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Wet and Dry Scrubbing 271

a contaminated airstream (32,36–38,40–48). The most commonly used odor control
processes include (1) wet scrubbers, (2) regenerative thermal oxidizers, and (3)
bioscrubbers.
The single- and multiple-stage wet scrubbers have been discussed in this example,
although different types of wet scrubber (other than packed towers), other packing
products (in addition to Q-PAC), and other scrubbing liquors (per previous discus-
sion) may also be used. Example 25 introduces a totally different type wet scrubber
(47) for hydrogen sulfide removal.
Regenerative thermal oxidizers (RTOs) are introduced in another chapter of this hand-
book. Chemical porcelain heat recovery media (manufactured by Lantec Products)
have revolutionized the design of RTO units. Thermal oxidation was once thought to
be practical for odor and VOC control only when airflow rates were large (>25,000
scfm). Lantec’s new Multi-Layered Media (MLM) has reduced the size and fabrica-
tion costs of a RTO to the point where a unit as small as 800 scfm is now practical.
Because of the low-pressure-drop characteristics of MLM, electric power consump-
tion and hence operating cost has been reduced for a RTO unit threefold to fivefold.
All Operating and manufacturing costs have been reduced as well, because of the non-
plugging characteristics of MLM heat recovery media.

Bioscrubbers are also called biofilters or biofiltration units, which are discussed in
another chapter of this handbook series. Conventional wet scrubbers such as this exam-
ple will predictably consume large quantities of chemicals in WWTPs. Per previous
discussions, chemical costs often dictate consideration of additional stage scrubbing.
Odor-causing compounds such as hydrogen sulfide, mercaptans, DMS, and DMDS
often require chemicals for treatment as well as an additional scrubber stage. The bio-
scrubber (37,44,45), on the other hand, utilizes a dense biofilm to control these mal-
odorous sulfur compounds. A biological substrate has recently been demonstrated (41)
as an effective (99.9% + removal) odor control method with greatly reduced chemi-
cal costs.

Example 20

Removal of carbon dioxide from an air emission stream by wet scrubbing is presented in
this example. Please note the following:

1. The environmental engineering significance of carbon dioxide removal.
2. A feasible wet scrubbing system for removing carbon dioxide from an air emission
stream.
3. The chemical reactions involved in scrubbing carbon dioxide using a scrubbing liquor
of 25% sodium carbonate and 4% caustic (concentrations by weight percentage).

Solution
1. The environmental engineering significance of reducing carbon dioxide emissions
has been noted previously by Wang and Lee (49). Greenhouse gases such as carbon
dioxide, methane, and so forth caused global warming over the last 50 yr. Average
temperatures across the world could climb between 1.4°C and 5.8°C over the com-
ing century. Carbon dioxide emissions from industry and automobiles are the major
causes of global warming. According to the United Nations Environment
Programme Report released in February 2001, the long-term effects could cost the
world about 304 billion U.S. dollars a year down the road. This is the result of the