Treating uncertain water
A treatment train covering how much to store, source choice, settling, filtering, disinfection and clean storage.
Critical caution: Boiling does not remove fuel, salt, pesticides, metals or radioactive contamination.
- Time
- 30–90 minutes
- Difficulty
- Moderate
- Outcome
- Produce microbiologically safer drinking water
Survival lens: The fieldcraft principle is to reduce uncertainty in stages. Do not depend on one miracle device: protect the source, remove sediment, apply a validated treatment and prevent recontamination.
Before you begin
- Reject water exposed to fuel, pesticides, saltwater, industrial chemicals or radioactive fallout
- Choose bottled water first
- Keep raw and treated containers separate
What you need
- Clean covered containers (food-grade)
- Pot and heat source
- Cloth or sediment pre-filter
- Validated water filter if available
- Plain unscented bleach and a dropper
- Labels and marker
Step-by-step procedure
- Plan the quantity. Queensland Get Ready advises packing at least 10 litres of drinking water per person for a three-day kit; CDC advises at least one US gallon (about 3.8 litres) per person per day for at least three days. Heat, illness, pregnancy, breastfeeding and heavy work increase need. Add water for animals, hygiene and cooking.
- Choose the least contaminated source. Commercially sealed water is safest. Next consider known-safe stored tap water, then protected rain or groundwater. Surface water requires treatment and may still contain chemicals.
- Clarify cloudy water. Let sediment settle, pour the clearer water off carefully and pass it through a clean cloth, paper towel or coffee filter. This improves later treatment but does not disinfect.
- Filter for the right hazard. Use a certified filter according to its instructions. Many portable filters remove bacteria and protozoa but not viruses, salt or dissolved chemicals.
- Boil when possible. Bring clear water to a rolling boil for 1 minute. CDC advises 3 minutes at elevations above 6,500 ft (about 2,000 m). Cool naturally while covered.
- Disinfect with bleach only when you cannot boil. CDC’s dose is for plain, unscented household bleach containing 5–9% sodium hypochlorite: 8 drops per US gallon (roughly 2 drops per litre). Double the dose for cloudy, coloured or very cold water, then wait at least 30 minutes before drinking. Bleach strength varies by country and brand—check the label and use the product’s own drinking-water directions or local health-authority advice if the strength differs.
- Store without recontamination. Use a clean, covered, narrow-neck container or tap. Pour rather than dip cups or hands. Mark treatment method and date.
- Keep a water ledger. Record litres collected, treated, used and reserved. Recalculate days remaining every evening.
How to check it is working
- No chemical, fuel or salt contamination suspected
- A full rolling boil was achieved, or the bleach dose matched the label strength and waited 30 minutes
- Treated water never contacted raw-water tools
- Container is covered, labelled and shaded
- Days of water remaining is calculated
Common mistakes
- Assuming clear water is safe
- Believing boiling removes chemicals or salt
- Using scented, splash-less or “colour-safe” bleach
- Guessing a bleach dose without knowing its strength
- Letting clean hoses, taps or ladles touch raw water
Sources and further reading
- CDC: How to make water safe in an emergency
- CDC: How to create an emergency water supply
- Queensland Government Get Ready: emergency kit
- WHO: Water, sanitation and health in humanitarian emergencies
How this guide was checked
Official-source aligned: Aligned with CDC emergency drinking-water guidance and WHO emergency WASH guidance
Independent specialist review: not yet completed. How review works.
Last updated: Nov 2026 · edition 1.0.
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