Page 689 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

644 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

In Europe, ozone was common from about 1906. By the 20.1.2.3 Supercritical Water Oxidation
1990s, ozone had been installed in over 1000 municipal water The phenomenon of supercritical fluid (SCF) was discovered
treatment plants, mostly in France and Switzerland, with a in 1821 by Charles Cagniard de la Tour, a French scientist
large number in Germany. Many installations were for taste (Jain, 1993, p. 806). The process was applied originally, about
and odor control and color removal, and were backed up with 1915, by the pulp and paper industry to oxidize waste lignin to
chlorination. A consequence of the breakup of organic mol- carbon dioxide and water (Hochleitner, 1996, p. 48).
ecules by ozone is that the products can serve as more readily
A state-of-the-art review of SCWO by Groves et al. (1985)
available substrate for microbes, which may then thrive in the listed few citations earlier than 1980. A patent by Professor
form of biofilms, for example, in the distribution system. M. Modell (Box 10.2) in 1982 and another in 1985 (Thornton
Higher doses of chlorine may be required to control such and Savage, 1992, p. 321) stimulated the 1980s interest in
growths (White, 1999, p. 1203). SCWO. Applications of SCWO included hazardous wastes,
for example, polychlorinated biphenols (PCBs) and obsolete
20.1.2.1.2 Permanganate
explosives, which are insoluble in water and are difficult to
Permanganate, in the form of potassium permanganate, treat; these compounds dissolve readily in supercritical water
KMnO 4 , has been used in drinking water treatment for taste (SCW) because of its extraordinary solvating powers. Treat-
and odor control since 1932 and for manganese removal since ment of organic sludges from municipal wastewater treatment
1935 (these were the earliest references in Babbitt and Doland plants was another application.
(1949, pp. 547, 549), an authoritative text of that period)). In 1987, Professor Earnest F. Gloyna, University of Texas,
Austin (Box 10.3), initiated a comprehensive and concerted
20.1.2.1.3 Chlorine Dioxide
research program with broad sponsorship and which was
Chlorine dioxide had been in use for taste and odor control by intended to bring out the potential of the process. To give an
1944 (Babbitt and Doland, 1949, pp. 536, 550). This oxidant idea of the magnitude of the problem of sludge generation and
did not come into use, to any great extent, however, until hazardous wastes, Gloyna and Li (1995) stated that in 1982
about the 1990s due mostly to the search for alternatives to the total U.S. sludge market consisted of 20 million tons of
chlorine and due to its effectiveness both as a disinfectant and dry municipal sludge and 19 million tons of dry industrial
as an oxidant, for example, for oxidation of Mn . In add- sludge. In 1989, Texas industries alone either released or
2þ
ition, chlorine dioxide has a variety of uses in wastewater stored 893 million tons of toxic wastes. They stated further
treatment (Noack and Iacoviello, 1992). that in the United States there were about 127,000 hazardous
waste generators and that at least 41,000 of these waste
20.1.2.1.4 Other Oxidants
streams were amenable to SCWO treatment.
During the early years of water treatment, that is, c. 1920– In 1995, the first SCWO plant was built at Austin, Texas, to
1950, aeration was a common unit process, with dissolved treat wastes from the Texaco Chemical Co. (Svensson, 1995,
oxygen being the oxidant for manganese. Chlorine was p. 16). At the same time, progress was made toward overcom-
another common oxidant for both iron and manganese control ing some of the problems associated with reactor operation,
and for taste and odors. such as sticky solids and corrosion from acids produced. A cost
estimate to process municipal wastewater sludge using the
20.1.2.2 Wet-Oxidation
MODECt process was about $220–$275=metric ton dry solids
A 1958 paper by F. J. Zimmerman in Chemical Engineering ($200=$250=U.S. ton).
introduced ‘‘wet-oxidation’’ as an alternative to incineration.
As a note, ‘‘wet-oxidation’’ means ‘‘wet air oxidation,’’ that is,
using air as a source of oxygen. The process was patented in 20.2 OXIDATION THEORY
1958 by the Sterling Drug Co. and later was called the Oxidation theory has a great deal of depth that is explored
‘‘Zimprot’’ process. The Zimmerman process operated at more fully in the body of literature and various texts
5008C < T < 7008C, 82 < p(reactor) <122 atm (Anon., 1965, (examples include: Langlais et al., 1991; Rice and Netzer,
p. 91). For reference, T c ¼ 3748C, p c ¼ 218 atm (T c and p c are 1982; Eckenfelder et al. 1992; Snoeyink and Jenkins, 1980;
critical temperature and pressure, respectively). Thus, while Benjamin, 2002). The intent here is to provide both a ‘‘road
the temperature of wet-oxidation, as described, is greater than map’’ for understanding the process and an introduction to
critical, the pressure is less. Wet-oxidation was considered as supercritical oxidation.
a means of volume reduction, with carbon dioxide, water, and
ash being the products of the reaction. The motivation for its
20.2.1 FUNDAMENTALS
adoption was the high mass fluxes of sludge from municipal
wastewater treatment plants, such as Chicago, New York, Los By definition, a redox reaction involves electron transfer. This
Angeles, etc., and the limitations on disposal (e.g., ocean is understood in terms of balancing an equation, for example,
outfalls). Chicago tried the Zimpro process for several by the method of half-reactions, or alternatively by the
years but discontinued it in 1970 due to high costs of oper- method of oxidation numbers. The thermodynamics of a
ation an instead transported its sludge to agricultural land redox reaction is the same as for any other, except electrons
in Indiana. are transferred.