There is an easy way to purify water for almost free, without using fire or filters except for a simple filter to remove visible particles. If you use clear glass bottles with the caps you can pour water into the bottles through a simple filter such as a shirt or fine screen, as long as the water comes through clear to the eye. Leave 2" to the cap. Then cap the bottle and put in direct sunlight for over 48 hours. This process will kill biological threats in the water such as Cholera, and typhoid.

This process does take a little while but can be done without taking up your time and with very little effort.
Especially useful in situations where fire may give your position away, or you do not have fire building materials.