LEGIONELLA - World Health Organization

Water treatment — risk factors
In the dynamic environment of a cooling tower system, water treatment chemicals do not perform
in the same way as they do in a controlled laboratory trial (England et al., 1982). Also, the
temperature and flow velocities of cooling tower water will vary at different locations within
the system. Many other parameters, such as pH, conductivity, total dissolved solids, suspended
matter and the biological mass within the system, can vary over a relatively short period,
affecting water treatment.
Disinfection — risk factors
Efficacy of disinfection depends on water quality parameters such as pH and turbidity, which
may compromise the disinfection process.
Applied microbial control programmes never sterilize cooling water systems. Even if enough
chemical or other agent could be added to achieve sterilization, the system would rapidly become
recolonized with microorganisms, since cooling systems are open to the environment. The most
significant practical consequence of attempted sterilization would be selection in biofilms of
increasingly tolerant microbial communities comprising the survivors of the applied antimicrobial
treatment (Russell, 2000).
Biofilms — risk factors
Cooling towers and evaporative condensers typically move large quantities of air, and are excellent
air scrubbers or washers. Thus, dirt, dust and other particulate matter enter the cooling tower
water in the evaporative cooling process, as large amounts of air are moved through the unit.
Depending on location, the quantity of such material added to the cooling water can be substantial
(e.g. several kilograms per day).
Organic matter and other debris present in the air can therefore accumulate in the cooling
water. This material may serve as a nutrient source for the growth of microorganisms, including
legionellae. Diverse biofilms, which can support the growth of legionellae, may be present on
all wet or moist surfaces throughout the system; for example, on heat exchangers, the fill, the
sump and pipes (Geary, 2000; Donlan, 2002).
Temperature — risk factors
The typical temperature of the water in a cooling tower ranges from 29 °C to 35 °C at the
heat exchanger, and from 22 °C to 28 °C at the cooling tower. These temperature ranges are
conducive to the growth of legionellae and their hosts.
Design and materials used in construction — risk factors
Stagnation of the system or areas of stagnant water (e.g. deadlegs) prevent proper chemical
treatment of the system, and allow legionellae and their hosts to proliferate.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS