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What is Swimming Pool Water Superchlorination?

Super-chlorination is simply an extra dose of chlorine

Superchlorination is simply an extra dose of chlorine - a kind of shock treatment - to burn out bacteria, algae, nitrogen compounds, and especially ammonia, all or some of which have not been elimated by routine chlorination.
Three to five times the normal dose of chlorine is added to the pool as directed on the product instruction packet.
This may be required when Increased bathing loads and increased algae growth occur during the summer months or when unheated water reaches a temperature of 30 deg C or higher.

When is it needed?

You may need to superchlorinate when bad odours or algae (green or coloured water) are visible, when bathers complain of eye irritation, or after the pool has had a period of heavy use.

The best time to superchlorinate is after sundown, to avoid chlorine waste through dissipation by the sun's ultraviolet rays. You can do it at other times, but do not allow anyone to use the pool until the residual chlorine level drops back to normal (which, in most pools, is below 3.0 ppm). The product directions should give you the amount of time needed before you can safely use the pool again.

If this information is not provided, ask a pool service company how long you have to wait before swimming can be allowed. If at any time between scheduled superchlorination treatments the chlorine residual drops to 0, the water should be superchlorinated promptly.

Chlorine Residual of Zero

A reading of 0 is often, in fact, below 0, which means that some chlorine demand has accumulated in the water, and bacteria and algae growth will develop rapidly. In hot weather a residual reading of 0 can turn the pool water cloudy and green within a day or two.
One word of caution: don't superchlorinate when the pH level is low (acid). If you do, any copper dissolved from the pool equipment or plumbing by the acidic water may be oxidised by the large dose of chlorine and cause black stains on the pool finish.

A common misunderstanding

 Most people believe that if the water has a strong chlorine odour, too much chlorine has been added.

In fact , this is an indicator of too little free residual chlorine in the water. Pool water contains ammonia, which it gets from fertilisers blown or washed in from lawns and gardens, urine, and natural body oils and perspiration. Ammonia reacts with chlorine to formchloramine (combined chlorine), which is the major cause of burning eyes, skin irritation, and the unpleasant chlorine odour. The odour is particularly pungent if pH is low. Chlorine in an uncombined or free state is practically odourless. So when people complain of burning eyes and that "chlorine smell", realise it is because there is too little free residual chlorine in the water, not too much.
So, if skin and eye irritation occur, or if there is a distinct chlorine odour around the pool, the problem is too little chlorine, not too much. The solution is to superchlorinate the water to get rid of the chloramines.

So:-

Adjust the pH to between 7.2 and 7.4 for maximum chlorine efficiency, then superchlorinate.