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How to Filter or Purify Water in Freezing Temperatures

How to Filter or Purify Water in Freezing Temperatures

Water filtration and water purification are as important in freezing weather as they are the rest of the year. Giardia, bacteria, and cysts don’t go to sleep when temperatures dip near or below freezing, nor do beavers, mice, deer, rabbits, and all the other animals of the forest that can contaminate the water supply with organisms that cause water-borne illnesses in humans. The only thing that does change is the effectiveness and convenience of different water treatments and purification methods.

What are the pros and cons of the water filtering or water purification techniques that backpackers normally use in warm weather, when temperatures get frosty and dip below freezing?

  • Water filters that use hollow tub filtration technology like the Sawyer Squeeze, the Katadyn BeFree, Platypus Gravity Works, and others break when they thaw after being frozen, even if only partially frozen. Once this happens, there’s no way to know or test whether they’re still effective or whether they’ve been compromised.
  • A pump filter like the MSR Guardian ($350) can withstand a limited amount of freezing/thawing and is a good option if you afford it. But other pump filters like the Katadyn Hiker Pro or the MSR Miniworks are ruined if they freeze.
  • Ultraviolet purifiers like the Steripen can fail if their batteries freeze or lose power in cold temperatures. While lithium-ion batteries won’t freeze like alkaline batteries, their discharge rates can drop too low for cold-weather operation.
  • Liquid chemical purification drops like Aquamira or liquid bleach can freeze and become useless.
  • The reaction time of chemical purification tablets like Katadyn Micropur, AquaTabs, or Potable Aqua slows down in cold water, although they are not prone to freeze-thaw issues like their liquid counterparts.
Water filters can clog or break if they freeze making them a poor water treatment or purification method in cold weather.
Many gravity filters can clog or break if they freeze making them a poor water treatment and purification method in cold weather.

So what is the best water treatment solution when backpacking in the early spring or late autumn, the so-called “sweater seasons” when temperatures straddle freezing or dip below it? As a rule of thumb, we’d recommend bringing at least two techniques with you, in case one fails to work.


Boiling certainly works, but it does require that you carry more cooking fuel. The CDC recommends bringing your water to a roiling boil for 1 minute (3 minutes above 6500 ft of elevation)in order to make it safe for drinking. That will use up a lot of fuel if you have to carry it and isn’t a great option if you need to process a large quantify of water.

If you choose this option, you need to make sure you carry the right stove and fuel for cold weather use. Upright canister stoves work pretty reliably down to 20 degrees (f), but can become unreliable below that because it’s too cold for the fuel to vaporize. Inverted canister stoves, like the Kovea Spider, can work down to 0 degrees (f), because they burn canister fuel in its liquid form and not as gas. Finally, white gas (liquid fuel) stoves will burn down to -40 (f) but aren’t as easy to use as canister stoves.

Combine Heat and Chemical Purification

Another option is to bring the temperature of your water up to the point where its warm to the touch, pour it into a one-liter water bottle, treat it with a chlorine dioxide water purification tablet like a Katadyn Micropur, and stick it in your sleeping bag or an insulated water bottle cover to keep it while the chemicals purify is, just as they would in warmer temperatures. Micropur destroys viruses and bacteria in 15 min., Giardia in 30 min., and Cryptosporidium in 4 hrs. This would be a time-consuming process, but it’s reasonably robust as long as your stove doesn’t fail.


You can also use a Steripen if all of the water you need to treat is in liquid form, as long as you carefully babysit the batteries and keep them warm using your body heat. For example, you can suspend a Steripen from a lanyard around your neck to keep it warm. This is much faster than combining heat and chemical purification, but it relies on a battery power device, so you probably want to bring a secondary method along. I’ve heard so many stories about Steripen’s bonking in cold weather, that I wouldn’t bet the farm on it without bringing a reliable backup.

Cold-Proof Water Filters and Purifiers

The MSR Guardian pump purifier uses a tougher class of hollow tube filtration technology that can be thawed and reused. The Platypus QuickDaw can also be frozen and used again (it even has a self-test process to check for efficacy).

ElectroAdsorptive Purifiers

RapidPure and Grayl make purifier-grade water filters, meaning that they can remove viruses, in addition to bacteria and protozoa. Their filters use a technique called electroadsorption where relatively large filter pores are overlaid with a positively-charged mesh that latches onto germs like little magnets. The larger pore size is easier to force water through, unlike other popular hollow-fiber filters. All of their purifiers still use filter media, but their pore size is much larger, and therefore much less susceptible to damage by freezing.

