Chemical Engineering September 2018 - 41

Cover Story
120
100
80
60
40
20
6.0
6.4
7.0
FIGURE 2. The graph shows
the dosages of bleach and
chlorine dioxide required to
treat water at different pH
levels
7.6
7.8
8.0
Temp = 68o F
NaCIO
versus
CIO2
8.2
8.4
8.6 pH
mental releases and sensitization of operators.
Chlorine gas is highly toxic, making it
a powerful disinfectant for water, but hazardous
to humans, who must handle it in its
gaseous form. As a result, operators require
intense use of personal protective equipment
for chlorine gas. For containment,
chlorine gas requires high-pressure cylinders
that come with an inherent risk of potentially
large releases. It is important to note that
safety and security legislation in many areas
is expected to eventually force chlorine gas
users to switch to a less hazardous program.
Utilizing bleach for microbial control requires
a steady stream of bleach deliveries,
each of which carries a risk of spills and
worker exposure. Both bleach and chlorine
gas add significant amounts of chloride
ions to cooling water, increasing the risk
of operator sensitization when exposed to
cooling water.
In contrast, chlorine dioxide adds very little
FIGURE 3. At pH levels greater
than 7, chlorine gas is a less
effective biocide, while chlorine
dioxide maintains efficacy
100
90
80
70
60
50
40
30
20
10
7.0
40
7.5
8.0
8.5
pH
9.0
9.5
free chloride. For the same level of antimicrobial
effectiveness, bleach requires a greater
volume than chlorine dioxide. This may be
a factor in reducing handling and exposure
risks. Chlorine dioxide is generated on-site,
as needed, so there is a need for a ClO2generation
system, but no need for chlorine
gas storage. This can be a factor in lowering
the risk of a large-scale release. On-site
generation equipment for chlorine dioxide
requires integrated safety devices to monitor
system parameters to ensure safe production
of the chemical.
CIO2
CI2
10.0
Performance. After safety, process performance
is the next key area for which
the choice of microbial control program
has an impact. In many cases, the three
biocidal programs being compared for
biocidal efficacy are dictated by system
conditions and the particular requirements
will determine the antimicrobial program.
In low-demand systems with near-neutral
pH, bleach and chlorine gas are effective
biocides. Chlorine dioxide is especially effective
in challenged water systems. Chlorine
dioxide is well known to control and
remove biofilms from industrial water systems
as the compound migrates, via molecular
diffusion, from the bulk water into
the biofilm, where it deactivates microbes
that form the biofilm.
In addition to microbial control, the impact
of the water-treatment strategy on scale
prevention, corrosion inhibition, and fouling
reduction also must be considered. Bleach
and chlorine gas both increase chloride and
hypochlorite ions in the cooling water system.
This can increase the rate of corrosion
within the system, leading to shorter-thanexpected
asset lifetimes. Chlorine dioxide is
not aggressive toward other cooling water
chemicals that may be present in the system,
such as azoles for corrosion inhibition
and polymeric scale inhibitors.
Finally, due to the broad spectrum of chlorine
activity, chlorine gas and bleach can create
taste concerns, which can be especially
troublesome in process applications in the
food, beverage and healthcare industries.
Water quality. Incoming water quality is
important to understand, because the pH
range of the water system will determine
the activity rate of the microbial control program.
Bleach is most effective at a pH range
of 5.5-7.5, and it tends to increase the pH
of a system over time. When working with
bleach, some operators will treat with acid to
maintain the optimal pH range. This works,
but also introduces additional hazardous
chemistries (acid) into the plant. Chlorine
dioxide is effective at a broader pH range
(pH 5-11) and requires less intervention to
maintain a consistent pH (Figures 2 and 3).
The effects of pH are discussed further in the
next section.
Compliance. Regulatory compliance is a
strong consideration when determining the
best path forward for microbial control. In
the U.S., all biocides must be used in accordance
with the U.S. Environmental Protection
Agency and state environmental agency
guidelines. Certain industries also have U.S.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
SEPTEMBER 2018
Percent active biocide
Pounds of product, lb
http://WWW.CHEMENGONLINE.COM
Chemical Engineering September 2018

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