The availability of extensive, reliable data varied significantly from process to process, as outlined below. For combustion processes, extensive data are available on costs, and well-verified data are available on energy and emissions. Less consistent data are available on landfilling, and few data have been found on collection, separation, and remanufacturing and on composting.
Data on collection and transportation and cost data for all technologies involve special problems. They are therefore discussed separately in a later subsection of this summary.
In general, the data for rapidly completed processes(such as combustion) are much more extensive than data for processes that occur slowly (such as degradation in landfills). The original data used for energy and emissions form mass burning and combustion of RDF are quite reliable because the performance of those systems can be accurately measured. Data on the slower processes like landfilling are suspect because little reliable information is available on energy use and production and environmental releases generated over long periods.
Among the slower processes, the best data appear to be those on landfill gas generation; however, individual sources report widely varying rates of production from different landfills. The least accurate estimates used in the study are on the amounts and composition of water releases from landfills containing MSW or ash. Some of the data on the composition of the leachate reflect measurements made by researchers following strict quality assurance procedures, and those data seem reliable. However, all the sources report samples taken on a single occasion or over relatively brief periods of time. No studies quantifying water releases over long periods were found, and the method used in this study to extrapolate emissions over 20 years from individual measurements is speculative.
Composting is a relatively slow process. Data on composting are incomplete, and researchers have neither accurately measured composting emissions, as they have for combustion emissions, nor developed sophisticated models, as they have for landfills.
Recycling of MSW through curbside collection of recyclables or separation of mixed waste is a relatively new and changing approach. Recycling also involves many more processing steps than landfilling or combustion. Collection is a major contributor to the energy and emissions profiles for recycling. The limitations on the collection and transportation data used in this study (as outlined below) strongly affect the quality of the recycling data as well. There is currently no complete or consistent accounting of the amounts of MSW collected for recycling and the amounts actually recycled. The energy and emissions from the recycling (remanufacturing) processes themselves are not well characterized, and they will vary depending on the products made from the recycled material. Published estimates of the energy required for recycling and manufacture from virgin resources appear to be high-quality data, but they reflect processes in use in the mid-1970s. Available data comparing emissions from remanufacturing wild those from manufacturing virgin materials are so inadequate that they are not included in the report, although the differences may be significant.
Less Commonly Used Technologies
Two of the less commonly used options-anaerobic digestion and gasification/pyrolysis-are not used commercially in the United States. The data on those options presented in the report are therefore based on pilot plants. They do not provide an adequate basis for comparisons with other processes.
The third less commonly used option-cofiring of RDF with coal-is a commercial process, although it is used at only a few facilities. Reliable data on energy production are available for cofiring, but few studies of emissions have been made.
Collection and Transportation
The estimates of amounts of material collected and of energy and emissions for collection and transportation used in this study are based on the experience of a single community. In addition, the data provided by the community were not independently verified. Thus, the collection and transportation data in this report are intended to provide a basis for making order-of-magnitude estimates of the effects of altering the collection procedures used in a community, and for comparing the sources and magnitudes of emissions from collection with those from process steps. The estimates cannot be expected to be representative of other communities. Data are included on energy required for transportation of collected ferrous metals, aluminum, and glass to the point of remanufacture.
The cost estimates are adequate only for making order-of-magnitude comparisons and identifying-trends. Although all data found in the literature were updated to a single year using an appropriate inflation index, any other factors, such as the impact of different technologies, make direct comparisons impossible. Differing accounting systems also make comparative costs difficult to determine. Better estimates of relative capital and operating costs could be developed by designing reference plants for each technology and estimating costs of those plants on a consistent basis.
Contents and Organization of the Report and Appendixes
Table of Contents