How to Use Calcium Hypochlorite to Purify Water

When I was writing my book, Survival Mom, I was a real stickler for accuracy. Chapter 2 is all about water storage and purification, so I went to water expert Alan Martindale who is the Water Quality Supervisor for the City of Mesa in Arizona with all my unanswered questions about water safety.

I asked Alan about the use of calcium hypochlorite, aka pool shock, for purifying water because I wanted an expert to weigh in on this popular survival and prepper topic. I’ve written directions for its use in purifying water here.

Here is what Alan had to say:

“Lisa, I would agree that calcium hypochlorite can be used to disinfect water.  As you mention, the key is to make sure it is intended to be used for purification of drinking water.  The standard for this is National Sanitary Foundation (NSF) approved.  Calcium hypochlorite can be purchased in several concentrations. Although 65% is most common, you will also see 78% available chlorine.

Need to be real careful with chlorine concentrations.  You shouldn’t really drink much over 2ppm (parts per million) or it can cause diarrhea.  The maximum contaminant level (MCL) set by the EPA for chlorine is 4ppm.

Figuring chlorine concentration is based on weight.  Cal-shock 65 is 65% available chlorine so 1 pound = 0.65 pounds chlorine.  0.65 pounds (10.5oz) chlorine will treat 60,000 gallons of water to an initial residual to 1.3ppm!  To treat one gallon to 1.3ppm you would use 0.65/60000=  0.00017oz calcium hypochlorite.  1 ounce will treat about 5700 gallons.  These numbers are hard to understand and apply at small quantities, like a few gallons.

I read on one website that one granule, the size of a period, would treat one gallon and 1/8th level teaspoon would treat 55 gallons to 5ppm.  Sounds about right but I can’t confirm!

A huge issue associated with chlorinating is the “chlorine demand” of the water being treated.  If one water source has more contamination than another, it will take more chlorine to make it safe. Could be two or three times more.  Many emergency sites recommend having some test equipment to verify chlorine residuals. A pool test kit or test strips will work just fine.

Now having said all this, I probably complicated your issue.  Bottom line, calcium hypochlorite will work great but, it will take more experience and care to get safe results than using unscented laundry bleach.”

Pool test kits are inexpensive and can be purchased just about anywhere pool chemicals are sold. I recommend that everyone who owns calcium hypochlorite and is planning to use that as a way to purify water have at least one pool test kit on hand. That will be the best way we amateurs can determine that the chlorine level of our purified water is safe to drink.

As Alan states, using store-bought bleach is easier (8 drops per 1 gallon of water for purification purposes), but bleach has a shelf life. It begins to lose its effectiveness after several months, thus the popularity of calcium hypochlorite in survival circles.

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