Environmental Engineering FOURTH EDITION

396 ENVIRONMENTAL ENGNERDJG

Although wet scrubbers are very efficient, and can trap gaseous pollutants as well as
very fine particulate matter, they have disadvantages as well. Scrubbers use a great
deal of water that either requires further treatment or has limited use after being used
to scrub dirty gas. In places like the Colorado Basin, where water supplies are limited,
use in a scrubber may have a very low priority among other uses for available water.
Moreover, scrubbers use energy and are expensive to construct as well as to operate.
Finally, scrubbers usually produce a visible plume of water vapor.

Electrostatic Precipitators

Electrostatic precipitators are widely used to trap fine particulate matter in applica-

tions where a large amount of gas needs treatment and where use of a wet scrubber

is not appropriate. Coal-burning electric generating plants, primary and secondary

smelters, and incinerators often use electrostatic precipitators. In an electrostatic pre-

cipitator, particles are removed when the duty gas stream passes across high-voltage

wires, usually carrying a large negative DC voltage. The particles are electrically

charged on passage past these electrodes and then migrate through the electrostatic

field to a grounded collection electrode. The collection electrode can be either a

cylindrical pipe surrounding the high-voltage charging wire or a flat plate, like

that shown in Fig. 20-10. In either case, the collection electrode must be period-

ically rapped with small hammer-heads to loosen the collected particles from its

surface.

, I. .. _. - Cleangasout

Negative electrode connected

to electrical power source

Negatively charged wire

Grounded collecting plate

with positive charge

I

Dirty gas in

Hopper to discharge

Figure 20-10. Flat-plate electrostatic precipitator. (Courtesy of the American Lung

Association.)