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The Sustainable Process Index

The Sustainable Process Index (SPI) developed by Krotscheck and Narodoslawsky [1-3] is based on the assumption that a sustainable economy builds only on solar radiation as natural income. Most natural processes are driven by this radiation on earth's surface and for the conversion of radiation into products and services surface area is needed. Surface area is a limited resource in a sustainable economy because earth has a finite surface. Therefore area is a convenient measure for the SPI, the more area a process needs to fulfil a service the more it 'costs' from an ecological sustainability point of view.

Human activities exert impacts on the environment in different ways. On the one hand they need resources, energy, manpower and area for installations. On the other hand besides to the intended goods they produce emissions and waste. Consequently the SPI includes all these different aspects of ecological pressure on the environment. Therefore the total area Atot for sustainable embedding of human activities sustainably into the ecosphere is calculated by.

Atot = AR + AE + AI + AS + AP [m2] (1)
AR = ARR + ARF + ARN [m2] (2)
AI = AID + AII [m2] (3)

The areas on the right hand side are the "partial areas" that refer to the impacts of the different productive aspects. AR, the area required for the production of raw materials, is the sum (equ. 2) of the areas to provide renewable raw material (ARR), fossil raw material (ARF) and non-renewable raw material (ARN). AE is the area necessary to provide process energy including electricity. AI, the area to provide the installation for the process, is the sum (equ. 3) of the direct use of land area (AID) and the area for provision of buildings and process installations (AII). AS is the area required for support of staff and AP is the area for sustainable dissipation of emissions and waste products into the ecosphere.

The SPI method is based on the comparison of natural flows with the flows generated by a technological process. The conversion of mass and energy flows into area bases on two general "sustainability principles"[4]:

Principle 1: Anthropogenic mass flows must not alter global material cycles; as in most global cycles (like the carbon cycle) the flow to long term storage compartments is the rate defining step of these dynamic global systems, flows induced by human activities must be scaled against these flows to long term stores.

Principle 2: Anthropogenic mass flows must not alter the quality of local environmental compartments; here the SPI method defines maximum allowable flows to the environment based on the natural (existing) qualities of the compartments and their replenishment rate per unit of area.

For some resource flows like area consumption or the cultivation of renewable resources ARR the conversion is easy. Area consumption is based on the amount of area needed for growing a certain renewable resource, which is well known. The assumption here is that in a sustainable agriculture the process of growth and harvest closes the global cycles (e.g. the carbon cycle) on the field without changing local environmental compartments. However the effort for the activities of agriculture (energy and material input for planting, cultivating, harvesting and stoting) has to be factored in order to take into account the whole life cycle.

The area needed for fossil resource provision is based on the first principle as well, meaning that these flows are linked to the process of replenishing long term storage of carbon. The SPI method here takes into account the process of sedimentation in oceans, as this process takes out carbon from the dynamic global cycle into a long term storage compartment.

The conversion of non renewable resources to area is more difficult. Because no global cycles exist for non renewables, their use is inherently dissipative. Therefore the impact for these materials is generally separated into two parts: the provision of the material and the dissipation of the resulting emissions and wastes. The provision is taken into account within the raw material area ARN. It takes into account the impact of the whole life cycle to provide a non renewable material to the factory gate. Wherever no full life cycle for these materials is available, the energy input for mining and refining (as this usually provides the largest impact) is taken as a proxy. If even that is unknown, a first estimation of consumed area is made via the retropagatoric method. This uses the ratio of product value to energy input to estimate the ecological impact. The conversion of emission of processes in air, water and soil are calculated using the second principle. For most substances their existing concentration in (ground) water and soil is known. For these two compartments a replenishment rate can be defined, for ground water this is the seepage rate (depending on local precipitation). For soil the replenishment by decomposition of biomass to humus (best measured by the production of compost by biomass) is taken as a measure of renewal of this compartment. So area for "sustainable dissipation" of emissions is calculated by the amount of area needed to replenish enough ground water or soil that is able to absorb the amount of a given substance in the emission flow of a process without exceeding its natural concentration level in the respective compartment. Emissions to the compartment air are treated slightly different, as there is no natural replenishment rate for this compartment. Here the natural exchange of substances between forests and air per unit area (which is known for most airborne substances) is taken as a base of comparison between natural and anthropogenic flows. For a given emission flow (e.g. the gas effluent from a stack) all dissipation areas for the constituent substances flows to their final compartment are calculated. Only the largest of these dissipation areas however is taken into account, because all emissions with lower area consumption may be dissipated in this area without violating principle 2 in any other compartment.

For the purpose of technological optimisation the impact per good or service unit is of interest. This is represented by the overall footprint of a product atot (equ. 4).

atot = Atot / NP [m2.a/unit] (4)

NP is the number of goods or services supplied by the process in question for a reference period. In general this reference period will be one year, as most natural and engineering flow data are available on a yearly base. The area derived from a specific process to provide a specific good or service can be related to the area that is statistically available to a person. This relation represents the "cost" in terms of ecologic sustainability of this particular good or service, the SPI (equ. 5)

SPI index = atot / ain [cap/unit] (5)

where ain is the area per inhabitant in a given region. The lower the SPI the lower is the ecological impact of providing the good or service on the ecosphere.

The results of a SPI analysis contain diverse information. The SPI as calculated by equ. (5) gives an indication of the "cost" in terms of ecological sustainability of a given product or service. The number indicates what fraction of the overall "ecological budget" of a person is used to provide this good or service The partial areas in equ. (1) to (3) allow the identification of the largest contribution to the overall impact in terms of impact categories. The evaluation of the contribution of different steps to the overall footprint in (equ. 4) allows to identify the step in the life cycle that is the most problematic from the view point of sustainable development and that is the premium target for technological optimisation. Finally the inspection of the largest partial area caused by any step in the life cycle offers the possibility to identify optimisation potential in an in-depth technological optimisation.

All partial footprints calculated by the mass and energy inputs and outputs of all processes along the life cycle, are added up (according to the methodology described in the method section), and result in the overall ecological footprint atot for a unit of a desired product or service. To assure a better overview of the origin of the contributions to the ecological footprint of a process, the inputs have been divided into categories. There are 7 different categories:

  •  Area consumption
  •  Non renewables consumption
  •  Renewables consumption
  •  Fossil C consumption
  •  Emissions in air
  •  Emissions in water
  •  Emissions in soil

Within the software these categories are discerned in a colour code. Area consumption, consumption of fossil carbon, renewable and non renewable resources are impacts whose partial footprints are summarized along with the partial footprints of intermediates used by the process. The dissipation of substances in water, air and soil are impacts where only the largest footprint of a specific flow (e.g. the emission from a certain process in the process chain) is added to the overall footprint, according to the general SPI method.

[1] Krotscheck, C., M. Narodoslawsky (1996). The Sustainable Process Index - A new Dimension in Ecological Evaluation. Ecological Engineering 6/4 (1996) pp. 241-258
[2] Narodoslawsky M, Krotscheck C. The sustainable process index (SPI): evaluating processes according to environmental compatibility. Journal of Hazardous Materials 1995; 41(2-3): 383
[3] Krotscheck C, Narodoslawsky M. The Sustainable Process Index A new dimension in ecological evaluation. Ecological Engineering 1996; 6(4): 241
[4] SUSTAIN (1994) Forschungs- und Entwicklungsbedarf für den Übergang zu einer nachhaltigen Wirtschaftsweise in Österreich