SectionHiker.com Backpacking Gear Reviews and FAQs 
Most hiking and backpacking water filters must be replaced if they’ve been frozen. When it gets too cold to keep a water filter from freezing, your best bet is to switch to chemical purification using chlorine dioxide in tablet form (Katadyn chlorine dioxide tablets) or purifying your water with ultraviolet light using a Katadyn Steripen. While boiling your water will also work, it takes too much fuel to be practical unless, of course, you’re melting snow at the same time and have plenty of fuel to spare. There are several notable exceptions to this rule including the MSR Guardian, the Platypus QuickDraw as well as the water purifiers from RapidPure and Grayl, which we discuss further below.
Hollow Fiber Tube Filters
The majority of backpacking water filters sold today are made using hollow fiber tube technology, including the Sawyer Squeeze, Sawyer Mini, Katadyn BeFree, the Platypus GravityWorks Filter, and the HydroBlu Versaflow. If you look inside a hollow fiber filter, you’ll see many tiny tubes packed together. Each of these hollow fibers has tiny pores in its sidewalls that let clean water pass through, but trap contaminants such as bacteria or protozoa. This style of filtering is called exclusion since contaminants are excluded from the clean stream of water that passes through the filter.
When you use a hollow fiber tube filter, some water is trapped inside the filter. When it freezes, it expands inside the hollow fiber tubes and splits them so that unfiltered water will flow through, ruining the filter. When backpacking in cold weather, you want to keep a hollow fiber filter close to your body, so that your body heat will prevent it from freezing. I pack mine in a plastic bag and keep it in my chest pocket or sleep with it at night in my sleeping bag.
Ceramic Filters and Other Exclusion Media
A ceramic filter like the MSR Miniworks will also break when frozen because water trapped in the filter will expand, cracking the pores in the filter element. This is also true of the Katadyn Hiker Pro and the General Ecology First Need which are made with glass fiber, and the Katadyn Vario which is made with a combination of ceramics and glass fiber.
MSR Guardian
The MSR Guardian Water Purifier and the Platypus Quickdraw are the only hollow fiber-based filter that can withstand being frozen and reused after they have been completely thawed. MSR and Platypus (both divisions of Cascade Designs) engineered the product so that they can accommodate the expansion of frozen water without destroying the hollow fiber tubes used in the filter element. Both of these filters have self-test modes that should be used to check the filter’s integrity before reuse.
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 electroabsorption 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 Ultrapress Purifier Bottle
- RapidPure Intrepid Bottle Purifier
- RapidPure TrailBlazer Gravity Purifier System
- 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. In any event, I do recommend carrying Katadyn Micropur Chlorine Dioxide Tablets as a backup even if you do carry a filter in cold weather because they always work, as long as your water is sill in liquid form.
Last Updated: September 2022.
See Also:
- Ultraviolet Water Purification 101
- Cold Weather Water Treatment and Purification
- How Much Water Do You Need for Day hiking?
- How to Prevent your Water Filter from Freezing in Cold Weather
- Hiking and Backpacking Hydration Systems: Pros and Cons
I don’t suppose there’s any way to tell if a hollow-tube filter has been frozen or not. I have one Sawyer Squeeze that was possibly frozen, I just am not sure. I’ve replaced it with a pump style anyway, but I hate to just toss it if it’s still performing.
Use it and wait 2 weeks. If you get Giardia it’s probably busted. Sorry.
There has to be a better way. Maybe a color dye that should get trapped by the filter? We’d need to find something that’s visible to the eye and just slightly bigger than what the filter should catch.
Platypus GravityWorks has a Filter Integrity Test to determine if freezing has damaged the hollow fiber filters.
https://d1l67pfsx3wblg.cloudfront.net/pdf/GravityWorks_Ins.pdf
A. Backflush a minimum of 1/2 L of water.
B. Remove Clean Hose.
C. Blow air and check.
– Cannot blow through filter = Fiber OK.
– See steady stream of bubbles = Fiber broken.
STOP! REPLACE CARTRIDGE.
I try to force air through my dry filter, if it feels nearly impossible, I use it. It’s not scientific but I don’t like the idea of throwing out a filter every time I can’t be sure whether it froze or not. The majority of time I hike, it may be subjected to freezing temps. If you want to look up the science behind it and possibly develop a better test, Google “air testing hollow fiber filters”
I was wrong, the filter should be wet not dry. It’s been awhile since I’ve done it.
You are basically describing a bubble point test: https://scottlab.com/bubble-point. If you used compressed air and had a pressure gauge you should be able to reliably determine if the filter is good. You might have to measure a known good filter.
Have you let your MSR Guardian freeze though? I just don’t trust it for use in the winter. If the winters keep on going like they have, we’ll have running water year round, so a filter that can survive repeated freeze/thaw cycle would be ideal.
I wish MSR could half the weight of the Guardian — I carry it for two people, but for one it’s a little too much. Its ability to filter at a fast rate out of puddles has been super useful during this drought year though, where so many water sources have been unreliable.
Are there any limitations regarding water and/or air temperature that may prevent either chemical or ultraviolet light purification systems from working properly?
UV Light purification systems can be limited by water turbidity.
