Page 695 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Another concern was whether alum coagulation interfered
BOX 20.1 REACTIONS OF FIVE with the oxidation process. Figure 20.3b shows the respective
OXIDANTS WITH Fe 2þ AND Mn 2þ treatment train profiles for the three forms of Mn, that
is, dissolved, colloidal, and particulate, respectively, for
A not uncommon need in oxidation is to remove Fe 2þ or
C(ClO 2 ) ¼ 0.85 mg=L. As seen, the dissolved form is reduced
Mn 2þ from raw water sources used for drinking water
to the C(Mn ) 4 mg=L after reaction with ClO 2 ; the col-
2þ
such that they do not cause nuisance effects in domestic
loids and particulates are most likely assimilated into the alum
water uses (due to their oxidation by dissolved oxygen),
floc after rapid mix. Filtration reduces all the colloid and
causing ‘‘red’’ water and ‘‘black’’ water, respectively.
particulate forms to negligible concentrations. As a negative
Both may cause palatability deterioration, that is, bad
point, about 0.90–0.95 fraction of chlorine dioxide reverts to
taste, stained fixtures, and stained laundry. The tabular
chlorite ion, which, along with chlorate ion has been associ-
summary of reactions that follows is for five oxidants,
ated with health effects.
giving oxidation reactions for oxygen, ozone, HOCl,
ClO 2 , and KMnO 4 (Bablon et al., 1991, p. 139). The
20.2.2.6 Titanium Dioxide
first, for oxygen, gives reactions of Fe 2þ or Mn ; these
2þ
reactions may occur during treatment, in the distribution Titanium dioxide, TiO 2, has been speculated to be an effective
system, or as the water is withdrawn at a household. The catalyst for producing hydroxyl radicals in oxidation for water
next four oxidants are those that may be used during treatment. Its photo-efficiency, however, for degrading con-
treatment to remove Fe 2þ or Mn 2þ as Fe(OH) 3 (s) or taminants was found to be only 0.03–0.11 (i.e., in producing
MnO 2 (s), respectively. The second column shows the hydroxyl radicals). Professor J. R. Bolton (University of
stoichiometric ratio of oxidant to target ion. Of the four Western Ontario) considered these efficiencies so low as to
oxidants, HOCl has not been used to any extent in make TiO 2 -based water treatment systems economically
practice. Ozone, ClO 2 ,orKMnO 4 have been shown to unfeasible (Wilson, 1996, p. 29).
be effective. Of these three, ClO 2 was the method of
choice for at least two utilities in Colorado for Mn 2þ
20.2.3 SUPERCRITICAL WATER OXIDATION
removal because of lower residual concentrations and
about the same or faster kinetics (Gregory, 1996). SCF is a media in which chemical reactions, involving many
difficult-to-treat substances, may be carried out toward
achieving the ‘‘ideal’’ products of carbon dioxide and water.
Oxygen
This kind of reaction is possible because of the extraordinary
2Fe 2þ þ 1=2O 2 (aq) þ 5H 2 O 0.14 mg O 2 =mg Fe
properties of SCF. The particular interest here is SCW.
! 2Fe(OH) 3 (s) þ 4H þ
2Mn 2þ þ 1=2O 2 (aq) þ H 2 O 0.29 mg O 2 =mg Mn
20.2.3.1 Critical Point
! MnO 2 (s) þ 2H þ
The Figure 20.4 shows a phase diagram for water that indicates
Ozone
both the triple point, and the critical point (see glossary).
2Fe 2þ þ O 3 (aq) þ 5H 2 O 0.43 mg O 3 =mg Fe
The solid lines show the phase equilibria: the thin line from
! 2Fe(OH) 3 (s) þ O 2 (aq) þ 4H þ
the triple point that slants slightly toward the left is the solid–
Mn 2þ þ O 3 (aq) þ H 2 O 0.88 mg O 3 =mg Mn
! MnO 2 (s) þ O 2 (aq) þ 2H þ liquid equilibria; the line from the triple-point to the right
HOCl depicts the gas–liquid equilibria. Finally, at T 3748C, p
218 atm (22,089 kPa), ‘‘supercritical’’ water exists. At this
2Fe 2þ þ HOCl þ 5H 2 O 0.64 mg HOCL as O 2 =mg Fe
point, the character of the water changes. At the critical point,
! 2Fe(OH) 3 (s) þ Cl þ 5H þ

the gas density and the liquid density are equal (Silberberg,
Mn 2þ þ HOCl þ H 2 O 1.30 mg HOCL as O 2 =mg Mn
! MnO 2 (s) þ Cl þ 3H þ 1996, p. 451). The shaded area (upper right) illustrates a

portion of the supercritical region. Operating points in
ClO 2
practice are higher than both the critical temperature and the
Fe 2þ þ ClO 2 (aq) þ 3H 2 O 1.20 mg ClO 2 =mg Fe
! Fe(OH) 3 (s) þ ClO 2 þ 3H þ critical pressure. For example, dissociation of urea to nitrogen

Mn 2þ þ 2ClO 2 (aq) þ 2H 2 O 2.45 mg ClO 2 =mg Mn gas, carbon dioxide, and water requires T(operating) > 6508C
! MnO 2 (s) þ 2ClO 2 þ 4H þ (Timberlake et al., 1982).

KMnO 4
20.2.3.2 SCWO In-a-Nutshell
3Fe 2þ þ MnO 4 þ 2H 2 O 0.94 mg KMnO 4 =mg Fe

! 3Fe(OH) 3 (s) þMnO 2 (s) þ 5H þ The density of SCWranges 0.1–0.5 g=mL and islow enoughand
3Mn 2þ þ 2MnO 4 þ 2H 2 O 1.92 mg KMnO 4 =mg Mn the temperature is high enough such that there is no hydrogen

! 5MnO 2 (s) þ 4H þ bonding (Timberlake et al., 1982, p. 1). The dielectric constant is
reduced, for example, 3–10, and the water becomes nonpolar
and an excellent solvent for organic compounds. At T > 5008C,