The US environmental protection agency does indeed recommend using regular chlorinated bleach as an emergency water purifier in the event of flooding, etc, where the regular supply is contaminated. They suggest a ratio of
1 : 3800 (call it 4000)
=1cc per 4 litres
= 50cc's (or 1/5th of a cup) in a 200 litre water butt.
= 1 cup per large 1000 litre tank
To disperse it in a large tank or butt I guess it would be best to mix it in a bucket of water first, then tip that in and stir.
When the tap water stops flowing, Regular Clorox Bleach isn't just a laundry-aid, it's a lifesaver. Use it to purify water, and you'll have something to drink. It's the same in any natural disaster. As the shock wears off and the days wear on, the biggest demand is for drinking water. Time after time, relief crews hand out free Clorox Bleach with simple instructions: use it to kill bacteria in your water and you'll have purified water to drink. Here are the general guidelines. First let water stand until particles settle. Filter the particles if necessary with layers of cloth, coffee filters, or fine paper towels. Pour the clear water into an uncontaminated container and add Regular Clorox Bleach per the below indicated ratio. Mix well. Wait 30 min. Water should have a slight bleach odor. If not, repeat dose. Whether you use Clorox Bleach in an emergency or for everyday chores, it's always an environmentally sound choice. After its work is done, Clorox Bleach breaks down to little more than salt and water, which is acceptable anytime.