Sustainability: Humanity Perspective - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 743

Sustainability refers to processes and objects or matters. Sustainability is incompatible
with monotonous increase or decrease of amounts of a matter. Sustainability exists between
competing forces of increase and decrease. Monotonous increase of the amount of one matter
leads to the exhaustion of a limited surrounding (that contains the matter) or depletion of
source that provides the increase of the matter, while monotonous decrease of the amount
of a matter leads to the eventual exhaustion of the matter. In bioprocesses, sustainability is
compatible with steady state.

Example 15-1. Bottomless Lake.
Medicine Lake in the Jasper National Park of Canada is a typical glacier lake, except that
there is no visible draining outlet. Water from upstream pouring into the lake in a typical
summer day at 0.1 m 3 /s. In the summer, the lake is full of water. However, in the winter,
the lake is completely dry. The Natives call it the Medicine Lake because of this unique
feature. We learned that for the lake to be sustainable, one cannot just have an inlet supplying
water into the lake without water leaving the lake. Upon close examination, one finds that
water is leaking out from the porous bottom. Assume that the rate of water leaking out of

the Lake is 0.032 p m 3 /s with the water level in the lake h in meters. Further assume that
the evaporation loss of water from the lake surface is 0.001 m 3 /s. Determine the sustainable
level of water in the lake in the summer.
Solution. The sustainable level of water in the lake is the level of water at steady state.
Material balance of the liquid water around the lake (rate of increasing water amount and
rate of decreasing lake water is balanced) leads to
Q F Q E Q Evaporation ¼ 0

(E15-1.1)

where Q F is the rate of water flowing into the lake from upstream, Q E is the rate of water leak-
ing out of (or exiting) the lake, and Q Evaporation is the rate of liquid water evaporated into the
air (as vapor). Substituting the flow rates, we obtain

p h

0:1 0:032 0:001 ¼ 0 (E15-1.2)

Solving Eqn (E15-1.2), we obtain

0:1 0:001
0:032

2 m

h ¼ ¼ 9:57 m (E15-1.3)

The sustainable (liquid) water level is impressive in the summer.

15.2. SUSTAINABILITY OF HUMANITY

There is only one type of sustainability: the sustainability of human in existence that is of
interest in this chapter. This significantly narrows the definition of sustainability. However,
sustainability is necessarily neither unique nor quantitatively invariable. Since the 1980s
sustainability has been used more in the sense of human sustainability on planet Earth
and this has resulted in the most widely quoted definition of sustainability and sustainable
development, that of the Brundtland Commission of the United Nations on March 20, 1987:
“sustainable development is development that meets the needs of the present without