How to make water safe to drink in an emergency: boiling, bleach, tablets, filters

After a flood, an earthquake that damaged pipes or a failure at the treatment plant, tap water can become unsafe. Authorities usually announce this through the media or phone alerts. If your bottled water runs out, you need to know how to make water drinkable yourself.

First: clear the water

If the water is cloudy, let it settle for a few hours, then pour it slowly into another container through a few layers of gauze, a clean cotton cloth or a coffee filter. Particles shield germs from chlorine and UV, and clog filters quickly. Clearing doesn’t make water safe; it just prepares it for treatment.

Method 1: boiling

The safest method available to anyone. It kills bacteria, viruses and parasites.

1. Bring the water to a rolling boil (big, continuous bubbles).
2. Keep it boiling for 1 minute. Above 2,000 meters (6,500 feet), 3 minutes.
3. Let it cool covered and store it in a clean, closed container.

Boiled water tastes flat. To improve it, pour it back and forth between two containers a few times to aerate it.

What it doesn’t do: remove chemicals, heavy metals or nitrates.

Method 2: unscented bleach

Useful when you can’t boil. Use plain bleach, unscented and with no additives, with the concentration shown on the label.

Hypochlorite strength	Drops per liter	Drops per gallon (3.8 L)
5-6%	2 drops	8 drops
8.25%	2 drops	6 drops

For cloudy or very cold water, double the dose.

1. Add the bleach and stir.
2. Wait 30 minutes.
3. The water should smell slightly of chlorine. If it doesn’t, repeat the dose and wait another 15 minutes.

These amounts are the ones recommended by the US Environmental Protection Agency (EPA). Use a clean dropper kept just for this.

What it doesn’t do: it is less effective against some parasites such as Cryptosporidium, and it doesn’t remove chemicals.

Method 3: purification tablets

Tablets based on chlorine dioxide or NaDCC are easy to use and keep in a go bag. Follow the dose and waiting time on the pack exactly (usually 30 minutes to 4 hours, depending on the product and water temperature). Chlorine dioxide tablets also work on parasites resistant to ordinary chlorine.

Method 4: filters

• Portable filters (straw, bottle or pump) with 0.1 to 0.2 micron pores remove bacteria and parasites. Most don’t remove viruses, so for water from populated areas (contaminated with sewage), combine them with bleach or tablets.
• Kitchen filter jugs with carbon improve the taste of tap water but don’t make it safe if it’s microbiologically contaminated.
• Under-sink reverse osmosis removes almost everything, including nitrates, but needs mains pressure and often electricity.

More on filter types and how to choose one in the water filters guide.

Which method to choose

Situation	Recommended method
Authorities say tap water is unsafe, you have gas or power	Boiling
No way to heat water	Tablets or bleach
Spring or river water, while hiking	Portable filter, possibly plus tablets
Well water after a flood	Don’t drink it until it’s disinfected and tested
Suspected chemical contamination	Don’t use it at all; find another source

Well water after floods

A flooded well must be pumped out, cleaned and disinfected, and the water tested before drinking. Local health authorities can usually test it and advise on disinfection. Until then, use bottled water.