There was a really interesting study on merely taking water, butting it into a clear water bottle and laying it in the sun. It was a group in India. The results were really good. But it took multiple hours (6 – 48 hours depending on weather) and the water had to be relatively clear (again, UV rays vs. water turbidity).
Of course that does nothing to get rid of chemicals in the water. But neither do the hollow filter systems. The only thing that can do that would be reverse osmosis or boiling and recapturing the steam.
Not sure that there’s any requirements for chemical treatment; but I don’t believe there are any
For UV, if you get clear water (like found in the Sierra’s), then the UV pen should work fairly quickly. If you get it out of a river, filter it through a couple layers of a handkerchief first, then use the UV filter.
Not really. Chlorine dioxide just takes longer in cold water (chemical reaction – it’s about 2X) and you can’t purify ice with UV.
MSR also had the MIOX purifier that produced a chemical treatment dose using bit of salt water and battery power. I have one and like it…but they don’t make it anymore. There is a newer product using the same technology the H2go, I haven’t tried that one yet. Chemical treatment might get a bad rap but it is how municipal water supplies are treated.
Here’s a very interesting article about the long term use of the Sawyers.
https://ferguskane.com/blogs/wp-content/uploads/2017/04/murray2015.pdf
About 4 paragraphs in is a really good cleaning instruction set.
Then there’s this document in which Sawyer refutes the ‘claims’ in the above study:
https://sawyer.com/wp-content/uploads/2017/06/UNC-Handout_2015-10-21.pdf
It includes analysis from a college I’ve never heard of a reviewing the process used stating some concerns of the conclusions due to that process. I agree with some of their issues, but not all.
Please note, tthe Tufts study only tested the Sawyers because they were in Honduras and that’s what was there (Sawyer is agressive about donating these to Third Wold countries, so they will likely be the most prolific out there). But you can expect that any company using the hollow fiber filter system will experience similar results. I’m not knocking Sawyer. I’ve known of these filters since before they were commercialized through Sawyer. I own two and will continue to use them without hesitation.
But it’s clear that further, long-term studies should be performed for this tech. I can’t find anything newer than these 2015 articles.
After one of my trips where our water froze, I emailed Sawyer and asked. The rep said that they can’t confirm that freezing damaged the tubes, which I thought was kind of interesting. But it makes sense, since 1) I can’t confirm it froze, 2) If it did, was there enough water in there to burst the fiber, 3) It would take an electron microscope to verify and I don’t have one in my back pocket.
Anyway, at the price of the Squeeze (or other versions/brands), you could probably call it a consumable. Use it for a couple years and if there’s even a hint that the thing is not working right, trash it and get a new one.
Do you filter melted snow?
I haven’t worried about it in the past but wondered if thar should be done.
Good question. You still have to purify it. Most people boil it to kill any organisms when melting snow and to heat it as hot as possible so it won’t freeze again (when stored in an insulated container). If you don’t want to do that (because it uses more fuel) you can also use a UV Steripen or chlorine dioxide.
When I have a question about my filter, I fill a water glass and add food dye. I then filter that water into another glass. If it is clear, the filter isn’t compromised. At least that’s my thought process.
That works as long as the size of your food coloring molecules are 0.1 microns or less.
The food color molecules would need be larger than 0.1 microns for this to be a legitimate test. If they are smaller they will pass through anyway regardless of the integrity of the filter. Ideally, to test you really need a identifiable (eg colored) substance that is the size of the smallest thing you want to filter out. I can’t find a definitive micron size for common food dyes. Synthetic dyes are much smaller than 0.1 micron (~0.001- 0.003). Blue Indigo dye is apparently within the middle range of bacteria but smaller than Giardia and mostly smaller than Crypto according to an Osmonics Inc. chart I found. While not a food dye it apparently has low oral toxicity. While I’m definitely not recommending its use for this, it does appear to have some potential as a useful test.
I put my filter and reservoir in a dry bag in the foot of my sleeping bag.
Great article Philip. However I believe there is a lot of intentional paranoia promoted by commercial interests to sell us fear. When I was a a Boy Scout in the 60’s we never filtered our water and never got sick. I’m positive the water was better back then but I believe the source of water is more important than filtration. How far are you from the source of a spring or melted snow runoff ? I filter my water now ( BeFree) but I pay more attention to my source. If in doubt I filter and follow up with chemical sterilization. Never been sick from bad water. Nock on wood.
Thank you for this article, especially since I’m headed out to camp soon. Also, thank you Community for educated responses.
In regards to the post about fear mongering of commercial products:
As an ER nurse, I see several cases of gastrointestinal infections caused by a multitude of bugs. Nearly all are contracted from dirty hands. E coli, Clostridioides difficile, to name 2. So yes, your filter, frozen or not, will not save you from bugs on your hands.
I have also personally contracted giardia and cryptosporidium. I don’t recommend those infections. But I lived… I got them from dirty hands and likely lack of attention to where I was holding my “clean” gravity tube when filtering from less than ideal conditions.
I like the old school response. Boil or chemical treat in winter. For intermittent freezing temps in autumn, keep that filter from freezing. If in doubt, replace the filter. Believe me, ER nurses want to help sick people but wiping up the diarrhea that was tracked all over the room is not on top of our “fun things to do” list.
Good topic. Thanks for the article.