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AERATION AND AIR STRIPPING 5.15

90
Removal efficiency - 95%
Water temperature - 55°F
80 L
Trichloroethylene
(320)
Number in parentheses
70
indicates the Henry's
"~ 60 / Chloroform
_c"
,2-Dichloro LawcOnstantethane at 55 °F.
~_ 50
d~
1
~ 40
(29)
~ 30
20
10
i I ! I I I I I | I I i ! I
20:1 40:1 60:1 80:1 100:1 120:1 140:1
Air:water ratio, cubic feet:cubic feet
FIGURE 5.5 Effect of type of VOC on packed column design. (Source: AWWARF/KIWA Report, 1983.)

where Xi/Xo = ratio of influent to effluent liquid-phase concentration
R] = dimensionless stripping factor
_ (H)(G) where G is gas flow, (lb mol/h)/ft 2 [(g mol/h)/m 2]
(p,)(L)
The ability of a particular VOC to be stripped may be determined from Henry's law
constant. The higher the constant, the more easily the VOC is removed by air stripping.
The effect that the type of compound to be removed has on packing depth and air-to-
water ratio is shown in Figure 5.5. An air-to-water ratio of about 20 : 1 is required to
achieve 95% removal of trichloroethylene (TCE) with 15 ft (4.6 m) of packing medium
that is 1 in. (2.5 cm) in diameter. For 95% removal of a less volatile compound such as
1,2-dichloroethane, an air-to-water ratio of about 120 : 1 is required for a column with the
same size and depth of packing.
Desired removal efficiency also affects the design of a packed column. Figure 5.6 il-
lustrates the relationship between air-to-water ratio and packing depth to achieve various
efficiencies for TCE removal. About 6 ft (2 m) of 1-in. (2.5-cm) packing medium is re-
quired to achieve 80% removal of TCE with an air-to-water ratio of 20 : 1. To achieve
99% removal with the same packing and air-to-water ratio, about 20 ft (6.1 m) of pack-
ing would be required.
Water temperature must also be considered in designing a packed column. Most
groundwater supplies have a water temperature of about 55 ° F (13 ° C). However, water
temperature may be as low as 45 ° F (6 ° C) in northern regions and as high as 75 ° F (24 °
C) in some warm regions: The relationship between water temperature and removal effi-
ciency is illustrated in Figure 5.7.
Design Considerations. A diagram of a typical packed column installation is shown in
Figure 5.8. In an installation to treat groundwater, the water is generally pumped directly