LEGIONELLA - World Health Organization

Box . Po nts to be noted when clean ng and d s nfect ng
When cleaning and disinfecting a cooling tower or evaporative condenser, it is important to:
• minimize creation of aerosols
• where possible, remove drift eliminators, inspect fill and packing, and clean and repair
or replace as required
• clean all water filters and strainers associated with the distribution system
• check water distribution nozzles or troughs, and gutters, and clean or replace as required
• use a low-pressure spray of a combination detergent and oxidizer (e.g. sodium
hypochlorite) for cleaning; do not use high-pressure washers on plastic packing
or eliminators (if high-pressure washers are to be used on other parts of the tower,
the washing should be covered, to contain most of the splashing)
• manually clean the tower, packing, sump, eliminators and distribution system; ensure
that the cleaning is timed to minimize the risk of exposing individuals in the vicinity
of the tower, and that personnel wear positive-pressure high efficiency particulate
absorbing (HEPA) filter masks (respirators) during the procedure
• remove high-density plastic film pack for cleaning, if recommended by relevant
authorities — some authorities recommend this, because it can be difficult to check
after cleaning that the interstices in the pack are free of dirt and scale (HSC, 2000),
but there are no published studies comparing the effectiveness of cleaning in place
with removing the pack for cleaning
• ensure that records of the procedures are kept, and they include the date and the
signature of the responsible party or authority.
Spray drift — control measures
The effectiveness of drift eliminators varies, depending on their design and condition —stateof-the-art
eliminators are significantly more efficient than older designs. The eliminators should
be inspected regularly (at least every six months) and either cleaned and disinfected or replaced,
as necessary. Shorter intervals between inspection and cleaning may be advisable for systems
in which heavy fouling is a chronic occurrence, or where highly susceptible populations are
likely to be exposed (HSC, 2000).
5.4.2 Monitor control measures
This step involves defining the limits of acceptable performance and how these are monitored.
The results of tests such as those listed above allow corrective actions (discussed in Section 5.5.3,
below) to protect public health to be taken.
Legionella populations in cooling systems are highly variable, and elevated concentrations
occur sporadically in most cooling towers (Bentham, 2000), meaning that single measurements
show only a snapshot of the microbial situation. Since it is not possible to monitor Legionella
concentrations continuously, other strategies must be used to maintain concentrations as
consistently low as possible; one such strategy is to prevent situations that stimulate growth
of the bacteria.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS