( 10 times more soluble in water than
chlorine) it remains as a gas in solution and will not dissociate. The
primary means of disinfection and
sanitation is by oxidation.
● While chlorine is affected by the presence of organic-laden contamination
(i.e., ammonia), ClO2 is not affected by
these compounds.
● Chlorine dioxide is a discriminate
oxidizer. Therefore, dosage feed rates
are much lower compared to chlorine
and the other nonoxidizing biocides.
Chlorine dosage feed rates, by contrast,
are higher due to the susceptibility
of chlorine to be an indiscriminate
oxidizer. Higher dosage feed rates
can lead to higher (hypo-induced)
rates of corrosion.
● Chlorine dioxide reacts very selectively
with sulfides and phenols (aromatic
hydroxides). This is why is works so
well in odor-control situations. Not
only does it kill the anaerobic bacteria
causing the issue, it oxidizes the sulfide
to sulfate.
Generation Methods
Why hasn’t ClO2 been used before in
district cooling applications? Primarily due
to issues of generation methodology, as
well as economics. Since it cannot be manufactured and then transported, ClO2 must
be produced on site using generation
equipment – traditionally ‘wet-chem’ systems that combine chlorine gas, liquid
sodium chlorite and acid. The ‘two wet-chem
process’ blends liquid 25 percent sodium
chlorite and chlorine gas. The ‘three wet-chem process’ consists of blending liquid
25 percent sodium chlorite, 12. 5 percent
sodium hypochlorite and 15 percent
hydrochloric acid. The ClO2 generated
is delivered into an adjacent ‘day tank’
for output delivery via a pump manifold
assembly.
Chemical generation has its drawbacks.
Users have to handle, transport, store and
report the use of these chemicals. The on-site safety measures for wet-chem processes can be cumbersome and expensive. Moreover, many wet-chem systems
are batch systems that can’t produce a
continuous or on-demand ClO2 supply – or
sufficient supply to meet the needs of large-volume district cooling systems and TES
tanks. There is also the potential for mak-
Chlorine dioxide production
was revolutionized in 2002 with
the advent of electrochemical
generation – or ‘e-chem’
technology.
ing an impure product should one of the
chemical precursors run dry. In addition,
this generation method cannot be automated with any degree of certainty regarding
the concentration of ClO2 produced.
Chlorine dioxide production was revolutionized in 2002, however, with the
advent of electrochemical generation – or
‘e-chem’ technology. This process creates
ClO2 utilizing reverse osmosis permeate
water, electricity and a single chemical
precursor, sodium chlorite, a common salt.
Figure 2. Chlorine Dioxide Generation in the University of California, Irvine District Cooling System.
E-Chem Technology
on Campus
E-chem technology is being used to
resolve problematic biofilm issues in a
number of university campus district
cooling applications. The University of
California, Irvine (UCI) is the largest of
the university networks currently using
an e-chem generator from PureLine
Treatment Systems. The UCI district cooling
system (fig. 2) has a total water volume of
around 7 million gal, including 4 million
gal within the TES tank and 2. 8-3.0 million
gal in the distribution system.
Source: PureLine Treatment Systems.
