Inventories: basic principles
The fundamental idea underlying the calculation of environmental
inventories is simple. Any group of industrial operations can be regarded as a system by
enclosing them within a system boundary. The region surrounding this system boundary is
known as the system environment. This system environment acts as a source of all the
materials and fuels inputs to the system and as a sink for all outputs from the system.
Diagrammatically this concept can be represented
as shown below in Figure 3 where the system is represented by the shaded box.
Figure 3. Schematic diagram of a system showing inputs and outputs.
An environmental inventory for this system is therefore simply a list
of the quantities of all of the inputs which pass from the system environment, across the
system boundary into the system and all of the outputs which pass from the system across
the boundary and into the environment. When the inputs are all derived from raw materials
in the earth and the final products are all waste materials returned to the earth, the
inventory is referred to as a life-cycle inventory. It is important to note that in a true
life-cycle system there are no usable products — only waste.
It is no part of the inventory analysis to make value judgements about
the relative significance of the different inputs and outputs; instead the analysis aims
to provide the quantitative data upon which judgements can subsequently be made. It will
however be clear from the above description of an industrial system that the inputs to the
system are the parameters involved in discussing conservation problems while the outputs
are the parameters of interest in discussing pollution problems.
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