Understand Water Treatment Basics

• Tell storage, filtration, disinfection and safe drinking water apart
• Follow CDC boiling guidance correctly, including at altitude
• Understand emergency bleach disinfection and its limits, using published amounts
• Recognise water that must never be drunk regardless of treatment
• Know what to do during a boil-water advisory

• Bottled water

  Standard

  Commercially bottled water — the first choice in every advisory.

  Low cost

  Whatever sealed bottled water your store still has; buy early.

  Already have

  Your existing stored supply, which is exactly what it is for.

• A way to boil

  Standard

  A pot and any working heat source.

  Low cost

  A camping stove used outdoors only.

  Already have

  A gas hob, a kettle, or an outdoor grill — never a fuel-burning stove indoors.

• Unscented household chlorine bleach

  Standard

  Plain unscented liquid household chlorine bleach labelled for disinfection, with the sodium hypochlorite percentage shown on the label.

  Low cost

  The smallest bottle sold; you need drops, not cups.

  Already have

  Bleach you already own — only if it is unscented, contains no added cleaners, and is not past its date. Bleach loses strength with age.

• Cloth or filter for cloudy water

  Standard

  A coffee filter or fine cloth for pre-filtering sediment.

  Low cost

  A clean tea towel or paper towel.

  Already have

  Any clean tightly-woven cloth.

1. 01Separate the four words that get used interchangeablyStorage keeps safe water safe — it treats nothing. Filtration physically removes particles and, depending on the filter, some germs — it is not automatically disinfection. Disinfection kills or inactivates germs using heat or chemicals — it removes no chemicals. Potable drinking water means water safe to drink, which requires that nothing harmful of any kind is present. A step is not a guarantee.
2. 02Use bottled water first, alwaysIn any advisory or contamination event, CDC and Ready.gov both put commercially bottled water first. Treatment is what you do when safe water isn't available — not a preferred alternative to it.
3. 03Boiling — the most reliable method you can do at homeCDC guidance: if water is cloudy, first let it settle and filter it through a clean cloth, paper towel or coffee filter. Bring the clear water to a rolling boil for one minute. At elevations above 6,500 feet (about 2,000 metres), boil for three minutes. Let it cool naturally and store it in clean sanitized containers with tight lids.
4. 04Understand what boiling does and does not doBoiling kills bacteria, viruses and parasites. It does not remove chemicals, fuels, solvents, pesticides, heavy metals or most toxins — and because water evaporates as it boils, it can actually concentrate them. Never boil water you suspect of chemical contamination.
5. 05Disinfecting with bleach, when you cannot boilUse only plain unscented liquid household chlorine bleach suitable for disinfection, and read the label for the sodium hypochlorite percentage — dosing depends on it. CDC's published amounts for clear water: for 5–9% bleach, add 2 drops per quart or litre, 8 drops (about ⅛ teaspoon) per gallon, and ½ teaspoon per 5 gallons. For cloudy water, filter and settle it first, then double the amount. Stir well and let it stand for at least 30 minutes before use.
6. 06Check the result, and know when to stopAfter 30 minutes the water should have a slight chlorine smell. If it does not, repeat the dose and let it stand another 15 minutes before use. If it still has no chlorine smell after that, discard it and find another source. Do not keep adding bleach beyond this — more is not safer.
7. 07Know what bleach disinfection cannot handleChlorine is less effective against some parasites, notably Cryptosporidium, and it does nothing about chemicals. Where parasites are a concern, boiling or a filter rated to remove them is the appropriate step. Never use scented bleach, colour-safe bleach, bleach with added cleaners, or pool chemicals to treat drinking water.
8. 08Filters — read what the filter actually claimsA filter only does what its label and certification say. Many remove sediment and improve taste while removing no viruses at all. For parasites, look for a filter certified to remove Cryptosporidium and Giardia — typically an absolute pore size of 1 micron or smaller. For water of unknown quality, public-health guidance is to filter and then disinfect the filtered water, not to rely on filtration alone.
9. 09Water you must not drink at allNever drink flood water, or water you suspect contains fuel, sewage, industrial or agricultural chemicals, or unknown contamination — even after filtering, boiling or bleach. Those methods do not remove chemicals. Flood water can also carry sewage; do not use it for drinking, cooking, washing food, brushing teeth, washing dishes or washing hands. Wait for official guidance or use bottled water.
10. 10During a boil-water advisoryFollow your utility's instructions exactly, and treat it as covering more than drinking: use bottled or properly boiled water for drinking, ice, brushing teeth, washing produce, preparing food, mixing infant formula and pets. Throw out ice made during the advisory. Keep following it until the utility says the water is safe again — not when it looks or tastes fine.
11. 11Wells and rainwater are not exceptionsA private well is not automatically safe: test it regularly, and after any flooding have it inspected, disinfected and tested before drinking. Rainwater collected from a roof picks up whatever is on that roof — bird droppings, dust, roofing chemicals — so it is not drinking water without appropriate treatment, and local rules on collection vary.

Verified guidance · CDC — Making Water Safe in an Emergency