1.1 Framework for Defining Sustainability Indicators

The commonly understood three dimensions of sustainability as shown in Fig. 1 are largely interrelated, i. e. what is good for the environmental is also good for society, or what is good for the economy can be usually good for society, and so on.

As shown, sustainability exists at the intersection of the three domains represent­ing the economy, environment, and society. We can state that a system becomes more sustainable when all three domains, as represented by the intersection of the three domains, show improvement as a result of a human intervention. This Venn diagram also facilitates identification of the dimensions of the metrics to be used to evaluate relative sustainability of a selected system. For instance, any indicator or

Fig. 1 Generally accepted model for sustainability

metric that represents all three dimensions, such as energy use, will be a 3D or 3D metric. Similarly other metrics could be 2D or 1D, depending on how many domains are represented.

When the task is to compare the relative sustainability of a system against alter­natives, we need to consider the following actions in sequence: define the system, identify the metrics to be used and their dimensionality, prioritize them in terms of their significance, obtain values of those metrics for the competing alternatives, and compare them to arrive at a decision. A small set of indicators is ideally preferable because it simplifies analysis, and sometimes allows a decision by visual inspection of the values of the metrics. Though not always possible, it is advisable that the metrics are deemed to be necessary and sufficient, independent of each other, and are quanti fi able.

In this work the framework previously used by Martins et al. [10] is applied for the sustainability evaluation of microalgae biofuels, taking into account the biofuel supply chain stages that include: microalgae cultivation and harvest, biomass pro­cessing, oil extraction and pre-treatment, biodiesel production and blending, distri­bution, and final use. In practice, one needs to first clearly define the system boundaries with identified supply chain. This process allows identification of the indicators to be used and the kinds of data to be collected for calculating their values. Then, all the relevant inputs and outputs (energy, water, materials, product, by-products, wastewa­ter, gas emissions, solid wastes, etc.) should be identified and quantified, in order to be able to calculate the values of the selected metrics. Finally, when the values of the metrics for all possible alternatives are available, a decision on relative sustainability can be made either by inspection or by use of computational tools.

For deriving sustainability indicators and to evaluate microalgae biodiesel through its supply chain, the following sequential procedure (Fig. 2) can be applied:

1. System boundary definition, including inputs and outputs (energy and mass fluxes) through the supply chain.

2. Identi fi cation of the most relevant environmental, economic and societal impacts that ought to be considered and explicitly included in the indicators to be selected, as well as the data required for their calculation.

3. Selection and prioritization of an adequate set of sustainability indicators based on technical input and data availability. All the relevant inputs and outputs should be identified and quantified, in order to be able to calculate the indicators values.

4. Calculation of the chosen indicator values for sustainability evaluation. The 3D, 2D and 1D metrics are calculated based on the inventory analysis of the process.

5. Interpretation and decision making. Decisions for improving the process are made based on the results of the indicators calculation and on the consideration of other issues, for example cost analysis.