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Figure 12.2 Two ways of defining system boundaries between physical economy and environment in LCA: (a) with narrow system boundaries, (b) with extended boundaries

Given the system boundaries with the environment, the next step concerns the specification of processes which will be analyzed and those which will be left out. A recent development concerns a distinction between foreground and background processes, the former being analyzed in the usual detailed way, the latter being approached using input-output analysis as approximation (Hendrickson et al. 1998). The next step concerns the construction of the model and the gathering of data about the different inputs and outputs of the processes. The model must quantitatively relate the different processes to each other, using the magnitude of the functional unit as reference. A fundamental difference from ERA

and SFA is that in LCA processes are included to the extent that they contribute to the defined functional unit; in contrast, in ERA and SFA they are always taken into account to their full magnitude.

A specific issue regarding the construction of the inventory model concerns the so-called 'multiple processes'; that is, processes which provide more than one function. Main examples are co-production, meaning that one unit process produces more than one product, combined waste treatment and recycling. In LCA this is called the 'allocation procedure' (see Figure 12.3). A general framework for allocation is developed in the ISO standard (ISO 14041). However, this still permits different calculation procedures based either on physical characteristics as the guiding principle (Azapagic and Clift 1999, 2000), on system extension (Weidema, 2001) or on economic principles (Huppes 1993). A more detailed harmonized set of rules is an important aim for future life cycle inventory development. The inventory analysis concludes with the compilation of the inventory table, the total list of the extractions and emissions connected with the product systems investigated. If a study is only performed up to the inventory analysis, it is called an LCI, that is, a life cycle inventory study.

The next phase concerns life cycle impact assessment, or LCIA. This phase interprets the extractions and emissions of the inventory table in terms of environmental issues, and it aggregates these data for practical reasons; a list of 50 or 100 entries cannot be dealt with in decision making. In the 1970s impact assessment was in fact done in an implicit way, by defining a number of broad, inventory-based parameters, which were thought to be indicative for the total spectrum of impacts. Examples of such parameters include net energy consumption, total input of resources and the total solid waste output (Hunt et al. 1974). A more recent example of this approach concerns the MIPS method (material input per service unit; Schmidt-Bleek 1993a, 1993b), in which the total material input of a product system is quantified. These approaches are time-efficient, and can lead to robust results. However, such a small number of inventory-based indicators is not very discriminatory and neglects various types of impact.

Since the mid-1980s, different methods for aggregating substances into a surveyable number of categories have been in development. Here guidance is being given by the ISO standard 14042. In this standard a stepwise procedure is defined that separates the scientific and the normative (that is, value-based) steps as much as possible. A number of impact categories are defined, together with the underlying characterization models; that is, the models for the aggregation of the extractions and emissions within the given impact categories. Here generally a 'problem theme approach' is followed, as originally proposed by CML in the Netherlands (Heijungs et al. 1992). The categories are defined as much as possible on the basis of resemblance in the underlying environmental processes, for instance all substances leading to an increase in infrared absorption and thus to possible climate change. But, clearly, value choices also are involved in characterization modeling (Owens 1997). Table 12.1 presents a list of impact categories, developed by a working group of SETAC Europe, as a structure for the analysis of the impacts and as a checklist for the completeness of the different types of impacts to be considered. A main distinction is made between input-related categories ('resource depletion') and output-related categories ('pollution').

The impact assessment phase also includes a number of optional steps. One of these concerns normalization, which involves a division of the results by a reference value for each of the impact categories, for instance the total magnitude of that category for the

Co-production Combined waste treatment

Co-production Combined waste treatment

Recycling

Note: Horizontal arrows indicate flows from and to the environment; vertical arrows indicate flows from and to other product systems.

Figure 12.3 Allocation of environmental burdens in multiple processes

Note: Horizontal arrows indicate flows from and to the environment; vertical arrows indicate flows from and to other product systems.

Figure 12.3 Allocation of environmental burdens in multiple processes given area and moment in time. Thus the relative contribution to the different impact categories can be calculated, owing to the given product system. Another concerns weighting, being a formalized quantitative procedure for aggregation across impact categories, resulting in one environmental index. Such environmental indices are very practical to use, particularly in the ecodesign of products; they enable a fast comparison between materials which all have their environmental characteristics expressed in one single number.

Table 12.1 Impact categories for life cycle impact assessment

A. Input-related categories ('resource depletion')

1. extraction of abiotic resources deposits such as fossil fuels and mineral ores funds such as groundwater, sand and clay

2. extraction of biotic resources (funds)

3. land use increase of land competition degradation of life support functions biodiversity degradation due to land use

B. Output-related categories ('pollution')

4. climate change

5. stratospheric ozone depletion

6. human toxicity (incl. radiation and fine dust)

7. ecotoxicity

8. photo-oxidant formation

9. acidification

10. nutrification (incl. BOD and heat)

Flows not followed up to system boundary input-related (energy, materials, plantation wood output-related (solid waste, etc.)

Note: glob = global; cont = continental; reg = regional; loc=local. Source: Based on Udo de Haes et al. (1999).

The last phase of LCA, according to ISO, is life cycle interpretation. During this phase, the results are related to the goal of the study as defined in the beginning. This includes the performance of sensitivity analyses and a general appraisal. A sensitivity analysis is of great importance for checking the reliability of the results of the LCA study with regard to data uncertainties and methodological choices. This can also lead to a new run of data gathering if the goal of the study appears not to be reached satisfactorily.

The next two sections will discuss two aspects which are relevant for usefulness of analytical tools like LCA. These are the choice of the model parameters, and the different ways to deal with uncertainty.

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