The grey water footprint of a process step is an indicator of the degree of
freshwater pollution that can be associated with the process step. It is defined
as the volume of freshwater that is required to assimilate the load of pollutants
based on natural background concentrations and existing ambient water quality
standards. The grey water footprint concept has grown out of the recognition
that the size of water pollution can be expressed in terms of the volume of water
that is required to dilute pollutants such that they become harmless (Box 3.4).