25 May 2005
organisms such as clams, snails, mussels, or similar species are referred to as
macrobiological organisms. The presence of any biological growth can be detrimental to
cooling tower operations. Problems include fouling, corrosion, and loss of efficiency.
These problems can lead to downtime, higher operating cost, and even premature
replacement of equipment. Additionally, some bacteria are pathogenic and can pose a
risk to human life.
Algae. The term "algae" refers to algal, microbiological, tiny, stringy blue
and blue-green plants, which are usually found growing in masses on top of and on
sides of cooling towers. Algae grow only in sunlit areas. They will slough off and
become part of the suspended matter in the circulating water, a situation which may
cause fouling and plugging of water sprays. Algae also provide a breeding place, and
are a nutrient, for bacteria.
Bacteria. The term "bacteria" refers to a large group of one-celled
microorganisms. Bacteria can grow in either the absence or presence of sunlight. There
are several ways to classify bacteria, including "aerobic," meaning those living in the
presence of oxygen, and "anaerobic," meaning those living in the absence of oxygen. In
a cooling water system, one can categorize bacteria as either "planktonic" or "sessile,"
which are terms that describe whether the bacteria are, respectively, either free floating
or found growing on surfaces (stickers). Categories of bacteria are described below.
Table 4-3 shows types of bacteria and their growth conditions.
Planktonic Bacteria. Planktonic bacteria are suspended in the water,
sometimes referred to as "free floaters" or "swimmers," and are aerobic bacteria that
thrive in an oxygenated environment. They are not harmful to the cooling system since
they do not cause deposits or corrosion, but they can provide nutrients for other
microorganisms; in addition, some planktonic bacteria such as Legionella Pneumophila
are pathogenic and can present a significant human health risk.
Sessile Bacteria. Sessile bacteria are stickers, or non-swimming bacteria,
and can cause deposits and corrosion. Sessile bacteria types include slime-formers and
anaerobic (corrosive) bacteria. Slime-formers can grow and form gelatinous deposits on
almost any surface in contact with the cooling water. These deposits can grow so large
that they restrict water flow and interfere with heat transfer; they also may promote
under-deposit corrosion. Feeling the sides of the cooling tower basin just below the
water level is one way to detect the presence of slime-formers. Usually if there are slime
formers in the system, you can feel deposits. Anaerobic bacteria thrive in oxygen-
deprived environments and often establish colonies beneath slime deposits or under
other types of deposits. One type of anaerobe is sulfate-reducing bacteria (SRB), which
produce hydrogen sulfide, a chemical that is very corrosive to metals. This type of
corrosion attack is very localized and can result in pipe and tube failures. The presence
of SRB should be suspected in a water system if the underside of a slime layer is black
or if you detect the odor of rotten eggs. Any type of microbiological corrosion is referred
to as microbiologically influenced corrosion (MIC). Bacteria cause most of the MIC
found in cooling water systems. Use surface microbiological measurements to monitor