Small Cabin

Small Cabin Forum
 - Forums - Register/Sign Up - Reply - Search - Statistics -

Small Cabin Forum / Off-Grid Living / How much chlorine to use to purify water?
Author Message
# Posted: 16 Jul 2015 00:00 - Edited by: bldginsp

I've seen a lot of numbers tossed around for how much bleach to use to purify water. Some of those numbers seem like guesses, and I don't want to guess with bleach. I collect rainwater at the moment and want to clean the water in my 2500 gal. tank when friends come, so I'm wondering what the right amount is to use. So, I tried to research it and it wasn't easy. All I could find was that municipal water supplies shoot for 4ppm (parts per million) chlorine to kill bacteria. In swimming pools you are supposed to use 15-20ppm I think. And some people say if you are treating a tank of water that has stood for some time you should use more than normal. How much more? And what is 'normal'? No solid answer.

So anyway here's my math for shooting for the 4ppm that is used in city water. If we used the metric system the math would be easier, but we still use the King's measures, so here they are:

1gal = 4qts
1qt= 2pts
1oz.=500 drops

500 drops to the oz. is a standard that is used in the pharmaceutical industry. I guess in some cases pharmacists actually count drops for prescriptions.

Here goes:

4ppm=1/250,000. So we are trying to get one part bleach to 250,000 parts water. But regular household hypochlorite bleach in a bottle is not 100%, it is 8.25 percent. So says the label. But the label also says that only 7.25% bleach is actually available. To further confuse the issue, everybody seems to assume that regular household bleach is only 6%. I don't know why, perhaps someone can enlighten me. Anyway, assuming that Clorox regular bleach is 6%, in order to achieve a ratio of 1/250,000 bleach to water we divide 250,000 by 17 because 6x17= 100. In other words, Clorox off the shelf is 17 times weaker than 100% bleach, so we have to add 17 times as much to our mix. 250,000/17 is about 15,000. So by adding 6% Clorox at a ratio of 1/15,000 we achieve 4ppm actual chlorine in the water. Right? I think. Correct me if I'm wrong.

Based on that, here's hard numbers:

15,000 gal. water, add 1 gallon 6% Clorox.
3,750 gal, add 1qt.
1,875 gal, add 1pt.
937 gal, add 1 cup.
117 gal, add 1 oz. (2 tbsp.)
5 gal, add 22 drops
1 gal, add 4-5 drops

Those are basic numbers to achieve a baseline of 4ppm, you might add a bit more if you think your water might be iffy.

It pays to be careful. Just throwing in 'a splash' into a 5 gallon container can give you far, far more than is necessary, and bleach is, after all, a poison. Splashes are for martinis, if I'm going to treat small amounts of water with bleach I'll invest in an eye-dropper.

Math look good?

For convenience, here's an averaged chart:

15,000 gal, 1 gal 6% Clorox hypochlorite unscented bleach
3,750 gal, 1 qt.
2500 gal, 1-1/2 pt.
1875 gal, 1 pt.
1000 gal, 1 cup
500 gal, 1/2 cup
250 gal, 1/4 cup
100 gal, 1 oz. (2 tbsp.)
50 gal, 1 tbsp.
5 gal, 22 drops
1 gal, 4-5 drops.

# Posted: 16 Jul 2015 14:31 - Edited by: razmichael

Is this for drinking water?

The problem with bleach is that the concentration needed will very much depend on the amount of crud in the water. Another problem is that it does degrade fairly rapidly (over months) so the concentration on the bottle may not really be the actual strength, hence why there is often some guess work. This is also why I mix my own on a regular basis.
You also need to be careful comparing city drinking water concentrations and this water is processed well and the amount of residual chlorine is, in part, to maintain safety rather than the initial cleanup. The good thing about the chlorine is that it will disperse fairly quickly in the water (especially if exposed to air) so a levels will come down. You will however find lots of discussions on the dangers of the residual chemicals.

Your numbers match fairly closely to the two references I added to a recent thread (and these two match many other references). Almost all will talk about adding more if the water is dirty or you feel the risk is higher hence the guess work. Although not likely an issue, you also need to be aware of the limitations of Chlorine when dealing with certain cysts and spores.

EPA suggest 6 drops per gallon (8.25%) so slightly more than your estimate.
Army gives 2 drops per qrt with 4-6% so, again, just slighly higher than your calculations. This also states to ten drops/qrt if unsure about the strength and to double things for really dirty water.

I tend to put more than required in my lake water because of the added risk, and then let it sit longer, however this is only for washing dishes, showers etc. We bring in our drinking water plus I have a sawyer filter if needed (has not been).

