The Sustainable Process Index (SPl)

(Source:  Hertwich et al., Evaluating the environmental impact of products and production processes: a comparison of six methods. Science of the Total Environment, 1996 Vol. 196, Issue 1, 13-29.)

The SPI estimates the area that would be required to operate a process sustainably, based on renewable resource generation and toxic degradation; an extension of the dilution volume approach. The SPI is basically based on an operational definition of sustainability: a sustainable economy utilizes resources at or below the rate at which they are created, and it produces only waste streams that can be dissipated in the environment without threatening the life-support system or human health, and without accumulating in the environment Dilution volumes and resource generation rates are used to relate a particular activity or product to an area: where the surface area of the planet is seen as the ultimate constraint for human activity. 

The area required for a particular production process consists of the area required for raw materials and energy production, the area of the installation, the living area of the process staff and the dilution area for waste dissipation. This area is divided by the per capita area of the region in which the production takes place, so that the index measures how many persons’ life support capacity the process of interest requires. 

The valuation is implicit in the goal of sustainability and the ambient concentrations used to estimate dilution areas. This causes problems when fossil fuels and minerals are considered: it takes millions of years to generate coal, petroleum or natural gas via sedimentation and geophysical conversion processes.