LEGIONELLA - World Health Organization

Isolation of Legionella from any system is likely to occur occasionally (Bentham, 2000).
Conducting comprehensive system risk assessments after positive test results should reduce
the incidence of such results. Periodic review of the number of positives recorded should be
used to assess whether a consistent level of control has been achieved and whether overall
control is improving. After the introduction of a new WSP or significant modification to the
system, the microbial test parameters described above should be applied. Verification of the
other control measures determined for the WSP should precede microbial testing.
With the fan off, the water flow throughout the tower should be viewed for unrestricted flow
from inside. If possible, drift eliminators should be examined for damage or excessive drift,
from both inside and outside.
The condition of the internal structure of the tower should be examined, with the fan, the
water pump and any dosing and filtering equipment switched off. Any deterioration of materials
(wood, metal, etc.) should be noted, particularly of the fill, the drift eliminator, the basin and
the water distribution system; a check should also be made for visible microbial growth.
More detailed inspections should be undertaken if the plant is shut down completely during
annual inspection. This provides the opportunity for examining the interior of pumps, sections
of pipework and heat exchange equipment.
5.6 Surveillance
The system assessment should be independently reviewed periodically (e.g. every two years) and
after any major changes to the system or management. The review should be undertaken by
a formal, competent authority.
The tower should be inspected under normal working conditions by an independent surveillance
team, wearing appropriate safety equipment to prevent the inhalation of aerosols. A number
of items can be examined externally; for example, the team might look for signs of microbial
growth, algae, water leaks, splashing and blockages or restrictions at air inlets. Where chemical
dosing equipment is installed, it should be examined for correct operation and for adequate
stock of chemicals.
Details of maintenance should be recorded to identify performance trends and for prompt
attention to faults reported by operational staff or building occupants. Maintenance procedures
should be constantly monitored to ensure adherence to clearly defined objectives. Any changes
to plant operation or modifications should be recorded in the maintenance manual.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS