LEGIONELLA - World Health Organization

Where temperature controls (discussed below) cannot be maintained, an alternative means
of control needs to be implemented; for example, where legionellae multiply in warm areas
of cold-water systems. The effectiveness of control measures for Legionella depends on many
variables. Physical systems such as ultraviolet (UV) and filtration may be satisfactory if fitted
near the point of use, but they are not dispersive; that is, they do not form a residual level of
treatment throughout the water system and therefore will not affect biofilms harbouring
Legionella downstream of their point of use. Criteria for a universal acceptable level of effectiveness
have not been defined, but might include a required log reduction of Legionella in water and
an effect on biofilms (e.g. reduction of formation or growth of biofilms).
Applying alternative control techniques requires detailed consideration of the extent and complexity
of the system, and of the composition of the water. Where alternative measures are implemented,
monitoring is needed to ensure that controls are adequate and maintained (see Section 4.5).
Tap diffusers reduce water use but can increase aerosol production. Therefore, in high-risk
areas, such as hospitals, diffusers should not be installed (and facilities should consider removing
diffusers that are already in place). Mixing valves should be as close to the shower outlet as
possible, and shower fittings should be detachable so that they can be routinely cleaned and
disinfected.
Distribution systems — control measures
Control of legionellae should begin at the design stage of the water system. There are many
different designs for modern plumbed water systems that supply hot and cold water in buildings.
Systems may be gravity fed, with a storage tank for cold water fed by the mains supply, or
they may be pressurized, with no intermediate storage tank. Hot water is supplied from a water
heater, calorifier, boiler or plate heat exchanger, depending on the scale of the system. Cold
water is distributed either directly from the mains supply or via a cold-water storage tank.
Pipes should be as short as possible. In complex systems, regulating valves should be used to
control flow. Deadends should be avoided in both the design and construction phases, and in
existing systems they should either be removed or regularly flushed.
Standard system fittings should include devices to prevent backflow on heat production systems,
and purge valves to prevent scaling and corrosion and facilitate monitoring. These should be
installed at appropriate locations in the system, according to national standards.
Construction materials — control measures
The materials used to construct piped water distribution systems should be compatible with the
chemical quality of water (after a corrective treatment) and should minimize bacterial growth.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS