Page 688 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

20 Oxidation

Chemical oxidation is one of the pervasive reactions that insoluble precipitate, such waters are likely to cause ‘‘red-
occurs throughout the natural environment. It is also one of water’’ and ‘‘black-water’’ problems in the distribution system.
the unit processes in water treatment, albeit not a main one. The use of oxidation for cleavage of organic molecules, for
As applied to organic compounds, such as the substances example, natural organic matter (NOM), came on the scene in
classified as hazardous wastes or organic sludges, the goal is, in the 1970s as a means to cause more effective adsorption or
general, to ‘‘mineralize’’ the compound, that is, with carbon microbial degradation. As hazardous wastes became an issue,
dioxide, water, and ions (e.g., Cl ), as products. More often, beginning from about 1980, stimulated in the USA by the

however, the products are compounds that are of lesser molecu- 1980 Comprehensive Environmental Response, Compensa-
lar weight and possibly more biologically reactive. As an tion, and Liability Act (CERCLA), oxidation was one of the
example, ozone is used sometimes as pretreatment for activated alternative approaches explored to cause a molecular trans-
sludge treatment. Also, ozone may be used as pretreatment for formation of synthetic organic molecules.
granular activated carbon (GAC) adsorption, since the resulting
lower molecular weight organic molecules may adsorb better.
Inorganics, for example, Mn , may be oxidized for removal as 20.1.2 HISTORY OF OXIDATION TECHNOLOGY
2þ
oxide particles, for example, MnO 2 ; the intent is to cause the An early application of oxidation was for taste and odor
reaction to occur within the treatment plant so that removal control. Oxidants used included ozone, chlorine dioxide, and
takes place, as opposed to within the distribution system. permanganate. About 1960, faced with the growing problem
of biosolids, that is, sewage sludge, wet-oxidation was
20.1 DESCRIPTION patented but never saw wide application in practice. The
successor technology, supercritical water oxidation (SCWO),
Oxidation is a means to transform an element to a different
came about in the early 1980s as the issue of hazardous wastes
‘‘oxidation state’’ or a compound from one species to another.
In a reactor, an oxidizing agent is added in order to ‘‘reduce’’ became prominent, but has not been implemented widely.
the element or compound in question. Oxidation and reduc- 20.1.2.1 Oxidation Based on Electromotive Potential
tion, by definition, occur simultaneously, that is, the oxidized
element or compound looses electrons to the reduced element The ‘‘Advanced Waste Treatment Research’’ program of the
or compound which gains electrons. This simultaneous oxi- United States Public Health Service (USPHS) (active 1961–
dation–reduction is termed a ‘‘redox’’ reaction. Redox reac- 1968) explored extending the use of oxidants from such
traditional areas as taste and odor control to ‘‘refractory’’
tions include the formation of a compound from its elements,
all combustion reactions, reactions in batteries that generate organics, that is, organic molecules that were not removed
an electric current, and the generation of biochemical energy. by biological treatment, and industrial wastes constituents
For synthetic organic compounds (SOCs), of which thousands such as sulfite or cyanide (Anon., 1965). The program recog-
have been synthesized, the ‘‘ultimate’’ reaction has been to nized the important role of the hydroxyl radical, that is, OH . ,
form water and carbon dioxide as products, an elusive quest. and examined different approaches to generate the radical.
The upshot, however, was that oxidation was expensive com-
pared to other approaches, for example, adsorption by GAC.
20.1.1 APPLICATIONS OF OXIDATION TECHNOLOGY
Applications of the oxidation process include taste-and-odor 20.1.2.1.1 Ozone
control in drinking water, cleavage of organic molecules so The first ozone plant in the United States was at Whittier,
that they may be more amenable to biodegradation, as a Indiana, constructed in 1940. In 1980, there were only five
coagulation aid, etc. Also, in water treatment, ferrous iron, ozone plants; all were for taste and odor control. By 1996,
Fe , may be converted to ferric iron, Fe , and Mn 2þ may be some 130 water treatment plants used ozone for drinking
3þ
2þ
converted to Mn . The respective hydroxide and oxide of water (Kaminski and Prendiville, 1996, p. 62). The largest
3þ
these trivalent ions are insoluble in water and so may be was the (600 mgd) Los Angeles plant, which was for use as a
removed by settling or filtration. The divalent ions, Fe , coagulant aid. The Milwaukee drinking water treatment plants
2þ
and Mn , are found in natural waters having low ‘‘redox had ozone installed at the headworks of the plant, and was
2þ
potential,’’ that is, with a reducing character, such as some completed in 1998; the purpose was to reduce the probability
groundwaters and some reservoir or lake waters, perhaps at of having viable cysts or oocysts in the drinking water (motiv-
deeper levels that may be devoid of oxygen. Without proper ated by the occurrence of a cryptosporidosis outbreak in April
treatment, that is, oxidation and removal of the resulting 1993 in which some 500,000 persons became infected).
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