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Water Purification in Winter

Sleep with your water filter on Cold Nights

If you plan on drinking water from natural sources or melt it from snow in winter, you still need to filter or purify it before drinking it. If you’ve read someplace that you don’t need to purify water from melted snow, you are mistaken.

The problem with winter is that traditional methods of water filtration and purification don’t work very well.

  • Filters freeze and can crack, completely losing their integrity.
  • Chemicals such as chlorine dioxide tablets take a lot longer to work in cold water because chemical reactions work better in warmer temperatures.
  • Battery operated devices like ultraviolet lights are risky to rely on because batteries lose energy much faster in cold weather, even when not in use, and shouldn’t be relied solely for survival.

Your best bet for purifying water in winter is a liquid fuel (white gas) stove. If you’re melting snow or if you’ve found a liquid source, bring it to a roiling boil for a full 3 minutes if you are above 2,000 meters in altitude (1 minute below 2000 meters) to kill all the giardia, cryptosporidum, and bacteria in it.

If you are low on fuel or trying to preserve it, bring your water up to a boil, pour it into a water bottle, and add one chlorine dioxide tablet for each liter of water. This is a good system if you use hot water bottles in your sleeping bag at night to stay warm, because the bottles will stay insulated while the chlorine dioxide is doing it’s thing. Chlorine dioxide tables, like Katadyn Micopur, which is what I use, require 60 degree (F) water to work within their published guidelines: 15 minutes to kill protozoa and bacteria, and 4 hours to kill cryptosporidium cysts. The only downside with this method is that it takes a lot of time, and is probably best done after you rehydrate at the end of the day, after boiling a liter or two of water with your stove.

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  1. Do you mind if I offer a small correction?

    The 10 minute boiling rule is often quoted but it isn't backed up by science. Remember that boiling water doesn't get hotter the longer it boils. If you need to kill something that can't be killed immediately in boiling water then you need a pressure cooker, which raises the boiling point.

    Fortunately Giardia and the other things hikers have to worry about can be killed by temperatures even lower than boiling. The following site has some authoritative scientific statements on the subject.


  2. Darn – I should have checked that. But I don't like your source all that much, so I went and looked this up at the CDC. To quote:

    "Boiling can be used as a pathogen reduction method that should kill all pathogens. Water should be brought to a rolling boil for 1 minute. At altitudes greater than 6,562 feet (greater than 2000 meters), you should boil water for 3 minutes."


    I'll edit the post to correct this. Thanks for calling me on that one Heber!

  3. The survival topics site is similar to the good ole' fashioned approach of Tyndallization where you heat materials just enough to kill the living bacteria several times (typically 3days) in a row. One of the things that they used to do in biological labs before the invention of filters. It would be really tricky in the woods so I like the "make sure it actually boils" approach of the CDC.

  4. When I read the survival topics advice I was reminded of beer brewing, where you typically bring your wort up to 160-170 degrees to sterilize the water. I like the CDC approach better, particularly since some critters are very hard to kill when they are encased in bio films and not freely suspended in water.

  5. Out here in the West as long as no stock animals have been near, in winter I just bring the water to a boil and use it for food. I drink mostly hot beverages as well so easy that way ;)

  6. boiling water is best, we all prefer something hot during winter right

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