Place and Time - Sustainable Design: The Science of Sustainability and Green Engineering - page 156

moving and changing in a bacterium may be very different from processes at
the lake or river scale, which in turn are different from processes that affect a
contaminant as it crosses the ocean. This is simply a manifestation of the first
law of thermodynamics: Energy or mass is neither created nor destroyed, only
altered in form. This also means that energy and mass within a system must
be in balance: What comes in must equal what goes out. Engineers measure
and account for these energy and mass balances within a region in space through
which a fluid travels. Recall from Chapter 2 that such a region is known as a control
volume and that the control volumes where these balances occur can take many
forms. Figure 2.3 illustrates several ways in which mass balances (reactors) apply
to environmental processes. So within any control volume, one can calculate the
balance. Mass balance, for example, is

quantity of
mass per unit volume
in a medium

rate of production or loss
of mass per unit volume
in a medium

=

[total flux of mass] +

(4.1)

or, stated mathematically,

dM
dt =

M in − M out (4.2)

where M is the mass and t is the specified time interval. If we are concerned
about a specific chemical (e.g., environmental engineers worry about losing good
ones such as oxygen, or forming bad ones such as the toxic dioxins), we would
need to add a reaction term ( R ):

dM
dt =

M in − M out ± R (4.3)

However, within these reactors are smaller-scale reactors (e.g., within a
fish liver, on a soil particle, in the pollutant plume or a forest, as shown in
Fig. 4.2). Thus, scale and complexity can vary by orders of magnitude. So
the bottom line is that green engineering must make use of the tools that
chemical engineers provide, especially the thermodynamics of mass and energy
balances.