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How many drops of bleach to make water safe to drink, for any container; how many purification tablets; how much gas it takes to boil; and how long the water you have stored will last your household.
From the guide UK Crisis Survival: water →
A last resort when you can't boil or filter. Use only plain, unscented bleach with no detergent, thickener or fragrance (most "thick" bleaches are unsuitable), and check the strength on the label.
Bleach doesn't remove chemicals, fuel or heavy metals, and works less well on some parasites (such as Cryptosporidium). Never treat flood water, water from radiators or central heating, or water near industrial sites for drinking. There is no official UK dose: this uses the US CDC emergency dose (2 drops of 5–6% bleach per litre, doubled for cloudy or very cold water). A standard dropper gives about 20 drops per ml.
Follow the pack exactly: usually wait 30 minutes, longer if the water is cold. Chlorine dioxide tablets also kill most parasites.
Boiling is the best method: a full rolling boil kills bacteria, viruses and parasites. Let it cool, keep it covered, and use it within 24 hours.
Assumes about 12 litres per 220 g cartridge, boiling from cold with a lid on a camping stove. Wind, cold weather and boiling without a lid use much more. Use a winter-grade propane/isobutane mix below about 5 °C, and only cook with a window open and a carbon monoxide alarm.
| Source | Typical amount | Use |
|---|---|---|
| Bath, filled at the start | 150–200 L | Washing and flushing; drink only after treatment (or use a bath water bladder) |
| Hot-water cylinder | 100–200 L | Drain from the bottom tap with the heater off. Usually drinkable after boiling |
| Cold-water tank in the loft | 50–200 L | Washing; treat before drinking |
| Toilet cisterns (not the bowl) | 6–9 L each | Washing and flushing; treat before drinking. Not with blue cleaning blocks |
| Radiators and heating pipes | – | Never drink: chemical inhibitors |
| Method | Kills or removes | Doesn't |
|---|---|---|
| Boiling (best) | Bacteria, viruses, parasites | Chemicals |
| Tablets | Bacteria, viruses; chlorine dioxide also most parasites | Chemicals |
| Filters (0.1–0.2 micron) | Bacteria and parasites | Most don't remove viruses: boil or add a tablet too for town water |
| Bleach (last resort) | Bacteria and viruses | Some parasites; chemicals |
| Sunlight (SODIS) | Bacteria and viruses: clear bottles in full sun for 6 hours (2 days if cloudy) | Slow; needs clear water and sun |