Environmental Engineering Reference
In-Depth Information
is no further use within the system). Materials that may have no further use
within the generating process (or firm) but could be reused within the sys-
tem as a whole may be considered “residuals.” A Type I flow of particular
concern in most economies involves dispersive use of toxics—that is, uses
of toxic materials that inevitably result in their dispersal into the environ-
ment. Lead in gasoline is a classic example of this type, as is cadmium in fer-
tilizers containing mined phosphate.
A more complex Type II system arises as scarcity (or growing popula-
tions) makes a Type I system inadequate. Feedback and internal cycling
loops develop, in large part through the evolution of new organisms—or,
in the case of an economy, new technologies, products, or services. Accord-
ingly, flows of material into, and waste out of, the system diminish on a per-
unit basis (e.g., per weight of biomass supported, or per dollar of economic
activity). Internal reuse of materials can become quite significant. In an
economy, while the internal material use within the system may remain
high, the velocity of materials through the system is reduced. Material man-
agement systems, either planned or spontaneous, become more prevalent.A
common example of a Type II system in many economies today is the life
cycle of most cars. The post-consumer car is first stripped of useful sub-
assemblies, which are then reconditioned if necessary and recycled as used
parts. The remaining hulk is shredded, and the steel (some 75 percent by
weight of the automobile) is recovered for recycling. This is an internal
material loop.The remaining plastic, glass, and miscellaneous materials and
liquids, known as “fluff ” or “ASR” (automobile shredder residue), are then
landfilled. Landfills, which would be both environmentally and economi-
cally more efficient if conceptualized and designed as residual storage facil-
ities as opposed to heterogeneous waste disposal sites, currently are not part
of material cycling streams. Accordingly, the fluff stream offers an opportu-
nity to shift from a Type I to a Type II system, from treating ASR as a waste
to managing it as a residual.
A Type III system is one in which full cyclicity has been achieved. In
many cases a Type III system should be a goal, but not always. For example,
local energy recovery from plastics might well be preferable to shipping
lightweight plastics long distances to reformulating facilities, because of the
environmental impacts of the transportation required in the latter case.
Materials recycling must also be considered broadly, in conjunction with
related technologies and natural systems. Thus, for example, production of
energy from biomass feedstocks should be evaluated in the context of the
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