As I see it you have only 3 distinct water treatment options for winter.
1. boiling creek water or melting snow to a boil
2. UV ray treatment (SteriPen) Cold water does not affect its effectiveness BUT you should try to keep the batteries warm.
3. Chlorine dioxide tablets (Katadyn Micropur tabs) Here very cold water requires you to let it dissolve over 4 hours for max effectiveness unless you put it in warmed water.
Taking both #s 2 & 3 seem prudent. I do it all year long.