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Survival · Water

How to find and purify water when you cannot trust the tap

Field methods for finding, purifying, and storing water: boiling, filters, chemicals, and solar stills. Titles from 413 offline guides — the full protocols stay in the app.

§ 01

The direct answer

You need about 3 litres of drinking water per person per day at rest in a temperate climate, and 4 litres or more if you are walking, clearing debris, or in heat. That number fails before food does. Treat every untreated source as contaminated: surface water, rain off a dirty roof, melted snow, and well water after flooding.

Four methods cover almost every field case. Boiling: a rolling boil for 1 minute at sea level, 3 minutes above 2,000 m, kills pathogens. Filtration: a 0.1–0.3 micron filter removes protozoa and most bacteria; it does not remove viruses unless the filter is rated for them. Chemicals: unscented household bleach at ~2 drops per litre (5–6% sodium hypochlorite), 30 minutes wait, or iodine tablets per the packet. Solar stills and condensation: slow, low-volume, useful when no fuel and no tablets remain.

Order of work: settle sediment, filter or cloth-strain, then disinfect. Cloudy water wastes chemical dose and clogs filters. Do not drink seawater, urine, or blood — they raise the water you owe. The app holds 413 water articles behind this page: desert seeps, tropical vines, raft stills, CERT storage. Titles below. Protocols stay offline.

§ 02

Method comparison — one decision table

Pick the method you can actually run with what you have. Time and kill-range matter more than preference.

Boiling
Kills bacteria, viruses, protozoa. Needs fuel + pot. 1 min rolling boil (3 min above 2,000 m).
0.1–0.3 µm filter
Protozoa + most bacteria. Fast. Viruses usually pass unless the filter says otherwise.
Bleach / chlorine
~2 drops unscented 5–6% bleach per litre; 30 min. Weak on some cysts (Crypto).
Iodine tablets
Follow the packet. Not for long-term use or pregnancy. Taste is a feature — it means dose happened.
Solar still
Low yield (often <0.5 L / still / day). Last resort when you have sun, a pit, and plastic.
§ 03

In the offline library — titles only

12 titles · 413 articles in this cluster · full text in-app

  1. 01Water Storage and Purification for Emergencies: CERT-Recommended Methods
  2. 02Finding Safe Water in the Tropics: Animal Signs and Natural Indicators
  3. 03Desert Water Sources: Identifying and Evaluating Potential Supplies
  4. 04Water Procurement at Sea: Rain, Dew, Solar Stills, and Desalting Kits
  5. 05Constructing a Belowground Solar Still: Step-by-Step Instructions
  6. 06Building an Aboveground Solar Still: Collecting Water from Plants
  7. 07Using Polluted or Salt Water in a Solar Still: Safe Water from Unsafe Sources
  8. 08Solar Still and Condensation Techniques for Water Collection in the Jungle
  9. 09Improvising Water Containers in the Wild: Step-by-Step Guide
  10. 10Extracting Water from Vines, Roots, and Palms: Safe Methods and Dangers
  11. 11Avoiding Poisonous Plants When Building a Solar Still
  12. 12Hot Weather Sea Survival: Sun Protection and Hydration Strategies

Titles only. Full text, regional variants, and tables are in the offline library — they are not published on this site.

§ 04

Related

Rationing calculator →Fire →Offline library →

All nine topics: survival index. Educational reference — disclaimer.

Then get it on the phone

You’re reading this with a signal. The situation it describes is one where you won’t have one.