Legionella: a major risk for cooling towers
Legionnaires' disease is a severe lung infection caused by the bacterium Legionella pneumophila. Cooling towers (also known as evaporative cooling towers, or ECT) are among the primary contamination vectors because they produce aerosols — fine water droplets that can be inhaled several hundred metres away.
Studies estimate that 60 to 80% of cooling towers contain Legionella. The question is not whether the bacteria are present, but whether their concentration reaches dangerous levels.
French regulations: ICPE 2921
Cooling tower regulations in France fall under ICPE category 2921 (Installations Classified for Environmental Protection), revised by the decree of 14 December 2013:
- Mandatory registration for towers with capacity ≥ 3,000 kW
- Declaration with inspection for towers with capacity < 3,000 kW
- Action threshold: Legionella pneumophila concentration must be maintained permanently below 1,000 CFU/L
- Shutdown threshold: if concentration exceeds 100,000 CFU/L → immediate shutdown + declaration to the Prefecture
- Mandatory testing: minimum monthly for Legionella
- Annual cleaning: minimum one mechanical and/or chemical cleaning intervention per year
- Permanent preventive treatment: mandatory throughout operation, aimed at reducing biofilm and limiting Legionella
- Maintenance log: mandatory register of all maintenance operations and test results
The link between limescale, biofilm and Legionella
Scale (calcium carbonate deposits) plays a direct role in Legionella proliferation:
- Colonisation surface — Scale creates a rough, porous surface that provides an ideal habitat for biofilm, a layer of micro-organisms protected by an extracellular matrix.
- Bacterial protection — Biofilm formed on scale protects Legionella from biocide treatments. Bacteria can be up to 1,000 times more resistant to chlorine when embedded in biofilm.
- Nutrient source — Scale traps organic matter that feeds micro-organisms.
- Reduced biocide effectiveness — Treatment products cannot reach bacteria buried beneath layers of scale and biofilm.
By reducing scale, LIMPEO reduces the surface available for biofilm, which limits Legionella proliferation and improves the effectiveness of biocide treatments.
Notable outbreak cases
| Location | Year | Cases | Deaths | Identified source |
|---|---|---|---|---|
| Pas-de-Calais, France | 2003–2004 | 86 | 21 | Petrochemical cooling tower |
| Murcia, Spain | 2001 | 800+ | 6 | Cooling tower (largest documented outbreak) |
| New York City, USA | 2015 | 138 | 16 | Cooling towers (led to new regulation) |
| Orange County, USA | 2017 | 22 | – | Theme park cooling towers |
All of these outbreaks were linked to poorly maintained cooling towers with high levels of scale and biofilm.
The role of LIMPEO in prevention
LIMPEO, designed by MAGIIC SAS, uses electromagnetic waves to transform calcite (adherent scale) into aragonite (non-adherent micro-crystals). By reducing scale by (scientific studies), LIMPEO:
- Reduces the colonisation surface for biofilm in cooling circuits
- Improves the effectiveness of biocide treatments by eliminating protective scale layers
- Reduces the frequency of chemical descaling operations
- Increases concentration cycles, reducing blowdown by 50 to 70%
- Extends the lifespan of components (packing, valves, pumps)
Important: LIMPEO does not replace regulatory biocide treatment. It complements it by acting on scale and biofilm, making the overall treatment programme more effective. See also our article on the ROI of anti-limescale treatment in industry.
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FAQ — Legionella and cooling towers
What is Legionnaires' disease?
Legionnaires' disease is a severe lung infection caused by inhaling water droplets contaminated with Legionella pneumophila. It presents as severe pneumonia with a mortality rate of 10 to 15%. Immunocompromised individuals, smokers and the elderly are particularly vulnerable.
What is the regulatory threshold for Legionella in France?
The decree of 14 December 2013 (ICPE 2921) sets the action threshold at <1,000 CFU/L of Legionella pneumophila. Above 100,000 CFU/L, the operator must immediately shut down the tower and report the situation to the Prefecture.
How does limescale promote Legionella growth?
Scale creates a rough, porous surface that supports biofilm. This biofilm protects Legionella from biocide treatments (up to 1,000 times greater resistance to chlorine). By reducing scale, LIMPEO removes the biofilm's support structure and improves the effectiveness of disinfectants.
Does LIMPEO replace biocide treatment?
No. LIMPEO is a complement to regulatory biocide treatment, not a substitute. It acts on scale and biofilm to make the overall treatment programme more effective. Preventive biocide treatment remains mandatory in accordance with the decree of 14 December 2013.
How often must Legionella testing be carried out?
The decree requires testing at minimum monthly for Legionella pneumophila. The operator must also maintain a maintenance log documenting all maintenance operations, test results and corrective actions.
What are the penalties for non-compliance?
Failure to comply with ICPE 2921 regulations may result in administrative sanctions (formal notice, suspension of operations), fines, and in the event of an outbreak, criminal liability for the operator.