DATA SOURCES AND ASSUMPTIONS
To ensure the inclusion of the best available data, the research for this study included a detailed review of a variety of sources, both primary and secondary. A hierarchical set of criteria were used to select data for inclusion in the data base; the order of preference was as follows:
Estimates that are based on assumptions, limited data, or ambiguous data are printed in italics in the basic worksheets for individual technologies. Data calculated from those original estimates, however, such as the estimates presented for the various integrated strategies, do not appear in italics.
- 1. Recent compliance test data were used whenever possible to certify air emissions from combustors; all dioxin/furan estimates are derived in accordance with the U.S. Environmental Protection Agency's (EPA's) requirements for Total PCDD/PCDF measurements; for leachate composition, preference was given to data based on sites that follow appropriate quality assurance/quality control procedures.
- 2. When only data from secondary sources were available, at least two sources were sought (although not always found); competing data from different sources proved to be a valuable method for determining which values were representative. Authors of reports were sometimes questioned about ambiguous data.
- 3. When no published data were found, information was sought through personal communications with current users of the technology (e.g., results of a program for curbside collection of yard waste were provided by a community that operates such a program).
- 4. When data based on assumptions were used to calculate a given input or output, the "reasonableness" of the assumptions was confirmed by knowledgeable researchers. Those researchers generally favored data from actual operations over data derived from models, although models of landfill performance were generally well regarded.
Although specific assumptions and sources of data are identified in the footnotes on the basic worksheets, two important general comments should be noted:
The data base is designed to calculate the correct transportation energy requirements and corresponding emissions for the proportions of MSW that are collected as MSW, as curbside recyclables, as separately collected yard waste, and as commercial waste. Each of these collections uses different trucks, different loadings, and different routes. As each hypothetical ton of MSW is apportioned to the appropriate collection and transportation methods, the overall energy and emissions are adjusted accordingly.
- 1. The data base expresses energy for transportation in British thermal units (Btus). The transportation energy estimates are based on the Btu value of the fuel and the mileage for each type of truck. Estimates of transportation emissions are derived from prevailing regulatory standards. However, the regulatory standards are based on operations that involve many fewer stops and starts than are required for collecting MSW, and the standard truck engines use no power to operate rams and lifts. Therefore, emissions four times as large as the regulatory limits were assumed for the study.
- 2. Estimates of routing miles, truck loadings, and mileage for collection and transport are based on the experience of one community-Palo Alto, California (population 57,000). Palo Alto is an affluent community that has had an aggressive curbside recycling program since 1978. A year-round curbside compost collection program was initiated in 1990.
- 3. Estimates of electricity used in a process are reported in Btus required to
generate the electricity in an efficient and up-to-date fossil-fuel-fired plant.
(1) For example, use of glass as a sand substitute in an asphalt slurry seal; this limitation is not intended to imply that alternative uses are disadvantageous.
(2) Tellus Institute has developed a model, called "Waste Plan," that can provide such correlations.
Table of Contents