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Cold Weather Water Treatment and Purification

Near freezing temperatures complicate water treatment and purification because filters break when they freeze, batteries are prone to failure, and chemical reactions slow down dramatically.
Near freezing temperatures complicate water treatment and purification because filters break when they freeze, batteries are prone to failure, and chemical reactions slow down dramatically.

Water treatment and purification are as important in cold weather as they are the rest of the year. Protozoa, bacteria, and cysts don’t go to sleep when temperature 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.

In winter, when you have to melt snow to make drinking water, the most convenient and foolproof way to purify your water is to boil it. But what about if you backpack in cold weather when temperatures are intermittently near or below freezing? This is the time when many three-season water treatment and purification methods become inconvenient to use, unreliable, or fail altogether.

  • Gravity and pump filters like the Sawyer Mini, the Platypus GravityWorks, or the Katadyn Hiker Pro are prone to clogging or cracking when they freeze and thaw (they don’t have to freeze all the way through to fail, mind you.)
  • 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 bleach can freeze and become useless.
  • The reaction time of chemical purification tablets like AquaTabs or Potable Aqua Iodine slows way down in cold water, although they are not prone to freeze thaw issues like their liquid counterparts. There’s also some evidence that the effectiveness of chlorine dioxide and iodine drops in cold water, despite long contact times.
Water filters can clog or break if they freeze making them a poor water treatment or purification method in cold weather.
Water 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 “shoulder seasons” when temperatures straddle freezing?

Boiling certainly works, but it does require that you carry more cooking fuel. Technically, you only need to bring your water up to 160 degrees fahrenheit to kill water-borne organisms, but since that’s difficult to know without bringing a thermometer, you’re probably best off bringing it up to a visibly gentle boil. While using a stove is probably faster, a wood stove or campfire can be good weight saving options, particularly in the evenings when you have time to kill.

Another fuel-saving 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 an Aquatab or Katadyn Micropur, and stick it in your sleeping bag or an insulated water bottle cover to insulate it while the chemicals purify your water…just as they would in warmer temperatures.

The best option is to 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. One of my readers, Ben Weaver, wears a Steripen Opti on a lanyard around his neck at all times to keep the batteries from freezing.  I think this is an excellent strategy although I’d still recommend having a backup if the batteries fail. The other advantage of using a Steripen is that it takes a lot less setup time than heating your water in a stove, so it’s good when you’re on the move.

While these tactics work for neutralizing organisms in cold water, they won’t be effective if you need to remove chemical contaminants from your water like heavy metals or fertilizer. There’s really no option other than bring a mechanical purifier like an MSR Guardian or General Ecology First Need in that case, although freezing of the unit is still a concern.

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25 comments

  1. phillip,

    Thank you for this and other cold weather articles. I live in NC, and hike in the moutains. It does get cold down here, but it is different than what you experience in the NE. Keep up the articles coming and thanks for the good tips.

  2. Phillip,

    Is there an agreed “best practice” for dealing with mechanical filters near freezing temps? Is it possible to flush them enough that the filter element (in something like the MSR Guardian or Sawyer Mini) doesn’t generate micro-cracks. (Eg., is even the humidity from blowing through them going to add enough water vapor to freeze and crack them?)

    Thank you for sharing your experience!

    • Yeah – leave them at home. I wouldn’t risk it.

      MSR claims that the Guardian can survive a thaw after being frozen, but also warns not to use it or try using when it is frozen. In other words, it’s useless in freezing temps. No other filter manufacturer makes the same claim, but then again no other manufacturer sells a filter that costs $350.

  3. Great article! In winter, for weekend warrior trips, I use a “lightsaber”, aka Steripen. I use a shoelace, tied in a loop and double looped around my neck, and then route the flap of the steripen pouch over the shoelace. Basically, I wear the Steripen as a pendant hahaha. It’s against my bare skin, and kept warm. I ONLY notice when I put it on. Never noticed it while hiking, OR even sleeping! After 5 years of using this technique I’ve never experienced a problem, and I will hike subzero F.
    Ben in Lancaster, PA

    • Excellent strategy Ben – I’m going to add that to my list of suggestions. Heck, I may use this myself, along with on elf my other backups. Thx!

