A store that has gone stagnant, been left open, had something die in it or simply sat unused for a year is not automatically ruined. Shock chlorination is the standard way to recover it: dose the whole volume hard, leave it, then flush it out.
This is a different operation from the routine bleach dosing on the water page. That treats a jug for drinking, at around 2 drops per litre. This treats a whole system at roughly fifty times that strength, and the water is then thrown away rather than drunk.
When this is the right answer
Use it when the container itself needs disinfecting, not just the water in it:
- A tank or butt that has stood stagnant, or that you are commissioning for the first time.
- After anything got in: an animal, flood water, a burst that let dirty water back into the system.
- After a WaterBOB or bath store has been drawn down and you want to refill it into a clean vessel.
- Annually, as maintenance on any long-term store.
It is not the answer to chemical contamination. Chlorine deals with living things. If something has been spilled into a store, if it smells of fuel or solvent, or if flood water carrying oil has reached it, chlorination does nothing useful and the correct action is to empty it, clean it mechanically and start again.
The dose
The arithmetic, so you can adapt it to any bleach you actually have: 5% bleach is 50,000 mg of chlorine per litre. One litre of it in 1,000 litres of water gives 50 mg per litre, which is 50 ppm. If your bleach is 10%, halve every quantity above. If it is 3%, use roughly 1.7 times as much.
Doing it
- Clean it mechanically first. Chlorine cannot reach through sludge. Drain the tank, scrape and brush out sediment and biofilm, and rinse. This step does most of the work; the chlorine finishes it.
- Part fill, add the bleach, then fill. Adding bleach to a part-filled tank and then topping up mixes it far better than pouring it onto a full one, where it sinks and sits.
- Wet every internal surface. Splash or brush the dosed water up the walls, over the lid and around the inlet. Anything the chlorine does not touch is not disinfected.
- If the tank feeds pipework, draw the chlorinated water through it. Open each tap in turn until you smell chlorine, then close it. The pipes and taps are part of the system and recontaminate a clean tank the moment you use them.
- Leave it 12 to 24 hours.
- Drain, refill with clean water, and flush until the chlorine smell is gone. Repeat the refill if it lingers.
Getting rid of the water afterwards
This is the step that is skipped most often and it matters.
Do not put shock-chlorinated water into a septic tank. It kills the bacterial culture the tank depends on, and you will be dealing with a failed septic system on top of everything else.
Do not pour it on the garden or into a watercourse. At 50 to 200 ppm it will kill plants outright, and it is genuinely damaging to a stream or pond, where it kills fish and invertebrates at concentrations far below what you are holding.
Discharge it slowly to the foul sewer if you are on mains drainage. If you are not, spread it thinly over bare ground well away from any water, a soakaway or vegetation you care about, and let it break down. Chlorine dissipates fairly quickly in contact with soil and air, but the point is to avoid dumping the whole volume in one place.
If you have a lot of it, letting the tank stand open for a day or two before disposal drops the concentration substantially at no cost.