Humans can survive weeks without food but only three to five days without water. On a hike or camping trip you may not reach a fresh surface source before your own supply runs out, and the water you do find can carry bacteria, viruses, parasites, and chemicals. Every purification method has pros and cons, and none of them does everything. The practical skill is knowing which problem each one solves: dirt, microbes, or chemicals.
This guide groups the methods that way. Start with the principle that runs through all of them: clear the water first, then disinfect it.
Step one is always clearing the water #
Boiling, tablets, UV, and sunlight kill microorganisms, but they do not remove mud, and they work worse in cloudy water. If the only water you find is dirty, pre-treat it. Seven unique ways to find water in the wild covers where to look, and the food and water hub covers storing it.
Prefer water that is dirty but clear. Muddy water is the worst case, and a homemade filter can handle the first pass. If all you have is muddy water, filter it before you do anything else.
Sedimentation. Let the water stand in a container for at least two days so particles settle. Pour the clearer water off the top into another container, taking care not to stir the sediment, then run it through a sand filter. This only separates particles. It does not make the water safe.
Cloth pre-filter. Pouring through a clean cloth or a fine mesh strainer removes large debris. A T-shirt works. One field method ties shirts one above the other on a tripod of three sticks, with a container beneath. Pour water through, and repeat if it is still cloudy.
Sand filter. Cut a bottle into a cone shape, with the neck down. Put pebbles or cotton in the neck, covering about 3 inches, and fill the rest with sand. Pour the water through into a clean container. If it still looks cloudy, run it through again. It is cheap, effective, and worth practicing at home first.
Activated charcoal. Charcoal adsorbs impurities, which are physically trapped in its porous structure. You can buy filters containing it, or pack activated charcoal powder into cloth or mesh and pour water through. Replace or regenerate it regularly. Charcoal does not remove dissolved salts or heavy metals.
Killing microorganisms #
Boiling #
Boiling is the easiest and most dependable method, and you already know how to do it. Water boils at 212°F (100°C) at sea level. Get a rolling boil and hold it for at least one minute, which kills bacteria and parasites. At higher altitudes boil it longer, because the boiling point is lower. The survival source recommends a longer, conservative boil of several minutes in the field, then letting the water cool before you drink.
Boiling has limits. It does not remove chemical contaminants like heavy metals or pesticides, and it does not remove dirt, which is why you clear the water first.
Purification tablets and drops #
Tablets and drops are what you can pack without a fire. The common active ingredients are iodine, chlorine, and potassium permanganate. They are effective at killing microorganisms when dosed correctly. Chlorine tablets handle bacteria, viruses, and parasites. Iodine works similarly and may be more effective against certain organisms such as cryptosporidium. Chlorine dioxide drops are another option.
Always follow the manufacturer’s instructions, since dose and contact time differ by product. The usual routine is to add the specified amount, mix thoroughly for at least 20 seconds, and wait about 20 minutes before drinking. Tablets may leave a taste or odor, which fades if you let the water stand open. They do not remove chemicals or particles, and people with certain health conditions or sensitivities should check before using iodine.
Iodine drops #
Where you carry liquid iodine, the field guidance is five drops per container if the water is clear and ten if it is dirty. Shake, then leave the container in a shaded, dark place for about an hour, since iodine is light-sensitive.
Household bleach #
For emergencies, unscented household bleach with no additives can disinfect water. Add approximately 1/8 teaspoon (about 8 drops) per gallon, mix well, and let it stand 30 minutes before drinking. A similar emergency approach uses 8 drops of 3 percent hydrogen peroxide per gallon, mixed well and left for 30 minutes.
Chlorine dioxide #
Chlorine dioxide is sold as drops or tablets and is effective against a wide range of pathogens. The amount depends on the clarity and temperature of the water. Shake to mix, let some treated water run over the bottle threads, and leave the bottle in the shade for thirty minutes before drinking.
Solar water disinfection (SODIS) #
SODIS needs only clear plastic bottles and sunlight. Fill the bottles with clear water, lay them in direct sunlight with no shade nearby, and leave them for about six hours on a sunny day or two days when it is cloudy. UV radiation kills bacteria, viruses, and parasites. The water must start out clear for UV to penetrate. SODIS works for small quantities and does not remove chemicals or improve taste, and it fails in poor weather, so start before your stored water runs out.
