How to make water safe to drink: boil, bleach, tablets, filter, UV

Which method for which water

The CDC ranks them plainly: boiling is the best way to kill germs in water, and the next best is to filter it and then disinfect what comes through.² What you start with decides the rest.

• Tap water under a boil notice: boil it, and use the boiled water for everything Health Canada lists: drinking, preparing food, brushing teeth, ice cubes, washing fruit and vegetables, hot and cold drinks.⁶
• A stream or lake on the trail: the CDC says drinking straight from even a clear mountain stream is generally not safe.² Boil it, or filter it and then disinfect it.
• No stove and no gas: bleach or tablets, by the dose and the wait below.
• Cloudy water: clear it first. Chemicals and UV work worse on cloudy water; boiling, the WHO says, works even on turbid water.¹⁴
• Water that may hold fuel or chemicals: none of these methods. See the last section.

Step by step

1. Clear it

   Let cloudy water settle, or strain it through a clean cloth, paper towel or coffee filter, then draw off the clear water.¹

2. Treat it

   Boil it for the time your authority gives, or add bleach or tablets at the dose it or the label gives.¹

3. Wait

   Time 30 min

   Let boiled water cool. After bleach, the CDC’s wait is at least 30 minutes before you drink it.¹

4. Store it

   Keep it in clean, sanitised containers with tight covers.¹

Boiling: how long, and at what altitude

That is the American rule, and Ready.gov asks for the same “one full minute”.⁵ Ireland’s water utility asks for a minute too.¹³ Germany, Sweden and the drinking water regulator for England and Wales count no minutes at all,¹¹ while Argentina’s medicines and food agency says 3 minutes¹⁶ and Mexico’s health ministry 5.¹⁵ The tables under “Where you live” set them side by side. Germany’s federal environment agency, the UBA, tells households to bring the water to a rolling boil once and let it cool slowly for at least 10 minutes, a kettle being the practical way to do it;⁷ Sweden says to boil until it bubbles vigorously with large bubbles.⁸

Boiling leaves nothing in the water to protect it afterwards, the WHO notes, so it can be contaminated again in a dirty jug.⁴ It also tastes flat. The CDC’s fix is to pour it from one container to another and let it stand for a few hours, or to add a pinch of salt per litre.¹

Bleach: the official dose, per litre and per gallon

Household bleach works where there’s no flame, if it’s the right bleach. The CDC says to follow the label’s instructions for drinking water first; if there are none, read the sodium hypochlorite strength under “active ingredient”, typically 5–9% in the United States.¹ It has to be plain bleach, with no scent, no colour-safe formula and no added cleaners,³⁵ and the EPA wants it fresh: kept at room temperature for less than a year.³

Double the dose for cloudy, coloured or very cold water, say the CDC and the EPA.¹³ After the wait the EPA expects a slight smell of chlorine; Ready.gov adds that if a second dose and 15 more minutes still give none, the water should be thrown away and another source found.³⁵

The table hides a disagreement. For bleach of about the same strength, Health Canada puts about 0.2 ml into a litre and the CDC 0.1 ml.⁶¹ Even a drop isn’t a fixed amount: two drops are 0.1 ml to the CDC and 0.2 ml to Health Canada. Follow the authority where you live, and measure its millilitres with a clean medicine dropper, Health Canada’s tool for the job, rather than borrowing another country’s drop count.

Tablets, filters and UV: what each removes

The CDC’s two pages don’t quite agree on chlorine dioxide: the emergency page says the tablets kill Cryptosporidium if you follow the instructions correctly, the hiking page calls them somewhat effective and says to filter first.¹² Tablets of every kind are made for a set amount of water, so the packet’s dose is the dose.¹³

Iodine splits the American agencies. The EPA gives five drops of 2% tincture of iodine per quart or litre, ten if the water is cloudy or coloured, then at least 30 minutes;³ Ready.gov says iodine, and camping-shop treatments that aren’t plain 5.25–6.0% sodium hypochlorite, are not recommended and should not be used.⁵ The CDC says to avoid iodine-treated water if you’re pregnant, have thyroid problems or are hypersensitive to iodine, and that nobody should drink it for more than a few weeks at a time.¹

In the shop, a “filter” strains out protozoa and bacteria and a “purifier” also deals with viruses: those are REI’s definitions.⁹ The CDC is stricter about what a filter does. Most portable filters remove parasites but not viruses or bacteria. One with an absolute pore size of 1 micron or less, or certified to NSF standard 53 or 58 for cysts, removes parasites; 0.3 micron or less removes bacteria too. Reverse osmosis filters remove viruses too, and salt with them.² So read the pore size on the box, and after filtering add a chemical disinfectant, the CDC’s step for the viruses and bacteria that get through.¹

A UV pen or bottle can kill parasites, bacteria and viruses in small amounts of clear water, the CDC says, and it says to filter the water first.² The sun does a slower version, and the CDC’s instructions for it are clear plastic bottles of clear water laid on their side in the sun for 6 hours if it’s sunny, or 2 days if it’s cloudy. Its wording for the result is careful: it improves the water by reducing the number of germs.¹

What no method fixes

The EPA adds heavy metals, salts and most other chemicals to what boiling and disinfection leave in. It says not to use water with floating material, a dark colour or a questionable smell, and that flowing water is generally better than still.³ And never use the water from radiators or a heating boiler for drinking, cooking or washing, the CDC says.¹

Where you live: the minutes, the dose and the label

The rule changes at the border, so find your own row and leave the others alone.

The UK’s bleach figure needs its note. It comes from a script the four Chief Medical Officers wrote for broadcasters to read out in a national power cut, which they present as their own advice, not government policy. If you can’t boil the water and have no in-date purification tablets, it says to add “approximately one-quarter of a teaspoon of unscented, household bleach for every 10 litres of water (approximately one bucket)” and leave it to stand for 15 minutes.¹² The script doesn’t say how strong the bleach is, and strength is what every other dose in this article hangs on. It does say that it’s important not to use too much, and that water with a very strong chlorine smell should stand uncovered for 2 hours before you drink it.¹²

Bleach isn’t one product worldwide, so a dose only works for the bottle it was written for.

• United States: 5–9% sodium hypochlorite in the CDC’s table, 6 or 8.25% in the EPA’s.¹³
• Spain: lejía is graded by grams of active chlorine per litre, not by a percentage. Only a bottle labelled apta para la desinfección del agua de bebida (fit for disinfecting drinking water) is meant for it, and its maker must print how much to add to reach 3 mg of active chlorine per litre of water. Bottles marked no apta are not.¹⁰
• Anywhere else: the CDC’s first instruction works for any bottle: follow the label’s own directions for drinking water, and if it has none, find the sodium hypochlorite strength before you use anybody’s table.¹

Whatever you make up with water, a pouch included, the CDC lists preparing food among the things untreated water shouldn’t be used for.¹ Chicken & Rice takes 370 ml, a Porridge & Fruit breakfast 250 ml, and the whole 72-Hour Food Reserve 3.1 l, all of it counted in how much water to store per person. With the power off, heating that water is the hard part, and how to cook without power sets out what may be lit where; with cold water the meals take 20–30 minutes depending on the meal, listed dish by dish in freeze-dried meals with cold water. On a trek every litre for a meal comes from a stream and gets treated first, and the water per day is part of food for a multi-day hike.

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