Three methods. Know which one the water actually needs.
No single method handles every contaminant. Filtration removes sediment, bacteria, and protozoa but not viruses. Boiling kills everything biological but doesn't remove particulates or chemicals. Chemical treatment (chlorine or iodine) kills most pathogens given enough contact time but works slower in cold or cloudy water. Combine methods — filter cloudy water before boiling or dosing it — rather than relying on one alone.
Boiling
| Elevation | Time at a rolling boil |
|---|---|
| Below 6,500 ft | 1 minute |
| Above 6,500 ft | 3 minutes — water boils at a lower temperature with less atmospheric pressure |
Chemical treatment — bleach
Use plain, unscented liquid chlorine bleach, 5–9% sodium hypochlorite, with no added fragrance, dye, or surfactant.
| Water condition | Dose per liter | Wait time |
|---|---|---|
| Clear | 2 drops | 30 minutes |
| Cloudy or cold | 4 drops | 30 minutes |
The water should have a faint chlorine smell afterward. If it doesn't, repeat the dose and wait another 15 minutes before drinking.
Chemical treatment — iodine
Follow the dose printed on the specific product — potency varies by formulation — but a common rule of thumb is 1 tablet per quart (about 1 liter), with a 30–60 minute wait depending on water temperature (longer in cold water). Do not use iodine treatment if pregnant or if you have a thyroid condition; consult a doctor first if either applies.
Filtration
A 0.1–0.2 micron hollow-fiber filter removes essentially all bacteria and protozoa (Giardia, Cryptosporidium) along with sediment, but does not reliably remove viruses, which are smaller than the filter pores. In areas with a realistic risk of human sewage contamination, follow filtration with boiling or chemical treatment.
Storage
Store treated water in food-grade, opaque containers in a cool, dark location out of direct sunlight. Pre-treated tap water stored for emergencies should be rotated roughly every 6 months.
Frequently asked questions
How much bleach do I add per liter of water?
Using plain, unscented liquid chlorine bleach (5–9% sodium hypochlorite, no added fragrance or additives): 2 drops per liter for clear water, 4 drops per liter for cloudy or cold water. Mix, then wait 30 minutes before drinking. The water should have a faint chlorine smell; if it doesn't, repeat the dose and wait 15 more minutes.
How long do I need to boil water to make it safe?
A rolling boil for 1 minute kills bacteria, viruses, and protozoa at elevations below about 6,500 feet. Above that elevation, boil for 3 minutes, since water boils at a lower temperature with less atmospheric pressure.
Do water filters remove viruses?
Most portable filters (0.1–0.2 micron) remove bacteria, protozoa, and sediment but not viruses, which are too small for the filter pores. Pair filtration with boiling or chemical treatment in areas where human sewage contamination is a realistic risk.
Full context and sourcing: Off-Grid Water Purification Guide. Need a filter? See the vetted pick in the Survival Gear Hub.
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