Clear water isn't safe water. Thirst kills in three days. Bad water can do it in one.
ACT
- Choose before you treat. Running water rather than still water. Upstream of houses and herds. A spring coming out of the ground rather than a river.
- Clear it. Strain it through a tight cloth folded several times, then let it settle. Pour off the clear water and throw away what's at the bottom. A better filter is described in Filtrer avec ce qu'on a.
- Bring it to a rolling boil for one minute. Three minutes above 2,000 meters (6,500 feet). It's the only method that never fails — as long as you can keep a fire going, see Making and keeping a fire.
No fire: a filter in wild country with few people around, a filter plus a disinfectant anywhere people live.
Nothing at all: a clear bottle, clear water, full sun, six hours, set on a surface that reflects light.
ADAPT
You have fire. Boil it, no debate. But count your fuel: bringing a liter to the boil takes wood, three times a day, for each person. That math decides where you set up camp as much as access to water does, and it weighs on the choice described in Building an emergency shelter.
You have no fire. The filter becomes your main tool. Back it up with a disinfectant as soon as you're near people.
You're in a city. A water heater holds dozens of liters of clean, protected water that almost everyone forgets: drain it from the tap at the bottom of the tank. Toilet tanks too — the tank, not the bowl — as long as no cleaning tablet was dropped in. A building's pipes keep water in their lowest parts: open a tap on the top floor to break the vacuum, and another one at the bottom to collect it.
It's raining. Collect it: it's the only resource that comes to you without exposing you. See Collecting and storing rainwater.
You're in the mountains. Above 2,000 meters, boil for three minutes. Melted snow is low in minerals and burns through a huge amount of fuel: melt it into a little liquid water, never in a dry pot, or the pot scorches before the snow melts.
There are many of you in one place. Treating water isn't enough on its own anymore: keeping waste separate and washing hands matter even more. See Hygiène et transmission des maladies.
You're not sure. Water you haven't treated isn't a reserve. It's a decision you've put off.
UNDERSTAND
Clarity tells you nothing about what lives in the water. Almost every surface water on the planet carries something that can put you on the ground within twenty-four hours. And diarrhea isn't an inconvenience: it's dehydration on fast-forward, at a time when you can no longer rehydrate. It's the first killer, before cold, before hunger.
Boiling inactivates everything alive, without exception: bacteria, viruses, protozoa, worms. No supplies, no expiry date, no dose to remember. It isn't the duration that kills germs, it's the temperature: at 100 °C inactivation is almost instant, and the recommended minute is only a safety margin in case you misjudge the boil. The Institut Pasteur goes up to five minutes to cover the hepatitis A virus, which survives heat better than other waterborne germs.
What boiling doesn't do: it removes no heavy metals, nitrates, pesticides or fuel. It even concentrates them very slightly, since part of the water leaves as steam while the pollutants stay. Water that's chemically suspect is still suspect after boiling. When the doubt is chemical, you don't treat it: you find another source.
Filters have a limit few people know about. Most portable filters stop bacteria and protozoa, but not viruses: viruses are ten to a hundred times smaller than the filter's pores. In the wild, the main risk comes from bacteria and parasites, and a good filter is enough. Near people, the risk of viruses from sewage takes over, and a filter alone is no longer enough.
A filter clogs, gets backflushed, and above all it freezes. A cartridge that has frozen cracks from the inside with nothing to show for it: the water trapped in its pores expands and forces the walls apart. It still seems to filter, and it no longer stops anything. In cold weather, it sleeps against your body.
For chemical treatment, one rule: the dose printed on the package is the authority, never a rule you memorized. Concentrations vary from one product to another, and mistakes go both ways. Just remember three things. Contact time matters as much as the dose, and it doubles in cold water. Cloudy water cancels everything, because the particles physically shield the germs. And silver alone keeps water that's already safe from going bad, but it never makes water safe.