Environmental Engineering Reference
In-Depth Information
Moreover, land is not just considered a recipient of solar influx, but it is also thought of as having
an attached environmental capital of water and air that surrounds that land (Krotscheck et al., 2000).
Process indicators
Intensity of manufacturing
Intensity of manufacturing metrics falls into the category of input/output indicators. These
metrics are ratios of the impacts, which are placed in the numerator, to the outputs, which are
placed in the denominator (Tanzil and Beloff, 2006). Some typical impacts are water use,
energy use, type of energy (renewable or nonrenewable), materials use, toxic release, green
house gas emissions, and solid wastes (Sikdar, 2003b). The denominator is usually the quantity
of product produced expressed in terms of mass, money, or any other suitable unit. Metrics
expressed as ratios are convenient for the purpose of comparing alternatives among different
products or processes (Tanzil and Beloff, 2006).
Some key operation metrics are KWh/unit of product or KWh/revenue, liters of water/unit
of product, tons of raw material/tons of product, air emissions (CO
2
, SO
2
, NO
x
)/unit of product,
and solid waste/unit of product (Friend, 1998). Readers with more interest in other indicators
may consult Krajnc and Glavi
(2003), who present a comprehensive list of ratio type indica-
č
tors of sustainable production.
Metrics that characterize the amount of energy and water per unit of product that takes
place along the whole life cycle are also called “embedded energy” and “embedded water,”
respectively. For instance, a slice of bread (30 g) has 40 liters of embedded water, an egg 135
liters, and hamburger (150g) 2400 liters (Hoekstra and Chapagain, 2006).
Recycling metrics
With the used of input/output analysis, recycling metrics can be established to account for the
recycling effort. For the food industry, recycling indexes are useful to account for the use of
recycled materials in the area of packaging (e.g., aluminum, glass, metal, and cardboard). In
reference to recycled materials use, there are two indexes available, the Virginity Index and the
Recycling Rate, shown in the following equations:
Virgin Materials Input
[4.2]
Virginity Index
=
Total Materials Inputs
Consumption of Recycled Materials
[4.3]
Recycling Rate
=
Total Materials Inputs
A similar metric can be established for water recycling as:
Volume of Water Recycled
[4.4]
Fraction Water Recycled
=
Total Water Consumption
There is another recycling metric based on material flow that expresses the recycling
efficiency of an industry in terms of the potential to recycle (Bailey et al., 2004):
Mass Recycled
[4.5]
Fraction Recycled
=
Mass Potentially Recyclable
