Methods for controlling Legionella growth in piped water systems and cooling towers

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Controlling Legionella Growth in Water Systems

Strategies for control of Legionella growth in water depends on water chemistry, temperature and the use of the water system.

Legionella growth in water systems can be controlled by using chemicals and modifying the environmental conditions (e.g. by controlling nutrient levels, controlling temperature, and preventing low flow and stagnation).

In some water systems, chemical control of Legionella growth may not be safe because of the system’s design. Therefore, a chemical control strategy should take into account system design, operating parameters and water chemistry (including the potential for production of disinfection by‑products).

The table below shows methods that can be used to control Legionella growth in water systems and the advantages and disadvantages of each method.

Method Advantages Disadvantages
Keeping temperature <20 °C
  • Simple, effective and easily monitored
  • Little significant growth of Legionella
  • Only really applicable to drinking
    water systems
Keeping temperature >50 °C
  • Simple, effective and easily monitored
  • Does not eliminate legionellae
  • Requires circulation temperature to be near 60 °C
  • Difficult to maintain temperatures in old systems
  • Requires protection against scalding

Periodic flushing with hot water at 50–60 °C (usually an essential part of control by high temperature, above)

  • Simple, effective and easy to monitor
  • Not applicable in cold-water systems
  • Requires protection against scalding
  • Must be maintained and inspected to achieve consistent control
  • Recolonization occurs within days
Dosing with sodium hypochlorite
  • Proven, effective disinfection technique
  • Simple to use
  • Relatively cheap
  • Formation of trihalomethanes
  • Needs protection (e.g. carbon filter) for dialysis patients
  • Toxic to fish
  • Affects taste and odour
  • Not stable, particularly in hot water
  • Increases corrosion of copper
Dosing with monochloramine
  • More persistent than chlorine
  • Simple to use in mains distributions
  • Penetrates into biofilms
  • Needs protection (e.g. carbon filter) for dialysis patients
  • Toxic to fish
  • Affects rubber components
  • No commercial kit available for dosing small water systems
Dosing with chlorine dioxide
  • Proven disinfection technique
  • Simple to use
  • Formation of chlorite
  • Needs protection (e.g. carbon filter) for dialysis patients
  • Safety considerations (depending on method of generation)
Dosing with hydrogen peroxide
  • Simple to use
  • Weak disinfectant
  • Suspected of mutagenicity
Copper and silver ionization
  • Effective when prescribed concentrations are maintained
  • Frequent monitoring of copper and silver needed
  • Pretreatment needed (pH, hardness)
  • Increased concentrations of copper and silver in water
Anodic oxidation
  • Disinfection demonstrated
  • Pretreatment needed (depending on effect of pH and hardness)
  • Effect on Legionella in biofilms not known
UV (ultraviolet) disinfection
  • Proven disinfection technique
  • Simple to use
  • Effective only at point of application; no control downstream (no residual)
  • Not suitable for turbid waters
  • No effect on biofilm formation
Ultrafiltration at point of entry to the building or system
  • Physical disinfection barrier
  • Effective removal of biomass and particles
  • No inactivation of Legionella downstream of the filter within system
  • Effect on formation of biofilms and sediment not known
Point-of-use filters
  • Physical barrier
  • Easy to install (may require some modification of the outlet)
  • Suitable for hot and cold-water systems
  • Good for use in systems exposing high-risk patients
  • Only suitable at point of use
  • Must be replaced regularly
  • Particulates in water may reduce flow and operational life
  • Expensive
Pasteurization heat with flushing
  • Disinfection barrier
  • Useful as short-term remedial measure
  • Simple to apply in hot-water installation
  • Transient effect on Legionella
  • No limitation of biofilm formation
  • Scalding risk
Non-oxidizing biocides
  • Proven technique for cooling systems
  • Not suitable for potable water systems
  • Most not applicable to spa pools
  • Resistant populations may develop
  • Need to alternate two different biocides
  • Often concentrations cannot be readily monitored
  • Difficult to neutralize for sampling purposes

Source: LEGIONELLA and the prevention of legionellosis. World Health Organization 2007.

Note: MBL is certified for Legionella analysis. Click CDC Legionella ELITE certification for details.

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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at
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