How to sterilize water? Simple:

You don't need to ‘sterilize’ water. Sterilization is the destruction of all microorganisms in, on and around an object. What is needed, is disinfection (killing of pathogenic (disease causing) organisms).

Disinfection can be done many ways, including filtration, heat, ozonation, and chemical disinfection.

Despite many stories to the contrary, simply boiling water will disinfect it. At any elevation you're likely be at the boiling point of water is high enough to kill (or denature) anything in the water. You don't need to boil it for any particular length of time, just get it boiling at a good rolling boil.

Filtration is a good method, you should use a filter that has an absolute rating of 0.2 micron diameter or LESS (0.1 micron). Personally, I use iodine crystals (Polar Pure ™ first, then filter the water.

Chemical disinfection is the use of various chemicals (usually a halide like chlorine or iodine) in the water. It's usually a quick, economical and effective method.

Here is a summary of water disinfection chemical usage based on the Wilderness Medical Society Practice Guidelines 2nd Edition, edited by William Forgey MD (page 63):

For chemical disinfection, the key is the concentration of halogen, in parts per million (halogen to water):

Concentration of Halogen
Contact time Contact time Contact time
@ 5oC / 41o F @ 15o C / 59o F @ 30o C / 86o F

2 ppm 240 minutes 180 minutes 60 minutes
4 ppm 180 min 60 min 45 min
8 ppm 60 min 30 min 15 min

How to get the desired concentration of halogens, for various products:

Iodine tablets, also known as:
tetraglycine hydroperiodide; EDWGT (emergency drinking water germicidal tablets); USGI water purification tablets; Potable Aqua (trade name); Globaline (trade name):

4 ppm – ½ tablet per liter of water 8 ppm – 1 tablet per liter of water.


NOTE: These tablets should be gunmetal gray in color when used –
if rust colored, they are useless:

The free iodine has combined with atmospheric moisture. The bottles should be kept well sealed and replaced often. Checking the tablets in the bottle just exposes them to moisture in the air.


For 2% iodine (tincture of Iodine) (gtts=drops)
4 ppm – 0.2 ml (5 gtts) 8 ppm – 0.4 ml (10 gtts)

NOTE: Tincture of Iodine should NOT be used as a wound treatment, so this is not a good option for a 'dual use' item.

10% povidone-iodine solution (Betadine ™)

NOTE: Solution only, NOT SCRUB - Scrub has soap in it

4 ppm - 0.35 ml (8 gtts) 8 ppm – 0.7ml (16 gtts)

Saturated (in water) Iodine crystals (Polar Pure ™)
4 ppm – 13 ml 8 ppm – 26 ml

Iodine crystals in alcohol
0.1 ml / 5 ppm 0.2 ml / 10 ppm

Halazone tablets (Monodichloroaminobenzoic acid)

4 ppm – 2 tabs 8 ppm – 4 tabs

NOTE: The old Vietnam era chlorine tabs are decades out of date. Chlorine tabs decay even more rapidly than iodine tabs. Not recommended.

Household bleach (Chlorox ™)

4 ppm – 0.1 ml (2 gtts) 8 ppm – 0.2 ml (4 gtts)

Note: Plain chlorine bleach offers a relatively economical method of treating large (gallons) of water at a time. 4 liters is approximately 1 gallon.

For very cold water contact time should be increased.

If drinking this water after disinfection, flavoring agents (drink mixes, etc) can be added: This must be done AFTER the period allocated for disinfection (the disinfecting agent will bind to the organic material and not work).