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.
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).
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.
- Grayl Geopress Purifier Bottle
- Grayl Ultralight Purifier Bottle
- RapidPure Intrepid Bottle Purifier
- RapidPure Scout Inline Hydration Pack Purifier
- RapidPure Universal Bottle Purifier Adapter
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.SectionHiker is reader-supported. We independently research, test, and rate the best products. We only make money if you purchase a product through our affiliate links. Help us continue to test and write unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.