Environmental Engineering Reference
In-Depth Information
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Chlorine
Organic Chemicals
THMs
Figure 3.1
Trihalomethanes.
Trihalomethanes are created in the water itself as byproducts of water
chlorination. Chlorine (present in essentially all U.S. tap water) combines
with organic chemicals to form THMs (see
Figure 3.1
). They include chloro-
form, bromodichloromethane, dibormochloromethane, and bromoform.
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The abbreviation mg/L stands for milligrams per liter. In metric
units, this is used to express the weight of a chemical dissolved
in 1 liter of water. One liter is equal to about 1 quart, and 1 ounce
is equal to about 28,500 milligrams, so 1 milligram is very small
amount. About 25 grains of sugar weigh 1 milligram.
Note:
THMs are known carcinogens—substances that increase the risk
of getting cancer—and they are present at varying levels in all public tap
water.
Inorganic Chemicals
Several inorganic substances (particularly lead, arsenic, mercury, and cad-
mium) are of public health importance. These inorganic contaminants and
others contaminate drinking water supplies as a result of natural processes,
environmental factors, or, more commonly, human activity. Some of these
are listed in
Table 3.2.
For most inorganics, MCLs are the same as MCLGs,
but the MCLG for lead is zero.
Microorganisms (Microbiological Contaminants)
This group of contaminants includes bacteria, viruses, and protozoa, which
can cause typhoid, cholera, and hepatitis, as well as other waterborne diseases.
Bacteria are closely monitored in water supplies because they can be danger-
ous and because their presence is easily detected. Because tests designed to
detect individual microorganisms in water are difficult to perform, in actual
practice a given water supply is not tested by individually testing for spe-
cific pathogenic microorganisms. Instead, a simpler technique is used, based
