Disinfection of Swimming Pools and Spa Pools

Infections are caused by many pathogenic microbes. Disinfection is a process which kills microbes but not their spores and aims to reduce transmission of disease. Pools kept above the recommended disinfection concentrations and within levels pH quickly kill microbes and prevent disease transmission.

Last updated: 10 February 2010

Infections transmitted in swimming pools

Infections are caused by the transmission of certain pathogenic micro-organisms also known as germs. Transmission in swimming pools and spas occurs when germs are introduced into the swimming pool water, mainly by bathers, and other bathers come into contact with them. There are many infections that may be transmitted by swimming pools and spas. The infections mainly infect the skin, ears, eyes, noses, lungs and the gastro intestinal tract. The infections can give rise to symptoms such as pusy sores on the skin, conjunctivitis in the eyes, runny noses, sore ears and burst ear drums, pneumonia, vomiting and diarrhoea.

The most serious risk to health is when there is a faecal accident in the pool. Faecal contamination is mainly caused by infants who are not toilet trained. Parents must take great care with their child's hygiene and ensure that they do not swim if they have had diarrhoea in the previous two weeks. Certainly, no person who has had diarrhoea is the previous two weeks should swim in a pool or spa.

It is important to minimise the entry of germs into a pool. Swimmers should always go to the toilet and shower before swimming. Cosmetics should not be applied or be removed before swimming.

There are some situations which may allow germs to thrive. These include dirty pools, warm pools, aerated pools, pools with too little disinfectant, pools with too much stabiliser and pools where the pH is too high. A combination of these situations can easily lead to infection transmission.

Disinfection

Disinfection is a process which kills germs, but unfortunately not their spores. The aim of disinfection is to reduce the transmission of infections. Disinfection is not an instantaneous process but takes time. The higher the concentration of disinfectant the more rapidly it kills. At the minimum recommended concentrations of chlorine and bromine, the kill time is about one minute or less for most germs.

Most germs are easily controlled by disinfection but Cryptosporidium oocysts and Giardia cysts, which are types of spores, are resistant to disinfectants. Their transmission needs to be controlled by preventing these germs and their spores entering into pool or spa. There is a separate fact sheet about Cryptosporidium and Giardia.

Disinfectant above the recommended minimum concentrations must be present in the pool water at all times. Disinfection must be provided by a disinfectant which leaves an effective residual in the pool that is not harmful to swimmers.

Suitable disinfectants

Chlorine and bromine based disinfectants are the only satisfactory pool and spa disinfectants. They also oxidise and destroy some organic chemicals which may enter the pool. These disinfectants provide a readily measurable residual in the pool water. Each disinfectant has its advantages and disadvantages and pool operators should consider any disinfectant or disinfectant system carefully before use.

Dosing of disinfectants

All public swimming pools and spas must, as a minimum, have metered dosing equipment to continuously dose the disinfectant. Automatic dosing equipment should be used where a fee is charged for any purpose at a public pool. Dosing systems must be active while the pool or spa is open and for at least one hour before and after the pool is open.

Efficient chlorine pools operate on the chemical process known as breakpoint chlorination which can only be reliably and constantly achieved with dosing equipment.

Disinfection factors

• Increasing pH decreases disinfection power so that above pH 7.6 chlorination is ineffective and above pH 8.0 bromination is ineffective.
• Stabiliser (cyanurate) should not exceed 50mg/L as disinfection becomes ineffective and chlorine levels should be increased if stabiliser is used. Stabiliser is not suitable for use with bromination.
• Pools must be equipped with a circulation system and filters capable of producing clean, clear water. Filtered water is easier to disinfect, reduces contaminants and produces less by-products. Circulation systems must be active while the pool or spa is open and for at least one hour before and after the pool is open.

Oxidisers

Most of the disinfection power of chlorine and bromine is consumed in oxidising organic matter instead of disinfecting germs. Oxidisers are often used to oxidise or burn up organic matter and decrease the oxidising demand on chlorine and bromine.

Ultra violet light and ozone

UV light and ozone systems may be used in addition to, but not instead of, chlorine and bromine to disinfect and oxidise swimming pool and spa pool water. UV light must be located in the circulation system and not in the pool. Ozone, unless low dose generation is used, must be dosed in the circulation system and quenched using activated carbon before the water returns to the pool.

Further Information

Guidelines for Disinfecting Public Swimming Pools and Spa Pools specify the required disinfection concentrations and other chemical parameters for public swimming pools and spa pools operations.

Public Swimming Pool and Spa Pool Code of Practice provides detailed explanations and information on disinfection, pool chemistry, risk assessment and other issues relevant to swimming pool operation.

Both the Guidelines and Code of Practice may be found at the NSW Health swimming pool website