Emergency Water Purification: Boiling, Chemicals and Filters

Water that looks clean can carry the thing that ends your emergency in a hospital. The three tools available to you — heat, chemicals and filtration — each cover different threats, and knowing which gap each one leaves is the whole skill.

Why this is the highest-value skill on the site

Waterborne illness is the classic secondary disaster: the event knocks out the supply, and dehydration from vomiting and diarrhoea does the actual harm. Getting water right removes the most likely serious outcome of nearly every scenario.

What you need

• A pot and a heat source for boiling
• Purification tablets with a known dose and stated contact time
• A filter rated to 0.1 to 0.2 microns for bacteria and protozoa
• Cloth or a coffee filter for pre-filtering sediment

Treat the water

1. 01

   Pre-filter first, always. Pour through cloth to remove silt and organics. Sediment shields microbes from chemicals and clogs filters within minutes.

2. 02

   Boil when you have fuel. A rolling boil for one minute kills bacteria, viruses and protozoa. Add two minutes above 2000 metres. Boiling is the most reliable method there is.

3. 03

   Use tablets when fuel is short. Follow the dose and, critically, the full contact time — usually 30 minutes, longer in cold or cloudy water.

4. 04

   Filter when the water is dirty but the source is not sewage. A 0.2 micron filter removes bacteria and protozoa such as giardia and cryptosporidium, but not viruses.

5. 05

   Combine for unknown water. Filter to clear it, then chemically treat or boil. Filtration plus disinfection covers every category of pathogen.

6. 06

   Store treated water separately in labelled, clean containers, and never let the dirty container touch the clean one's mouth.

The mistakes that make people ill

• Skipping the contact time on tablets because the water "looks fine"
• Filtering flood or sewage-contaminated water and drinking it untreated
• Cross-contaminating clean containers with dirty hands or lids
• Assuming a filter removes chemicals — it does not

Common questions

Is rainwater safe to drink? Usually much cleaner than surface water, but roof runoff picks up bird droppings and debris. Discard the first few minutes of flow and treat the rest.

How much household bleach? Only unscented, plain sodium hypochlorite, and only if you know the concentration. Purpose-made tablets remove the guesswork and are the better answer under stress.

Where kit is worth the money

A squeeze or gravity filter and a strip of purification tablets together weigh almost nothing and cost less than a night out. They cover the one failure mode that turns a manageable emergency into a medical one — which makes them the most sensible money in the entire preparedness budget.