UFC 3-240-13FN
25 May 2005
4-4.5
Algae Control. Algae can be controlled by two techniques: chemical
methods and physical methods. Since algae require sunlight to survive and grow,
covering the upper hot water decks of cooling towers with plywood can often control
algae deposits. Chemical methods consist of using oxidizing and non-oxidizing biocides,
which can control algae to various degrees (see Tables 4-6 and 4-7). The effectiveness
of oxidizers is considered only fair while that of several non-oxidizers is in the range of
very good to excellent. One of the more effective biocides for algae is terbutylazine, a
triazine product (see Table 4-7).
4-4.6
Bacterial Control. Accepted industry practice for bacterial control is the
use of oxidizing and non-oxidizing biocides that are specific for the type of bacteria. The
most cost-effective microbiocide programs for medium and large cooling towers use an
oxidizer as a primary biocide and one or more non-oxidizers selectively as a secondary
biocide (see Tables 4-6 and 4-7). Smaller cooling systems often use one or more non-
oxidizing biocides, although dry oxidizing biocides are also used commonly. The most
overlooked aspect of bacterial control is maintaining a system kept clean of deposits
and SS (i.e., dirt, silt, sand, corrosion products) through the use of filters and periodic
wash-down procedures. Clean systems reduce the demand for chemical and
microbiological control. Table 4-8 shows accepted industry guidelines for a bacterial
control program with the use of a test kit.
Table 4-8. Guidelines for Bacterial Control in Cooling Towers
Bacterium Type
Colony-Forming Units/ml
< 104
Total aerobic bacteria
Sulfate-reducing bacteria
Undetectable
< 106 Undetectectable
Surface microbiological
4-4.6.1
Bacterial Control with Oxidizing Biocides. Bacterial control with
oxidizing biocides can be accomplished by either continuous feed or slug feed of the
oxidant. A continuous-feed process typically maintains 0.1 to 0.3 ppm of free halogen in
the return water to the cooling tower. A typical slug-feed process adds treatment
chemicals periodically to give 0.5 to 1.0 ppm of free halogen in the return water to the
cooling tower for a period of 2 to 4 hours, 3 times per week. Halogen refers to the group
of elements including chlorine and bromine. "Free" halogen refers to the measured
residual of halogen available for disinfection. Stabilized Halogen technology is generally
controlled on a total halogen residual. For continous feed, control at 0.5 to 1 ppm total,
and for slug feed control at 2.4 ppm for a period 2-4 hours, 3 times per week.
4-4.6.2
Bacterial Control with Non-Oxidizing Biocides. Bacterial control with
non-oxidizing biocides uses one or more biocides as shown in Table 4-7. Usually
different non-oxidizers are added on an alternating schedule; they are slug-fed every
other week for optimum effectiveness. Each time you use a non-oxidizing biocide, it is
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