A Guide to a Clean Sanitised Swimming Pool


When left untreated, bacteria and algae multiply which can cause health problems such as ear, nose and throat infections. The only way to keep them under control is to sanitise the water regularly and filtrate the water. Also see A Guide to Maintaining Swimming Pool Water Quality.

Whether you choose to sanitise your pool using chlorine, a saltwater chlorinator, ozone or bromine, maintaining the correct chemical balance is essential.

Dead bacteria are removed by the filter and the “residual sanitiser” left in the water will kill any new bacteria immediately. Most bacteria will be killed when exposed to a “free chlorine residual” as low as one milligram per litre or one part per million (ppm).


Chlorine is one of the most effective known chemical treatments for the sanitisation of large volumes of water, which makes it one of the most popular treatments for swimming pools.

Chlorine is available in granular, liquid, or tablet form and can be stabilised (contains a stabiliser that helps protect the chlorine from the degrading ultra violet rays of the sun) or unstabilised. It can also be generated by a salt-water chlorinator (see below).

The sun’s rays remove unstabilised chlorine from the water very quickly which means larger amounts of chlorine is needed to maintain the appropriate ‘free’ chlorine.

  • As salt and liquid chlorinators become more popular, granular chlorine is used less as a means of regular chlorination. It is the most time consuming method (needs to be done manually) and puts the most strain on the cleaning capacity of the pool filter. The chlorine works better if it is thrown straight into the water.

  • Liquid chlorine is a great way to chlorinate a pool as the water stays crystal clear as there are no residues. A downside is that the chlorine is easily affected by heat and does not last long in the water. To combat this, pools that are liquid chlorinated should have a chlorine stabiliser level of between 30 and 50 parts per million.

  • Chlorine products in tablet or stick form, are highly concentrated and contain a stabiliser which means you only have to add small amounts to your pool to achieve the desired protection level. Note: Care must be taken as excessive use can lead to stabiliser levels getting too high (see Stabilising a Pool below).

The following terminology will help you to understand what the chlorine is doing:

  • Free Chlorine Residual – is "free" to latch onto and break down new wastes in the pool. A normal chlorine test will measure this.
  • Combined Available Chlorine – has already latched onto existing wastes and is poor at breaking down new waste.
  • Total Residual Chlorine – is the total of ‘free’ chlorine and combined available chlorine in the pool.

The amount of ‘free’ chlorine in a pool that is stabilised should be 2ppm below 26º and 3ppm above 26º.

The amount of ‘free’ chlorine in a pool that is unstabilised should be 1ppm below 26º and 2ppm above 26º.

The amount of chlorine required depends on pool size, filter cycle times, water temperature and the amount and frequency of swimmers.

Note: Never mix chemicals – even different types of chlorine – fire and/or explosion may result. When adding chemicals, use small amounts, run the filter and test the effect after several hours. Adding large amounts of chemical to achieve large changes can result in large problems. Also see A Guide to Swimming Pool Chemical Safety.


Another sanitiser, bromine, works similar to chlorine. Bromine is available in tablet form and when dissolved in water, produces a compound that’s an effective sanitiser at levels of between 4.0 and 6.0ppm. Bromine will combine with swimmer waste to form bromamines which are not irritating to swimmers in the way that chloramines (see Chlorine Odour below) are. Note: Periodic superchlorination may still be required.

Salt Chlorinators

Salt chlorinators make chlorine from common salt, and come in various sizes. Make sure you select one that is able to produce sufficient chlorine for your needs. Note: you may still need to add extra chlorine from time to time to maintain a satisfactory ‘free’ level.

Also see A Guide to Swimming Pool Salt Chlorinators.

Stabilising a Pool

Unless you have an indoor pool, the water needs to be stabilised to counter the loss of chlorine due to the effects of the sun’s ultraviolet rays. Some types of chlorine i.e. granular, liquid as well as saltwater chlorination produce unstabilised chlorine. They therefore need a regular addition of stabiliser (either neat cyanuric acid or a compound) to prevent unnecessary chlorine loss.

Tablets or stick chlorine have an inbuilt stabiliser to prevent them from dissipating.

Regardless of the type of chlorine, an initial charge of stabiliser, up to 50ppm, may be necessary. Stabilising the water saves you money through having to use less chlorine.

Stabilisers are dissolved and dispersed more effectively if introduced near the pool water inlet and should be dispensed slowly over a period of days.

Note: A stabiliser does not evaporate off; once in the pool it stays there and is only lost through excessive splashing or when the pool is emptied. A separate cyanuric acid testing kit is available from pool suppliers. This test is not normally part of the routine testing kit.

If too much stabiliser is added to the water it holds on to all the ‘free’ chlorine leaving none to break down bacteria and body wastes. The pool will then go green. This process is called Chlorine Lock and the only remedy is to empty most of the water out of the pool and start again.

The optimum cyanuric acid level should be between 30 and 50ppm with a maximum recommended level of 60ppm

The pool water can be tested by your local pool shop professional who can provide advice on how much stabiliser to add or if there is too much already in the pool.

Chlorine Odour and Superchlorination

Swimmer waste, such as perspiration, body oils, urine, cosmetics, sun lotions and other debris from the atmosphere build up over time and cause cloudy and dull water.

These wastes also cause the unpleasant “chlorine” smell and irritate swimmers’ eyes and skin. Most people think it’s too much chlorine in the pool when in actual fact there is not enough.

Chlorine combines with nitrogen in these wastes to form “chloramines” – smelly, irritating compounds. Superchlorination is the only way to get rid of them.

Otherwise known as “shocking” or oxidising, superchlorinating every three to four weeks with a pool oxidiser will rid the pool of chloramines and return the pool to a bright, sparkling body of water, free of wastes. Some automatic controllers continually burn off chloramines and you will not need to superchlorinate as often.

If in doubt seek advice from your local pool shop professional.

Heated Pools

Heated pools require more chlorine than non-heated pools because the chlorine is used quicker in hot water. Note: Stabilised chlorine products should not be used in indoor heated pools as the sanitiser effectiveness is greatly reduced.

Handy Hints

  • Keep a log book on the chlorine and pH levels and record how much of each chemical is added to the pool. Over time this will be a useful guide in getting to know the amount of each chemical required.

  • By adding chlorine after sunset the new chlorine will then be in contact with the bacterial and body wastes for as long as possible before people swim in the pool.

  • When you add water do it via the skimmer box so it will then go through the filter before entering the pool. If this is done at sunset then the chlorine has all night to act on the new water before swimmers enter the pool. This is very important if bore water is used.
  • Amoebae causing meningitis are very active at higher temperatures, so when the pool is above 26°c make sure that there is a little more chlorine in the pool.


There is pool control equipment available that will automatically measure levels of chlorine and pH of pool water to help make light work of maintenance. This equipment constantly checks and adjusts chlorine and pH levels, reducing the need for more frequent superchlorination.  

Liquid pool controllers can also reduce pump running times as they work more quickly and efficiently. For other alternative methods of sanitising a pool please see A Guide to Swimming Pool Salt Chlorinators and A Guide to Swimming Pool Ozonators and Ionisers.


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