Environmental Engineering Reference
In-Depth Information
TABLE 6.2
Common Types of Initiators, Propagators, and Scavengers Found
in Wastewater, and Values of
k
tot
for Ozone Depletion in Water
Containing Different Species, M
Initiator
Promoter
Scavenger
Hydroxyl ion
Aromatic
Bicarbonate ion
Ferrous ion
Alcohols
Carbonate ion
DOCs (humic and fulvic acids)
DOCs (humic acid)
k
tot
(s
−
1
)
[M]
pH
0 (none)
4.0
0.15
±
0.02
50 mM PO
2
−
4
0.072
±
0.006
4.0
7
μ
M
t
-BuOH
0.055
±
0.004
4.0
50
μ
M
t
-BuOH
4.0
0.02
±
0.002
Source:
From Staehelin, J. and Hoigne, J. 1985.
Environmental Science and Technology
19, 1206-1213.
E
XAMPLE
6.12 W
ASTEWATER
O
XIDATION USING
O
ZONE IN A
C
ONTINUOUS
R
EACTOR
Consider a diffuse aerator reactor operated in the continuous mode (Figure 6.23). A
given influent feed rate of water (
Q
L
,m
3
/s) is contacted with an incoming gas stream
of ozone at a concentration
[
O
3
]
i
g
(mol/m
3
) at a volumetric flow rate of
Q
G
(m
3
/s). We
are interested in obtaining the extent of ozone consumption within the reactor if “pure”
water is used and also in the presence of other solutes. Consider a CSTR, for which a
mass balance for ozone in the aqueous phase within the reactor gives
d
t
=
Q
L
(
[
O
3
]
0
−[
O
3
]
)
+
K
w
aV
L
[
O
3
]
∗
−[
O
3
]
−
k
tot
V
L
[
O
3
]
, (6.119)
where
[
O
3
]
0
=
0,
K
w
a
isthemasstransfercoefficientforozonefromgastowater,
[
O
3
]
∗
isthesaturationconcentrationofozoneinwater,and
k
tot
isthefirst-orderdecomposition
of ozone as described earlier. At steady state since d
[
O
3
]
/
d
t
=
0, we have
[
O
3
]
[
d
[
O
3
]
V
L
1
=
+
(Q
L
/V
L
+
k
tot
)
·
(
1
/K
w
a)
.
(6.120)
O
3
]
∗
1
With increasing
k
tot
, the ratio decreases indicating that the exit ozone concentration
is lower and more of ozone is consumed within the CSTR. Consider a typical ozone
wastewater oxidation reactor with
Q
L
=
2500 m
3
/h and
V
L
=
1500 m
3
.
K
w
a
for ozone
istypically0.03 min
−
1
(Roustanetal.,1993).FromStaehelinandHoigne(1985),
k
tot
=
3 min
−
1
at pH 4 in the presence of 50 mM phosphate. Hence the ratio is
[
O
3
]
ss
/
[
O
3
]
∗
=
0.0098. If the partial pressure of ozone
P
O
3
in the incoming gas phase is known, then
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