Page 694 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Oxidation 649

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Thus, only solutes that have a reaction rate constant, plant was expanded to Q(1989) 0.64 m =s (15 mgd), at
100 L=mol=s, which gives t 1=2 ¼ 11.5 min, may be which time KMnO 4 was added as a pretreatment oxidant to
k O 3
degraded within a reasonable time. For example, if address the foregoing problems. During plant trials, THM
u(reactor) ¼ 23 min, the degradation fraction is 0.75 of the concentrations were reduced from about 80 to 10 mg=L;
target compound that will be degraded; if u(reactor) ¼ 35 min, color was reduced from 20 to 7; the chlorine dose was reduced
the degradation fraction is 0.88. Only about 20 compounds from 19 to 12 mg=L (Zawacki, 1992).
meet this criterion among the 67 tested by Hoigné and Bader
(1983a).
20.2.2.5 Chlorine Dioxide
Conclusions from the foregoing are as follows: kinetic data
from a reference list may be used to evaluate whether a Chlorine dioxide is a strong oxidant, on the same order as
o
particular compound may be oxidized by ozone, that is, chlorine, with E (ClO 2 ) ¼þ1.15 mV (Table 20.1). In addition
within a feasible reaction time and to a significant degradation to its use as a disinfectant, chlorine dioxide has found a place
fraction; ozone degradation is not feasible for many organic as one of the useful oxidants in water treatment for oxidation
compounds. of Mn 2þ or Fe 3þ (Gregory, 1996, 1997) either of which may
Ozone is also considered for oxidation of iron and manga- occur in groundwater sources or from surface water sources
nese, that is, (the latter taken from water depths where a reducing atmos-
phere may be present). Without treatment, Mn 2þ oxidizes
readily when O 2 is present and forms MnO 2 , a cause of
2Fe 2þ þ O 3 (aq) þ 5H 2 O ! 2Fe(OH) 3 (s) þ O 2 (aq) þ 4H þ
‘‘black water’’ in the distribution system, or in the case
(20:15) of Fe , oxidation causes ‘‘red water’’ These are palatability
3þ
and nuisance issues that may result in bad taste, stained
Mn 2þ þ O 3 (aq) þ H 2 O ! MnO 2 (s) þ O 2 (aq) þ 2H þ
fixtures, and stained laundry. The U.S. Environmental Protec-
(20:16) tion Agency (USEPA) secondary maximum contaminant
levels for iron and manganese are 300 and 50 mg=L,
respectively.
20.2.2.3 Hydroxyl Radical
Oxidation of Mn 2þ or Fe 3þ results in an insoluble pre-
As seen previously, the hydroxyl radical, . OH, is the most cipitate, for example, MnO 2 or Fe(OH) 3 (or Fe 2 O 3 ), as the
powerful oxidant among those listed in Table 20.1; the case may be, with removal by coagulation-settling-filtration.
hydroxyl radical is the hydroxide ion minus one electron. As a case in point, Horsetooth reservoir in northern Colorado
The hydroxyl radical has been recognized as having a central is a source water for two adjacent water treatment plants,
role in redox reactions since the 1960s with the work of the Soldier Canyon WTP, Fort Collins (serving the urbanizing
Advanced Water Treatment Research (AWTR) program of area surrounding the city) and the City of Fort Collins WTP,
the USPHS (Anon., 1965). As illustrated in Figure 20.1, the serving the city itself. Both plants draw water at about 65 m
OH . radicals may be generated by ozone dissolution. Gener- (200 ft) depth where anoxic conditions are prevalent during
ation may be also by UV radiation of peroxide, H 2 O 2 , or other the late summer months and before the fall overturn. Conse-
methods, for example, by electrical energy or titanium quently, from about June through November Mn 2þ is com-
dioxide. mon at concentrations ranging as high as 450 mg=L
(Gregory, 1996, p. 1-1). Levels of manganese as low as
20.2.2.3.1 Radiation by UV 20 mg=L have caused problems and, therefore, concentrations
The UV photolysis of ozone in water yields hydrogen perox- 10 mg=L has been the goal in treatment. The City of Fort
ide. The latter reacts in turn with UV radiation to form Collins WTP used KMnO 4 as an oxidant from 1991 to 1998
hydroxyl radicals. The sequence is (Topudurti et al., 1993) and had experienced less than satisfactory treatment results.
At the same time, the Soldier Canyon WTP had used chlor-
ine dioxide as an oxidant to remove Mn , which was
2þ
(20:17)
O 3 þ hn þ H 2 O ! H 2 O 2 þ O 2
effective. Before switching to ClO 2 , however, the City of
H 2 O 2 þ hn ! 2OH . (20:18) Fort Collins embarked on an in-house research program
consisting of bench- and pilot-scale experiments to investi-
gate more formally the kinetics and other characteristics of
20.2.2.4 Permanganate three oxidants, that is, permanganate, KMnO 4 , ozone, O 3 ,
A common oxidant in water treatment, most commonly used and chlorine dioxide, ClO 2 , with respect to Mn 2þ oxida-
tion. Figure 20.3a shows the Mn 2þ concentration time for
through trial and error, is potassium permanganate, KMnO 4
(Zawacki, 1992; Ma and Graham, 1996). As a case in point, each of the three oxidants. As seen, chlorine dioxide was
3
the Collier County WTP, Q(1985) 0.17 m =s (4 mgd), had the most effective oxidant, reducing C(Mn ) 2 mg=L
2þ
water quality issues such as hardness of 280 mg=L, 7–15 mg after 5 min. The initial concentrations of oxidants were
H 2 S=L, color units of 17–20, and high levels of NOM. The C(ClO 2 ) ¼ 4 stoichiometric amount needed, that is, 0.5
plant used lime softening to reduce hardness and chlorine mg=L; C(KMnO 4 ) ¼ 3 stoichiometric amount needed, that
oxidation mitigation of taste, odor, and algae. In 1989, the is, 0.36 mg=L; C(O 3 ) ¼ 1.5 mg=L.