Environmental Engineering Reference
In-Depth Information
Lakes and other lakes in the northeastern United States and southeastern Canada often contain a
concentration of mercury which is deemed unhealthy, especially for pregnant women. Children
born to mothers who consumed mercury-laced fish may show birth defects or mental retardation.
In addition to coal-burning power plants, municipal incinerators are sources of mercury, because
batteries, switches, and fluorescent bulbs that are tossed into the garbage may contain mercury,
which, when incinerated, escapes into the atmosphere. Lead may also cause mental retardation and
other brain and central nervous system effects. Part of the lead enters surface and groundwater via
atmospheric deposition. The lead used to originate from leaded gasoline. With the phasing out of
leaded gasoline, lead deposition and lead-related diseases are on the decline.
9.3.4.2 Polycyclic Aromatic Hydrocarbons
PAH are organic compounds consisting of two or more fused benzene rings. Naphthalene has two
rings, anthracene three, pyrene and chrysene four (in different arrangement), benzopyrenes (there
are several) five, perylene six, and coronene seven. Some of the PAH are known or suspected
carcinogens, notably benzo(a)pyrene. PAH are a product of incomplete combustion. Whenever we
see a sooty smoke, we can be sure that it contains PAH, because PAH adhere to soot particles. PAH
in the vapor or particulate phase are carried by winds and subsequently deposited on land and water
in the dry form, while those scavenged by precipitation are deposited in the wet form. Because
PAH are unstable (they decompose in sunlight, or by oxidation with atmospheric oxidants), they do
not travel as far as metals, and they are deposited closer to the combustion sources. Because PAH
are very hydrophobic, they do not readily incorporate into rain droplets; therefore dry deposition
predominates over wet deposition, especially near polluted cities. Rivers, lakes, and coastal waters
surrounded by urban-industrial areas are especially affected by PAH deposition from the myriad
of combustion sources in those areas. Soot particles with adsorbed PAH settle in the sediments
of these waters. Bottom feeding fish and shellfish that live in the sediments may ingest PAH
and are often found with cancerous lesions and other diseases. Because PAH dissolve in fatty
tissues, these organisms may bioaccumulate PAH and transfer them to the food chain, including
humans.
The irony is that PAH are not emitted as much from big centralized combustion sources as
from small, dispersed sources. Large power plants, industrial boilers, and municipal incinerators are
easily controlled for preventing emissions of products of incomplete combustion, of which PAH are
a part. Basically, the control involves “good engineering practice,” which is combustion in excess
air, thorough mixing of fuel and air, high flame temperature, and sufficient residence time in the
combustion chamber. Under those conditions, practically all the carbonaceous matter burns up into
CO
2
and H
2
O, and no organic molecules or radicals are left which may recombine to form soot and
PAH. Most of the soot and PAH emissions come from residential and commercial furnaces, wood
stoves and fireplaces, open fires, barbecues, aircraft jet engines, gas turbines, and, most copiously,
internal combustion engines, such as diesel and gasoline-fueled trucks and automobiles. We are
all familiar with the cloud of soot emanating from a diesel truck under accelerating drive, when
the fuel-to-air ratio becomes very rich, or the black trail left behind a jet aircraft during take-off.
If we want to minimize PAH emissions, we must find better ways of controlling these dispersed
stationary and mobile sources. Cigarette smoke also contains PAH; the latter are probably the cause
of lung and throat cancer of smokers.
