How to Purify Water in an Emergency: 4 Methods Compared

When the tap is unsafe or the supply is cut off, you need a way to make water safe. Four methods work in most situations, and each has strengths and limits. Always start with the clearest water you can find, and know that no method makes chemically contaminated water safe.

Step zero: clear the water

If the water is cloudy, let it settle and pour it through a clean cloth, paper towel or coffee filter. Clearer water makes every treatment method work better.

1. Boiling

Boiling is the most reliable method and needs no special equipment beyond a pot and a heat source. The CDC advises bringing water to a rolling boil for one minute, or three minutes at elevations above about 6,500 feet. Let it cool, and store it in a clean, covered container. The drawbacks are the fuel and time required, and the flat taste, which improves if you pour it back and forth between containers.

2. Filtering

A portable filter removes bacteria and protozoa and is fast and easy. Check the product to see whether it also handles viruses. Follow the maintenance and backflushing instructions, and replace the filter element when it is due. Do not let a filter freeze, since ice can damage the membrane.

3. Chemical treatment

Chlorine dioxide tablets or drops are light and inexpensive. Follow the label for dosage and wait time, which differs by product and by the organisms targeted. Cold or cloudy water usually requires longer contact time. Tablets are an excellent backup in any kit, since they take almost no space.

4. Ultraviolet (UV) light

UV purifiers use light to inactivate bacteria, viruses and protozoa in clear water. They are fast, but they need batteries, work only in clear water and do not remove particles or chemicals. Pre-filter cloudy water first.

Comparing the methods

Method Good for Limits

Boiling Any situation with fuel and a pot Time, fuel, flat taste

Filtering Fast treatment on the move May not remove viruses; can clog or freeze

Chemical Backup kit, travel Wait time, taste, effectiveness varies

UV Fast, clear water Needs batteries; clear water only

Layering for safety

In an emergency, combine methods for extra margin. For example, filter cloudy water and then treat it with chemicals or boil it.

What to avoid

• Do not rely on water that is colored, smells of fuel or chemicals, or comes from a possible chemical spill.
• Do not drink floodwater.
• Do not use bleach unless emergency guidance for your situation gives specific instructions, since the wrong product or dose can be dangerous.

Store it safely

Use clean, food-grade containers with tight lids and label them with the date. Keep them cool and out of sunlight.

Next step

Shop water treatment gear.