How does a Brita filter work?
April 18, 2004 10:29 PM   Subscribe

Brita filters: How do they work? There is much information about the fact that you need to keep the filter (partially comprised of activated charcoal, from what I can gather) in contact with water at all times. How does activated charcoal actually filter impurities? [More Inside]

I am not a water tester, chemist or molecular biologist. So, I apologise if the terms I am using here are not, in fact, the technical terms. From what I can decipher, if the activated charcoal filter is in contact with water, the 'pores' stay saturated enough so that impurities are trapped within the molecules, and there is little room for oxygen, a key component to microbial growth. If it dries out, there is, as a friend put it, “a nice little matrix of available oxygen and water for bacteria to grow.” Then, as you pour the next bit of water through the filter, the impurities are simply washed into this new pitcher of water. Is this what is going on? I have a battle between me (the Brita filler-upper) and a friend (who says that if the filter is in contact with the water all the time, the impurities are leaking into the pitcher, and thus we should let it dry out between refillings). Help my water taste better! I know it may be water under the bridge, but help me whet my appetite for a compelling conversation here!
posted by fionab to Technology (8 answers total) 1 user marked this as a favorite
Oh yes. I have read this How Stuff Works article on activated charcoal, along with many of the linked entries from that page, but none of them get down to the nitty-gritty of the actual contact with water necessity. Thank you MeFites!
posted by fionab at 10:38 PM on April 18, 2004

Ask your friend where the impurities go when the filter dries out. Then ask him where they go when you pour water back into the filter. By his logic, they get washed into the water when the water runs through, regardless whether the filter stays wet or dry.

If that's truly what he believes, he might as well stop using the Brita and drink straight from the tap.

Now the flip side of the coin is that there are plenty of water-loving bacteria. Indeed, bacteria hate dryness. So unless the filter is bone dry, it's a breeding ground for bacteria... provided the water is the right temperature and that they can get nourishment. Which, given the carbon is attaching all the organics, isn't likely until the end of the filter's life.

So the only important thing is that the filter be kept cold, bacteria-wise.

As for removing more impurities, the longer the filter is in contact with the water, the more it can remove. Stack a half-dozen filters end-to-end, and it'll be mighty clean by the time it dribbles out the far end!

Leaving the jug full enough to keep the filter covered would result in cleaner water: it's left in contact with the filter, which keeps absorbing impurities.
posted by five fresh fish at 1:48 AM on April 19, 2004

As I understand it, likely what I read at How Stuff Works myself, the carbon binds with the impurities. This being the case, they are not going to wash out just because the filter gets dry.
posted by Goofyy at 2:38 AM on April 19, 2004

In that case, why put the filter at the top of the container? Why not just coat the insides of the container in the carbon, or at least put it across the bottom?
posted by bingo at 6:21 AM on April 19, 2004

from the Brita site FAQ:

What is the best way to store my water filter whilst away on holiday?

The best way to preserve your cartridge whilst away is to remove it from the jug and place it in a sealed polythene bag in the fridge. On your return, prepare the cartridge as you would a new one, by soaking for a minimum of 20 minutes, place it back in the funnel, flush through twice before using as normal.

I'd assume that if the filter really needed to be wet they'd suggest just leaving it in the fridge in the carafe, or putting water in the plastic bag with the filter.

I don't think I've ever read anything on the packaging for the Brita filters about having to keep the filter submerged in water, though I have heard this rumor before.

Also, it's my understanding that Brita filters only remove minor impurities and are really only filtering the taste of the water and not really removing major harmful things.
posted by soplerfo at 7:15 AM on April 19, 2004

At one time in my life I was an aquatic chemist and took a few microbiology/toxicology classes. I think people here are confusing the stuff that activated charcol takes out with bad things that will cause you harm in water.

Activated charcol doesn't work as a microbial filter really, the carbon is basically a magnet for organic compounds. These organics are strongly drawn to the carbon, leaving your water and usually resulting in a cleaner taste. You could call them "impurities" because that's what they are, but keep in mind they're just organic compounds and not in any way, shape, or form bad microbial buggers.

You are told to change your brita filters often because bacteria like to camp on carbon and eat the organics trapped there, but it takes a while for them to build up (which is why you're supposed to swap out filters every 2 months or so). If left in long enough, you'll actually be exposing yourself to more harm by having a carbon filter than not at all.

If you want to kill actual bacteria or other nasty buggers from your water you'll need to look at other forms of filtration like ozone (kills everything alive in water), chemical (chlorine, iodine, etc), or UV radiation (also kills all).
posted by mathowie at 10:12 AM on April 19, 2004

We actually use a Brita Jug at home here in Korea (private, corporate water supply, I won't bother with the details other than to note it's better than the water you get in the cities), and I am gratified to read at the linked FAQ that 'The Brita water filter jug uses replaceable cartridges that reduce chlorine and other impurities such as lead, copper and aluminium that affect the taste of your tap water.'

It's mostly heavy metals that I was worried about. I'd note that this feature of the filtration would be in addition to the ones mathowie noted above...but my question in response to the FAQ quote would be 'reduce by how much?'
posted by stavrosthewonderchicken at 9:22 PM on April 19, 2004

In case anyone ends up here at some point looking for more Brita filtration info, here's the blog entry that I wrote on the topic.
posted by fionab at 10:33 PM on April 21, 2004

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