Making water safe: methods

General rule: clarify first, then disinfect. If the water is cloudy, let it settle and pass it through a clean cloth, paper towel, or coffee filter. Then apply one of the following methods. The safest approach in the field is to filter and then disinfect.

Which method do I use?

Follow the diagram from top to bottom. Each "No" takes you to the next question.

How to prefilter and let water settle

Cloudy water shields germs from chlorine and UV light, and clogs filters. Clarifying it first makes any method work better. None of these steps disinfects: after clarifying, always boil or disinfect.

1. Let it settle. Leave the water still in a clean container until the sediment settles at the bottom (several hours to overnight if it is very cloudy). Without stirring, move the clear water from the top into another container with a ladle or with a clean hose as a siphon. Leave the bottom behind.
2. Filter through cloth. Pass it through a clean cloth, a bandana, or a cotton T-shirt folded in several layers, a paper towel, or a coffee filter (EPA). Replace or rinse the cloth when it clogs.
3. Repeat until the water looks clear. If it is still cloudy, use the double dose of chlorine.

Layered filter (sand and charcoal)

If you have a lot of cloudy water, you can build a filter with a bottle or a bucket with holes in the bottom. From bottom to top: cloth, gravel, coarse sand, crushed charcoal (from your campfire, not commercial briquettes, which contain additives), fine sand, and another layer of cloth. Pour the water in at the top and collect it at the bottom; discard the first liters.

1. Boiling (the most reliable method)

1. Clarify if the water is cloudy.
2. Boil at a rolling boil (large, constant bubbles) for 1 minute. Above 6,500 feet (1,981 m) of altitude, boil for 3 minutes. If your water utility issues a boil-water notice, follow its instructions (some notices call for 2 minutes at a rolling boil).
3. Let it cool covered.
4. Store it in clean, sanitized containers with lids.
5. Improve the taste by pouring it back and forth between clean containers several times, or letting it sit for a few hours.

Boiling kills bacteria, viruses, and parasites, but does not remove chemicals, salt, or metals. You need fuel and a safe way to cook (see chapter 4 on carbon monoxide).

2. Chlorine (household bleach)

Use only regular, unscented bleach, with no "color-safe" label, no "splash-less" agents, and no additives. Check the label: it must say sodium hypochlorite and its concentration. Bleach loses strength over time; replace it every year.

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1. Clarify the water if it is cloudy (let it settle and filter through cloth).
2. Add the bleach with a clean dropper according to the table (source: EPA). Double the dose if the water is cloudy, colored, or very cold.
3. Stir and wait 30 minutes.
4. Smell it: it should have a slight chlorine odor. If it does not, repeat the dose and wait another 15 minutes.
5. If it tastes too much like chlorine, pour it back and forth between clean containers and let it sit for a few hours.

"tsp" = teaspoon. If you only have 1% bleach, the CDC says to use 40 drops (½ tsp) per gallon.

3. Disinfecting tablets or drops

• Follow the label to the letter: each product is calculated for a specific volume and waiting time.
• Chlorine dioxide: kills bacteria, viruses, and Giardia; it is the most effective chemical against Cryptosporidium if you respect the label's waiting time (usually several hours).
• Iodine and chlorine: kill bacteria and viruses, but do not work well against Cryptosporidium.
• Do not use iodine if you are pregnant, have thyroid problems, or are allergic to iodine. No one should drink iodine-treated water for more than a few weeks in a row.
• Cold, cloudy water needs more time or a higher dose: check the label.

4. Portable filters

Look at the label: absolute pore size or NSF certification.

• Pore size of 1 micron or less (or NSF/ANSI 53 or 58 "cyst reduction"): removes parasites such as Giardia and Cryptosporidium.
• Pore size of 0.3 microns or less: also removes bacteria.
• No common portable filter removes viruses. After filtering, disinfect with chlorine, chlorine dioxide, or by boiling.
• Reverse osmosis removes all three types of germs as well as salt, but it needs pressure and special equipment.
• Pitcher-type carbon filters improve taste; they are not disinfection methods.
• Do not let a hollow-fiber filter freeze: it can crack without you noticing.

5. Solar disinfection (SODIS)

A last-resort option when you have no fuel or chemicals. It uses the sun's UV radiation and heat.

1. Use clear PET bottles (like soda bottles) of 2 liters or less, without scratches or labels. Glass, PVC, and polycarbonate do not work.
2. Use clear water. If you cannot read large letters through the bottle, filter it or let it settle first.
3. Lay the bottles down in direct sun, preferably on a sheet of metal or a reflective roof.
4. Wait 6 hours with sun or up to 50% clouds; 2 days in a row if it is very cloudy. It does not work on days of continuous rain.
5. Drink straight from the bottle to avoid recontaminating it.

SODIS does not remove chemicals or sediment, and you cannot verify by eye that it worked. Use another method if you have one.

6. Distillation (removes salt)

It is the only home method that separates water from salt, heavy metals, and many nonvolatile contaminants: you evaporate the water and collect the condensed vapor. At home: boil the water in a pot with the lid upside down (handle pointing down) and a cup hanging from the handle to catch the drops; it is slow and uses a lot of fuel. In the field, use a solar still.

1. Choose a sunny spot with soil that looks damp (the bottom of a dry streambed, for example).
2. Dig a bowl-shaped hole about 3 feet (1 m) wide and 2 feet (60 cm) deep, with a deeper pit in the center where the container fits upright.
3. Place the container and, if you have one, a tube with one end reaching the bottom of the container and the other coming out over the edge of the hole.
4. Put nonpoisonous green vegetation around it (it increases the moisture).
5. Cover with clear plastic and seal the edges with dirt and rocks.
6. Place a rock in the center to form an inverted cone, with the tip about 16 inches (40 cm) below ground level and right above the container. The plastic must not touch the soil of the hole, or the soil will absorb the water.
7. Drink through the tube without opening the still. Cap the tube between uses.

Real-world yield: according to FM 21-76, each still produces 0.5 to 1 liter in 24 hours; a person needs at least three to meet daily needs. Digging in the heat uses more body water than you will get: build it during the cool hours. To use salt or contaminated water, dig a trench about 10 inches (25 cm) deep about 10 inches from the edge of the hole and pour the water there: the soil filters it and the still evaporates only the water.

Bag still (above ground): fill a clear plastic bag ½ to ¾ full with nonpoisonous green leaves, close it, place it on a sunny slope with the opening facing downhill and a small rock at the low point so the water collects there. Replace the leaves when they stop producing water. Never use poisonous plants: the liquid will be poisonous too.