Bleach dosing
How much plain unscented bleach to add, either to make water drinkable or to disinfect a tank. These are two different jobs at very different strengths.
Sodium hypochlorite, from the label. Household bleach is usually 5 to 6%.
20dropsinto 10 litres, then wait 30 minutes
- Roughly
- 1 ml
- Contact time
- 30 minutes
- Then
- It should faintly smell of chlorine
If it does not smell faintly of chlorine after 30 minutes, dose it again and wait another 30. No smell means the chlorine has been used up by whatever was in the water, and the water is not yet safe.
Plain unscented bleach only. Nothing thickened, scented or colour-safe. Bleach loses strength on the shelf, noticeably within a year.
How this is worked out
2 drops per litre of 5% bleach, doubled if the water is cloudy, then 30 minutes. Other strengths scale inversely: 10% bleach needs half as much, 3% needs roughly 1.7 times as much.
That works out at about 5 ppm. Shock chlorination, the other mode here, runs at 50 to 200 ppm, so ten to forty times this.
10 litres, 5% bleach, clear: 2 x (5 / 5) x 10 = 20 drops.
Every figure here comes from Water: disinfecting, which gives the reasoning and the sources.