Water treatment methods

The simplest method

CDC guidance is clear: bring water to a rolling boil for one minute at elevations below 6,500 feet, or three minutes above that elevation. A rolling boil means large bubbles breaking the surface continuously, not just small bubbles forming on the bottom of the pot.

Boiling kills bacteria, viruses, and parasites. It is effective against every biological pathogen of concern in drinking water. The common advice to boil for 5 or 10 minutes is outdated and wastes fuel, which may be in short supply during an emergency.

Boiling does not remove chemical contaminants, heavy metals, or dissolved solids. If your water source is contaminated by a chemical spill or industrial runoff, boiling will not help. Switch to bottled water or a different source.

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When you cannot boil

EPA's emergency disinfection guidance for regular unscented household bleach (8.25% sodium hypochlorite) is 6 drops per gallon of clear water, or 12 drops per gallon of cloudy water. Mix thoroughly and let stand for 30 minutes.

After 30 minutes, the water should have a faint chlorine smell. If it does not, repeat the dose and wait another 15 minutes. The chlorine smell means the disinfectant has done its work and there is residual capacity left over.

The 6-drop figure is calibrated for the current standard bleach concentration of 8.25%. Older references citing 2 drops per quart were based on the previous 5.25% formula. Using the old ratio with modern bleach results in under-treatment. Always check the sodium hypochlorite percentage on the label.

Do not use bleach with added fragrances, surfactants, or color-safe formulas. Only plain, unscented household bleach is safe for water treatment.

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Effective but conditional

UV purification devices (like SteriPEN) use ultraviolet light to inactivate bacteria, viruses, and protozoa. Most treat one liter in 60 to 90 seconds when the water is clear.

The critical limitation is turbidity. Cloudy or silty water blocks UV light from reaching pathogens sheltered inside suspended particles. Pre-filter or settle turbid water before UV treatment. If you cannot get the water visually clear, use boiling or chemical treatment instead.

UV treatment leaves no residual disinfectant, meaning the water can be recontaminated after treatment. Treat water just before drinking, and store treated water in clean, covered containers.

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When to use which

Boiling is the first choice when fuel is available. It works on any water clarity and kills everything biological.

Bleach is the backup when fuel is limited or unavailable. It requires clear water for reliable results and a 30-minute wait. Keep a small bottle of unscented bleach in your emergency kit.

UV is fast and effective for clear water but requires batteries or charging and does not work on turbid water. Best as a complement to other methods, not as a sole treatment.

No single method handles every scenario. The most resilient household keeps supplies for at least two methods.

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