LEGIONELLA - World Health Organization

It is often practical and highly effective to reduce the concentration of dissolved minerals, such
as calcium and magnesium, in make-up water before it enters the cooling system (water softening).
Water softening reduces the potential of the system to form biofilms, but may increase corrosion.
Reduction of organic load in the source water by chlorination or filtration (or both in concert)
helps to remove nutrients that could lead to legionellae proliferation. Chlorination used to
reduce the organic load may also serve to disinfect the water of its inherent microbial load.
Water treatment and water distribution — control measures
A system should be designed in such a way that water circulates through all parts of the system
that should be wetted whenever it is operational. Deadlegs on existing systems should be
removed or shortened (so that their length is no longer than the diameter of the pipe), or
should be modified to permit the circulation of chemically treated water.
Dirt, organic matter and other debris should be kept to a minimum, as water treatment chemicals
are generally more effective when the system is kept clean.
After stagnation of part or all of the system, system operation should always be coordinated with
full chemical treatment of the water. Similarly, when a cooling tower system has been shut down
for more than three days, the entire system (i.e. cooling tower, system piping, heat exchangers,
etc.) should be drained to waste, if practicable. Since it is often not possible to completely
eliminate all water from shut-down cooling systems, cooling water must be pretreated with
an appropriate antimicrobial regimen before system start-up (HSC, 2000; Guideline 12–
2000 in ASHRAE, 2000); that is, before activating the fans.
Corrosion inhibitors should be used to minimize corrosion of metal surfaces. Surfactants,
biocides and other chemicals should be used to control fouling due to scale, silt and microbial
growth. Use of these chemicals will help to maintain efficient heat transfer at metal surfaces,
ensure free flow of water throughout the system and prevent the proliferation of microorganisms
that are responsible for surface corrosion and degradation.
Disinfection — control measures
Because of the many factors that can compromise the disinfection process (outlined above),
it is advisable to vary the antimicrobial stresses applied in the cooling water microbial control
programme (McCoy, 1998), particularly in the case of non-oxidizing biocides. One practical
and effective means to vary antimicrobial stresses is to alternate between two non-oxidizing
biocides added as a single (“slug” or “shot”) dose, manually or automatically, at 3–4-day intervals.
Another effective approach is to alternate use of an oxidizing antimicrobial with a nonoxidizing
antimicrobial, to ensure that different modes of antimicrobial action are employed.
When varying antimicrobial stresses, performance-based monitoring is used to assess the
extent of microbial control achieved (McCoy, 2003).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS