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Swimming Pool Maintenance

Chlorine Shock Treatment


The idea of shocking a pool with a higher than normal dose of chlorine is not to kill bacteria that have become specially chlorine-resistant. This is a fallacy that is spread by some swimming pool manufacturers (I hope from ignorance) and this is a concern since it's not just oversimplification. One pamphlet I picked up stated the following:

There are literally thousands of bacteria and algae types growing in the domestic swimming pool. Some build a resistance to normal chlorine levels.

To knock them back to an acceptable level, ...

The way it reads, each swimming pool owner is breeding super-bugs – the kind we hear about infecting hospitals. This is ridiculous. The reality is much more boring.

Chemical Process

With normal use the chlorine in a swimming pool gets used up. That is, it reacts with bacteria and sweat, etc, to form chloramines. The amount of free chlorine is reduced and the amount of combined chlorine increases. It is these chloramines that have the strong chlorine smell.

Chloramines are about 25 times less effective as a sanitizer than free chlorine is. Initially monochloramines are formed. These are oxidised to bichloramines, which in turn are converted to trichloramines and then to nitrogen and chlorides – end of process. Normally the free chlorine does the oxidising of these chloramines as well as its normal sanitising job, but after heavy use there may not be enough free chlorine to finish the job. In such a case chlorine additional to the regular dose needs to be added to the pool.

Common sense tells us that the more a pool is used, the more chlorine will be needed to sanitise it. Also, the more chlorine will be needed to oxidise the chloramines thus formed.

Chlorine Ratios

A friend of mine used to own a 110,000-120,000L pool (ie, a large pool). He would shock it at the beginning of the swimming season to get it ready for swimming, and again half way through the season. And that was it. No shocking every week or two. With maybe half a dozen people swimming in it regularly there was still plenty of free chlorine in the pool because unless he had several large pool parties, the ratio of water to swimmers remains high enough for normal chlorination to be sufficient. (His pool was indoors and had a cover as well, both of which also help.)

The smaller the pool the more impact swimmers have on the free chlorine level, which is one good reason why spa pools need to have a higher level of free chlorine maintained in them than swimming pools do.

How Much Shock?

Free chlorine is easily measured using DPD1 tablets or test strips. Combined chlorine (the chloramines) is not as often measured, however, probably because people do not understand the point of measuring it. If you can measure it using DPD3 tablets or test strips, then do.

The amount of chlorine that should be added to shock a pool is about 10 times the amount of combined chlorine. This will normally require calculating the difference between the total chlorine level and the free chlorine level, since those are the two chlorine values normally tested.

If not, then after heavy pool usage pay particular attention to free chlorine levels. If the chlorine you add seems to be used up faster than normal there are two main causes.

  1. Sunlight is causing the chlorine to break down quickly. See the Stabilising page.
  2. The chlorine is being used up by oxidising chloramines.

In practice an extra dose (ie, a double dose that day) will do the job.

If you are shocking the pool every week then either you have very uneven swimming habits or you aren't adding enough sanitizer during the week. (If you only swim in the weekends, and then throw big pool parties, it's quite possible for a double dose of chlorine to be needed to cope with the extra activity.)

On to Non-Chlorine Shock Treatments.

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