If you melt snow in winter for drinking water, you still need to purify it before you can drink it. The easiest to do this is to let it boil for 1 minute if you’re below 6,562 feet (2,000 meters) of elevation or 3 minutes if you’re above 6,562 feet (2,000 meters). Boiling is the surest method to kill disease-causing organisms, including viruses, bacteria, and parasites. These are the CDC (Center for Disease Control) recommendations and I’ve always found them reliable.
Alternative Water Filtration/Purification Methods in Winter
Water filters are unreliable in winter because they can freeze, which will usually ruin the filter. They’re also difficult to keep from freezing when not in use if you’re camping outdoors in the cold. While we recommend carrying extra fuel for boiling your water, you can also use the following two methods to purify snowmelt or bring them as a backup in case your stove fails. No kidding: stoves can fail in winter.
Chlorine Dioxide Tablets
If you can’t boil snowmelt, perhaps because you’re low on fuel, you can use chlorine dioxide tablets, like Katadyn Micropur to purify liquid water. You can add a chlorine dioxide tablet (usually 1 per liter) to a water bottle as long as the water is in liquid form. Katadyn Micropur destroys viruses and bacteria in 15 min., Giardia in 30 min. and Cryptosporidium in 4 hrs. If you’re not worried about “Crypto”, then adding a tablet to lukewarm water will purify it pretty quickly.
Ultraviolet Light Purification
Ultraviolet light is another viable way to purify snowmelt, using a Steripen. Ultraviolet light doesn’t add any taste to the water and is even faster than using chlorine dioxide tablets. The only real issue with a Steripen is that you need to keep the batteries warm enough to use. Some people hang a Steripen around their neck on a string, under their baselayers to keep their batteries operational.
Stoves and Stove Fuel for Snow-Melting
When it comes to melting snow and purifying it, I’ve always opted for boiling because it’s simple and I already have the stove out to do it. While you can use a canister stove for snow-melting, they operate in fairly limited weather temperature range. For example, an upright canister stove will only work down to about 15 degrees F, an inverted canister stove will work down to about 0 degrees, while a liquid fuel stove (white gas stove) will work down to -40 F.
My preference is to carry a liquid fuel stove winter stove (MSR Whisperlite) because it operates reliably in all temperatures and because boiling water for that extra minute requires more fuel. One advantage of carrying liquid fuel is that you can carry just what you need since you fill your own fuel bottles. If you carry a canister stove, you can only buy or pack your fuel in canisters, which becomes a bulky hassle when you have to carry several of them. There are other reasons to carry a liquid fuel stove in winter, but that’s a topic for another FAQ.