Originally Posted by CarlF
I read that municipal water in the US has .5-1.0 ppm chlorine. If I'm doing my math right, 2 teaspoons (10 mil) of 5% household bleach per 100 gallons would be over 1ppm. That seems awfully little but lines up pretty close to Markj's number.
Our local water source considers 1.5-3.0 ppm for chlorine to be acceptable levels. The CDC recommendations comes out to about 160 ppm. Why the extreme difference?
My guess is because the water at your tap--with 1.5 ppm or so in it--has already been treated and sanitized, and this amount is simply what remains. Probably just enough to insure that no further contamination is likely as it passes through the system. The CDC recommendation, on the other hand, is for water that has not been previously treated and is suspected of some contamination.
Obviously there is a continuum here. A full cup in a normal sized water tank is going to be gross overkill. 1/4th of a cup in a 100 gallon tank may be more than you need, depending on just how much you trust the water source. If you are pretty sure the water is good, maybe you don't need to add anything, or maybe you just add a teaspoon or two per 100 gallons. If you are suspicious of the water, though, approximately 1/4th cup per 100 gallons would be the CDC recommendation.