UFC 3-240-13FN
25 May 2005
4-4.3
Fungi. The term "fungi" refers to classes of organisms made up of molds
and yeasts, some of which attack and cause wood decay in cooling towers. The control
of fungi requires special preservative treatment of wood. Fungi also produce deposits in
cooling water equipment.
4-4.4
Microbiological Control. The term "microbiological control" refers to
techniques used to minimize the presence of microbiological organisms in cooling
water. Chemical biocide treatment is the method used on government installations for
microbiological control in cooling water. Biocides that are used to control microbiological
growth fall into one of two broad categories: oxidizing and non-oxidizing microbiocides.
A cost-effective approach for control involves the regular use of oxidizers as a primary
biocide, augmented by selective use of non-oxidizing biocides. Important factors for the
effectiveness of any biocide include using a proper dosage and allowing adequate
contact time with the microbiological organisms. All microbiocides are toxic and must be
handled safely and with caution; use the MSDS for safety instructions.
4-4.4.1
Oxidizing Biocides. "Oxidizing biocides" is a term describing
microbiocides that oxidize or irreversibly "burn up" the bio-organisms. Oxidizing biocides
also destroy nutrients that the microorganisms require for growth. Avoid addition of
excess amounts (over-feeding) of oxidizing biocides because they are corrosive to
metal and wood in the cooling system and have the potential to destroy some scale and
corrosion inhibitors. The various oxidizing biocides are described below. Table 4-6
provides guidelines for selecting oxidizing microbiocides.
4-4.4.1.1
Chlorine and Chlorine Release Agents. Chlorine (Cl2) compounds are
the most effective industrial oxidizing biocides and the most widely used. Chlorine is
available as a chlorine gas, dry calcium hypochlorite (HTH), liquid sodium hypochlorite
(bleach), plus several other dry products that release chlorine. When chlorine is
introduced into water, it hydrolyzes to form hypochlorite ion (OCl-) and hypochlorous
acid (HOCl); it is the latter chemical that is the stronger oxidizing biocide. The presence
of hypochlorous acid is greater, proportionate to hypochlorite ion, at low pH levels. At a
pH of 5.0, hypochlorous acid exists almost exclusively. At a pH of 7.5, there are
approximately equal amounts of hypochlorous acid and hypochlorite ion. Figure 4-16
shows this relationship. Chlorine is effective, but to a lesser degree, as a biocide at a pH
of 7.5 or greater because the hypochlorite ion has about one-tenth the biocidal efficacy
of hypochlorous acid. A pH range of 6.5 to 7.5 is considered optimal for chlorine or
chlorine-based microbiological control programs. Above pH 7.5, relatively higher levels
of chlorine are required to be effective. Military installations seldom use gaseous
chlorine for treating cooling towers because of safety concerns, difficulty with controlling
the feed of the gas, and increasing concern for the environmental effects of escaping
residual chlorine gas. The most commonly used chlorine-based products are bleach
and HTH.
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