Abstract
Management standards, government initiatives, non-government organizations and corporate guidelines emphasize the strong need for the development of methodologies to facilitate the environmental comparison of process design and product alternatives.  This chapter helps address the need to support these "burden-based" assessments by presenting a discussion of metrics commonly used for the relative comparison of alternatives in eleven impact categories, such as ozone depletion, smog formation and acidification.  Each section of the chapter provides a historical introduction, an overview of the state-of-the-art, identifies key limitations and illustrates one common approach.  The illustrations are based on two alternatives for the production of 1,4 butanediol (BDO), considered both from a site-specific processing and a holistic product perspective.
 

Introduction
Comparison Metrics: Impacts associated with process or product changes can be considered in the context of natural resources, human health, ecological health, social welfare and economics.  All these factors are taken into consideration under the "sustainable development" umbrella.  In this chapter we focus on the first three and describe environmental comparison metrics (potentials, potency factors, equivalency factors or characterization factors) for eleven associated "impact" (sub)categories: global warming, stratospheric ozone depletion, tropospheric ozone (smog) creation, eutrophication, acidification, human carcinogenicity, toxicological impacts to humans, toxicological impacts to ecosystems, land use, water use and the depletion of fossil fuel resources.  Although these eleven subcategories are commonly addressed in available environmental comparison case studies, this list is not comprehensive.

Emissions or resource consumption data are typically multiplied by environmental comparison metrics to help represent their relative importance within a given impact category.  A significant number of methodologies are available that can be used to derive environmental comparison metrics in application domains like product life-cycle assessment (LCA), process design and for "regulatory" prioritization. ^{e.g. 1,2} These methodologies range in complexity (data intensity, knowledge requirements), comprehensiveness (breadth or scope of representation), sophistication (relevance to and depth of representation of the environmental mechanisms) and accuracy (uncertainty inherent to the model and associated with input data).  As a result, the selection of a methodology often remains subjective and strongly influenced by resource availability (in-house knowledge, input data availability, etc.).

The metrics illustrated in this chapter are commonly used in relative comparison applications or as target (objective) functions for design "optimization."  No endorsement of the approaches or suggestion that they are the "best available practice" is intended.  The results do not indicate that an actual impact will occur but often reflect differences in implicit concern, as in the precautionary principle.  In some cases the metrics may reflect intrinsic differences at a common midpoint in a cause-effect chain (environmental mechanism), as illustrated in Figure 1 for acidification.

Figure 1:

Midpoints and endpoints in the simplified cause-effect chain for acidification

There is a tendency to define indicators at common midpoints to help minimize (parameter or data) uncertainty.  For example, this modelling preference would imply that there is no need to forecast specific effects associated with global warming.  Comparison can be made in terms of radiative forcing and half-life differences (inherent chemical properties).  However, comparison at midpoints may not always account for all factors in a cause-effect chain (increasing model uncertainty), as is the case of acidification metrics in Figure 1, and can result in complications associated with the resolution of trade-offs, if any, across impact categories like acidification and global warming.

Comparison using endpoint metrics may increase the number of categories requiring consideration, for example the specification of different types of impacts to human health associated with ozone depletion, but may provide a set of metrics that are more readily comparable across categories using available "valuation" tools due to their "observed" nature.  For example, human health effects endpoints are often associated with different levels of perceived severity.  Although considered a developing approach and with additional data requirements, the results for different human health endpoints can be combined into a single human health measure using approaches like QALYs (Quality Adjusted Life Years) and DALYs (Disability Adjusted Life Years).³  Commonly however differences in severity in the context of toxicological impacts to human and ecological health are ignored (effects inherently considered equal).

"Holistic" Assessment: Life-cycle assessment (LCA) is one framework for "holistically" evaluating the inputs and emissions associated with all the stages in a product's life-cycle from cradle to grave (raw material acquisition, manufacturing, use and disposal).  Similar to site or process specific comparison of process designs, an LCA consists of iterative steps.^4-6 Having established the product system boundaries and aims of the assessment (goal and scope definition), associated resource consumption and emissions must be identified and tabulated in a data sheet (inventory analysis).  These inventory data are then considered according to the substance's potential contribution within an impact category, such as global warming and ozone depletion.  Associated elements include:

• Selection of impact categories, category indicators and characterization models

• Assignment of inventory analysis results to impact categories (classification)

• Calculation of category indicators using associated characterization factors (comparison metrics) derived using characterization models (characterization)

Calculating the magnitude of category indicator results relative to reference information like geo-specific emissions (normalization), sorting or ranking impact categories (grouping) and aggregating indicator results across impact categories (weighting or multi-objective decision making) remain controversial topics that cannot be addressed in sufficient detail in this chapter.

It should be noted that the inability to compare design alternatives in terms of "actual" impacts can be compounded in life-cycle assessments by additional factors, including:

• the inventory data reflecting only a portion of the total inputs/emissions at given sites

• the unknown and changing location of many sites in a life-cycle

Case Study: The US EPA ⁷ commissioned a life-cycle case study to help explore the implications of BDO (1,4 butanediol) production from bio-derived feed stocks compared to natural gas.  Two system boundaries are addressed in this chapter:

• the processing stage (the gate-to-gate perspective) 

• and the cradle-to-gate life-cycle 

The product life-cycle stages addressed for the BDO alternatives are summarized in Figures 2 and 3. ⁷ As the final product is functionally identical in both life-cycles, a "cradle-to-gate" approach (from extraction of the raw materials to the final deliverable product) was selected.  The functional unit, the performance basis used to help consistently compare alternatives, was defined as one US pound of BDO produced and ready for shipment.  This functional unit and the inventory data can be adjusted to reflect annual production rates for the US, thus being more relevant from a national emissions and resource consumption perspective.  However, this multiplication will not influence the results in a relative comparison context and will still not provide an absolute indicator of "actual" impacts.

It should be noted that the BDO life-cycle analysis represents a "snap-shot in time" and all the stages are subject to change, particularly the developing bio-based life cycle.  It remains open to debate whether such a "static" comparison is always appropriate, although practical alternatives are currently limited in availability and it can be difficult to forecast future developments that may alter the outcome (improvements in available technologies, novel processes, alternatives in reaction chemistry, solvent substitutions, etc.).

Figure 2:

Simplified life cycle flow diagram for BDO derived from natural gas (energy consumption and associated processes not shown) adapted from US EPA7

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Figure 3:

Simplified life cycle flow chart for BDO derived from corn glucose (energy consumption and associated processes not shown) adapted from US EPA7

 

Summary
In this chapter we briefly outlined many considerations associated with current "state-of-the-art" metrics that are used in environmental comparison applications and illustrated the importance of comparing design alternatives from a life-cycle, as well as site-specific, perspective. It is clear that our ability to readily compare alternatives in product life cycle assessment (LCA), as well as in the context of more restricted scopes like the direct comparison of on-site emissions associated with process design alternatives, remains limited. Despite these limitations and the significant debates that ensue, decisions must ultimately be made to help focus our finite resources (time and money) in an effort to reduce the potential of negative consequences associated with chemical emissions and the depletion of natural resources.

It is important to note that the methodologies and the metrics presented in the case study are commonly used but may not necessarily be considered the "state-of-the-art". The case study represents a "snap-shot in time" and all the stages are subject to change, particularly the developing bio-based life cycle. Updates and more detailed information can be found in the attached references and through the ongoing research activities of organizations like SETAC.