An industrial ecology (ecosystem) has been defined to
exist on three levels [6]. These levels are characterized
by the amount of recycling or reuse of material that is
within the system (or the system’s “openness”). The first
level is a completely closed system with no material or
energy leaving the system. The third level is a completely
open system with little material or energy, once
consumed, remaining within a system. The second level
is characterized by some factor of energy and material
is reused within the system. The second level seems to
be the most applicable model for actual systems. It is
within these industrial ecosystems models that green
supply chains will play a critical and practical role.