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276 Lawrence K. Wang et al.

It is important to know the detailed relationship between the scrubbing process and
the stripping process. The reader is referred to another chapter, “Gas Stripping,” of
this handbook for a more detailed explanation of the stripping process than that given
here. This chapter places emphasis on scrubbing process design and applications.
Nevertheless, the reader should understand both the similarities and dissimilarities of
the two processes.

For instance, if a packed tower reactor (Fig. 1b) or another reactor (Fig. 1a,c,d) is
available, an environmental engineer may wish to use the same reactor both as a
scrubbing process and a stripping process. In each instance, the scrubber or stripper
will have two separate streams: (1) gas stream and (2) liquid stream.
It is a scrubbing process if (1) the gas stream is the target contaminated air emission
stream from which one or more airborne pollutants (such as SO , H S, HAPs, VOCs,
2 2
SVOCs, PM, heavy metals) will be removed by the reactor and (2) the liquid stream
is the scrubbing solution (such as water with or without chemicals depending on the
airborne pollutant(s) that need to be removed).
It is a stripping process if (1) the gas stream is the scrubbing agent (such as air with
or without gaseous chemicals depending on the waterborne pollutants to be removed)
and (2) the liquid stream contains the targeted pollutant (such as ammonia, chlorine,
VOCs) that will be removed by the reactor. Normal instances of use of stripping tow-
ers is potable groundwater remediation, other contaminated groundwater treatment, or
some other water pollution control need.

This discussion of the difference between scrubbing processes and stripping pro-
cesses is more than an academic exercise. The optimum performance of a scrubber
or a stripper tower most often depends on the correct selection of packing media
with which to fill the tower. A given packing may perform better in promoting
mass transfer in a scrubbing (gas film controls) process as opposed to promoting
mass transfer in a stripping (liquid film controls) process. The opposite is true as
well: A packing media may be better suited to enhancement of mass transfer in a
stripping process and be less effective (less efficient, larger HTU value) in a scrub-
bing process.
2. The possibility of a combined wet scrubbing and gas stripping process is presented in
the following discussion. A combined wet scrubbing and stripping process has been
attempted by Wang and colleagues (48,49) for groundwater decontamination and
reuse. The contaminated groundwater contains high concentrations of total hardness
and volatile organic compounds (VOCs). An industrial plant near the contaminated
site is discharging an air emission stream containing high concentration of carbon
dioxide and is in need of additional industrial water supply.

It has been demonstrated by Wang et al.(48,49) in a small pilot-plant study that a com-
bined wet scrubbing and stripping process system using the aeration or tray tower
technology is technically feasible for achieving (1) reduction of CO from the air
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emission stream by scrubbing (i.e., groundwater is the scrubbing solution) and (2)
reduction of VOCs by simultaneous stripping (i.e., the carbon dioxide gas is the strip-
ping gas). Thus, the air emission stream and the groundwater stream treat each other.
After treatment, the former is free from CO , whereas the latter is free from VOCs.
2
The treated groundwater that is free from both VOCs and hardness may be recycled
for the in-plant application as the industrial water supply. The treated air emission
stream free from CO is discharged into the ambient air.
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