When to disinfect water with bleach
After a hurricane, a flood or a boil-water notice, tap or well water may carry bacteria and viruses. The safest option is bottled water; the next best is boiling for one full minute (three minutes above 6,500 feet). When you cannot boil water, the US Environmental Protection Agency (EPA) and the CDC recommend plain household bleach as a back-up. This calculator gives the number of drops for any volume and bleach strength.
The EPA dose
The EPA table is based on regular unscented bleach:
| Water | 6% bleach | 8.25% bleach |
|---|---|---|
| 1 quart or 1 liter | 2 drops | 2 drops |
| 1 gallon | 8 drops | 6 drops |
| 2 gallons | 16 drops (¼ tsp) | 12 drops (⅛ tsp) |
| 4 gallons | ⅓ teaspoon | ¼ teaspoon |
These amounts add about 6 milligrams of chlorine per liter. The calculator keeps that dose and adjusts the number of drops for the strength printed on your bottle, assuming a standard drop of 0.05 mL. If the water is cloudy, colored or very cold, the dose is doubled.
Worked example
You need to treat 1 gallon of clear water with 8.25% bleach. The calculator adds 0.29 mL of bleach, which is 6 drops, matching the EPA chart. For 5 gallons, use 29 drops, a little over ⅓ teaspoon.
Step by step
- If the water is cloudy, let it settle and pour it through a clean cloth, paper towel or coffee filter.
- Use a clean container and a clean dropper.
- Add the bleach, stir well and let the water stand for at least 30 minutes.
- Smell it. It should have a faint chlorine odor. If not, add the same dose again and wait 15 minutes more.
- If it still does not smell of chlorine, do not drink it.
Choosing the right bleach
Use bleach whose label lists sodium hypochlorite at 6% or 8.25% and says it is suitable for disinfection. Avoid scented, color-safe, splash-less and gel products, which contain additives, and old bottles: bleach loses strength over months, especially when warm.
Limits
Chlorine does not kill every germ. Cryptosporidium is highly resistant, and Giardia needs a longer contact time in cold water. Bleach does nothing against chemical contamination such as fuel, pesticides or floodwater runoff. If you suspect chemicals, use another water source.