LEGIONELLA - World Health Organization

Disinfection — control measures
To control Legionella numbers in the distribution system, a disinfectant residual should be maintained.
Monochloramine residual (currently available only for mains distribution systems) appears to be
effective against Legionella in biofilms, and may be more effective than chlorine (Kool et al., 1999).
Biofilms — control measures
A critical objective of any strategy to prevent the proliferation of Legionella in plumbing systems
should be to minimize the development of biofilms hosting Legionella.
Routine cleaning of storages and control of nutrients in source water will reduce nutrient load
and so help to reduce biofilm formation and growth.
Temperature — control measures
Temperature is critical in Legionella control. Consequently, water temperature should, as far as
possible, be measured and registered.
Control measures for water temperature include the following:
• For recirculating hot-water systems, the temperature of the water leaving the heater
should be not less than 60 ºC and the temperature of the return should be not less than
50 ºC. Very small differences between the temperature at the outlet of the heater and the
returning water may indicate shortcuts in the circulation.
• For non-circulating hot-water systems without storage tanks, the length of the pipes connecting
the heating device with the taps should be as short as possible.
• The temperature of hot water at the tap should reach its maximum value within one
minute, and the temperature of the cold water within two minutes (HSC, 2000).
• The temperature of hot water reached within one minute at the tap should not be less
than 50 ºC, except where thermostatic mixer valves are installed.
• The temperature of cold water at the tap should not exceed 25 ºC. Where possible, the
temperature should be less than 20 ºC, to reduce growth of legionellae. Where cold-water
supplies are routinely above 20 ºC, the water should be treated as a warm water supply.
• Where fail-safe thermostatic mixer valves are installed, the cold-water temperature should
not exceed 25 ºC and the hot should not exceed 50 ºC immediately before the valves.
• Temperature increases of cold-water pipes, reservoirs and treatment devices should be prevented
by appropriate insulation and sufficient distance between cold pipes and hot-water pipes
or heating equipment.
• In systems in which water temperature at the tap cannot be maintained at 50 ºC because
of the risk of scalding a susceptible population (e.g. in an old people’s home), alternative
means of control should be implemented. Alternative measures include the use of biocides
or periodic flushing (superheating) of the system with a return (and tap water) temperature
of at least 60 ºC. This measure requires stringent safety measures to prevent scalding.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS