Basis for the Comparisons: a Life-Cycle Analysis
The best available data have been converted to a consistent basis for comparisons. In compiling data about net energy requirements and environmental releases, a life-cycle assessment approach was used that generally followed a typical life-cycle assessment practice(1) As applied to a given MSW management option, a life-cycle assessment is a comprehensive, quantitative description of the energy and materials used and the wastes released in all steps of the option.
The data for each option and strategy are reported on the basis of one ton of MSW, set out for collection. In the strategies that used curbside collection of recyclables in combination with a disposal technology such as landfilling or combustion, the energy and emissions for both curbside collection and the disposal technology are based on one ton of material left at the curb; that is, for example, if about 14% of total MSW is separately collected for recycling, energy and emissions are reported for the sum of 280 pounds left for curbside collection of recyclables and 1,720 pounds left for disposal.
Energy and Emissions
In calculating energy data, a ton of waste is followed through all transportation(2) and processing operations to its final disposition (e.g., recycling and remanufacturing, combustion with energy recovery, or landfilling operations with gas recovery). Emissions data are presented for all steps except remanufacturing, as discussed above.
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.
For consistency, the same 20-year period was used in considering all other emissions from the landfill, including the gas not recovered and the leachate (liquid that leaks from the landfill). The leachate from the ash from combustion processes was therefore also followed for 20 years, although releases to the air during combustion are accounted for when the MSW is burned. Landfill emissions will continue for a period longer than the 20 years considered in this analysis.
Other factors complicate the life-cycle analysis of materials separation, collection, and recycling. Recycling of suitable components of MSW involves five steps:
Data on capital and operating costs for the individual options were converted to 1991 dollars per ton of daily capacity to provide a consistent basis for cost comparisons. The PEPCOST Index, which was designed to make such conversions for SRI International's Process Economics Program, was used.
A data base was constructed that includes the energy and emissions data for each waste management option and for each step in a comprehensive MSW management strategy: collection, processing, disposal of residues, and, if appropriate, recycling. Because a community ultimately chooses and implements a strategy that includes at least the first three of these steps and may choose a strategy that incorporates several individual options, the data base combines the energy and emissions for each component in proportion to its contribution to the overall strategy for treatment of the waste.
The data base is available in electronic form for analyzing various possible MSW management strategies. Users can change variables in the data base (e.g., transportation distances, volume of recyclables collected, truck fleet fuel consumption) to reflect a particular community's circumstances.
The United States generated 180 million tons of municipal solid waste in 1988 (3). MSW is estimated to be growing at rates of 0.75% to 1.5% per year-i.e., at the same rate as population growth to twice the rate of population growth.
Today, 69-73% of MSW is landfilled, and landfill gas is recovered for energy at about 128 of the nation's larger landfills; 17% of it is burned, 94% of that amount (or almost 16% of total MSW) for energy recovery. Estimates of the percentage of MSW that is recycled vary significantly; the U.S. Environmental Protection Agency (EPA) and the Office of Technology Assessment (OTA) have published estimates of 10-14%. Composting accounts for a small percentage of waste treatment.
The EPA has set a national voluntary goal of reducing the quantity of MSW by 25% through source reduction and recycling by 1992, and at least 21 states have adopted laws to mandate or encourage separation of recyclable materials from MSW. The quantity of waste recycled by programs under community control is not well documented.
Collection and Transportation
MSW management includes curbside collection of the waste, transportation of the waste to a landfill or a processing facility (e.g., a combustor or a materials recovery facility), and possibly transportation of the residue from processing to a landfill. Although many models of collection and transportation requirements for various types of collection programs have been developed, it proved difficult to find actual data on energy and emissions for these steps. Accordingly, this study used data on transportation energy requirements supplied by one community. The city had operated a curbside collection program for recyclables for many years, and it initiated a program for curbside collection of yard waste about a year before this study began. It is not necessarily typical of other communities.
The community supplied data on actual tonnages collected by each truck in each of the three separate collection programs; the number of trucks operated and the number of miles traveled by each truck; and the fuel consumption on each route. Fuel use per ton of material picked up on each route was lowest for collecting household and commercial MSW. About 2.5 times more fuel was used to pick up a ton of separated recyclables, and about 600 times more fuel was used to collect a ton of yard waste (because of the small quantities collected on each route in that program).
To develop the estimates presented in this summary, these fuel use rates were converted to energy use per ton of MSW at the curb, and then apportioned according to the amounts set out. The energy and emissions results are extremely sensitive to the amount collected by each truck. Therefore, energy use per ton of material collected increases as additional curbside collection programs are implemented.
No direct emission measurements for MSW collection or curbside collection vehicles have been made during actual operation. Emissions from collection and transportation were therefore estimated on the basis of the actual fuel use by assuming that the emissions per unit of fuel met the maximum permissible emission limits for heavy-duty diesel truck engines operating according to a specified EPA procedure that simulates freeway and city ddving. When these engine limits have been compared to actual emissions from vehicles under the same load and speed conditions, the results vary by 20-50% for emissions of different types; for example, the operating vehicles emit larger quantities of hydrocarbons and particulates, but smaller amounts of nitrogen oxides and carbon monoxide than the tested engines. The duty cycle of the MSW packer trucks in these tests is quite different, in terms of stop-start frequency and compactor operation, from the typical duty cycle for the trucks modeled by the EPA. Therefore, in developing emissions estimates for this study, the emissions limits were increased by a factor of four to provide a better approximation of actual emissions.
Status of the Major Waste Management Options
Open landfills have been used as a waste management method for centuries. Rules and regulations for construction and operation of solid waste landfills were established by the Resource Conservation and Recovery Act (RCRA) of 1976 as a way to reduce the number of open dumps common at the time. Since then, landfill requirements have become more stringent. Careful enclosure of MSW, by providing liners underneath it, covering the landfill with dirt ("daily cover") at the end of each day, installing gas collection systems, and capping the landfill when it is filled, permits the collection of between 30% and 85% of the methane, carbon dioxide, and other organic gases generated by the waste. Those gases can be burned for energy recovery if the quantity generated is large enough to justify the expense of the equipment. More than 100 landfills recover landfill gas for energy. The majority produce electricity, but in a few locations, the gas is used for process heat, or it is upgraded to pipeline quality and sold.
Although only about 160 of the nation's approximately 6,000 operating landfills are operating or plan to operate landfill gas-toenergy plants, the energy and emissions data in this report are based on landfill with gas recovery. The largest landfills (about 200 have a capacity of more than 1,000 tons per day) are more likely to include the energy recovery facilities, and those landfills now receive more than 40% of all MSW landfilled in the United States. In comparison with facilities that either collect landfill gas and flare it or allow the gas to escape into the atmosphere, landfill gas-to-energy operations reduce environmental releases of methane while providing an energy benefit.
Most landfills reach capacity because they fill up or reach practical height limits, rather than by reaching a weight limit. Therefore, efforts to reduce the amount of space that MSW occupies can extend the life of a landfill. Combustion and recycling programs can help to reduce waste volume. Other options include:
Like landfilling, open burning has been used for centuries to dispose of waste. In the United States, combustion of MSW to recover energy in the form of saleable electricity was first practiced in about 1902, in New York City.
Many newer plants now recover energy. In modern plants, energy can be recovered in the form of hot water, steam, and electricity, or in some combination of those three forms. Until the 1970s, MSW combustors included little, if any, air pollution control equipment. The units of the 1950s and 1960s were generally marked by bad odors and smoke. They were primarily operated only to reduce the volume of the waste. Since the early 1970s, increasingly stringent environmental controls have been applied; as a result, today's combustors produce less air pollution.
Two options commonly used for combustion are:
In a mass burn facility, pretreatment of the MSW includes inspection and simple separation to remove oversized and noncombustible items and unacceptable components such as obviously hazardous or explosive materials. The MSW is then fed into a combustor, where it is typically supported on a grate or hearth. Air is fed below and above the grate to promote combustion. Mass burn plants can be large facilities, with capacities of 3,000 tons of MSW per day or more; however, they can be scaled down to handle the waste from smaller communites, and modular plants with capacities as low as 25 tons per day have been built.
RDF production begins with inspection of the MSW, removal of buLtry or hazardous waste, and shredding of the remaining MSW. Noncombustible materials are often separated as well. The shredded RDF is most frequently burned above a traveling grate. RDF preparation and direct firing cannot be performed economically in small plants, and the minimum size of an RDF plant tends to be large. If RDF is compressed into pellets or cubes, it can be used in existing, conventional furnaces with grates. A few operating facilities now produce such pellets or cubes at one location for sale or use at another.
The energy produced by both mass burning and RDF combustion is generally used for electrical power generation. MSW combustion can thus eliminate the need to mine, burn, and dispose of the residue of some of the coal or oil that would otherwise be used to generate electricity.
Regulatory requirements for control of MSW combustion have grown increasingly stringent since they were first implemented in the 1970s. For both types of options, federal regulations governing all facilities with capacities greater than 2S0 tons per day set limits on a range of pollutants, including acid gases, metals, and dioxins/ furans. The EPA is developing comparable requirements for units with capacities of less than 250 tons per day. State and local requirements may be more stringent and may apply to even smaller combustors. Current regulations for the larger plants are more stringent than those governing fossil fuel plants.
