UFC 3-240-13FN
25 May 2005
important to maintain an adequate dosage for 24 hours to enable sufficient contact time
for maximum effectiveness.
4-4.7
Legionnaires' Disease. Legionnaires' Disease (Legionellosis) is a
respiratory disease (atypical pneumonia) that is caused by infection of susceptible
individuals who have inhaled a fine water mist containing the bacterium known as
Legionella Pneumophila. Water in a cooling tower can become infected with the
bacterium if an inadequate microbiological control situation occurs. The presence and
density of Legionella Pneumophila bacteria cannot be detected by standard
microbiological testing methodologies. Instead, cooling water samples must be sent to a
laboratory that has been certified to conduct the required tests. If the presence of the
bacteria in cooling water is established, proper disinfection steps are required. A
procedure known as the Wisconsin Protocol, developed by the Wisconsin State Health
Department, has proven effective. This protocol requires the addition of high dosages of
chlorine (10 ppm free residual) at a pH of less than 7.5 for 24 hours, flushing the
system, then repeating. Additional testing for Legionella is required to determine the
effectiveness of the procedure. Maintaining a clean, microbiologically free cooling water
system and using effective water treatment is preferable to dealing with remedial efforts.
The Cooling Technology Institute (CTI) and the American Society of Heating,
Refrigerating, and Air Conditioning Engineers (ASHRAE) have published position
papers on the prevention of Legionella; these can be downloaded from their respective
websites: http://www.cti.org/ and http://www.ashrae.org/.
4-4.8
Macrobiological Control. The term "macrobiological control" refers to
control of larger biological organisms such as mussels, clams, and snails. They can
exist in cooling systems that use seawater or river water as makeup. The first line of
defense is using mechanical prevention with strainers and filters on the intake water to
prevent infiltration into the cooling water system. Control methods within the cooling
system include thermal shock and chemical treatment with oxidizing and non-oxidizing
biocides.
4-5
CORROSION IN COOLING SYSTEMS. The term "corrosion" (in a cooling
water system) is defined as the electrochemical deterioration of a metal that is in
contact with cooling water. Corrosion occurs when an electric current flows from one
part of the metal (anode) through the water (electrolyte) to another part of the metal
(cathode). Corrosion takes place at the anode only. The cathode is the driving force of
the corrosion action. Forms of corrosion associated with industrial water systems are
illustrated in Figure 4-17.
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