BASES FOR COMPARISONS

For analysis of energy balances, the MSW management strategies can be divided into two or three 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, followed by transportation to the next step. The next step is either landfilling or 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. Energy is always needed for collection (e.g., to pick up the MSW) and transportation, and additional energy is always required for disposal as well as processing (in a landfill, a materials recovery facility, or a combustion plant). The analyses of the integrated strategies presented in Sections 5 through 8 break down energy balances for the individual steps, to indicate the approximate amount required for each one. In this section, however, energy balances are provided for each complete strategy.

The data for each technology and strategy are reported on the basis of one (1) 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 covered by the comparisons is 20 years. That unusually long period was chosen to permit comparisons of energy recovery from landfill gas collection with that for combustion of MSW in a waste-to-energy facility. Gas forms very slowly in a landfill, and choosing a shorter time frame for the analysis would underestimate the amount of energy that might be recovered from the waste. A period longer than 20 years was not considered because gas production in landfill-gas-to-energy operations may fall to an uneconomic level within that time, and current commercial practice is to close the energy recovery operations when they have operated for 20 years or less (CEC, 1991). The use of a 20-year period underestimates gas production and may underestimate landfill gas recovery, but the available published data are inadequate for extrapolating beyond that time period.

Gas recovery for energy production is not widely practiced. Fewer than 160 of the nation's approximately 6,000 landfills have such facilities. However, gas recovery is a commercially viable, beneficial option that is used as a benchmark for comparison with other technologies.

The elements of transportation considered in this report include the various collection steps (i.e., energy required 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, truck loadings, and energy requirements for 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 was included in the study and is covered in this report, but the results of the analysis have been excluded from the electronic data base. Energy requirements for trips to remanufacturing facilities are not provided in the data base because the amounts involved are relatively small compared to the energy requirements for remanufacturing (see Table 7.2 in Section 7). Energy requirements for transportation of waste paper may be significant, however.

Energy savings from remanufacturing curbside-collected glass, metal, and paper are included in the data base. Those estimates are also provided in the summary figures below.

Energy consumed in transportation is reported as fuel consumed. Conversion and transportation to the point of use consume about 19% of the Btu content of crude oil converted to gasoline, or about 11% for diesel fuel (DeLuchi, 1991). This factor is not considered in the analysis.

ENERGY REQUIREMENTS FOR INDIVIDUAL TECHNOLOGIES

Table 2.1 summarizes the types of energy required and recovered or saved by the major MSW management technologies. Elements of the energy balance that are excluded from this analysis are also listed. Energy requirements for collection and for disposal (if relevant) are combined under the heading, "Transportation."

Net Energy Balances for Selected Strategies

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