LEGIONELLA - World Health Organization

• periodic or continuous bleed-off or blow-down (excessive bleed-off should be avoided,
because it will result in a loss or dilution of water treatment chemicals, and thus reduce the
effectiveness of the treatment programme)
• addition of chemicals to the water at a rate sufficient only to maintain predetermined chemical
concentrations and a stable total bacteria count below a predetermined acceptable level
• regular checks of tower components
• cleaning of wetted surfaces
• water treatment to minimize corrosion and scaling, and to provide biological control
• routine cleaning and disinfection
• regular visual inspections for general cleanliness
• cleaning of the sump of the unit when any build-up of dirt, organic matter or other debris
is visible or found through sampling (effective means of removing particulate matter include
side-stream “filtration”, coupled with strainers, cartridge filters, sand filters, centrifugalgravity-type
separators or bag-type filters).
The aim of the maintenance programme is to ensure optimal thermal performance and also
to minimise the risk of disease, through a combination of mechanical maintenance and total
tower cleanliness. The programme should cover regular water treatment, inspections and cleaning,
and should be implemented as soon as the cooling tower starts to operate. Box 5.3 provides
an example of a corrective action procedure for emergency disinfection. The type of situation
that might lead to corrective action is finding that monitoring results suggest the tower is out
of control; for example, if results show a failure of biocide dosing, or repeated high counts of
total viable bacteria or legionellae.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS