Selecting an approach for managing a community's municipal solid waste (MSW) is a difficult, technically complex process. The problem is compounded by a lack of comprehensive sources of current data on the various possible approaches to MSW management. In general, the best available data are for systems that include environmental controls. Thus, extensive data have been published on air emissions from the combustion of waste, and significant amounts of data are available on air emissions and leachate from landfills. Few data exist on composting or on curbside collection, separation, and remanufacturing of recyclable materials. In addition, very few life-cycle assessments of waste management alternatives have been published. The National Renewable Energy Laboratory (NREL) began a review for the Department of Energy (DOE) to determine what is already known and establish a consistent basis for comparing the environmental releases, energy use and production, and economics of waste management options.
This study was initiated to compile publicly available data on the five major options commonly used for MSW management today:
Many communities will use more than one option to manage MSW. Such combinations of options are identified here as "integrated strategies." For example, some communities offer curbside collection of recyclable materials in addition to collection of the remaining MSW for landfilling or combustion. Some communities collect yard waste for composting, as well. This report provides the data needed to compare the wide variety of integrated strategies. Realistically, it was expected that some information would be unavailable, and that some published data would require validation. One goal of the study was to identify missing information and to define additional research needed to improve the options for managing MSW.
This report was intended to help communities make informed decisions by giving them consistent data describing their possible choices. The scope of the study excluded a number of factors that a community may wish to consider, such as ecological impacts, health risks, local social values, and the regulatory requirements of specific jurisdictions. Because the report focuses on options for managing waste that is set out for collection, it does not discuss programs designed to reduce the amount of waste to be picked up for disposal, such as source reduction and backyard composting.
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 wellverified data are available on energy and emissions. Less consistent data are available on land filling, 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 the degradation in landfills). The original data used for energy and emissions from 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, and the limitations on the collection and transportation data used in this study 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 with 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 d ird less commonly used option-refiring 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. No data were found on energy required for transportation of collected ferrous metals, aluminum, glass, or paper to the point of remanufacture.
The cost estimates are adequate only for making order-of-magnitude comparisons and identifying trends. Although all the data found in the literature were updated to a single year using an appropriate inflation index, many 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 the costs of those plants on a consistent basis.
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