Water filters that you attach to your faucet are known to be good for filtering out heavy metals like lead and disinfectants like chlorine. But they’re not designed to filter out bacteria that can grow in the filter itself.
Nancy Love is an author of a new study in the journal Environmental Science: Water Research & Technology that examines the bacteria that can grow in activated carbon filters. She’s the Borchardt and Glysson Collegiate professor in the department of civil and environmental engineering at the University of Michigan.
She says that her study, which tested faucet filters with Ann Arbor city water, corroborates previous studies that found water filters support bacterial growth.
“The count of the bacteria coming out of the filter increases relative to what was going into the filter. And we see that those counts can increase up to 100 times. It doesn’t mean the bacteria are harmful. There are a lot of, you know, there are bacteria in drinking water and they’re completely harmless. But the counts go up,” she says.
Love and her team found that not only were bacterial levels going up, but also that filters could support different forms of bacteria.
“We used some new, novel methods to show that we changed which bacteria were present in what was coming out of the filter relative to what was going in,” she says.
Love says there is the possibility that opportunistic pathogens - bacteria that are harmless to healthy people but could pose a threat to people with compromised immune systems - could grow in filters.
“We haven’t proven that part yet, but what the study suggests is that we need to take a closer look at these filters and different waters and see whether or not that is occurring or can occur,” says Love.
Love says researchers still aren’t sure how much of a danger bacterial growth in filters poses. She says she and her colleagues are currently studying this issue in Flint, although results aren't in yet.
Love hopes that by studying more water systems, scientists will learn more about any potential risks.
"I would say, given the aging infrastructure in the United States, given changing populations in cities where you may have a shrinking population, which means the water takes longer through the pipes or may sit in the pipes longer, I think those factors need to be taken into consideration when we look at point-of-use filters and water quality," she says. "We can't just say we did this study in one place, and it applies to every place."
But she says, no matter what, it's a good idea to maintain your faucet filter by changing filter cartridges regularly according to the manufacturer's directions.
Beyond that, Love says whether you have a filter on your faucet or not, it's a good idea to flush your water every morning to clear out biofilm, the bacteria that grow on surfaces when water sits in pipes.
“And so, overnight, when you’re sleeping, you have about eight hours of water just sitting in your pipes, that’s when the biofilm grows the most. So when you turn your faucet on first thing in the morning, that first slug of water that comes out will have higher bacteria counts than if you let it flush for a bit of time,” she says.
Love recommends turning your filter onto bypass, and flushing for 15 to 30 seconds before turning the filter on, and then running water through the filter for five to 15 seconds before drinking it.