Environmental Engineering FOURTH EDITION

382 ENVIRONMENTAL ENGINEERING

SMOKE AND OPACITY

Visible smoke from a stack is often the only immediate evidence of a pollution violation
external to an industrial source. Effluent gases cannot be sampled and analyzed without
a complicated system that always involves notifying the emitter, but smoke issuing
from a stack can be seen. The opacity of a smoke plume is thus still the only enforcement
method that may be used without the emitter’s knowledge (and, often, cooperation).
Therefore, many regulations are still written on the basis of visually estimated smoke
density.
The density of black or gray smoke is measured by visual observation and esti-
mation of smoke opacity - the amount of light blocked by the smoke. Although this
method might appear to have very large inherent uncertainty, it has been used enough so
that the reproducibility of observations is fairly reliable. Some data on reproducibility,
developed by EPA, are:

0 For black plumes (133 sets at a smoke generator), 100% of the sets were read

with a positive error of less than 7.5% opacity; 99% were read with a positive error of

less than 5% opacity. (Note: For a set, positive error = average opacity determined by

observers’ 25 observations - average opacity determined from transmissometer’s 25

recordings.)

0 For white plumes (170 sets at a smoke generator, 168 sets at a coal-fired power

plant, 298 sets at a sulfuric acid plant), 99% of the sets were read with a positive error

of less than 7.5% opacity; 95% were read with a positive error of less than 5% opacity.

A positive observational error associated with an average of 25 readings may thus be

established at 5%.

A typical opacity standard is 20% opacity, which is a barely visible plume, with

allowances for 40% opacity for very short periods of time. Modem practice consists of

training enforcement agents to recognize opacities by repeated observation of smoke

of predetermined opacity.

CONCLUSION

As with water pollution, the analytical tests of air quality can be only as good as the

samples or sampling techniques used. Moreover, the prevailing analytical techniques

leave much to be desired in both precision and accuracy. Most measurements of

environmental quality, including these, are reasonable estimates at best.

PROBLEMS

19.1 An empty 6-in. diameter dustfall jar weighs 1560 g. After sitting outside for

the prescribed amount of time, the jar weighs 1570 g. Report the dustfall in the usual

manner.