Sunday, February 17, 2008

Emergency Disinfection of Drinking Water: How long do you REALLY need to boil water to purify it?

There are so many conflicting guidelines for emergency disinfection of drinking water, that many people may decide to boil water longer than is necessary, just to be on the safe side.

In most crisis situations fuel may be in very short supply, so knowing exactly how long to boil water to safely purify it may prove to be helpful.

The United States Center For Disease Control and Prevention (CDC) and the US Environmental Protection Agency (EPA) recommends boiling water for 1 minute, or 3 minutes above 2 kilometers (6562 feet) to disinfect water for drinking.

The CDC recommends boiling water for one minute to kill most organisms. One minute is long enough to kill all major harmful waterborne bacteria and protozoa including Esherischia coli, Salmonella, Shigella sonnei, Campylobacter jejuni, Yersinia enterocolitica, Vibrio cholerae, Legionella pneumophila, Cryptosporidium parvum, Giardia lamblia, Entamoeba histolytica and other pathogens.

Waterborne viral pathogens such as hepatitis A, which is known to be one of the more heat-resistant viruses, are also inactivated by one minute of boiling.

If you can't boil water for drinking, here is what the CDC recommends:

"If you can't boil water, you can treat water with chlorine tablets, iodine tablets, or unscented household chlorine bleach (5.25% sodium hypochlorite). If you use chlorine tablets or iodine tablets, follow the directions that come with the tablets. If you use household chlorine bleach, add 1/8 teaspoon (~0.75 milliliter [mL]) of bleach per gallon of water if the water is clear. For cloudy water, add 1/4 teaspoon (~1.50 mL) of bleach per gallon. Mix the solution thoroughly and let it stand for about 30 minutes before using it. Treating water with chlorine tablets, iodine tablets, or liquid bleach will not kill many parasitic organisms. Boiling is the best way to kill these organisms."

Unfortunately liquid bleach has a fairly short 'shelf life', but granular calcium hypochlorite lasts a long time. The EPA has easy to follow guidelines for using it for emergency disinfection of drinking water. Several methods are discussed here, including using granular calcium hypochlorite to disinfect water. Calcium hypochlorite is the main ingredient in 'pool shock', a powdered product commonly sold to disinfect swimming pools.

Caution: Use only 'high test'
calcium hypochlorite that contains a minimum of 70 percent, preferably 78% available chlorine.

Here is what the EPA says about using granular calcium hypochlorite to disinfect water:

Make a concentrated 'stock' solution (liquid bleach):

Add and dissolve one heaping teaspoon of high-test granular calcium hypochlorite (approximately ¼ ounce) for each two gallons of water, or 5 milliliters (approximately 7 grams) per 7.5 liters of water. The mixture will produce a stock chlorine solution of approximately 500 milligrams per liter, since the calcium hypochlorite has available chlorine equal to 70 percent of its weight. To disinfect water, add the chlorine solution in the ratio of one part of chlorine solution to each 100 parts of water to be treated. This is roughly equal to adding 1 pint (16 ounces) of stock chlorine to each 12.5 gallons of water or (approximately ½ liter to 50 liters of water) to be disinfected.

Mix the treated water thoroughly and allow it to stand, preferably covered, for 30 minutes. The water should have a slight chlorine odor. If not, repeat the dosage and allow the water to stand for an additional 15 minutes. If the treated water has too strong a chlorine taste, allow the water to stand exposed to the air for a few hours or pour it from one clean container to another several times.

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This article is an excerpt from Mrs. Tightwad's Handbook #1: HOW TO SURVIVE DISASTERS AND OTHER HARD TIMES. For more information, see the left sidebar on this site:

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