Methods of water purification include boiling, filtering, ultraviolet light and chemicals. All of them have pros and cons. Since 2005 my principal means of treatment has been chlorine dioxide (eg. Aquamira). Your choice ultimately depends on two main factors: personal preference and the conditions into which you are venturing.
Pros: Effective at killing all nasties; good option in emergencies when filter is clogged or chemicals are lost; best option in extreme cold when all water sources are frozen.
Cons: Very time consuming; uses up a lot of fuel, which equates to extra weight you really don’t want to be carrying.
Pros: Tastes good; chemical free; maintainable on trail; generally durable if cleaned regularly and used in quality sources.
Cons: As not all water sources are created equal, pretty much all filters require regular maintenance in order to avoid clogging; set up and pumping large amounts is time consuming; doesn’t kill viruses; heaviest of the different options.
Pros: Lightweight, compact, effective against Giardia, viruses and protozoa.
Cons: Doesn’t kill crypto; taste isn’t great (although you can get used to it); regular use for long periods is said to be harmful to your thyroid; long waiting time (20-30 minutes; double that for extremely cold water).
Pros: Lighter than filters; tastes good; fast working.
Cons: Doesn’t work well if the water is murky; not “on-trail” maintainable; battery life variable.
Pros: Lightweight, compact, effective against Giardia and crypto.
Cons: Long wait time (20-30 minutes); said to take up to 4 hours to be completely effective against crypto.