A comprehensive picture of a technology's environmental impact requires both an energy assessment and a life-cycle inventory. The analyst must catalog the available energy consumption data and assemble the data relating to solid-waste and water and air emissions. Even with this information, it may be difficult to assess the relative merits of two or more sets of inventory data. Four hypothetical scenarios demonstrate this difficulty (Figure 5). System B has the same energy consumption as System A but produces significantly lower quantities of all three wastes. To compare System B with System C, which has higher volumes of all three waste streams but much lower energy consumption, requires a judgment call or an "eco-points" rating system of some kind. Does the fact that System C has a lower energy consumption offset the higher emission rates? System D, which has both the lowest energy consumption and the lowest emission rates, is most readily rated the best of the four. System B or C could be considered better than A and worse than D, but some kind of aesthetic, ecological, or resource rating system is needed in order to decide.
The scenarios presented in Figure 5 are relatively simple. Even so, the products are difficult to rank. If we increase the level of detail in each of the waste streams to the point of quantifying the particular components of the solid-waste streams (e.g., suspended solids, dissolved solids, biochemical oxygen demand [BOD], for emissions to water; particulate matter, sulfur dioxide, carbon monoxide for emissions to air) the difficulty of ranking technologies increases enormously. How does a technology with high sulfur dioxide emissions and low suspended solids and BOD rate relative to one in which negligible sulfur dioxide is discharged and the aqueous waste stream is high in suspended solids and BOD? Is there a way to assess or weigh equivalent impacts of an air emission and a water emission? For air emissions, such assessments can be based on human toxicity or photochemical-smog-forming potential (Hocking, 1985), but ratings of pollutant categories again require judgment calls (i.e., become subjective).
Finally, there is the difficulty of weighing the significance of the aesthetic, or the personal-preference, factor in technology choices. In Figure 5, System A, which could represent one of the reusable cups, has a high aesthetic rating but also high energy costs and emission loadings. System D, which could represent a disposable cup, has a low aesthetic rating but ecologically favorable low energy costs and low emission loadings. Again, it is a difficult choice to make. Perhaps the best solution is to continue to provide a diversity of cup technologies for the diverse applications to be met. In this way, a reusable cup that begins to become energy competitive after enough uses can fulfill both a reasonable resource expenditure and aesthetic needs.
Alternatively, in situations where little or no reuse is likely, selecting the appropriate disposable cup and accommodating consumer preferences (i.e., insulated for hot drinks or noninsulated/clear for cold drinks) would be the best choice.