POWER September 2015 - 42

WATER TREATMENT
Winning the Cooling Tower
Trifecta: Controlling Corrosion,
Scale, and Microbiological Fouling
In cooling water chemistry for power plants, it is not enough to control one or
two of the major chemistry issues. Successful treatment requires simultaneous
control of corrosion, scale, and microbiological fouling. These
three are so strongly tied to each other that if one is allowed to go out of
control, the other two soon will be.
David Daniels
T
here is a synergistic relationship among
the three major cooling water treatment
issues: corrosion, scale or deposit formation,
and microbiological fouling. In order
to control one, you need to control all three
(Figure 1). And, to make things harder, sometimes
the treatment strategies used to fight
one side of this triangle can actually wind up
enhancing another side.
Take, for example, the common use of
bleach as an oxidizing biocide. At the proper
cooling water pH, bleach can be an excellent
biocide. Its cost and availability certainly
make it the most commonly used. However,
bleach is inherently corrosive. It is a nondiscriminating
oxidizer. It will oxidize carbon
steel as quickly as it will oxidize biofilms.
Then, the roughened up surfaces of the corroded
carbon steel have many more nooks
and crannies and crevices available for microbiological
colonies to be established in
places where they can hide from the bleach.
Bleach may also oxidize treatment chemicals
used to minimize scaling or corrosion.
Then there's the relationship between
scaling and biocide effectiveness. Scaling
deposits in condenser tubes and in the cooling
tower provide excellent surfaces for biofilms
to attach and microbiological colonies
to develop. The biofilms consist primarily of
exo-polysaccharides, which are " sticky " and
will collect deposits and debris to use as a
food source and to create a shelter to protect
themselves from the elements and, in particular,
biocides such as bleach. Some research
has even shown that the biofilm structure itself
creates surface conditions that promote
incipient crystal formation and accelerate
growth.
Of course, the fact that microbiological
species accelerate corrosion is well documented.
It even has its own name- microbio42
www.powermag.com
POWER
| September 2015
logically influenced corrosion (MIC)-and it
is ubiquitous. Biofilms cause can cause general
oxygen pitting by establishing a concentration
gradient between the fully oxygenated
cooling water and low-oxygen or even fully
anaerobic conditions under the biofilm. The
anaerobic conditions found at the base of the
well-established biofilm promote the growth
of anaerobic bacteria such as sulfate reducing
bacteria. These bacteria convert sulfate in the
cooling water to sulfide and produce an acidic
hydrogen sulfide molecule that is very corrosive
to a variety of steel alloys, including
stainless steel and even copper alloys, which
are generally more immune to attack from
the aerobic bacteria. Other bacteria that grow
in biofilms metabolize metals directly for energy
or use metallic corrosion products.
Now that we see how these three factors
work together to cause trouble in cooling
systems, let's attack each individually and
review aspects of a good cooling water program
that achieves control of all three.
How to Control Biofilms in a
Cooling System
Besides the specter of MIC chewing on the
1. Scaling, biofouling, or corrosion?
Answer: Yes. Courtesy: M&M Engineering
Associates Inc.
metallic parts of your cooling system (Figure
2), accumulations of biofilms also inhibit
heat transfer in the condenser tubes and can
accumulate on cooling tower fill to the point
where it can collapse the fill support and
cause significant damage to a cooling tower.
The key to maintaining control over microbiological
populations in power plant cooling
water is to provide a consistent level of treatment
throughout the year (see sidebar). That
is, the treatment needs to be consistently effective.
The actual concentrations of biocide
required to provide this level of treatment
may change significantly, depending on cooling
water conditions and the environment.
Some power plants find that they can shock
treat the cooling water system for an hour a
day during most of the year and cut back to
once or twice a week during the winter while
still properly controlling biofouling in their
system. Other power plants not only require
daily or even continuous feed of bleach but
also need to add additional nonoxidizing biocides
during the summer months.
The most common (though still imperfect)
method for determining that you are adding
sufficient oxidizing biocide to control micro2.
When being green is bad. Control
of microbiological fouling is critical. Courtesy:
M&M Engineering Associates Inc.
http://www.powermag.com

POWER September 2015

Table of Contents for the Digital Edition of POWER September 2015

Contents
POWER September 2015 - Cover1
POWER September 2015 - Cover2
POWER September 2015 - Contents
POWER September 2015 - 2
POWER September 2015 - 3
POWER September 2015 - 4
POWER September 2015 - 5
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