I just noticed that Sawyer says:
"Can I freeze the filter?
While we have no proof that freezing will harm the filter, we do not have enough proof to say it will not harm the filter, therefore we must say that if you suspect the filter has been frozen, to replace it -- this is especially true with a hard freeze."

That has me wondering just how big of a worry freezing really is. It also makes me way less likely to fret over an accidental freeze. Has anyone actually ruined a filter that way? If so how wet was it and how well frozen?

It would really be nice to know if a well shaken out filter is really all that susceptible to damage from freezing. It sounds to me as if it may not be.

Edited by staehpj1 on 01/29/2013 12:13:01 MST.

I would still take the necessary steps to prevent the filter from freezing. I place mine in a freezer grade gallon Zip-lock and place it in my sleeping bag when temps are expected to be at or around freezing. I believe the Sawyer uses hollow fiber elements to filter out the contaminants. If the filter would freeze, you risk the chance of these fibers stretching, thus allowing larger particulates through the "clean" water, or bursting altogether.

My take from considerable reading (here and elsewhere) is that there's no way to tell whether or not the filter has been compromised by freezing. I suppose you could run contaminated water through it and check under a microscope for little nasties, not that you'd have a microscope out there in the field. That's why Sawyer says to replace it if it freezes. I believe this is true of any filter, although the type that Sawyer uses is more vulnerable. (Someone please correct me if I'm wrong here!)

Since the actual filter is very small, it can easily be put in a sandwich bag and stuck in the bottom of your sleeping bag should you suspect a frosty night.

If it's just a light frost outside (say 31*F) the filter won't freeze unless you left it out in the open on the ground outside your shelter. If you have lots of ice in your water bottle in the morning, and the filter is next to the water bottle, undoubtedly the filter has been compromised.

Of course winter travel at 0*F is another story. I would assume that in those circumstances you'll be melting snow for water and leaving the filter at home. Otherwise you'll have to protect the filter from freezing 24/7, which might be difficult.

Yeah, avoiding freezing is probably prudent, but Sawyer's language makes me think that it may be less of an issue that I had assumed. I have never heard anyone say that after their's froze it failed the "blowing air through it" test (which sawyer doesn't endorse). If someone actually has had one fail after freezing I'd like to hear about it. If mine accidentally freezes and I still can't blow air through it I don't think I will worry about it.

Pete, could you elaborate on the "blowing air" test? I used mine in winter and I was pretty careful to keep it warm, but it doesn't take much for water that's sitting just above 32 degrees to freeze in 20 degree temps while being filtered.

Even a single ice crystal forming inside of the membrane of the Squeeze and it should go into the trash.

This is an *absolute* one-micron filter, not a nominal one-micron filter.

Pete - I read the same thing and had the same conclusion

I've taken mine out a couple times when it got down to 30 F and took a bit of precaution and I'm pretty sure it's okay

Yeah, if you blow on it and air flows through, it's obviously no good

I just leave mine in a pint zip top bag. That also provides a little freeze protection. If it's 30 F I just leave it overnight on the ground, with some stuff on top, next to the water bottles. Drips of water never came close to freezing so I'm sure it's okay. Also shake it out vigurously.

When you backwash the filter, water is at high pressure and the plastic tubes stretch a little. I think that would be the same as if they froze. If the whole unit was full of water and frozen there would be much bigger stretching and possibly breaking.

I wish they'de test this more rather than putting that statement out. Shake out. Freeze. Repeat a number of times. Then test it. There must be some way to test this.

If the pores were stretched a little bigger, maybe it wouldn't make any difference. It's not clear what filtering is actually required. If you just want to filter out Giardia, for example, they're bigger so it wouldn't matter.

Here's a pic of a MSR Sweetwater filter I cut in half (notice the crack the length of the cartridge) after realizing something was dreadfully wrong. By that time, I had already savored a good bit of water filtered from where a feces ridden cow trail entered the creek I was paddling. 2 pints of bovine waste sloshing around in my belly and 60 more river miles (3 days) to go. What could go wrong?

Actually, I had intentions of cutting it in half once I got home from that trip but procrastinated for 3 yrs! At any rate, I remember it freezing hard the night before I realized something was wrong with it...hard enough that water in my water bottle froze.

I can not with any certainty say freezing cracked this cartridge. I can say that I'm careful with my gear and this filter was never dropped. I also never took measures to prevent it from freezing.

Based on the water bottles I have inadvertently cracked in the freezer (so I could have ice for car camping) and seeing how frozen water will bulge out a styrofoam cup, if not break it (have a broken one in the freezer now used for ice massages), in my mind, it goes to reason that allowing a filter to freeze is not a good idea. I now sleep with my filters (inside a ziplock) if there's a chance of freezing temps.

EDITed for clarification.

Edited by rustyb on 01/29/2013 14:12:28 MST.

I do what I can to prevent my filter from freezing, but

You get freezing damage when a bigger dimension of water freezes and expands a larger distance. Like a water bottle. Or the crack in your filter.

If you have a tiny tube of flexible plastic that freezes, it's less likely to break.

