Watchful said:
I'd like to see a definitive source on this. I've seen people recommend everything from 4 to 5 to 7 to 8... and based on the amount of chlorine present in the bleach, even more.
Overuse or underuse of bleach can be dangerous. Can we get consensus on this?
I am an engineer who specializes in public water systems. A big part of what I do is design of small treatment plants, and I have a special interest in disaster-related water treatment.
There are some basic facts about bleach that need to be understood before determining what dose to use.
1) Chlorine bleach (sodium hypochlorite) degrades with time. The half life, i.e., the time it takes to decude to half its effectiveness, is usually about 3 months, but varies considerably, and depends upon exposure to air and storage temperature. Newly manufactured bleach (notice I didn't say newly purchased) has a concentration of around 5.25%. 3-month old bleach therefore should be treated as though it were 2.625%, so you'd need twice as much.
2) Chlorine reacts with lots of things, but you want to use it for bugs. If you have a lot of other things in the water, they "use up" a lot of chlorine before it can kill the bugs. It is entirely possible to use up all the chlorine with non-living suspended and dissolved components in the water, so nothing is left over for bugs. You cannot tell how much chlorine the water will use up just by looking at it. However, all other things being equal, cloudy or colored water (cloudy and colored are not the same) will require a much higher dose.
3) Water temperature matters. Chemical reaction rates proceed at much slower rates in cold water. You are trying to basically burn these bugs to death with a chemical agent (gross oversimplification, but you get the idea). The bleach burns them a lot slower in cold water, so you have to let it sit a long time, sometimes hours, to get the same effect for a given dose as on a hot day.
4) Water pH matters. At pH 6.5 and below, almost all of the chlorine is in the form HOCl (hypochlorous acid). At pH 8.5 and above, almost all of it is OCl- (hypochlorite ion). HOCl is the stronger of the two, but not for some things.
5) Brand of bug matters. Some bugs are really easy to kill, some are darned near impossible to kill. Some bugs are killed more easily at low pH; some things, like some viruses, are killed more easily at higher pH.
What I do in the backcountry is first filter my water through a GOOD filter like a Katadyn, then put a couple of drops of fresh, unscented bleach in each quart or liter and let it sit for a half hour. The filter removes most things that will make you sick, plus a lot of the constituents that use up chlorine before I dose it. Good-tasting, safe water.
If you have murky water and no filter you can add a pinch of alum to a quart, shake it up, and let the snowflakes settle. Then pour the clear part off into another container (the one you drink out of, and dose that with bleach. A few drops to the quart and a half hour is my gold standard.
What I tell the guys who run the plants I design is to use their lab equipment and maintain a certain "chlorine residual" that has been determined based on a chemical analysis of the water. Often times filtration and fancier methods are involved too.
The discrepancy in the recommended bleach doses is probably based on regional needs, personal experience, and judgement. None are "wrong" per se. The "correct" dose truly depends upon things which are just not knowable in the field.
Now, what I would do in a long-term backwoods situation is build myself a slow sand filter. Lots of drinking water has been made by doing little more than trickling the water through about 4 ft of fine sand at a low rate. There is a guy who did some work with less developed societies using shallow wells in fine sandy sediment. You can just about build a decent filter using ordinary soil and a hollowed out log.
I don't have the time today to write more, but some good resources are the AWWA bookstore (
www.awwa.org ) or google for "slow sand filtration," "chlorine demand," "chloramination," and such things. A lot of the AWWA books are geared toward the people with specific educations, but there is a wealth of information aimed at people like new water utility employees, etc. with some really high quality and also accessible information.
Scott
Also, like I said, pools use chlorine all the time to purify the water. It is a recognized use of chlorine to purify water on a large scale such as in disasters.
D ) just warn us about mere repetition not adding anything to the weight to be given to authority?