Environmental Engineering Reference
In-Depth Information
such as genetically modiied seeds, need not be eco-friendly. This distinction is crucial, as
understanding of this will help in creating sustainable cycles.
The question “what makes a cycle sustainable or unsustainable” can be answered when
one understands when should the cycle be “open” and when should it have to be “closed.”
In the cycle where we extract from nature and dispose waste into nature after use, the
rate of extraction should be lower or equal to the rate at which nature can replenish the
resource, for example, harvesting timber from forests. Another example is extraction of
crude oil, where humans are consuming in a few hundred years a resource that has taken
nature millennia to generate. At the same time, nature is unable to quickly reprocess and
manage the release of huge quantities of waste exhaust CO
2
gases. Such resource use and
waste generation is unsustainable and causes severe environmental disruption such as
climate change. This clearly indicates that our current lifestyle and most of the products
that we extensively use are unsustainable. Furthermore, in addition to the materials that
go into making a product, the energy going into the manufacture should also be from sus-
tainable sources. This energy is termed
embodied energy
. For example, the energy going into
the making of photovoltaic cells (that generate energy) takes nearly a decade to be recov-
ered through the energy generated from it. Unfortunately, the resource requirement and
embodied energy of modern-day living is so intense that it is just impossible for nature to
support even for a few more decades, unless our lifestyles change to a closed-loop resource
cycle. Current lifestyles, as we know, simply do not fall in this category. This is indeed a
challenge to scientists of today to make products that follow the closed loop and not the
open loop.
The following paragraphs discuss some examples of modern-day approaches that have
vitiated natural closed-loop systems resulting in vicious open systems, leading to dificult-
to-manage problems and unsustainability. Furthermore, solutions have been sought that
are also
open
and resulting in more complex problems. The current water crisis and severe
pollution is an indicator of a closed cycle becoming open. Figure 15.5 illustrates the nature
Farming
Clouds
Well water (3-10 m)
Rain
Biofertilizer
Lakes/ponds
Silt
Animal feed/
human feed
FIGURE 15.5
Closed-loop traditional farming cycle. Black arrows indicate water evaporation and condensation, which is a
part of the closed natural hydrological cycle.
