LEGIONELLA - World Health Organization

Identify control measures
Control measures are activities or processes applied to a system to prevent a hazard occurring.
Such measures are applied at control points, which are steps at which control can be applied to
prevent or eliminate a water safety hazard or reduce it to an acceptable level. Some plans contain
key control points; that is, points at which control is essential to prevent or eliminate a hazard.
Control measures for microorganisms in industrial systems have been described by Eggins &
Oxley (1982), and include:
• excluding the microorganism
• manipulating the environment to prevent colonization by, and limit growth of, the microorganism
(e.g. by controlling nutrient levels, controlling temperature, and preventing low
flow and stagnation)
• manipulating the environment to limit growth of the microorganism
• using a disinfectant (e.g. a biocide).
The remainder of this subsection discusses how these strategies can be used to control Legionella.
Any WSP would be based on a combination of control methods, rather than relying on any
single method.
Exclusion of microorganisms
In most systems, it is not practical to exclude legionellae or to prevent their periodic reintroduction,
because low numbers of Legionella may enter a building through piped distribution systems
or storage systems. Therefore, emphasis must be upon design and control.
Control of nutrient levels
Limiting the amount and type of nutrients (particularly organic nutrients) that are available
to the bacteria in the water system is an important control measure. Nutrient levels can be
controlled by:
• selecting materials that will not serve as substrates or provide nutrients for biofilm development
• ensuring that chemical additives used to control scaling, corrosion and microorganisms
are applied at appropriate and effective concentrations (Crespi & Ferra, 1997), and are
chemically compatible (i.e. nonreactive) with one another and with the system
• considering the properties of materials used in the water system (e.g. insulating properties,
potential for corrosion, interaction with chemical disinfection processes)
• ensuring that system design is appropriate and will prevent the accumulation of biofilms,
sediments and deposits (e.g. the design should eliminate deadends and stagnation, and
allow access to all parts of the water system for maintenance and cleaning).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS