chaosmagnet,

It is only with water inside that freezing causes problems for a filter. When dry there is no problem. With ceramic filters it cracks the filter element. My Katadyn has frozen many times, when dry, but not damaged. I suspect, the way the Sawyer is designed, that freezing would enlarge the openings in the hollow fibers.

deathwind,

In the US (unless you are downstream from human habitation) any one of the three will work just fine. Boiling works every time for bacteria, cysts and virus. Filtering works for bacteria and cysts (virus are a limited problem in the US). There are "filters" that work on virus (the Sawyer .02 series as an example). They are called purifyers. Tablets work for bacteria and virus, but only a few work on cysts and that requires time (hours in most cases).

Respectfully,

Jerry