Diarrhoeal incident response protocol for public swimming pools that use chlorine with cyanuric acid

Diarrhoeal incidents pose a particularly high risk to the health of pool users. Immediately closing the affected pool and undertaking appropriate remediation is the only way to prevent the spread of disease. Chlorine stabiliser (cyanuric acid) significantly slows the rate at which free chlorine inactivates or kills contaminants such as Cryptosporidium. Therefore, it is necessary to achieve a much higher free chlorine C.t value (chlorine concentration mg/L x contact time in minutes) for public swimming pools that use cyanuric acid compared to a pool that does not use cyanuric acid. Cyanuric acid may only be used in an outdoor chlorine-disinfected pool and must not be used in a spa pool.

Recommended remedial steps

Immediately close the affected pool and any other pools that are hydraulically connected and ensure staff involved in the response have appropriate personal protective equipment.
Remove as much of the faecal material as possible using a bucket, scoop or another container that can be discarded or easily cleaned and disinfected. Dispose of the faecal material in the sewer. Do not use aquatic vacuum cleaners for removing faecal material unless the vacuum waste can be directly discharged to the sewer and the vacuum equipment can be adequately cleaned and disinfected.
Adjust and maintain the pH to 7.5 or lower.
Ensure cyanuric acid is 15 mg/L or less (this can be achieved by partially draining and adding fresh water without chlorine stabiliser to the affected pool).

Once the cyanuric acid concentration is 15 mg/L or less, use unstabilised chlorine to hyper-chlorinate the affected water body(ies) by dosing the water to achieve a free chlorine C.t inactivation value of 31,500 mg.min/L (for example, raise and maintain free chlorine of 20 mg/L for 28 hours, or via alternative combinations of chlorine concentration and time that achieve the required C.t). There is limited evidence to suggest that the use of chlorine dioxide for hyper‑chlorination is effective in swimming pools where cyanuric acid is present.
Ensure that filtration and any secondary disinfection systems operate for the whole decontamination process.
If the filtration system incorporates a coagulation step, ensure the coagulant concentration is correct to enhance the filtration process.
After the hyper-chlorination process is complete, reduce the total chlorine to below 10 mg/L. Sodium thiosulfate can be added to neutralise excess chlorine.
Backwash filter media or replace the filter element as appropriate. Precoat filter media should be replaced.
Ensure the water is balanced.
Hygienically clean, disinfect or dispose of materials, tools, equipment or surfaces that have come into contact with contaminated water.
Record the incident and remedial action taken.
Re-open the pool.

For more information on diarrhoeal incident risk management of public swimming pools and spa pools, contact your local Public Health Unit on 1300 066 055 for advice.