How much bleach?

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    We need to "freshen" our 100 gallon freshwater tank. How much bleach should we add to a full (100 gallons) to achive this? Thanks.   Jack

Damn Yankee:
To Sanitize your tank use 1/4 cup bleech per 15 gal. To just freshen it up I'd go with maybe half of that amount. 3/4 to 1 cup.

from :

. . . . "Bleach will kill some, but not all, types of disease-causing organisms that may be in the water." . . .

If you are going to use it for drinking or cooking - 
"Add 1/8 teaspoon (or 8 drops) of regular, unscented, liquid household bleach for each gallon of water, stir it well and let it stand for 30 minutes before you use it".

A little more info from the same site:

Chemical Treatment
When boiling is not practical, certain chemicals will kill most harmful or disease-causing organisms.
For chemical disinfection to be effective, the water must be filtered and settled first.Chlorine and iodine are the two chemicals commonly used to treat water. They are somewhat effective in protecting against exposure toGiardia, but may not be effective in controlling more resistant organisms like Cryptosporidium. Chlorine is generally more effective than iodine in controlling Giardia, and both disinfectants work much better in warm water.

You can use a non-scented, household chlorine bleach that contains a chlorine compound to disinfect water.
Do not use non-chlorine bleach to disinfect water. Typically, household chlorine bleaches will be 5.25% available chlorine. Follow the procedure 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 table as a guide. (Remember, 1/8 teaspoon and 8 drops are about the same quantity.)

Available Chlorine Drops per Quart/Gallon of Clear Water Drops per Liter of Clear Water 
1% =10 per Quart - 40 per Gallon -or- 10 per Liter
4-6% =2 per Quart - 8 per Gallon (1/8 teaspoon) -or- 2 per Liter
7-10% =1 per Quart - 4 per Gallon -or- 1 per Liter

(If the strength of the bleach is unknown, add ten drops per quart or liter of filtered and settled water. Double the amount of chlorine for cloudy, murky or colored water or water that is extremely cold.)

Mix the treated water thoroughly and allow it to stand, preferably covered, 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, allow the water to stand exposed to the air for a few hours or pour it from one clean container to another several times.

You can use granular calcium hypochlorite to disinfect water.
Add and dissolve one heaping teaspoon of high-test granular calcium hypochlorite (approximately ¼ ounce) for each two gallons of water, or 5 milliliters (approximately 7 grams) per 7.5 liters of water. The mixture will produce a stock chlorine solution of approximately 500 milligrams per liter, since the calcium hypochlorite has 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 ounces) of stock chlorine to each 12.5 gallons of water or (approximately ½ liter to 50 liters of water) to be disinfected. To remove any objectionable chlorine odor, aerate the disinfected water by pouring it back and forth from one clean container to another.

You can use chlorine tablets to disinfect filtered and settled water.
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 or liter of water to be purified.

You can use tincture of iodine to disinfect filtered and settled water.
Common household iodine from the medicine chest or first aid kit may be used to disinfect water. Add five drops of 2 percent U.S. or your country’s approved Pharmacopeia tincture of iodine to each quart or liter of clear water. For cloudy water add ten drops and let the solution stand for at least 30 minutes.

You can use iodine tablets to disinfect filtered and settled water.
Purchase commercially prepared iodine tablets containing the necessary dosage for drinking water disinfection at drug and sporting goods stores. Use as stated in instructions. When instructions are not available, use one tablet for each quart or liter of filtered and settled water to be purified.

I'm going to assume Jack's use of the term "freshen" is intended to mean the same thing as "sanitize". As I see it, there are really only two levels of chlorine bleach quantity to consider.  The first is a stronger solution placed in the tank temporarily (and not consumed) which is performed as a sanitizing function.  The second level is the introduction of much smaller amounts into water that will be consumed, such as treating questionable water for drinking.  Kyle's post above with the "8 drops per gallon" is for the treatment of consumable water. But if you want to sanitize the tank and flush, you need higher amounts.

I don't see any benefit in using something in between, as you can't drink the stuff, and why not be sure you kill EVERYTHING in the tank.  Beach is cheap, so go for the full sanitization.

The Illinois Department of Health has published guidelines for disinfecting tanks intended for potable water. They indicate 2 pints of 5.25% chlorine bleach for every 100 gallons of water.  See link for more details:

Be careful what bleach you buy these days. Chlorox, and other brands, love to add extra ingredients (fabric softener, and other mystery color brighteners) so it is harder to find pure bleach today. Read the bottle carefully.

If, after sanitizing, flushing, and refilling your tanks you still have a bit of chlorine smell, add a couple tablespoons of baking soda to your tank. Dissolves and kills the chlorine taste.

Lastly, be aware that Chlorine bleach has a shelf life.  If you have bleach that has been sitting around your house for a couple years, it should be okay to use, but it has lost some of its strength.  You will need to use a higher quantity because it is no longer at the concentration percentage listed on the bottle.

Len Silva:
Laundry bleach is about 6%.  Liquid pool chlorine is about 10% and considerably less expensive.  The cheaper store brands are probably has fewer added ingredients.


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