Another thing is if a tube is enclosed, like a water pipe, and it freezes down the pipe, as it gets to the end it will put more and more pressure. If you have a water pipe that's open, it will freeze towards the opening and there will be no huge pressure.

Too long? Don't want to read? If you freeze your filter, you will likely be able to filter Giardia and Cryptosporidium, but not bacteria.


Here's what we're looking at here;

The fibers in the Sawyer have pores in them. They become saturated with water, and the water can permeate through the fibers out the other end of the filter but contaminants can't. What this means is, after the first use there will almost always be water in your filter. There is humidity in the air, so the only true way to get the water out of your filter would be a kiln (obviously, don't do this).

So you have a water droplet in a porous membrane and that water freezes. If the water is in the pore, it will expand and rupture the fiber. At 0.1 microns, we're not talking about a lot of durability in the material. It will rupture. However, if the water droplet is not near a pore, it will freeze unrestricted and cause no damage. It's impossible to predict.

So, you might damage some of your pores and you might not damage others. You could also have more damage in one area of the filter than another.


What size are the contaminants? Let's look at the following chart:

As you can see, the 0.1 absolute biologic filtration system claimed by Sawyer is a complicated thing. I can't say I understand it completely. Sawyer claims 7 log (99.99999%) of all bacteria and 6 log (99.9999%) of protozoa are removed, but most harmful bacteria are smaller than protozoa like Giardia. Perhaps the smallest bacteria are significantly less common than the larger ones, I don't know. According to our chart, taken from the Grant County Emergency Response website, our 0.1 micron filter is enough to claim to filter all bacteria, or virtually all- 7 log.


So, if you rupture a pore in your filter, chances are you will still be able to filter protozoa, since they are the largest thing in the water at 5 microns. Your pores could expand or rip to 50x their normal size and still hit things that are 5 microns large, and the expansion of water while freezing is not 5000%. You will lose the ability to filter most bacteria, but since bacteria are only really prevalent in water that has been contaminated by waste, you can likely survive with no ill effects for an extended period of time without worrying about the bacteria in the water as long as you get your water from good sources, like springs. The protists are the common threat, and the reason filters considerably less dense than 0.1 microns are commonly used in the outdoors. The EPA only requires a water filter to go to 3 log (99.9%) to be considered a filter for protozoa.

Bottom line: it's probably ok to keep using your sawyer filter after it freezes. The chances of you getting Giardia or Cryptosporidium through the filter, even after the fibers have ruptured, is impossible to measure but very very unlikely- The Sawyer would probably still be better than a Brita filter (which can't even filter most microorganisms) and it's going to be way safer than drinking straight stream water.

Your immune system first filters everything through the liver and spleen, and then attacks any foreign particles with white blood cells, proteins, antibodies, and even good bacteria. Your body can likely handle the .00001% that the Sawyer misses, and whatever it misses after having been frozen.


So, if I were you, I would keep using your sawyer after it freezes, but I would try hard not to let it freeze.

Edited by mdilthey on 01/29/2013 14:36:32 MST.

Oh crap. I can blow air through mine both directions easily. But it's been dry for almost a year, maybe this would account for that?

The air test seems bogus to me. Single molecules of CO2, O2, Nitrogen, Argon, etc. are all much smaller than 0.1 microns.

I don't think it matters if it dries, shouldn't be able to blow air through it, and probably holes are big enough for protozoa to go through too

Sorry Jerry, that doesn't hold up. A molecule of CO2, which you breathe out, is about 200 picometers across. There are 1,000,000 picometers in a micron. That means that 500 molecules of CO2 lined up end to end could still pass through the smallest pore in a Sawyer filter.

Edited by mdilthey on 01/29/2013 14:40:30 MST.

Ok everyone, go blow your filter!

When I try to blow through my Squeeze, no air goes through that I can detect. It's not the size of a molecule, it's a big enough opening to let through enough air to be noticed. Like I just punched a hole maybe 0.5 mm diameter and it is difficult to notice any airflow.

My full breath sails right through the filter easily.

0.5 mm is 500 x 500 diameters of a CO2 molecule : )

try it - put a hole with pin through a stiff card, like a credit card (but not the magnetic strip) and try to blow through it

I don't know where the blowing myth started. It's like everyone believes air particles are larger than water particles. They're both molecules made out of a small handful of atoms... It makes perfect logical sense that both would pass through, and the math is obvious as well.

A single bonded pair of Oxygen atoms (O2) is hundreds of times smaller than a virus, and a virus can pass through a Sawyer. It's thousands of times smaller than the 0.1 micron size of the sawyer. Use your heads!


If the Sawyer had one hole, blowing through the filter would be hard. it probably has hundreds of thousands of holes.

If your blowing thing worked as a test of the filter, WATER WOULD NOT GO THROUGH.


I could blow through my filter from Day 1, before I ever put water in it and before I took it outside.

Edited by mdilthey on 01/29/2013 14:50:34 MST.

Someone posted on one of the enuberable threads about filters, parasites, and Squeezes, that their filter manufacturer (not Sawyer) recommended the "blow through it" test to determine if it was freeze damaged.

Try blowing through a plastic card with a pin hole that's maybe 0.5 mm diameter.

Edited by retiredjerry on 01/29/2013 14:58:27 MST.