Environmental Engineering Reference
In-Depth Information
where
R
F
= ε
a
+
(
ε
w
/K
aw
)
+ ρ
b
K
SA
+
(
ε
N
/K
NA
)
. The number of pore volumes
flushed in time
t
is
R
F
ε
a
ln
[A]
g
[A]
0
.
Q
g
t
ε
a
V
tot
=−
N
PV
=
(6.228)
The volumetric flow rate of air is given using Darcy's law (Wilson and Clarke, 1994)
(A
x
ε
a
κ
d
)
(P
0
−
P
1
)
2
X
L
P
1
Q
g
=
,
(6.229)
κ
d
is the permeability of soil (Darcy's constant, m
2
/atm s),
P
0
is the inlet
pressure (1 atm),
P
1
is the outlet pressure (atm),
X
L
is the distance between the inlet
and exit (m), and
A
x
is the area normal to the flow (m
2
)
.
Note that
N
PV
is directly proportional to
R
F
and is logarithmically related to degree
of removal [A]
g
/[A]
0
.
where
E
XAMPLE
6.32 E
XTRACTION OF
V
APORS FROM THE
V
ADOSE
Z
ONE
Estimate how much
o
-xylene vapors can be removed from 1 m
3
of the vadose zone of an
aquifer that has a total porosity of 0.6 in 1 day. The water content is 10% and the xylene
content is 20%. Darcy's constant is 0.01 m
2
/atm s.A vacuum of 0.8 atm is applied across
a 10-m soil length. The soil organic fraction is 0.02 and bulk density is 1.4 g/cm
3
. The
density of xylene is 0.9 g/cm
3
and has an aqueous solubility of 160
μ
g/cm
3
.
To calculate
R
F
,
ε
a
=
(
1
−
0.1
−
0.2
)(
0.6
)
=
0.42,
ε
w
=
0.06,
ε
N
=
0.12.
K
aw
=
0.215,
K
oc
=
10
2.3
,
K
sw
= φ
oc
K
oc
=
4.0 P kg.
K
NA
=
(
0.9
)/(
160
)(
0.215
)(
0.001
)
=
26,
K
SA
=
4
/(
0.215
)
=
18. Hence
R
F
=
29.
Q
g
(
1
)(
0.42
)(
0.01
)
[
(
1
2
−
(
0.2
)
2
)/(
0.2
)
]
exp
(
0.001
)(
8.64
10
4
)/(
29
)(
1
)
0.001 m
3
/s. Hence
(
1
/
20
)
=
[
A
]
g
/
[
A
]
0
=
−
×
=
0.05. Thus removal from pore air is 95%.
As observed in Chapter 4, air-water partition constant
K
AW
increases with tem-
perature, whereas
K
SA
and
K
NA
decrease with temperature. Hence with increasing
T
, retardation factor decreases and the efficiency of soil vapor extraction increases.
Therefore, heat treatment of soils will accelerate soil-vapor stripping (Wilson and
Clarke, 1994).
6.4.2.3
Incineration for
Ex Situ
Treatment of Soils and Solid Waste
Thermal processes are used to destroy organic species in soils and solid waste.
There are two types of incinerators employed, namely, rotary kiln and direct flame
incinerators. Rotary kiln incinerators are ideal for soil and solid waste combustion.
The basic design and operation of a rotary kiln incinerator is described in detail by
Reynolds, Dupont, and Theodore (1991).
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