Civil Engineering Reference
In-Depth Information
Process Selection
All of the aeration techniques discussed thus far have been successfully used to reduce
contaminant concentrations in either full-scale or pilot-scale applications. Selection of
the appropriate air-stripping option depends on site considerations, the characteristics
of the contaminant, the desired removal efficiency, and cost.
Figure 9-15 sets out the circumstances under which the various air-stripping pro-
cesses are economically feasible. The two principal parameters controlling the selection
are Henry's constant and the desired removal percentage. Where removal efficiencies
do not have to exceed 90 percent, both diffused aeration and spray towers may be
cost-effective alternatives. Assuming that equal power is required for both of these
types of systems, the deciding factor is whether Henry's constant is greater or less
than about 1,000 atm.
For removals greater than 90 percent, a packed tower is probably the only alter-
native. The lines in Figure 9-15 that delineate the viable zone for packed towers
assume that the height of a transfer unit (i.e., HTU) is approximately 3 ft (1 m) and
that the maximum economical depth of packing is approximately 30 ft (9.1 m). For a
ratio of local mass transfer coefficients of
k
l
/
k
g
0.01 (where
k
l
is the liquid-phase
resistance and
k
g
is the gas-phase resistance) and a stripping factor greater than 5, it
can be shown that the maximum removal efficiency over a wide range of Henry's law
Diffused
Aeration
Spray Towers
90
Crossflow
To w e r
99
99.9
Packed
To w e r
99.99
99.999
Not Feasible
99.9999
NH
3
CHCI
3
PCE
CH
4
0.1
1.0
10
100
1,000
10,000
Henry's Constant (atm)
Fig. 9-15.
Diagram for selection of feasible air stripping process (Reprinted from
Occurrence
and Removal of Volatile Organic Chemicals from Drinking Water,
by permission. Copyright
1983, American Water Works Association and the American Water Works Association Research
Foundation.)
