If you own a water filter for backpacking or camping, it’s really important to prevent it from freezing in cold weather because it can seize up or crack when it thaws. Either of these conditions are bad news because you run the risk of drinking contaminated water than can make you sick if your filter has been compromised.
One easy way to prevent water filter freeze-up is to put your filter in a plastic ziploc bag and sleep with it in your sleeping bag so that your body temperature will keep it warm. I did this last weekend when I was hiking on the Appalachian Trail in Southern Vermont and nighttime temperatures dipped into the 30’s. The plastic bag prevents any water trapped in your filter from leaking onto your sleeping bag and keeps you nice and dry.
If daytime temperatures are also below freezing then you may need to change the way you make your water safe to drink, by boiling it or by using a portable ultraviolet light called a Steripen to kill any bacteria, cysts or viruses.The only issue with a Steripen is that is less effective in turbid water with floating particulates, so you’ll need to let your water sit for a while to settle out. Chemicals such as chlorine dioxide can also be an effective in cold weather but it takes them a lot longer to kill organisms because the chemical reaction is slowed by colder temperatures.