Page 668 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Disinfection 623

19.3.6.2 Characteristics of ClO 2
BOX 19.3 THE DISINFECTANT
At concentrations >10% in air, ClO 2 may be explosive; there-
RESIDUAL ISSUE
fore, it is generated onsite (Doull, 1980, p. 52). Based on the
calculation by Henry’s law, the concentration in the aqueous American disinfection practice for drinking water has
solution would have to be 8g=L (at 208C) in order to reach utilized chlorine, since its inception about 1910. Chlorine
the >10% concentration level in air, and would require an air– also maintains a residual in the distribution system. Thus,
water interface (Masschelein, 1992, p. 172). In fact, 4 g=L for a if say 2–3 mg HOCl is applied at the clear well, about
storage concentrate solution is the industry standard for refer- 0.5 mg HOCl may be measured in the distribution system.
ence 3000 mg=L stabilized solution is produced for commer- Usually, the system is monitored at various points and
cial use by CDG, LLC (Gregory, 2009, 2010). booster chlorine injections are added, if needed. Chlorine
Since chlorine dioxide gas is highly soluble, that is, H reacts, however, with organic substances in the finished
(ClO 2 ,208C) ¼ 1.0 mol=L=atm ¼ 84,500 mg=L=atm (calcula- water so that the residual becomes diminished.
tion from Note 8, Table H.5), the gas dissolves as quickly as it The premise of maintaining residual chlorine is that the
is produced. By comparison, chlorine gas solubility is H(Cl 2 , distribution system is protected from cross-connection
208C) ¼ 7283 mg=L=atm; in other words, by comparison, contaminations, that is, contaminants entering the system
chlorine dioxide gas is more soluble than chlorine gas by means of a negative hydraulic gradient (this may occur
(which is highly soluble). because of many kinds of activities, some of which may
As to other characteristics, ClO 2 decomposes upon expos- not have been anticipated by those who administer a
ure to UV light to produce ClO 3 , which in turn decomposes to system). Municipal plumbing codes, coupled with inspec-
chlorine and oxygen. In a weak acid solution, ClO 2 is stable at tions and unwavering enforcement, are intended to min-
concentrations <10 g=L. In a basic solution, however, that is, imize the occurrence of cross-connections. The frequency

pH > 8, ClO 2 hydrolyzes to form ClO 2 and ClO 3 . Solid of cross-connection occurrences depends upon the code
sodium chlorite is explosive on heating or on contact with and the diligence of the inspections and enforcement.
organic matter and is best stored in solution, for example, Despite all efforts, cross-connections seem to occur
300–400 g=L (Masschelein, 1992, p. 172). every so many years; either the code is violated or people
find ingenious ways, albeit unintentional, to cause a cross-
19.3.6.3 Reaction Alternatives connection. The efforts on the distribution side are
Chlorine dioxide may be generated from the dissolution of ‘‘slogging’’ and unglamorous, but are an essential part of
chlorine gas and sodium chlorite and then combining the two any potable water system. In addition to reducing cross-
to form ClO 2 . Another approach, that requires special caution, connections, there are many other aspects to managing the
is to bring solid sodium chlorite into contact with chlorine gas. distribution system, for example, having a program of
Sodium chlorite is a white crystal with strong oxidizing cap- flushing the mains and dead ends, providing for inter-
acity even in the solid form. Another reactant, less common, is changeability of parts, controlling corrosion, detecting
sodium chlorate, NaClO 3 . and fixing leaks, etc. Regardless of the sophistication
and effectiveness of treatment, the ‘‘game may be lost’’
1. Chlorine gas reacting directly with sodium chlorite in the distribution system if there is not in place an equally
solution: According to Masschelein (1992, p. 173), if effective program to minimize cross-connections.
chlorine gas is used, that is, added to the reactor, that The premise of the disinfectant residual has been ques-
is, with ClO 2 fed into the reactor at the same time, tioned from time to time in that the dosage is probably not

the reaction occurs directly, that is, enough to eliminate the public health risk. Further, the
disinfectant could attenuate the concentrations of indica-
Cl 2 þ 2ClO 2 ! 2ClO 2 þ 2Cl (19:28) tor microorganisms, such as coliforms while it is less

effective against say viral pathogens, that is, masking the
occurrence of a cross-connection while not reducing the
Equation 19.29 occurs because the reaction between
risk (suggested, c. 1984 by Henry Ongerth, retired from
the chlorine and the dissolved chlorite is faster than
California Health Department). Along this line, some
the hydrolysis of chlorine, that is, Equation 19.29,
have pointed to European practice. For example, in Berlin,
Cl 2 þ H 2 O ! HOCl þ H þ Cl . This direct method,

þ
since 1979, treated water has not been post-chlorinated
that is, Cl 2 with ClO 2 , is the most common in prac-

(Masschelein, 2002, p. 59).
tice and is done with a slight excess of chlorine, and is
about 0.95-fraction complete.
2. Chlorine gas forming hypochlorite solution to react
with sodium chlorite solution: Another approach, HOCl solution to form a ClO 2 solution (Aieta and
albeit not common, for the generation of chlorine Berg, 1986, p. 62). First, the reaction of chlorine gas
dioxide is to react chlorine with water to form a with water gives HOCl as a product, that is,
hypochlorous acid solution, that is, HOCl. A sodium
chlorite solution is brought into contact with the Cl 2 þ H 2 O ! HOCl þ H þ Cl (19:29)
þ