Environmental Engineering Reference
In-Depth Information
towers with dry towers when the former reach the end of their useful life,
and avoidance of wet towers in new applications.
The key to avoiding
Legionnaires' disease infections associated with wet cooling towers is for
building management to have a preventive O&M program in place. Elements
in such a program include periodic sampling of cooling tower water to
determine
b.
Cooling tower operation and maintenance.
levels present and biocidal treatments when
deemed necessary. Guidelines for the operation and maintenance of cooling
towers have been developed and published by the World Health Organiza-
tion (WHO).
L. pneumophila
i.
Biocidal treatment of cooling tower waters.
A number of biocides
have been evaluated for use in controlling
populations in
cooling tower water. Though many have been found to be effective under
laboratory conditions, they are often not effective under actual cooling tower
operation. As a result, it would be prudent for building management to
follow the recommendations of the Centers for Disease Control (CDC). Pop-
ulations of
L. pneumophila
in cooling tower water can be significantly reduced
in routine maintenance activities and following disease outbreaks. The CDC-
recommended procedure includes the establishment and maintenance of a
neutral pH with careful addition of acid, addition of hypochlorite sufficient
to maintain a free residual chlorine content of 100 to 250 ppm (w/v) for a
minimum of 48 hours, flushing and refilling the system, and, if necessary,
maintaining a free chlorine residual of 2 to 5 ppm until
L. pneumophila
L. pneumophila
populations have been reduced to acceptable levels.
survives low-level
chlorination in water treatment systems. As a consequence, hot water sys-
tems in hospitals, hotels, nursing homes, and possibly residences may be
subject to significant
c.
Potable water systems.
Legionella pneumophila
population development. Since such
waters are used for whirlpool spas and hot tubs, they pose potential exposure
risks as well.
Exposures from potable water systems can be reduced by the implemen-
tation of good system O&M practices. These would include periodic collec-
tion of samples to assess the presence of
L. pneumophila
L. pneumophila
and population levels.
Risk factors for proliferation of
L. pneumophila
include elevated water tem-
perature (>86° <140°F, >30°
<60°C), older hot water tanks, vertical configu-
ration, and elevated magnesium and calcium levels. Water with tempera-
tures >140°F (60°C) are typically free of
.
Several methods have been evaluated for use in preventing and control-
ling the growth of
L. pneumophila
in hot water systems. These include thermal
eradication and hyperchlorination. The “heat and flush” method (thermal
eradication) is the most widely used
L. pneumophila
control method in hos-
pitals. The method requires heating water to 158°F (70°C), followed by
flushing all faucets and shower heads with heated water. This procedure
L. pneumophila
