Air Pollution Control 399HEPA FiltersVery hazardous or toxic particulate matter of diameter less than 1 bm must sometimes
be controlled to better than 99.9%. A single stage of high efficiencyparticle attenuation
(HEPA) micropore or glass frit filters, through which the precleaned gas is forced or
sucked by vacuum, can achieve this level of control, and four to six HEPA filter stages
in series can achieve 99.9999% control. HEPA filters are used to control emission of
radioactive particles.CONTROL OF GASEOUS POLLUTANTSGaseous pollutants may be removed from the effluent stream by trapping them from
the stream, by changing them chemically, or by changing the process that produces
the pollutants.
The wet scrubbers discussed can remove pollutants by dissolving them in the
scrubber solution. SO2 and NO2 in power plant off-gases are often controlled in this
way. Packed scrubbers, spray towers packed with glass platelets or glass frit, cany
out such solution processes more efficiently than ordinary wet scrubbers; removal of
fluoride from aluminum smelter exhaust gases is an example of packed scrubber use.
Adsorption, or chemisorption, is the removal of organic compounds with an adsorbent
like activated charcoal (see Fig. 20-13).
Incineration (Fig.20-14), or flaring, is used when an organic pollutant can be
oxidized to C02 and water, or in oxidizing H2S to S02. Catalytic combustion is a variant
Figure 20-13. Adsorber for control of gaseous pollutants. (Courtesy of American Lung
Association.)