GTBL042-20 GTBL042-Callister-v3 September 21, 2007 19:48
Environmental and Societal Considerations • 793are further shaped, treated, and assembled into products, devices, and appliances that
are ready for the consumer—this constitutes the “product design, manufacture, as-
sembly” stage of Figure 20.1. The consumer purchases these products and uses them
(the “applications” stage) until they wear out or become obsolete, and are discarded.
At this time the product constituents may either be recycled/reused (whereby they
reenter the materials cycle) or disposed of as waste, normally being either incinerated
or dumped as solid waste in municipal land-fills—as such, they return to the earth
and complete the materials cycle.
It has been estimated that worldwide, on the order of 15 billion tons of raw
materials are extracted from the earth every year; some of these are renewable and
some are not. Over time, it is becoming more apparent that the earth is virtually a
closed system relative to its constituent materials and that its resources are finite. In
addition, as our societies mature and populations increase, the available resources
become scarcer, and greater attention must be paid to more effective utilization of
these resources relative to this materials cycle.
Furthermore, energy must be supplied at each cycle stage; in the United States
it has been estimated that approximately one-half of the energy consumed by man-
ufacturing industries goes to produce and fabricate materials. Energy is a resource
that, to some degree, is limited in supply, and measures must be taken to conserve and
more effectively utilize it in the production, application, and disposal of materials.
Finally, there are interactions with and impacts on the natural environment at
all stages of the materials cycle. The condition of the earth’s atmosphere, water, and
land depends to a large extent on how carefully we traverse this materials cycle.
Some ecological damage and landscape spoilage undoubtedly result during the ex-
traction of raw materials phase. Pollutants may be generated that are expelled into
the air and water during the synthesis and processing stage; in addition, any toxic
chemicals that are produced need to be disposed of or discarded. The final product,
device, or appliance should be designed so that during its lifetime, any impact on the
environment is minimal, and furthermore, so that at the end of its life, provision is
made for recycling of its component materials, or at least for their disposal with little
ecological degradation (i.e., it should be biodegradable).
Recycling of used products rather than disposing of them as waste is a desirable
approach for several reasons. First of all, using recycled material obviates the need to
extract raw materials from the earth, and thus conserves natural resources and elim-
inates any associated ecological impact from the extraction phase. Second, energy
requirements for the refinement and processing of recycled materials are normally
less than for their natural counterparts; for example, approximately 28 times as much
energy is required to refine natural aluminum ores as to recycle aluminum beverage
can scrap. Finally, there is no need to dispose of recycled materials.
Thus, this materials cycle (Figure 20.1) is really a system that involves interac-
tions and exchanges among materials, energy, and the environment. Furthermore,
it is imperative that future engineers, worldwide, understand the interrelationships
amongst these various stages so as to utilize the earth’s resources effectively and
minimize adverse ecological affects on our environment.
In many countries, environmental problems and issues are being addressed by the
establishment of standards that are mandated by governmental regulatory agencies
(e.g., the use of lead in electronic components is being phased out). Furthermore,
from an industrial perspective, it becomes incumbent on engineers to propose viable
solutions to existing and potential environmental concerns.
Correcting any environmental problems associated with manufacturing will in-
fluence product price. A common misconception is that a more environmentally