Environmental Engineering Reference
In-Depth Information
(see Section 5.2.9.4). For particle control on power plants, BACT is an electrostatic precipitator. For
other industrial categories the requirement may be a fabric filter (“baghouse”) or a spray chamber
scrubber (see Section 5.2.9.1).
Title IV of CAAA 1990 specifically addresses the acid deposition problem. By the year 1995,
the average emission level of all major power plants and industrial boilers was to be limited to
2.5 lb SO
2
per million Btu heat input; and by the year 2000, 1.2 lb SO
2
/MBtu. It is estimated that
in the year 2000 the U.S. emissions of SO
2
were roughly one-half of what they were in 1990, or
about a 10 million ton reduction. The utilities and industry could choose any method they wished
to achieve that emission reduction, including installing flue gas desulfurization technology, fuel
switching, seasonal fuel switching (e.g., using coal in the winter and natural gas in the summer),
and marketable permits. The latter means that if one source reduces SO
2
emissions by more
than the required quota, the excess can be sold to a source that does not wish to reduce that
pollutant.
For toxic pollutants, such as may be emitted from petroleum refineries, chemical manufactur-
ing, aluminum smelters, paper and pulp mills, automobile paint shops, dry cleaners, and so on, the
requirement is Maximum Achievable Control Technology (MACT). Depending on the pollutant
and the facility, MACT usually consists of adsorption, absorption, or incineration.
The BACTs and MACTs are not supposed to be cast in concrete for all times. As new tech-
nologies come into the market that prove to be more efficient and/or more economical, the EPA
may adopt, after due process, new technologies in lieu of the old ones. In any case, the introduction
of “technology forcing” as manifested by BACT and MACT is a revolutionary concept. It places
the onus on government to develop and define appropriate emission control technologies, rather
than merely setting an emission standard, and let the “sources” find the devices that meet the stan-
dard. Also, the federally required control technologies are uniform across the nation. Individual
states may impose even stricter control technologies, but never less efficient ones. This new way
of controlling emissions places emphasis not only on measurement of emission rates, but also on
monitoring and supervising the installation and proper functioning of the control devices.
9.2.2
U.S. Ambient Standards
The setting of emission standards has as its purpose ensuring that concentrations of air pollutants
in the ambient air remain at a sufficient low level so that the population at large—and, especially,
sensitive individuals, such as children and the elderly—will not suffer adverse health effects.
The appropriate indices for exposure to harmful air pollutants are the ambient concentrations and
the averaging time periods for which these concentrations prevail. Therefore, the U.S. Congress
mandated the EPA to promulgate ambient concentration standards, called the National Ambient
Air Quality Standards (NAAQS). The NAAQS stipulate the concentrations and the averaging
time periods for various air pollutants that should not be exceeded. In the United States, current
ambient standards exist for five pollutants: particulate matter (PM), sulfur dioxide (SO
2
), nitric
oxides (NO
x
, measured as NO
2
), carbon monoxide (CO), and ozone (O
3
). These are the so-called
criteria pollutants
. This is not to say that concentrations of other pollutants need not be curtailed.
Indeed, some toxic pollutants may be far more injurious to human health and biota than the
aforementioned five pollutants. The fact is that for the criteria pollutants a fairly well known dose-
response relationship has been established over years of clinical and epidemiological research,
while such a relationship may not be known for many other pollutants. For the other pollutants,
classified as
toxic pollutants,
there are no specified ambient standards, but their emission into the
