Environmental Engineering Reference
In-Depth Information
aeration is to increase the dissolved oxygen level in water. Oxygen enhancement is
essential for biological treatment of wastewaters and activated sludge and also for the
precipitation of iron and manganese from wastewaters. An illustrative example for
the case of absorption of oxygen into water is given below.
E
XAMPLE
4.17 O
XYGEN
A
BSORPTION IN A
W
ASTEWATER
T
ANK
The oxygen levels in wastewater will be lower than its equilibrium value because of
high oxygen demands. Improved oxygen levels in water can be achieved by bubbling air
(oxygen) through fine bubble porous diffusers (see Figure 4.2).This is called
submerged
aeration
. Large aeration units can be assumed to be homogeneous in oxygen concen-
tration due to the turbulent liquid mixing within the water. The oxygen absorption rate
(air to water) is given by
C
eq
i
w
−
C
i
w
,
N
i
=
(K
w
a)
·
(4.57)
where
C
eq
i
w
is the equilibrium saturation value of oxygen in water (mol/m
3
)
. This is
given by the air-water partition constant,
C
eq
i
w
=
P
i
/K
aw
·
C
i
w
is the concentration of
oxygen in bulk water (mol/m
3
)
. To account for other wastewater constituents (e.g.,
DOCs, surfactants, and co-solvents),
C
i
w
in wastewater can be related to the
C
i
w
in tap
water by a factor
β
for which a value of 0.95 is recommended.
K
w
is the overall liquid-
phase mass transfer coefficient for oxygen (m/s) and
a
is called the specific air-water
interfacial area provided by the air bubbles. It has units of m
2
/m
3
and is given by (6/
d
)
ε
g
, where
d
is the average bubble diameter at the sparger and
ε
g
is the void fraction.
The void fraction is defined as the ratio of air volume to the total volume
(
air
+
water
)
at any instant in the aeration vessel. It is a difficult quantity to measure. Hence
K
w
a
is lumped together and values reported.
K
O2
a
is temperature dependent. Eckenfelder
(1989) gives (
K
w
a)
=
(K
w
a)
20
◦
C
θ
(t
−
20
)
, where
θ
is a parameter between 1.015 and
1.04 for most wastewater systems.
K
w
a
is characteristic of both the specific physical
and chemical characteristics of the wastewater. It is generally estimated in the laboratory
by performing experiments on the specific wastewater sample (see Eckenfelder, 1989).
In a specific bubble aeration device,
K
w
a
was determined to be 0.8 h
−
1
for a
wastewater sample at 298 K. At 298 K
P
i
in air is 0.2 atm. The air-water parti-
tion constant for oxygen at 298 K is 0.774 atm/m
3
/mol. The latter two parameters
give us equilibrium concentration of oxygen in water,
C
eq
0.26 mol/m
−
3
(
×
10
−
4
mol/L). If the wastewater was completely mixed (homogeneous) and if it had
zero DOC, then the maximum rate of oxygen absorption is
N
max
i
i
w
=
=
2.6
0.208 mol m
3
/h.
If the total volume of the wastewater is 28,000 L (
=
1000 ft
3
)
, the oxygen absorbed
will be 5.8 mol/h (
=
=
93 g/h).
4.3 SOIL-WATER AND SEDIMENT-WATER EQUILIBRIUM
Contaminated sediments in the marine or freshwater environment exist in the United
States and other parts of the world. A few examples in the United States are the
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