Both the RapidPure and Grayl Purifiers can be used through multiple freeze/thaw cycles, although the manufacturers do recommend replacing them after they’ve been frozen twice.

Updated September 2022.

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  1. This may be a professional deformity on my part, but I do not see any reason why bleach or aquamira should be “useless” after freezing. You may want to double check your source for this.

  2. Interesting article, plenty of good information for decision making. I do venture out in winter, AT section hiking. I guess I am in good shape, I have the GRAYL Geopress and a spare filter. As a backup, chemical treatment in in my kit. Even in my wife’s car go bag there is a GRAYL Ultralite with spare filter.

  3. Just watched a GMC (on GMC video) blog of a hiker who’s through hike seriously interrupted due to a malfunctioning water filter. Poor fellow, did he ever get sick. Blood work and antibiotics. It seemed like such a small inconsequential leak. Nasty stuff..

  4. Thanks for the information as we head into winter! I keep my filter next to my body while hiking and in my sleeping bag (in a plastic pouch) at night. Or I lug 4 liters of water up a mountain on a day hike…

    For an excellent stove that burns well and efficiently at freezing temps and at elevation may I add the MSR Whisperlite? Yeah- it’s an old stove and is heavier than a canister but that sucker has boiled water in 5 minutes in 10 degrees (tested on my Maine porch last winter). It does NOT like wind, though. Nor do I in cold weather…

    • I also use a whisperlite (the classic version) in winter. It works best with a windscreen or if you dig a snowpit and use that to block the wind. I’ve found that a pie tin makes a very good stand for it so it doesn’t melt into the ground. Boil time is hard to measure because you’re usually not starting from room temperature.

  5. Beaver. Never beavers. Beaver is always singular, whether there is one of them, or two or more.

  6. Philip, what would method would you recommend for a day hiker that doesn’t want to carry four liters during the winter? Seems like a steri-pen wouldn’t have battery issues during single day use? Thanks!

    • That could work if your water sources are liquid. Mine aren’t usually or they’re unreachable. I just suck it up and carry 3 liters and prehydrate like mad at home. If you do use a steripen, I would keep it tied around your neck on a lanyard to keep it close to your body heat and bring a backup, like Micropur. Always bring a backup. Shit fails.

  7. For several years now I’ve been using a Sawyer which I carry in my pocket or wrap a hand warmer around and throw it in the foot of my bag at night. Kill two birds and all that.

    • I’ve tried doing that and found it to be a pain in the ass. In fact, I did it with the filter in the second picture above, which is what a Sawyer looked like in 2009 or thereabouts.

  8. It sound like Rapidpure technology is similar to the one used by General Ecology’s First Need, i.e. electrostatic absorption. Is there a connection between these two products/companies? Also in the past you reviewed First Need but nowadays this product is never mentioned anymore or compared to, even though it used to be the only trusted product for removing viruses and chemicals. I guessed that maybe it was due to weight, but MSR Guardian is about the same weight. Do you know why that is?
    Thanks in advance.

    Note: I posted this question elsewhere on this website but moved it to a more recent article (sorry for reposting)

    • 1) Totally different technologies. General ecology uses a dense glass fiber mesh.
      2) I chalk it up to an inept sales team, the fact that the outdoors is not their core business, and that the Guardian is a much better product because it is self-cleaning and can be frozen and still work.

  9. What are your thoughts on the Aquamira Frontier filter straw? I have been using this product for the past 5 years and have been very happy with it. I wasn’t aware of the dangers associated with frozen filters, but I always carry mine in a cotton pouch, which I carry on a cord around my neck and under my clothing to keep it from freezing.

    • In winter I like to boil my water to keep it from freezing and carry 2-3 liters of it when hiking. A straw doesn’t really fit into that equation very well, even when I boil it with a white gas stove – which purifies it. I guess the filter straw is ok for emergency use, but I’m not a sipper, but a gulper when I drink water any time of year. That’s why I batch process it.

  10. Does anyone make a insulated pouch to protect water filters air or electronics during winter hiking?

    • Yes and no. It’s easy to find insulated pouches and backpacks. The problem is the heat source and making it last long enough and at a consistent enough temperature as the external temp changes. That’s why sticking it into a plastic bag and putting it into a jacket chest pocket is the way to go. Your body is the heat source.

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