Environmental Engineering Reference
In-Depth Information
Process
Extraction
Open cycle
Closed cycles
Manufacturing
Use
Discard
Reconditioning
FIGURE 15.4
Material resource cycle (open loop and closed loop, where recycling is given importance, are shown).
varying time scales. Petroleum resources take millennia to get replenished, while oxygen
in the air gets replenished within hours (with abundant trees).
From the earth's resources, humans extract/mine required materials, process and reine
it, manufacture products, use these products, and eventually discard them. Most of what
is discarded goes into landills and also dumped into the oceans as in the Midway Islands.
This cycle is open ended and termed the “open cycle,” where one is not aware what hap-
pens to the material after it is discarded. As in the Midway Island, all the solid waste,
plastics in particular, disintegrate and loat on the ocean surface. These are consumed by
birds and ledglings, which barely survive a few days.
The other cycle that is more talked about today and what is becoming a necessity are
the cycles that are joined by the dashed white and dashed gray arrows (see Figure 15.4).
In these cycles, the material end use is reconnected with its original extraction. This con-
stitutes a “closed cycle.” Here, the processed and manufactured product is either (i) recon-
ditioned and reused; (ii) reconditioned, part of it remanufactured, and then reused; or
(iii) the material is reprocessed, manufactured, and reused. These “closed cycles” tend to
be sustainable and are becoming important because there is a limit to the availability of
raw materials, a limit to the rate at which we are consuming these materials, and a limit to
the carrying capacity of the waste/trash the earth can handle. However, it is not only the
material usage that needs to be sustainable but also the energy used to make the product.
Unfortunately, in today's industry, most of the energy comes from unsustainable sources,
something that cannot continue for long.
Sustainability is the key to mankind's future. Here one should be clear of what an eco-
friendly sustainable product is and what a non-eco-friendly unsustainable product is!
Eco-friendly products do not cause harm to any natural cycle, for example, harvesting
bamboo for construction. Non-eco-friendly products cause harm, to varying degrees, to
the environment, e.g., ozone layer depletion caused by use of chloroluorocarbons. It is
important to distinguish between what is “eco-friendly” and what is “biodegradable.” An
eco-friendly product, such as glass, need not be biodegradable. A biodegradable product,