UV purifiers #
Battery-powered UV devices work against bacteria, viruses, and protozoa without chemicals. Charge the device or load fresh batteries, put the lamp in a container of clear water, and stir while it runs so all the water is exposed. Turbid water or high organic content reduces effectiveness, and UV does nothing against chemicals or particles.
Filters and portable gear #
Commercial filters take the work out of the field. They come in several forms:
- Survival straws. Place the straw in the water for about a minute so water reaches the membrane, then drink through it like a straw. They are light and require almost no effort, though they treat only what you drink directly.
- Filter bottles. A bottle with a built-in filter. Fill it, squeeze or press the water through the filter, and drink. Easy for first-time wilderness trips.
- Pump purifiers. Portable pumps draw in dirty water and push it through ceramic or charcoal elements, sometimes with chemical treatment built in. You pump the water into a bottle. Some bottles include the pump.
- Carbon filters. Granular or solid activated carbon removes impurities and chlorine and improves taste. Pour the water through and carry on.
- Ceramic filters. Water passes through fine pores in a ceramic element. They remove many impurities, but ceramic does not remove pesticides or organic pollutants. In regions where those are a concern, use a carbon filter first.
- Reverse osmosis. A membrane removes a wide range of contaminants. It is more of a base-camp or home system than a trail one.
Pick a filter based on the contamination you expect, flow rate, ease of use, and maintenance. Filters need cleaning or replacement on schedule.
Distillation #
Distillation boils water, collects the steam, and condenses it. It removes most impurities, including bacteria, viruses, chemicals, minerals, salts, and heavy metals, which makes it one of the few options for salty or mineral-heavy water. It is slow and uses a lot of fuel, which is a poor fit for large volumes, and some volatile organic compounds can carry over.
Improvised stills use a pot with a lid, a solar still, or a nested pair of containers, with the condensate collected and returned to a clean cup. The survival source notes that a simple still may not fully purify the water, so treat distilled water with another method if you have doubts.
Evaporation traps and solar stills #
When you can’t find any water source, a solar still can pull moisture from the ground. In the survival source, you look for a dry riverbed, since it holds the most water, and dig a hole there, reaching damp ground. Put a cup in the middle, add non-poisonous plants, cover the hole with plastic, seal the edges with sand, and place a small rock in the center of the plastic so it sags toward the cup without touching it. Sun evaporates water from the soil and plants and it drips into the cup. The more holes you dig, the more water you collect. The water is condensate, but treat it as you would any field water.
Methods to avoid relying on #
The original 18-method list included some ideas that sound good and are weak in practice. Treat these with caution.
- Plants. Wrapping a leafy branch in a bag collects transpired water over several days. It’s water production, not purification of a contaminated source, and eating plants for antimicrobial effect is not a substitute for treating water. Misidentifying a plant can poison you.
- Heated stones. Dropping clean, hot stones into a pot, or heating water in a hollowed stone, can boil water if you have no pot. It is a last resort for boiling, not a separate method.
Matching methods to problems #
| Problem | What works | What doesn’t |
|---|---|---|
| Mud, silt, debris | Sedimentation, cloth, sand filter, carbon | Boiling or tablets alone |
| Bacteria, viruses, parasites | Boiling, tablets, chlorine dioxide, UV, SODIS | Sand or charcoal alone |
| Chemicals, pesticides | Activated carbon, distillation | Boiling, UV, tablets, SODIS |
| Salt | Distillation, reverse osmosis | Boiling, filters, tablets |
A sensible routine #
- Collect from the best source you can: clear, moving, or large standing water, taken from the surface rather than the bottom.
- Settle and strain to get rid of sediment.
- Filter through sand, charcoal, or a commercial filter.
- Disinfect: boil if you can, treat with tablets or drops if you can’t, use sunlight if you’ve got time and sun.
- Store in a clean, covered container.
A bottle filter, a few tablets, and a metal cup cover most trips. Add a camping stove and a spare method and you have redundancy. Water is the one resource you can’t improvise for long, so test your methods at home before you need them.