      • Ben – Is there one model of Steripen that is more “lanyard friendly” than another, that you’d recommend?

      • Philip, I use the adventure opti model, and the pouch has a belt loop on it, and the flap for lightsaber;)

        Would like to add, I ONLY use the Steripen when temps goes below 32f,…otheriwse, I exclusively use a Sawyer.

        I truly don’t notice the Steripen when wearing it. Others may,…its all personal :)
        Ben

        PS, If I remember tonight, Ill send you a pic of my setup, via email.

      • Keep forgetting stuff, hahaha! Yes, I ONLY use lithium ion batteries, AND have two brand new spares in my first aid bag, which I can bring the temp up in my pocket if needed.

  4. The Steripen ONLY accepts lithium batteries, type CR123A.

    • Huh? Not my model. I am reading my Classic 3 Steripen instruction sheet. Bought it last year from REI. Here is what is says:

      “Classic 3 runs on disposable alkaline batteries, lithium batteries, or rechargeable nickel metal hydride (NiMH) AA batteries with a 2300 mAh capacity. For best results, always use quality, name brand batteries, like Energizer or Sanyo Eneloop.”

    • It depends on the model. Classic model uses AA. The Adventurer and maybe others use CR123A. I just scored the compact and lighter Adventurer on Amazon for $40 ($90 list price). It provides only a claimed 50 cycles vs. 100-150 for the Classic, but the balance of weight, size, great price and my intended use make it a good choice for me. I have a Sawyer system and have liked it, but the Steripen will be great for cold weather trips, like the short backpacking I’m doing this weekend on the AT!

  5. Admittedly I am in the southeast (NC) so don’t experience the same level of temps to your area. However I often find myself out in sub freezing temps. I still carry a Sawyer filter and keep it as a constant companion in a ziploc bag in my pants pocket. My body heat keeps it from freezing, at least down below 10 degrees. It doesn’t bother me much at all and it’s easily found when needed.

    • What is your strategy for water bottles with your Sawyer in Winter temperatures? Do you keep one Sawyer sized thread bottle and rest wide mouth containers? Every had your inverted Sawyer attached container freeze? Could you get the cap off?

  6. I use a filter year-round. It stays in an inside pocket next to my body or wrapped in my sit pad and puffy in my pack. No problems as yet. I realize it’s not ideal but in locations where agricultural and industrial runoff are present, killing biological nasties isn’t enough.

  7. As always, thanks for sharing all this great info with the rest of us.

  8. “While these tactics work for neutralizing organisms in cold water, they won’t be effective if you need to remove chemical contaminants from your water like heavy metals or fertilizer. There’s really no option other than bring a mechanical purifier like an MSR Guardian or General Ecology First Need in that case, although freezing of the unit is still a concern.”

    I was reading on the MSR Guardian site and I don’t believe that it will remove heavy metal or chemical contamination though it resistant to freezing. Heavy metal is a concern in quite a few of the places I hike due to the numerous old mines in the area :(

  9. For gravity/squeeze filters (Sawyer, etc.), what about carrying a small container of nontoxic antifreeze designed for RV water systems? Just run a bit into the filter after each use and leave it in there, then flush it out just before the next use. Sure, it’s one more thing to pack, but is there any reason it might not work?

    • Where does the antifreeze go when your flush it out? The ground? Not exactly LNT…

      • I wonder if a high proof potable alcohol ( Everclear) would work as an anti-freeze. Some, myself included would debate how potable Evercleer truly is. The idea comes from the alcohol used in air brake systems on trucks.

  10. What is your strategy for water bottles with your Sawyer in Winter temperatures? Do you keep one Sawyer sized thread bottle and rest wide mouth containers? Every had your inverted Sawyer attached container freeze? Could you get the cap off?

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