LEGIONELLA - World Health Organization

The section on control measures for cleaning and maintenance (below) contains additional
information on the disinfection process.
Oxidizing biocides
Commonly used oxidizing antimicrobials for cooling water include chlorine, bromine, stabilized
bromine, combinations of bromine and chlorine, chlorine dioxide, peroxy compounds such
as hydrogen peroxide and peracetic acid, and ozone (Kim et al., 2002; McCoy, 2002).Oxidizing
antimicrobials are often effective when fed continuously using metering systems with small
pumps, and many towers are successfully treated with continuous dosing with chlorine or
bromine.
Shot-dosing of oxidants, which can also be very effective in microbial control, is an alternative
to unvarying application of oxidizing antimicrobials.
Non-oxidizing biocides
Non-oxidizing biocides are most effective when shot dosed. The maintenance of a continuous
residual of non-oxidizing biocides in the system will inevitably lead to the selection of resistant
microorganisms and loss of microbial control (Russell, 2000; 2002). Non-oxidizing biocides
are usually dosed at higher concentrations (15–50 parts per million [ppm]) than oxidizing
biocides, and may require longer contact times at these concentrations (4–10 hours).
Treatment programme
All biocides should preferably be fed via a metering system, and the appropriate dose calculated
on the basis of system volume and half-life (dilution rate) within the system (Kim et al., 2002).
“Blow-down” or “bleed-off” is the removal of some of the water periodically or continually, and
its replacement with fresh water, to control the continuous accumulation of dissolved solids
in the water. This process may be controlled by a conductivity controller that detects the
increase in conductivity due to the dissolved solids, and automatically regulates the rate to
hold a preset conductivity by triggering the operation of a solenoid drain valve.
Blow-down may be activated immediately before the addition of the biocide, to ensure that the
amount of suspended dirt in the water that might react with and neutralize the biocide is
minimized. Blow-down may then be stopped for a period after the addition of the biocide, to
ensure that the chemical is retained at a sufficient concentration for long enough to be effective.
In selecting a chemical treatment programme, the operating parameters and water chemistry
that may be unique to the system should be considered. A microbial control problem is rarely
resolved by the application of generic technologies. Any microbial control strategy will fail
without due attention to other control measures. Usually, the advice and the practical guidance
of a water treatment specialist are necessary.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS