Water Purification: Boil, Filter, Bleach, Tablets

During a boil order, a flood, or a long outage that knocks out a town's water plant, you may need to make your own drinking water safe. Stored water is the first line of defense (see water storage: how much and how to rotate), but knowing how to treat water is the backup for when you run out.

01Boiling: the gold standard

Boiling kills bacteria, viruses and parasites.

• Bring water to a rolling boil for at least one minute. At altitudes above about 5,000 feet, the EPA says boil for three minutes.
• If it's cloudy, let it settle and filter it through a clean cloth, paper towel or coffee filter first.
• Let it cool and store it in clean, covered containers.

The downside: it takes fuel. In an outage, that means a camp stove used outdoors, a grill outdoors, or a wood stove. Never use outdoor cooking equipment inside.

Boiling doesn't remove chemicals, fuel or heavy metals. If you think your water is contaminated with chemicals (floodwater near a factory, for example), boiling won't make it safe.

02Bleach: cheap and effective

Plain, unscented household bleach kills most bacteria and viruses. The EPA's emergency guidance:

• 6% bleach: 8 drops per gallon of water.
• 8.25% bleach: 6 drops per gallon.
• Double it if the water is cloudy, colored, or very cold.
• Stir and let it stand 30 minutes. It should have a slight chlorine smell. If it doesn't, repeat the dose and wait another 15 minutes.

Check the label for the strength, and only use bleach with no scent, no additives and no "splash-less" formula. Bleach loses strength over time, so rotate it.

Bleach is less effective against some parasites like Cryptosporidium. Boiling is better for those.

03Filters: good for parasites and bacteria

Portable water filters, gravity filters and filter bottles remove bacteria and parasites if their pores are small enough. Look for:

• A pore size of 1 micron or smaller for parasites like Giardia and Cryptosporidium.
• Smaller (around 0.1 micron or less) to remove bacteria too.
• Most filters don't remove viruses. If viruses are a concern (flooding with sewage), filter and then disinfect with bleach or boiling.

Follow the manufacturer's instructions for how much water the filter can treat before you replace it.

04Tablets

Water purification tablets, often chlorine dioxide or iodine, are light and easy to store. They're great for a bug-out bag or a car kit.

• Chlorine dioxide works against bacteria, viruses and many parasites, with a longer wait time (read the label; sometimes several hours for Cryptosporidium).
• Iodine isn't recommended for pregnant women, people with thyroid problems, or long-term use.

05What to keep at home

A good home setup:

1. Stored water for at least several days.
2. A bottle of plain bleach and a medicine dropper, with the measurements taped to the bottle.
3. A gravity filter or a good portable filter.
4. Tablets in your 72-hour kit and car kit. See 72-hour kit checklist.
5. A way to boil water safely, outdoors or on a wood stove.

06Other sources of water at home

Before you treat water from outside, use what's already in your house: your water heater tank, ice cubes, and the water in your pipes.

Never drink water from a radiator, a hot-water heating system, a waterbed, or a pool without serious treatment. Toilet tank water (not the bowl) is OK to treat if it hasn't had chemical cleaners added.

Sources (1)

EPA: Emergency disinfection of drinking water.

Photo: 2008-07-05 Water boiling in cooking pot by Ildar Sagdejev (Specious), CC BY-SA 4.0.

Related: Sanitation and Hygiene When the Water Is Off