LEGIONELLA - World Health Organization

Where holding tanks are used, they can be disinfected by filling with water and chlorinating
at 5 mg per litre free chlorine while maintaining the pH between 7.0 and 7.6. After one hour,
this disinfected water can then be added to the cooling tower as part of the routine cleaning
and disinfection procedure.
In emergency responses, systems must be cleaned, the water used for cleaning drained, and the
system refilled. If the water used to refill the system is not clean and does not contain a
disinfection residual, recontamination may occur, making it necessary to repeat the entire
cleaning procedure.
Biofilms — control measures
To facilitate penetration of an antimicrobial into biofilms and sediments, use of a compatible
and environmentally acceptable dispersant and/or detergent is strongly advised. There are many
acceptable surfactant (surface-active) chemicals of this type, including non-ionic, anionic and
amphoteric compounds (McCoy, 2003).
Temperature — control measures
Systems operating at the lower end of their working temperature range are likely to support
less Legionella contamination (Kusnetsov et al., 1997). Therefore, systems should be designed to
operate at the lowest possible temperature, to minimize legionellae growth (Bentham & Broadbent,
1993).
Design and materials used in construction — control measures
Cooling towers should be designed to:
• be easy to clean
• avoid the accumulation of sludge and deposits
• provide easy access for maintenance of internal surfaces, including the fill (Broadbent, 1996).
Connected tanks, filters and other devices must be scrutinized for their potential to support
the proliferation of Legionella. Materials should be non-porous, with easy-to-clean surfaces,
and should not provide nutrients for growth.
Cleaning and maintenance — control measures
A maintenance programme for the cooling system is essential, and the programme should be
recognized and monitored as an important control measure.
The following procedures are effective for maintaining a clean system:
• regular physical cleaning, judicious use of chemicals and prevention of the build-up of
dirt and dissolved solids in the circulating water
0 LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS