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Pool disinfectant selection

Useful information on pool disinfectant solutions.

How to choose a pool disinfectant

Disinfection occurs when sanitation of the pool water is achieved. This means that transmission of infections between persons or from the pool is minimised and that growth of algae and other nuisance organisms is inhibited. Therefore, the use of a pool disinfectant is key.


Chlorine Gas

Chlorine gas is one pool disinfectant method available to professional pool operators to disinfect large community pools. It usually requires dosing with an alkali, such as sodium bicarbonate or soda ash to maintain pH, as hydrochloric acid is formed when elemental chlorine gas is added to water. The Dangerous Goods (Storage and Handling) Regulations limit the use of chlorine gas to pools that have sufficient buffer distances from residences and places of public gathering to minimise the risk of injury, should there be a chlorine gas leak. For this reason gas cylinders and injection points are located within external buildings and not within the pool hall or attached plant rooms.



Sodium hypochlorite is the most versatile pool disinfectant and is widely used. It does pose chemical handling risks, however, particularly in a bulk handling and storage situation. Sodium hypochlorite is strongly alkaline and tends to keep high pH levels. It is generally used in combination with acid or carbon dioxide dosing. It is easily dosed by metering pump, therefore it is flexible in meeting demand. It is stabilised in a caustic solution, thereby having a shelf life of some weeks. Calcium hypochlorite is widely available and suitable for manually dosing pools following closure. It is useful for soft waters in maintaining hardness levels lost by dilution and backwashing. It is generally not used for metered dosing. Tablets are also available which have a number of applications. Lithium hypochlorite is shelf stable, non-scaling and highly soluble in water. It is ideal for use in spa pools. In outdoor situations, cyanuric acid should be used with hypochlorites to reduce chlorine loss from sunlight.


Salt Chlorination

Salt chlorinators use a low voltage electric current to convert chloride salt contained in the pool water into free chlorine, thus an efficient pool disinfectant. Salt-water pools involve minimal chemical handling and daily maintenance, are low in complexity and are therefore recommended for hotels and motels, caravan parks and apartment blocks where professional pool operators are not usually employed. A timer or an automatic sensor and control system can control the operation of a salt chlorinator. A residual of between 2,000–8,000 ppm of salt is maintained in accordance with manufacturers specifications which requires periodic topping up to maintain chlorine production rates. The output of the chlorinator is related to the size or number of the electrode plates. As the chlorine output is fixed, careful consideration of bather loads and chlorine consumption should be considered when installing systems. Scaling of the electrode plates may occur if there is too much calcium hardness in the water, however this should be considered in line with protecting the pool surfaces. The plates should be cleaned with acid periodically in accordance with manufacturer’s directions. In outdoor use, cyanuric acid should be present in salt chlorinated pool water.



Chlorinated Isocyanurates

This type of pool disinfectant (for example, trichlor, dichlor) combines chlorine with cyanuric acid stabiliser and is therefore suited to pools exposed to direct sunlight. In outdoor use a start-up concentration of 25 ppm of cyanuric acid is recommended. When levels of cyanuric acid increase to 100 ppm, more frequent dilution of the pool water is indicated to control cyanuric acid levels. Trichlor is used in many pools because of the ease of chemical storage and the simple dispensing methods using erosion feeders. It is suitable for hard, alkaline water, as it does not contain calcium and helps keep pH down. It is suitable for outdoor hotel/motel and caravan park pools as the tablets are easily handled. It has also been used in community pools but is not as flexible as gas/hypochlorite systems. Care should be taken to ensure that cyanuric acid does not reach excessive levels. This can be corrected by backwashing or draining the pool on a regular basis.


Chlorine—Chlorine Dioxide

Chlorine dioxide is also an effective pool disinfectant and oxidant and is not affected greatly by pH. Stabilised chlorinous oxide solutions that form chlorine dioxide when added to water are beginning to be used at low levels (0.2–0.3 ppm) in swimming pools to supplement chlorination. Chlorine dioxide must be used in conjunction with free chlorine under tight supervision. The presence of chlorine dioxide may affect the operation of automatic chlorination equipment and specialist advice should be sought before use. A potentially problematic by-product is chlorite formation or reconversion, which is controlled by continued free chlorine addition and periodic superchlorination. Chlorine dioxide may also be generated on-site by mixing hydrochloric acid with sodium chlorite. Chlorine dioxide treatments have been proposed as a treatment for pools affected by Cryptosporidium contamination.



Bromine has been popular for some years, particularly in warm water pools (notably for hydrotherapy), but is losing popularity in some circles. Bromine has been associated with instances of skin irritation, and some pools have had difficulty remaining within regulatory limits for total bacteria count. Bromo-chloro-dimethyl-hydantoin (BCDMH) has been popular in hydrotherapy pools, due to better retention of disinfectant in heated situations and the absence of chlorinous odours. Brominated pools do have a particular odour. Like trichlor, BCDMH is dispensed by erosion feeders and is acidic. Bicarbonate buffering is usually adequate to control pH where BCDMH is used. Bromide-oxidant activator systems use a reserve of sodium bromide in the body of the pool with an activator. In this system sufficient bromide ions are maintained in the pool water. Sodium hypochlorite, or ozone, when injected into the water forms hypobromous acid. After reaction with contaminants, the hypobromous acid reverts back to bromide and so the cycle restarts.

Bromine can be purchased as a preformed hypobromite/hypochlorite liquid. It can also be manufactured on-site by pre-mixing sodium bromide with sodium hypochlorite in line prior to injection into the water stream. These solutions invariably form a chloramine build-up after continued use. Hence, the sodium bromide/sodium hypochlorite system relies on dilution with fresh water to reduce chloramines.


Problems with Bromide / Ozone Pools

Bromide/ozone pools have had problems because the rate of bromine production is related strictly to the ozone production capacity. Reserve bromide ions react with ozone to form free bromine. As the system relies on the bromide reaction to prevent ozone getting into the pool, it may be unsafe to add chlorine to supplement bromine production while the ozone generator is on because ozone could escape into the pool. If the ozone system breaks down and it becomes necessary to add sodium hypochlorite to activate the bromide, problems with chloramines and other compounds may emerge. A minimum bromide level must be maintained within the system. The use of this type of pool disinfectant has been discontinued in some pools because of these issues.