Cooling Tower Legionella: Prevention & Control
Cooling towers are a known source of Legionella bacteria. This guide covers the conditions that promote Legionella growth, the regulatory requirements, and the water treatment programme that prevents outbreaks.
Critical Safety
Legionnaires' disease is a potentially fatal pneumonia caused by inhaling Legionella bacteria in aerosol form. Cooling towers are the most common source of outbreaks. In Australia, cooling tower owners have a legal duty of care to manage Legionella risk.
Why Cooling Towers Are High-Risk
- Warm water (25–45°C) — the ideal temperature range for Legionella growth
- Aerosol generation — the tower's drift creates fine water droplets that can carry bacteria
- Stagnant water areas — dead legs, redundant pipework, and basins with poor circulation promote bacterial growth
- Biofilm formation — slime layers on tower fill and basin protect bacteria from biocides
- Nutrient sources — sediment, scale, and organic matter provide food for bacteria
- Amenae — protozoa in the water can harbour and protect Legionella from treatment
Regulatory Requirements (Australia)
Each Australian state has regulations for cooling tower Legionella management. Key requirements include:
- Registration of all cooling tower systems with the state health department
- A Risk Management Plan (RMP) covering all aspects of operation, maintenance, and water treatment
- Monthly microbiological testing (Heterotrophic Colony Count) and quarterly Legionella testing
- Automatic dosing of biocide and a controlled dispersant system
- Regular cleaning and disinfection — at least every 6 months
- Maintenance of a logbook recording all test results, maintenance, and water treatment
- Notification of the health department if Legionella is detected above specified levels
Water Treatment Programme
- Biocide dosing: Continuously dose a biocide (chlorine, bromine, or non-oxidising biocide) to maintain a residual that kills bacteria. The dosing system must be automated and have failsafe alarms.
- Biodispersant: Add a biodispersant to penetrate and break up biofilm, exposing bacteria to the biocide. Essential — without it, bacteria inside biofilm are protected.
- Scale and corrosion control: Dose scale inhibitors and corrosion inhibitors to keep the tower clean — scale and sediment provide habitats for bacteria.
- Blowdown control: Maintain conductivity (TDS) within the set range to prevent scale and concentration of nutrients.
- Drift eliminator maintenance: Keep drift eliminators in good condition — they reduce the aerosol that escapes the tower. Damaged eliminators increase Legionella risk.
Response to Positive Legionella Test
- Notify: Immediately notify the state health department if required by regulation.
- Hyperchlorinate: Raise the free chlorine residual to 5–10 ppm and maintain for 4–8 hours. Circulate through the entire system.
- Clean physically: Drain, clean, and physically remove all sediment, biofilm, and scale from the tower basin, fill, and distribution system.
- Recommission: Refill, dose biocide, and retest before returning to normal operation.
- Investigate: Determine why the bacteria proliferated — was the biocide dosing inadequate? Was there a dead leg? Was the biofilm not controlled?
Pro tip: PlantLogica can monitor cooling tower water quality in real time — conductivity, pH, biocide residual, and temperature. When any parameter drifts outside the safe range, the system alerts maintenance staff immediately, preventing the conditions that lead to Legionella growth.