Consider getting a pool test kit (not expensive) to actually test the levels after a rest period. I use one when I shock my well.

# Posted: 16 Jul 2015 14:48 - Edited by: Bret
Another site to look at. The bleach I purchased was of the 8% variety. It's good to get these ratios locked in for safey sake. Thanks for bringing up the topic.

# Posted: 16 Jul 2015 16:43

Quoting: razmichael
The problem with bleach is that the concentrated needed will very much depend on the amount of crud in the water.

Right on; that is why there is no exact recipe.

Somewhere I have read that one should do the first dose then mix / shake well and let sit for 30 minutes. Then if the water does not smell of chlorine do some more and repeat.

# Posted: 16 Jul 2015 16:56

That's accurate MtnDon - there should be a slight smell of chlorine after a while which shows that it has all not be used up (the chemistry is much more complicated than that before anyone jumps in). There is a fairly large spread of concentrations that are considered safe and so many factors involved that, as you state, there is no exact recipe. If you really are concerned about safe water then better to err on the side of more chlorine and let it rest longer to remove more of the residual. I always laugh as I throw bleach into the take after lugging up lake water while I watch the boys out swimming and sucking in mouthfuls. Then they go to a pool and get all that chlorine. If drinking then you can run it through a carbon filter to get rid of most of the smell - or air it out longer.

# Posted: 16 Jul 2015 17:44

do you have to clorinate your lake water that you pump up to your holding tank? We have a 3 phase filtration system which filters down to .1 microns so we were thinking we could do without the chlorine....are we wrong??? this is our first place with a cistern system....other place has a mountain spring that we tapped into and just filtered and had the best water in the world....
any suggestions appreciated!!

# Posted: 16 Jul 2015 17:51

Check out
CDC for some background. Cryptosporidium or Giardia are the biggest general worry and, actually, chlorination has a harder time killing these so if you have a good .1 absolute, you are likely in good shape (assuming it is cleaned and working properly.

# Posted: 16 Jul 2015 20:32
Reply I used this web site to determine the amount of bleach to use in our lake water. Not used for drinking. We use it for showers and dish washing.

# Posted: 17 Jul 2015 08:04 - Edited by: bldginsp

Here's a link to a page at the EPA site that silverwaterlady suggested which gives instructions on emergency disinfection of water:

The numbers they give for added chlorine are a bit higher than what I gave above, but not much. And, they confirmed what Don said with this:

"Stir and let stand for 30 minutes. The water should have a slight chlorine odor. If it doesn't, repeat the dosage and let stand for another 15 minutes before use."

I was always under the impression that if you can smell the chlorine, you've added more than you need. Of course remember that these are emergency instructions when you know that water is compromised.

One other thing to be aware of is that when chlorine enters water it forms compounds called chloramines by combining with organic compounds in the water. Chloramines are actually what you smell, that's why swimming pools smell so much. And, chloramines are carcinogenic. Filtration avoids this all together, but as pointed out above, you have to get filtration down to 1 micron to get the giardia and cryptosporidia cysts, which are 4-6 microns in size.

Ultraviolet light disinfection will kill the cysts, but it must be accompanied by particulate filtration, otherwise some cysts will survive the exposure to ultraviolet by hiding in the shadow of the particulates. OSTTM (Or So They Tell Me)

The only filtration systems allowed by the local Health Dept. for an approved water source where I am are systems with elaborate filtration and ultraviolet exposure. They are expensive, require constant electricity, and require maintenance.

# Posted: 17 Jul 2015 09:51 - Edited by: groingo

I am in a similar pickle but my source is a stream since my well crapped out.
I gather and hold the water in 20 gallon containers, rather than adding chemicals I run all potable and wash water through a pre filter then the Sawyer purification filter and then into smaller 3 and five gallon containers.
The Mini Sawyer filter is good for 100,000 gallons using a gravity feed that gives me 3 gallons in 15 minutes.
Being that I use four gallons of water a week, it should last a long time.

# Posted: 17 Jul 2015 10:32

Quoting: groingo
four gallons of water a week,

Wow! Here in the arid SW we go through at least 1.5 gallon per person per day. In summer more.

# Posted: 17 Jul 2015 13:52

Don't forget the Berkey type filters, a simple gravity feed through ceramic filters that has been around for 200 years. They claim it removes cysts, but I've also heard otherwise. Definitely removes everything else, very easy to use.

Your reply
Bold Style  Italic Style  Underlined Style  Thumbnail Image Link  Large Image Link  URL Link           :) ;) :-( :confused: More smilies...

» Username  » Password 
Only registered users can post here. Please enter your login/password details before posting a message, or register here first.