308 ENVIRONMENTAL ENGINEERING
Figure 15-4. 2,3,7,8-Tetrachlorodibenzo-p-dioxin.reclaims some of their energy content. These types of waste may also be judiciously
blended with wastes having low energy content, such as the highly chlorinated organic,
to minimize the use of purchased fossil fuel. On the other hand, rising energy costs
should not be a significant deterrent to the use of thermal destruction methods when
they are clearly indicated to be the most desirable method on an environmental basis.
Air emissions from hazardous waste incinerators include the common air pollu-
tants, discussed in Chap. 18. In addition, inadequate incineration may result in emis-
sion of some of the hazardous materials that the incineration was intended to destroy.
Incomplete combustion, particularly at relatively low temperatures, may also result
in production of a class of compounds known collectively as dioxin, including both
polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF).
The compound in this class that has been identified as a carcinogen and teratogen is
2,3,7,8-tetrachlorodibenzo-p-dioxjn (2,3,7,8-TCDD), shown in Fig. 154.
TCDD was first recognized as an oxidation product of trichlorophenol herbicides
(2,4-D and 2,4,5-T, one of the ingredients of Agent Orange) (Tschirley 1986). In
1977, it was one of the PCDDs found present in municipal incinerator fly ash and air
emissions, and it has subsequently been found to be a constituent of gaseous emissions
from virtually all combustion processes, including trash fires and barbecues. TCDD is
degraded by sunlight in the presence of water.
The acute toxicity of TCDD in animals is extremely high (LD50 in hamsters of
3.0 mgkg); carcinogenesis and genetic effects (teratogenesis) have also been observed
in chronic exposure to high doses in experimental animals. In humans, the evi-
dence for these adverse effects is mixed. Although acute effects such as skin rashes
and digestive difficulties have been observed on high accidental exposure, these are
transitory. Public concern has focused on chronic effects, but existing evidence for
either carcinogenesis or birth defects in humans from chronic TCDD exposure is
inconsistent. Regulations governing incineration are designed to limit TCDD emis-
sion to below measurable quantities; these limits may usually be achieved by the
proper combination of temperature and residence time in the incinerator. Engineers
should understand, however, that public concern about TCDD, and dioxin in general, is
disproportionate to the known hazards and is a major factor in opposition to incinerator
siting.
Landf II IsLandfills must be adequately designed and operated if public health and the envi-
ronment are to be protected. The general components that go into the design of these