LEGIONELLA - World Health Organization

F gure . Dec mal reduct on t mes for L. pneumophila serogroup at d fferent temperatures
D (minutes)
1000
100
10
1
49 50 51 52 53 54 55 56 57 58 59 60 61
Decimal reduction time (D) = time in minutes to kill 90% of the population of Legionella
Source: data combined from Dennis, Green & Jones (1984); Schulze-Robbecke, Rodder & Exner (1987)
Control of microorganisms
T °C
Controlling protozoa is critical in reducing the risk of legionellosis; currently, the best way to
achieve this is to prevent the development of the biofilms on which the protozoa graze (Donlan,
2002). However, preventing biofilm development can be difficult, particularly in dynamic
water systems such as cooling waters and spas, where the water flow is disrupted and large
amounts of nutrients may enter.
Any strategy for microbial control depends on water chemistry, temperature and the use of the
water system. Ideally, microbial control will be achieved using the control measures described
above. However, this is often not the case because of the characteristics of different water systems.
For example, some industrial water systems are never disinfected because the cooling systems
are open to the environment and would quickly be reinoculated with microorganisms after
disinfection. In some water systems, chemical control of Legionella may not be safe because
of the system’s design. Therefore, a chemical control strategy should take into account system
design, operating parameters and water chemistry (including the potential for production of
disinfection by-products).
A WSP should take into account the unique features of the individual water system to which
it is to be applied. Microbial control strategies are unlikely to be effective if other control
strategies, notably flow rates, temperatures and maintenance, are neglected.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS