Not having my field hygiene and sanitation manual handy, how many drops of bleach does it take to purify a gallon of clear water??

Two drops per canteen (quart).

Thanks!

A reminder:  chlorine bleach does not kill Giardia!  If it's in an inactive cycle, you're ok with bleach, but I think you better stick with iodine if you want to kill everything any time of the year.  If it's feasable, boil the crap out of the water.  If you're backpacking and boiling is impractical, use iodine in combination with a filter.

From the EPA's [url=http://www.epa.gov/safewater/faq/emerg.html]Emergency Water Purification[/url] page:

[i]Chlorine and iodine are somewhat effective in protecting against exposure to Giardia, but may not be effective in controlling Cryptosporidium. Therefore, use iodine or chlorine only to disinfect well water (as opposed to surface water sources such as rivers, lakes, and springs), because well water is unlikely to contain these disease causing organisms. Chlorine is generally more effective than iodine in controlling Giardia, and both disinfectants work much better in warmer water.

CHLORINE METHODS

Chlorine Bleach: Common household bleach contains a chlorine compound that will disinfect water. The procedure to be followed is usually written on the label. When the necessary procedure is not given, find the percentage of available chlorine on the label and use the information in the following tabulation as a guide.

Available Chlorine Drops per Quart of Clear Water
1% 10
4-6% 2
7-10% 1
(If strength is unknown, add ten drops per quart of water. Double amount of chlorine for cloudy or colored water)

The treated water should be mixed thoroughly and allowed to stand 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, it can be made more pleasing by allowing the water to stand exposed to the air for a few hours or by pouring it from one clean container to another several times.

Granular Calcium Hypochlorite: Add and dissolve one heaping teaspoon of high-test granular calcium hypochlorite (approximately 1/4 ounce) for each two gallons of water. The mixture will produce a stock chlorine solution of approximately 500 mg/L, since the calcium hypochlorite has an 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 oz.) of stock chlorine to each 12.5 gallons of water to be disinfected. To remove any objectionable chlorine odor, aerate the water as described above.

Chlorine Tablets: Chlorine tablets containing the necessary dosage for drinking water disinfection can be purchased in a commercially prepared form. These tablets are available from drug and sporting goods stores and should be used as stated in the instructions. When instructions are not available, use one tablet for each quart of water to be purified.

TINCTURE OF IODINE

Common household iodine from the medicine chest or first aid kit may be used to disinfect water. Add five drops of 2 percent United States Pharmacopeia (U.S.P.) Tincture of iodine to each quart of clear water. For cloudy water add ten drops and let the solution stand for at least 30 minutes.

IODINE TABLETS

Commercially prepared iodine tablets containing the necessary dosage for drinking water disinfection can be purchased at drug and sporting goods stores. They should be used as stated. When instructions are not available, use one tablet for each quart of water to be purified.

WATER TO BE USED FOR DRINKING, COOKING, MAKING ANY PREPARED DRINK, OR BRUSHING THE TEETH SHOULD BE PROPERLY DISINFECTED.[/i]

Several comments:

1. The chlorine in liquid bleach tends to dissipate over time, even if the bottle hasn't been opened yet. Use fresh stock for best results.

2. Calcium Hypochlorite pool shock treatment powders like "hth Super Sock It" are considerably less bulky to store than an equivalent amount of liquid bleach, and probably aren't as likely to lose their potency over time.

Question: What about B-T-F Iodophor? Is it good for making drinking water?

The reason I ask, is that I used to use it when brewing beer. It is a better disinfectant than bleach. Iodophor takes 60 secs, bleach takes forever! Bleach needed to rinsed or else it left a taste. Iodophor leaves no taste and you don't need to rinse.

Iodophor needs to be at 12.5ppm to work as a disinfectant (1/2oz for 5 gallons).

Any Ideas?

Bleach is a pretty crummy water disinfectant, asnd despite what the EPA seems to think, is actually worse at killing Giardia and Cryptosporidia than iodine.  Cold water dramatically reduces the effectiveness of either agent as it slows down dispersion. Shaking the water being disinfected helps disperse the solution, but doesn't change minimum exposure time.

30 minutes is minimum exposure time for either Chlorine or Iodine, but independent tests have shown that chlorine treated water may still have live bacteria and organisms in it up to 8 hours after treatment.  Iodine is usually more completely sterilized.

Iodine tastes like crap.

Chlorine is more tolerable since it is in most tap water.

Boiling is the most effective anti-microbial treatment.  Merely bringing the water to a boil is enough. All organisms die at temps well below the boiling point of water, even at higher altitudes, so bringing it to a boil is enough to do the job and saves a lot of fuel.

I use water filters most of the time and recommend the PUR Hiker or Guide for continental US travel, and the MSR filters as well.  For international travel, filter first then treat with iodine to kill the viruses.

Guzzler,

We use Idophor for sanitizing tanks for juice storage here where I work. I'll have to check into it. Thanx!

Great! Try to find out if it can used to make potable water.

I did find that it has been approved for disenfecting Anthrax (not in spore form, but when it makes contact with skin and turns back into a bacteria)

Reading some of the web sites, I didn't know that Iodophor was such a good disenfectant. Beats the pants off of normal iodine or bleach. Something about an extra oxygenator.

The reason I prefer using the iodine before the filter is so the charcoal in the filter removes most of the iodine taste.  I don't think any flavor of Kool-aid can totally cover the taste of purification tabs, but it improves the taste somewhat.  Could that be the reason thye package powder fruit drink in MRE's?

Another good tip is if you're gathering water from a lake and have a boat, take it from the middle of the lake.  You'll collect less crud.