Key messages

  • Decontamination may be required in cooling tower systems linked to a case or cases of Legionnaires’ disease, as described in the Public Health and Wellbeing Regulations 2009.
  • The department considers this procedure to meet the requirements of the Regulations.
  • Other procedures may be used, provided that they meet the requirements of the Regulations.

Cooling tower system decontamination – background

Decontamination may be required in cooling tower systems linked to a case or cases of Legionnaires’ disease, as described in the Public Health and Wellbeing Regulations 2009.

Cooling tower system decontamination – procedure

The following process is considered by the department to meet the intent of the Regulations. Other processes can be used, provided that they meet the requirements of the Regulations.

  1. Follow all relevant occupational health and safety procedures, including the use of personal protective equipment.
  2. Cease any chemical treatment. Isolate any electrical equipment except the water treatment pump.
  3. Add a low-foaming, chlorine-compatible biodispersant to the recirculating water.
  4. Disinfect the system by dosing the water with either:
    1. a chlorine-based compound, equivalent to at least 10 mg/L of free chlorine for at least 1 hour, while maintaining the pH of the water between 7.0 and 7.6, or
    2. a bromine-based compound, equivalent to at least 20 mg/L of free bromine for at least 1 hour, while maintaining the pH of the water between 7.0 and 8.5.
  5. Add the disinfectant slowly, over 5–10 minutes, to a turbulent zone of the tower basin to promote its rapid dispersion. Use an anti-foaming agent if excessive foaming occurs.
  6. Switch off equipment and drain cooling tower to waste in a manner approved by the local water authority. The entire cooling water system should be drained (116). Use of a wet vacuum cleaner can make it easier to remove waste material from the basin floor.
  7. Refill with clean water and switch on the recirculating pump.
  8. Repeat step 4, but maintain the specified concentrations for 3 hours. Then switch off the recirculating pump. Drain the cooling tower system to waste in a manner approved by the local water authority.
  9. Inspect the drift eliminators, and clean, repair or replace them, as necessary. If the eliminators are moved, ensure that they are correctly installed on replacement. Suitable precautions should be taken to minimise the release of aerosols during cleaning operations.
  10. Thoroughly clean the internal shell, fill and tower sump by brushing and gently hosing all surfaces. Remove all debris. Avoid damage to the tower and accessories during this operation.
  11. Thoroughly internally clean all water filters, strainers, separators, water nozzles and fittings associated with the water distribution system.
  12. Reassemble all components and hose with clean water.
  13. Repeat step 4, but maintain the specified concentrations for 3 hours. Then switch off the recirculating pump. Drain the cooling tower system to waste in a manner approved by the local water authority.
  14. Refill with clean water and switch on the recirculating pump.
  15. Repeat step 4 if the water is not visually clear. Clean the water filters and strainers, and repeat step 13. Repeat this sequence until the water quality is satisfactory.
  16. Immediately reinstate comprehensive effective water treatment, including use of biocide(s) and anticorrosives, and scale control.
  17. Record all actions in the maintenance logbook.

Footnotes

116. Where this is not practicable, a very high bleed-off rate should be used during step 4. This will help to remove suspended particulate matter from the system and partially replace cooling water with clean make-up water.