Water: finding, filtering and purifying

Untreated water can carry bacteria, viruses and parasites. Filtering removes dirt and many germs. Disinfecting kills what is left. When in doubt, do both: filter first, then disinfect.

How much you need

Plan on about 1 gallon (3.8 litres) per person per day for drinking and basic hygiene. Heat, exertion, illness, children and nursing mothers need more. Store at least a 3-day supply, and two weeks if you can.

Finding water

Moving water is usually better than still water. Collect upstream of animals, camps and roads.
Rain collected off a clean surface is low in germs but still worth disinfecting.
Dig in the outside bend of a dry stream bed or at the base of green vegetation.
Morning dew can be wiped off grass with a cloth and wrung out.
Do not drink seawater or urine. Do not eat snow without melting it; it costs body heat.

Step 1: Clear it

Cloudy water makes every other method weaker.

Let it stand so dirt settles, then pour off the clear water.
Pour it through a clean cloth, coffee filter or paper towel.

A home-made filter of sand, charcoal and gravel makes water clearer, not safe. Always disinfect afterwards.

Step 2: Filter

Filter type	Removes	Does not remove
Microfilter, about 0.1 to 1 micron (most hiking filters)	Protozoa (Giardia, Cryptosporidium), most bacteria	Viruses, chemicals
Ultrafilter, about 0.01 micron	Protozoa, bacteria, viruses	Most chemicals
Activated carbon	Some chemicals, bad taste and smell	Germs (unless combined with a filter)

For Cryptosporidium and Giardia, look for a filter certified to NSF 53 or NSF 58 for cyst reduction or removal, or with an absolute pore size of 1 micron or smaller. Backflush or clean filters as the maker says, and never let a hollow-fibre filter freeze once it is wet: ice can crack the fibres without any sign.

Step 3: Disinfect

Boiling (best)

Bring water to a rolling boil for 1 minute. At high altitude, boil for 3 minutes: EPA says above 5,000 feet (about 1,500 m), CDC says above 6,500 feet (about 2,000 m). Use the lower figure to be safe. Let it cool on its own, covered. Boiling kills bacteria, viruses and parasites; it does not remove chemicals.

Household bleach

Use plain, unscented chlorine bleach that is less than a year old. Stir, then wait 30 minutes. The water should smell slightly of chlorine; if not, repeat the dose and wait 15 more minutes.

Bleach strength	Per 1 gallon (3.8 L)	Per 1 litre
6%	8 drops (about 1/8 teaspoon)	2 drops
8.25%	6 drops	2 drops

Double the dose if the water is cloudy, coloured or very cold. Chlorine does not reliably kill Cryptosporidium.

Tablets

Chlorine dioxide tablets work against bacteria, viruses and Giardia. Against Cryptosporidium they need a long wait (often 4 hours) and are only partly effective; follow the label, and filter as well when you can.
Iodine works against bacteria and viruses but not Cryptosporidium. It is not for pregnant women or people with thyroid problems, or for use for more than a few weeks.

Sunlight (SODIS)

If you have nothing else: fill a clear plastic (PET) bottle of 2 litres or less with clear water, lay it on its side in full sun for 6 hours, or 2 days if it is cloudy. It does not work on cloudy water or in glass that blocks UV.

Storing water

Use food-grade containers. Wash them with soap, rinse, and sanitise with 1 teaspoon of bleach per quart (1 L) of water.
Keep sealed water in a cool, dark place. Replace stored tap water every 6 months.

Sources

CDC: Making water safe in an emergency (US government, public domain)
CDC: Water treatment when hiking, camping or travelling (US government, public domain)
CDC Yellow Book: Water disinfection for travellers (US government, public domain)
EPA: Emergency disinfection of drinking water (US government, public domain)
Ready.gov: Water (FEMA, US government, public domain)