The single values that have been derived for this study do not provide an adequate basis for making fine distinctions between technologies. Every technology has a range of performance values that vary with the plant design, operations, and maintenance, and with the nature of the MSW, as reported in Sections 5 through 8. However, the large-scale differences between some technologies and strategies in these estimates can be used for making comparisons.
BASES FOR COMPARISONS
For consideration of environmental releases, MSW management can be divided into several steps. The first is collection, which can include a single collection of MSW, or separate collection of recyclables, or separate collections of both recyclables and yard waste. The next stage is processing, which might include any of the major or less common technologies included in this report. The last stage is disposal of the residues, usually in a landfill. Emissions always result from collection (e.g., to pick up the MSW), and additional emissions may result from disposal as well as processing (in a landfill, a materials recycling facility, or a combustion plant). The analyses of the integrated strategies presented in Sections 5 through 8 break down emissions for the individual steps, to indicate the approximate percentage required for each one. In this section, however, energy balances are provided for the strategies as a whole.
The data for each technology and strategy are reported on the basis of one (l) ton of MSW set out for collection. If recyclable materials were separated before curbside collection, the data are reported in proportion to the percentage of the original ton of unseparated MSW that was separated.
The time frame chosen for the comparisons is 20 years. As described in Section 2, that unusually long period was selected to permit comparisons of energy recovery from landfill gas collection with that for combustion of MSW in a waste-to-energy facility. For consistency, it is necessary to use 20 years as the period for considering all other emissions, although emissions are released at different rates, depending on the technology. Combustion releases particulates and gases as soon as the MSW is burned. Landfilling, on the other hand, can release gases for many years, and leachate perhaps for centuries (FR, l991i). Data on releases from sanitary landfills over centuries cannot be obtained because widespread use of these types of landfills did not begin until the 1970s (see Appendix F). The emission rate drops significantly, however, after the first 8-40 years (Augenstein and Pacey, 1991). Because the emissions generally appear to diminish to a low and steady rate within 20 years, monitoring releases over a longer period would be unlikely to affect the estimates presented here by more than a factor of two to five; given the small quantities involved, even a fivefold increase would have a minor effect in absolute terms.
The elements of transportation considered in this report include the various collection steps (i.e., for collection of MSW in a packer truck and separate curbside collection of recyclables or yard waste, if included in the integrated strategy), as well as transportation to a processing plant or landfill. The details of the assumptions about distances and emissions from transportation are explained in the data base in Exhibit II. Users of the electronic data base can vary these assumptions to fit local conditions, if they so desire.
Transportation of recyclable materials, such as glass and metals, from an MRF to another facility for remanufacture is excluded. Emissions for those trips are not provided in the data base because the releases involved are relatively small (see Section 7).
EMISSIONS FROM MAJOR INDIVIDUAL TECHNOLOGIES
The emissions from each technology differ in type, as shown in Table 3.1. Emissions that have not been quantified for this analysis are also listed. Emissions for collection and disposal operations, such as loaders, are combined under the heading, "Transportation."
Emissions from Integrated MSW Strategies
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