Santora, spent three years testing different types of filters under various conditions in Yellowstone and Grand Teton National Parks, and in the Teton, Spanish Peaks, and Beartooth Wilderness areas.

They determined that carbon filters alone produced "horrible" biofilm growth. Ceramic and glass filters grew "significant" amounts of slime. Carbon filters where an anti-microbial agent was bonded to the carbon had some bacteria growth, but no significant slime growth.

The study looked only at biofilm growth and not at a filter's efficacy in ridding water of bacteria, protozoa, and viruses.

"The primary concern with portable water systems is that they make water microbiologically safe to drink," said Jeff Haug, the Marketing Director at Katadyn North America, a company that sells filters with (Katadyn) and without (Exstream) anti-microbial agents in them. "Even though a product may utilize an anti-microbial agent, it does not mean that it's necessarily more safe than another. We consider anti-microbial agents an extra feature. When it comes to the level of microorganism reduction, it doesn't help. "

But before discounting Jordan's findings as inconsequential, consider that biofilm undeniably can clog filters and cause performance to be decreased. "Biofilms and muddy waters can and should clog a filter. If these things don't clog it, you have to question what the filter's really doing," said Todd Benevedes, a Product Line Manager at Mountain Safety Research, which makes ceramic filters. "If a filter is clogged, it's obviously been working ... although you certainly don't want to keep using it that way. Clean it."

Biofilm growing on ceramic filters can be cleaned off in the field, but the user has to take the time to do this. In an Internet survey of 200 backpackers conducted from fall 2000 through spring 2001, Jordan found that almost none of the respondents cleaned their filters regularly. That's an unhealthy practice, Jordan said.

Anti-microbial agents are usually silver-based and are regulated as pesticides by the Environmental Protection Agency. They do not do anything in the actual filtration process, but, as Jordan's findings show, do kill bacteria growing in the filter.

"You could leave our filter bottle in the sun in the back of a car for five years and the water in it would still be fine to drink. The anti-microbial agents we put in our polymer-carbon filter and in the plastic of the bottle itself would kill the slime," said John Ferguson, Founder and CEO of Safewater Anywhere.

Jordan did find bacteria in the carbon filter with the anti-microbial agent he tested, but it was dead.

"We don't know the potentially harmful health effects that go with having pathogenic microorganisms present in slimes on the filter," said Jordan. "Are filters merely slime factories that increase the potential for infection when they fail catastrophically?" For example, if a filter cracks in freezing temperatures, the user is not only allowing the microbes in unfiltered water to reach his treated water, but also a tremendous influx of microbes from the filter slimes to pass as well.

"I'm a fan of Aqua Mira, personally," Jordan said.