6. SANITARY LANDFILLS
The major purpose of sanitary landfills, which are the most common waste management technology employed in the United States(1), is the storage of MSW in a way that protects human health and the environment. This section reviews two types of landfills:
All MSW management technologies addressed in this report require landfills for their residues, and the amount of that residue is affected by the technologies used to extract materials and energy from the waste. Even recycling requires landfills to dispose of: the impurities separated in materials recovery facilities (MRF) or later at smelters (i.e., slag); paper sludge containing fiber that is too short for reuse; fillers and inks; or small pieces of mixed-color glass. In general, recycling delays and/or reduces requirements for land disposal rather than eliminating them.
- MSW landfills, which contain MSW as discarded and the residues that remain after various other MSW management technologies are applied. Although some MSW landfills also receive ash, that ash is increasingly kept in separate monofills.
- Ash landfills, a type of monofill, which are limited to the ash that remains after combustion of waste.
Design of Sanitary Landfills
A new sanitary landfill is subject to many design regulations set by the U.S. Environmental Protection Agency (EPA) under "Guidelines for the Land Disposal of Solid Waste" (CFR, l991b), by states, and by local communizes. All new landfills that accept more than 20 tons of MSW per day are subject to requirements for controlling emissions to groundwater. Because all landfills that contain wet organic material produce methane, the largest of such landfills are likely to have to meet additional existing and proposed requirements for controlling emissions into the air.
State-of-the-art landfills incorporate a liner system, a leachate collection system, a leachate treatment system, a cap system, gas recovery systems to recover energy or to flare the gas, landscaping, security, groundwater monitoring wells, and a groundwater plan. The landfills require about 30 years of postclosure monitoring, care, and planning for eventual community use.
One of the most critical parts of the design is the liner at the bottom of the landfill (CEC, 1991). Figure 6.1 illustrates a design that conforms with current regulations. Regulations require that during operations the new MSW must be compacted and covered daily with an inert material that prevents litter from blowing and from providing a refuge for animals and insects. A cross-section of an approved liner design is shown in Figure 6.2. Appendix F provides a more detailed description of the requirements for constructing and operating landfills.
All new landfills that accept more than 20 tons per day of MSW must, in accordance with EPA regulations, collect and dispose of leachate and minimize the infiltration of water. Reducing water content retards biodegradation, and as a result the conversion of organic waste to methane and CO2 is inefficient. Rathje indicates that landfills preserve waste for future generations (Rathje, 1989; Rathje, 1990), and Bogner and Spokas report preliminary evidence that landfills are providing a sink for carbon by removing it from the atmospheric CO2 cycle (Bogner and Spokas, 1992).
CROSS-SECTION OF COMPOSITE LINER AND LEACHATE COLLECTION SYSTEM
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Landfilling combines preservation of waste, low-temperature partial oxidation and reduction through biological activity, and limited dissolution of components in the waste. Traditional landfilling operations consist of a daily cycle of filling, compacting the fill with heavy equipment, and covering the fill with "earthen materials" (CFR, l991c).
Variations on this traditional method, as summarized below, are also being considered (see Appendix F for more detailed descriptions):
- Shredfill-If MSW is shredded before land filling, landfill density is increased by about 25%, fire hazards are reduced, and less leachate is produced because of the smaller volume and surface area of the shredfill compared with a normal landfill. With this method, often no daily cover is needed for litter or rodent control. Shredding is expensive, however. Two shredfill plants are now operating .
- Balefill-An alternative means of increasing the density of MSW entails baling the refuse before placing it in a landfill. When baling is feasible, density can be increased to as much as 1,700 pounds per cubic yard (compared with 1,250-1,300 pounds per cubic yard for a normally compacted landfill). Other advantages of this method include reduction of litter, dust, rodents, and leachate. If baling is handled at a separate location, traffic to the landfill could be reduced. When a landfill consisting of baled refuse is full, it can support light industrial buildings after 2 years of stabilization [2711; up to 50 years may be needed before a standard landfill can be returned to unrestricted use (Vesilind, in press). About 40 balefill installations are operating in the United States .
A new balefill technology, which is in use in Japan, Italy, Belgium, and the United Kingdom, but not in the United States, provides compression pressures ranging from 1,400 to 2,800 pounds per square inch and makes cubes 1 yard on a side that contain 2,000 pounds of MSW. It is reported that the cubes can be coated with concrete and used as construction material .
- Reusable or mining landfills-In response to difficulties in siting new waste management facilities, some communities are considering efforts to reuse old, filled landfills by mining out the old waste, separating the degraded portion from what some sources call the "fuel fraction," and adding new MSW to the reconstructed landfill. The degraded refuse is reused as daily cover. One advantage is that the costs of closure and postclosure care are avoided. New York State has identified up to 400 landfills for which mining could be considered (Thornloe, 1991). The technology works best if the new landfill contains only readily biodegradable materials and the biodegradation is accelerated, or "stimulated," by recirculating leachate into the landfill(2). With stimulation, some estimates indicate that a landfill could be mined and reused again after 910 years have passed.
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