Disinfection of swimming pools

Last updated: 01 January 2007

Disinfection

Disinfection must be provided in swimming pools to kill microorganisms (germs). There have been many outbreaks of illnesses associated with swimming pools and most of the outbreaks have occurred in pools with inadequate disinfection or following faecal accidents by infants who are not toilet-trained.

Diseases transmitted in swimming pools

The most common microorganism, which has caused large outbreaks in public swimming pools is Cryptosporidium (Crypto). This organism causes a diarrhoeal illness called cryptosporidiosis, which is spread when people swim when they have diarrhoea, or within two weeks of having had diarrhoea.

Unlike other microorganisms, Crypto is not easily destroyed by chlorine (unless a pool is shock-dosed) and therefore outbreaks of cryptosporidiosis sometimes occur in swimming pools, especially following faecal accidents by infants who are not toilet-trained.

Outbreaks of other illnesses, especially folliculitis (caused by Pseudomonas aeruginosa), respiratory illnesses (caused by Legionella and Adenovirus), gastroenteritis (caused by Giardia, Echovirus, Norovirus and Shigella), haemolytic-uraemic syndrome caused by E coli 0157 and pharyngo-conjunctivitis caused by Adenovirus have occurred through swimming in contaminated public swimming pools. However, most of these outbreaks occurred in pools with inadequate disinfection. Outbreaks of these illnesses should not occur in pools maintained with sufficient chlorine or bromine.

Chlorine based disinfectants

Chlorine works by oxidising or breaking down pollutants and inactivating microorganisms. A free residual of chlorine is needed in the pool water at all times to control microorganisms. Chlorine is most effective at destroying microorganisms when the pH is between 7.2-7.8. Above a pH of 7.8, chlorine is a weak disinfectant. pH should be maintained between 7.2-7.8 for adequate disinfection and bather comfort.

Most larger or council operated public swimming pools are automatically controlled to maintain correct pH and disinfectant levels.

Chlorine is available as sodium hypochlorite (liquid chlorine), calcium hypochlorite or lithium hypochlorite (granular, briquetted or powdered chlorine)

Guideline for minimum free chlorine (pH 7.2-7.8):
Outdoor pools - 1mg/L (or 3mg/L if stabilised with cyanuric acid)
Indoor pools - 1.5mg/L (for pools 26°C or less) or 2mg/L (for pools greater than 26°C)

Bromine based disinfectants

(Guideline 3.5 mg/L; 4.5 mg/L>26°C)

Bromine is mainly used in the form of Bromochlorodimethyhydantoin (BCDMH) to disinfect indoor swimming and spa pools.

There have been reports of skin irritations in pools disinfected with bromine and similar to chlorine, these problems should not occur in properly maintained pools. Correct water balance, good filtration, frequent backwashing and dilution with fresh water should prevent these problems. Sodium bromide systems that use an oxidiser (hypochlorite or ozone) to generate bromine are also used to disinfect swimming and spa pools. Systems using ozone to re-generate bromine require highly skilled operators to ensure system performance and safe and healthy conditions.

Ultra violet light and ozone treatment systems

UV light and ozone treatment systems are installed in aquatic complexes to assist with the destruction of chloramines (combined chlorine). Chloramines form when chlorine reacts with ammonia, which is derived primarily from urine and sweat. Chloramines (especially trichloramine) can cause irritation to the skin, eyes and respiratory tract, and are more likely to be a problem in poorly ventilated buildings.

UV light and ozone systems also provide additional disinfection, however they must be used with a residual disinfectant such as chlorine or bromine.

Pool disinfectants (sanitisers)

The Australian Pesticides and Veterinary Medicines Authority (APVMA) regulates swimming and spa pool sanitisers (disinfectants). A list of approved sanitisers is provided on the APVMA website.

Circulation and filtration

Good circulation and filtration of pool water is essential for good water quality and maintenance of microbiologically safe conditions.

Pools that have high turbidity (cloudy water) due to poor circulation and/or poor filtration are more likely to have high counts of microorganisms because the organic material may shield against adequate disinfection. When this occurs pools may contain high concentrations of microorganisms even when disinfection concentration is satisfactory.

The importance of clean pools

Chlorine works best when pools are kept clean - so the less dirt, grime, oil and urine that gets into the pool the better. Preferably, people should shower before entering a public pool, and parents should encourage children to use a toilet before swimming.

Heavily used public pools need efficient disinfection and filtration systems to effectively remove pollutants and kill microorganisms. Automatic controllers assist to maintain optimum disinfection provided they are routinely maintained.

Related Links

• Controlling chloramines in indoor swimming pools  http://www.health.nsw.gov.au/factsheets/general/control_chloramiines.html
• Healthy swimming pools  http://www.health.nsw.gov.au/factsheets/general/swimpool.html