The ash from MSW combustion and the residue from the scrubber (used to neutralize acid gases in the gas stream) are disposed of, often in landfills called "ash monofills'' that contain only ash. Modem plants using good combustion practices can reduce the volume of MSW by up to 90%. The leachate from ash monofills is normally smaller in volume than that from ordinary landfills, and the constituents of the leachate are also different.
Curbside Separation and Mixed Waste Separation and Recycling
Curbside separation and mixed waste separation and recycling permit a reduction in the amount of waste that must be handled by other MSW options. As outlined previously under "Methodology," the five steps in recycling are: (1) separating reusable materials from other waste; (2) transporting and processing (including remanufacturing) the separated materials for use as replacements for virgin materials; (3) managing the wastes from separation and recycling; (4) returning the materials to commerce; (5) selling the recycled products. At present, most recycling efforts focus on the following reusable materials: newsprint, cardboard, glass, aluminum, some tin cans, and some plastics (particularly plastic beverage containers).
Some of the statistics that indicate that recycling now manages 10% or more of the nation's MSW are reporting estimates that include the amounts of material diverted from the local landfill by separate collection of recyclables, bottle deposit laws, and separate collection of yard waste for composting. Data on the amounts of MSW that are finally remanufactured and returned to commerce have not been found; however, they are clearly lower than the total quantities collected because some of the material is used as fuel, some is lost during remanufacturing, and when market conditions are poor, some may be landfilled.
Communities that wish to include recycling in their MSW management strategies have several options for separating recyclables from other waste. They can offer convenient sites where residents can receive payment for containers (e.g., buy-back centers); provide dropoff centers that may accept a wide range of recyclable and compostable materials; implement curbside collection of recyclable materials separated by residents from other MSW; and/or process mixed waste to separate recyclables.
Either mixed MSW collected in a standard packer truck or recyclables collected separately at curbside can be sent to a materials recovery facility (MRF) for further separation and consolidation of the collected materials. MRFs can be divided into "low-tech" and "high-tech" facilities, depending on the amount of manual labor required. All MRFs rely heavily on manual labor to sort and separate grades of paper and glass bottles by color, and plastic bottles by resin type and color. Nearly all MRFs also use magnets for recovering ferrous metals, and many use balers for paper, crushers for glass, and flatteners for the aluminum cans. High-tech MRFs would generally also use additional shredders, screens, possibly air classifiers for separating heavy materials from lighter ones, and special eddy-current separators that can separate aluminum. Currenty operating MRFs have sufficient design capacity to process 1 million tons per year of recyclables. Another 3 million tons of capacity are scheduled to begin operation by 1993. If all the planned facilities actually become operational, they will have the annual capacity to process 2% of all U.S. MSW in 1993.
Many communities conduct curbside collection programs for recyclables but do not operate MRFs. No data on collection rates for those programs were found. Returning materials to beneficial use and finding markets for recycled product may present difficulties. Recent rapid growth in collection and separation programs has combined with a generally sluggish economy to drive down the prices paid for recyclable materials. Markets for waste paper have traditionally been highly volatile.
Composting is biological conversion of organic matter. As part of an MSW management strategy, communities can choose from two types of composting programs:
Yard waste composting is typically a relatively simple, open-air process. An optional first step is to "chip" the yard waste to reduce its size and promote the breakdown of organic matter. It is then set out in long piles that are periodically turned over to expose all the material to air. Alternatively, the piles can be placed on a porous pad that is connected to a blower to supply air.
MSW composting begins with separating the organic materials from the rest of the waste and shredding or grinding the organics (the remaining MSW, about 50% of the total, is usually landfilled). In some cases, the organics are then intially composted inside a vessel that provides mechanical agitation and forced aeration; in other cases, composting takes place entirely in the open. Enclosed composting can help to control odors through better control of aeration and temperature. In all cases, composting in a vessel is followed by additional open air composting.
Although composting has so far made a small contribution to managing MSW on a national scale, it could theoretically be used to process at least the 18-20% of MSW that is yard waste. About 1,400 composting programs are operating in the United States, but at least 500 of them are seasonal programs for leaves only. Only 16 operating plants compost an organic fraction of MSW, and 4 of those add sewage sludge. The number of operational composting facilities changes frequently. Compost made from MSW is more likely to be contaminated than compost made from separately collected yard waste, and commercial markets for MSW-derived compost are difficult to find. The compost made in some MSW composting plants ends up in landfills.
Status of the Less Common Options
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