POWER September 2015 - 43

WATER TREATMENT
to ClCleaning
or Maintaining?
I'm indebted to one expert in this field
for a statement I've used many times in
discussing cooling water treatment with
clients. George Licina of Structural Integrity
taught me: " It's just like your mother
told you-it's easier to keep your room
clean than to get your room clean. " The
same goes for your cooling water system:
Consistent application of the right amount
of biocide or biocides for the conditions in
your cooling water will keep your " room
clean " and help control scaling and corrosion
(Figure 3).
biological fouling is still the free chlorine test
(DPD test). This is a minimum and should
be performed during each chlorination
cycle, or daily for those that continuously
chlorinate. As conditions change (seasons
and temperatures) the amount of bleach required
to produce the desired free available
chlorine residual will change, and the plant
should respond by increasing or decreasing
the biocide feed rate accordingly. Note that
oxidation reduction potential (ORP) is not a
good indication of microbiological control in
cooling water. ORP is no substitute for testing
free available chlorine.
Getting the Full Benefit from Chlorine.
A full discussion of the various types
of biocides (both oxidizing and non-oxidizing)
and optimizing strategies for their
use is beyond the scope of this article. But
it would be good to say a few words about
the most common biocide used in power
plants: chlorine.
Previously, the most common source of
chlorine was chlorine gas shipped to power
plants as a condensed liquid in 1-ton cylinders.
It was evaporated and swept into the
cooling water lines as a gas. When chlorine
gas dissolved in water, it formed hydrochloric
acid and hypochlorous acid. The hydrochloric
acid lowered the cooling water pH
slightly, which enhanced the effect of hypochlorous
acid as a biocide. Obvious concerns
about safety were the primary reason
that power plants moved away from the use
of 1-ton chlorine cylinders to liquid (sodium
hypochlorite) commercial bleach delivery.
Commercial bleach is typically supplied at
12.5% or 10% chlorine. One pound of chlorine
gas makes 0.74 pounds of hypochlorous
acid, whereas 1 gallon of 12.5% bleach has
the potential to generate 0.875 pounds of hypochlorous
acid, if there is sufficient hydrogen
ion (low enough pH) in the cooling water
to form the acid.
September 2015 | POWER
3. Clean bill of health. A clean cooling
tower is a good sign that corrosion,
scale, and microbiological fouling are under
control. Courtesy: M&M Engineering Associates
Inc.
+ O2). In a good quality hypochlorite
solution, the formation of chlorate is the primary
decomposition pathway. A variety of
factors can accelerate the decomposition of
bleach. The most critical are time, temperature,
exposure to ultraviolet (UV) radiation,
and concentration:
■ Time. All bleach decomposes over time
(Figure 4). One source calculated that at
a 90F storage temperature, a 10% bleach
solution loses half its effective hypochlorite
concentration in 95 days. Although
you may use up a tank of bleach in a few
weeks, you may want to ask your supplier
how long your bleach has been in storage
prior to its arrival at your facility.
Commercial bleach is generated by bubbling
chlorine gas into a caustic solution.
Some extra caustic purposefully remains in
the bleach to attempt to slow the decomposition
of hypochlorite to chlorates. The typical
pH of commercial bleach is between 11.5
and 13.5. Bleach solutions that have a pH of
11 or less decompose very rapidly.
Therefore, in strong contrast to the use of
chlorine gas, the use of bleach has the potential
to increase the pH of the cooling water.
It is been mentioned numerous times that
hypochlorous acid (HOCl) is much more effective
as a biocide than the hypochlorite ion
(OCl);
therefore, bleach in particular loses
its activity as a biocide in high-pH cooling
waters.
Sodium hypochlorite solutions start decomposing
the minute they are manufactured.
They decompose primarily along two chemical
pathways: by the formation of chlorate
(ClO3
-) and by the release of oxygen (ClO■
Temperature. In 60 days at 68F storage
temperature, bleach will have lost about
17% of its hypochlorite, whereas at 90F
it will have lost about 55%. The ambient
temperature should be a consideration in
the placement of a hypochlorite storage
tank.
■ UV radiation. The UV rays in sunlight are
very effective at decomposing bleach solutions.
All bleach tanks should be made
of UV-impervious materials or coated in
such a way to exclude UV light. Better
yet, put them inside.
■ Concentration. The more concentrated the
bleach solution, the faster it decomposes.
If your site is not large user bleach, it may
be best to stick with the 10% bleach solution
instead of the higher concentration.
Other factors that can have an effect on the
degradation rate include the concentration of
salts (chlorides) and transition metals (Ni,
Cu, Co) that may get into the bleach from the
caustic used in its manufacture or in tank and
4. Bleach degradation rate versus time. Source: Bernard Bubnis, " Understanding
Bleach Degradation, " NovaCem Laboratories Inc., Oxford, Ohio
■ 68F ■ 90F
14
13
12
11
10
9
8
7
6
5
4
www.powermag.com
43
Wt%NaOCI
http://www.powermag.com
POWER September 2015

Table of Contents for the Digital Edition of POWER September 2015
