About Clorox, chlorine and water disinfection

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There's a lot of confusion regarding what kind of chlorine to use in disinfecting water, how much laundry bleach to use, what chlorine will do for you (and what it won't) and how to get the chlorine taste/smell out of the water again after you've disinfected it.

This is actually worthy of about four or five pages of text but I'll give a brief rundown here.

The proper kinds of chlorine products to use are those that have only chlorine as their active ingredients - no soaps, colors, scents, UV stabilizers or anything else. Some of these products can be plain dry pool chlorine, generally 60-70% calcium hypochlorite, which is very concentrated and has a long storage life but simply must be stored correctly; plain liquid pool chlorine, all I've seen has been in the ten percent range and it won't store as well as the dry stuff; plain laundry bleach (Clorox, etc.), generally about 5-5.25% percent sodium hypchlorite, and has a relatively short shelf-life so make sure your's is not more than a year old, preferably not more than six months old.

There are other chlorine products on the market but I'm not as familiar with them. Just make sure the only active ingredient is chlorine and nothing else.

The exact amount of any of these chlorine products to use depends on a couple of variables.

#1 - Has the water already been treated? If the water in question is municipal city water that's already been chlorinated once already then it will need very little, if any, additional chlorine. Adding a little more won't hurt anything but it's not necessarily getting you anything either. I'm the belt and suspenders type so I'll always add a little but it's not really necessary.

If the water is untreated but from a known safe source (my well, for instance), you're using clean containers, they'll be tightly sealed and kept in the dark, it's still not necessary to use chlorine but it can help to safeguard against any surprises that might happen while you're unawares. It is important to keep the caps and necks of these containers clean in storage.

If the water is of unknown cleanliness you certainly should treat it. This leads to variable #2.

#2 - What condition is the water in? Is it cloudy? Is it cold? The colder the water is, the more stuff it has suspended in it (cloudiness, turbidity) makes it more difficult for the chlorine to do it's job which requires more of it. This can sometimes be traded off by extending the contact time used.

With those two variables in mind you can then decide on how much chlorine to use in your water. Before giving specific recommendations and getting a number of people jumping up and waving their favorite authorities in my face let me tell you what you're trying to accomplish here.

Chlorine disinfects water by oxidizing the organics in the water (including microbial life, mostly). The point in adding so many drops of chlorine to a given quantity of water is to achieve a certain level of unreacted chlorine still left in the water after waiting the disinfection time out. Some sources, such as the Clorox company, start pretty high and others, like many governmental and NGO sources, start lower and work up. In achieving this you can have reasonable certainty that any lurking pathogens in the water have been killed off. Follow the directions and this will happen except for some hard cases like protozoal cysts like Cryptosporidium and Cyclospora and sometimes Giardia. No reasonable quantity of chlorine (or iodine)will reliably kill Crypto and Cyclospora spores but they're not as worrisome as other pathogens. If they really concern you, you'll have to either boil your water or use a proper filter. Despite what some will have you believe, chlorine will kill Giardia spores but you need longer contact times (an hour or more)and more careful use of chlorine to do it.

Having said all of that, here are two charts out of my book, The Prudent Pantry: Your Guide to Building a Food Insurance Program relating to laundry bleach and pool chlorine. They were developed from data found in FM21-10 Field Sanitation and Hygiene from the US Army, FEMA and Red Cross sources:

Chart #1 Plain, Fresh Laundry Bleach

Liquid bleach needed to achieve five parts per million in clear water at or above 40: F (4: C). If the water is cloudy or below 40: F (4: C) use double the amount:

1 quart 2 drops 1 gallon 8 drops 5 gallons 1/2 teaspoon 10 gallons 1 teaspoon 50 gallons 3 teaspoons (1 tablespoon) 100 gallons 7 teaspoons (2 - 1/3 tablespoons) 250 gallons 17 teaspoons (5 - 2/3 tablespoons)

Chart #2 Stock Solution Made from Dry Pool Chlorine

Stock solution to achieve five parts per million in clear water at or above 40: F (4: C). If the water is cloudy or below 40: F (4: C) use double the amount:

5 gallons 1 cup chlorine stock solution 10 gallons 2 cups chlorine stock solution 25 gallons 5 cups chlorine stock solution 50 gallons 1/2 teaspoon dry chlorine 100 gallons 1 teaspoon dry chlorine 250 gallons 2 teaspoons dry chlorine 500 gallons 3 teaspoons dry chlorine

Chlorine Stock Solution: Quickly dissolve one half teaspoon of dry chlorine into two and one half quarts of clear water. Stock solution should be kept tightly sealed in a cool, dark place and used within two to three days.

Mix the recommended amount of chlorine into the water, wait thirty minutes and smell carefully. If you don't smell chlorine mix in another dose of chlorine and wait another fifteen minutes before use.

If you're going to be treating really large amounts of water at one time you'd do well to invest in a good pool test kit so you can directly measure the amount of free chlorine there is in the water. A company by the name of Taylor makes one of the best kits.

Any suspended solids you can strain out of the water first will make it that much easier to disinfect. If the water is really murky try letting it sit for a couple of hours to overnight undisturbed in a container and carefully pour off the clearer water from the top.

Now all through this ever lengthening post I've used the word "disinfection" rather than "purification" because that's all chlorine, iodine, hydrogen peroxide, "Stabilized Oxygen", etc. can do for you. They'll kill microbial life in the water but they cannot remove chemical contamination. In order to do that you'll have to resort to the use of an appropriate activated charcoal filter (a research project in itself), distillation or methods even more exotic. If at all possible, try not to use any water you suspect may have chemical pollutants in it.

Chlorine, particularly liquid chlorine products like bleach, are sensitive to environmental conditions and do not easily lend themselves to use in situations like backpacking and other on-the-go type of events. For these situations iodine works with more certainty and ease of use.

Getting rid of the taste/smell of chlorine is as easy as just leaving an open container of chlorinated water on the kitchen counter over night, setting the open container in the sun for a few hours, pouring the water back and forth between two clean containers for a few minutes and so on and so on.

There is a Water Treatment FAQ on the Providence Cooperative site if you'd like to explore some of the other possibilities and I also cover water treatment, storage and location in great depth in The Prudent Pantry.


The Prudent Food Storage FAQ, v3.5


-- A.T. Hagan (athagan@sprintmail.com), December 15, 1999


Since you mentioned dry pool chlorine, this is a very strong oxidizer. Do not store near flamable liquids, do not spill framable liquids on it. It is sstrong enough for spontanious combustion when mixed with flamables.

-- goldbug (goldbug@mint.com), December 15, 1999.

Bleach begins to lose its power at six months. At one year, it has lost half its strength--use twice as much or until there is a faint smell of chlorine to the water.

-- Old Git (anon@spamproblems.com), December 15, 1999.

Sorry about the charts. They looked pretty good when I put them in but they lost all formatting when I posted.

Dry pool chlorine does have to be properly stored. The label directions will make it clear about how best to do so.

Laundry bleach should be kept tightly capped when not in use and not stored in the light, even in its opaque jug.


The Prudent Food Storage FAQ, v3.5


-- A.T. Hagan (athagan@sprintmail.com), December 15, 1999.

I think I read somewhere that you might be able to extend the useful life of Clorox by transferring it to tightly capped glass jars (e.g., mason jars). Is this true?

-- eve (123@4567.com), December 17, 1999.

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