Landfill
The term sanitary landfill was first used in the 1930s to refer to the compacting of solid waste materials. Initially adopted by New York City and Fresno, California, the sanitary landfill used heavy earth-moving equipment to compress waste materials and then cover them with soil. The practice of covering solid waste was evident in Greek civilization over 2,000 years ago, but the Greeks did it without compacting.
Today, the sanitary landfill is the major method of disposing of waste materials in North America and other developed countries, even though considerable efforts are being made to find alternative methods, such as recycling and composting. Among the reasons that landfills remain a popular alternative are their simplicity and versatility. For example, they are not sensitive to the shape, size, or weight of a particular waste material. Since they are constructed of soil, they are rarely affected by the chemical composition of a particular waste component or by any collective incompatibility of co-mingled wastes. By comparison, composting and incineration require uniformity in the form and chemical properties of the waste for efficient operation.
About 80 % of the solid waste generated in the United States still is dumped in landfills. In a sanitary landfill, refuse is compacted each day and covered with a layer of dirt. This procedure minimizes odor and litter, and discourages insect and rodent populations that may spread disease. Although this method does help control some of the pollution generated by the landfill, the fill dirt also occupies up to 20% of the landfill space, reducing its waste-holding capacity. Sanitary landfills traditionally have not been enclosed in a waterproof lining to prevent leaching of chemicals into groundwater, and many cases of groundwater pollution have been traced to landfills.
Historically, landfills were placed in a particular location more for convenience of access than for any environmental or geological reason. Now more care is taken in the siting of new landfills. For example, sites located on faulted or highly permeable rock are passed over in favor of sites with a less-permeable foundation. Rivers, lakes, floodplains, and groundwater recharge zones are also avoided. It is believed that the care taken in the initial siting of a landfill will reduce the necessity for future clean-up and site rehabilitation. Due to these and other factors, it is becoming increasingly difficult to find suitable locations for new landfills. Easily accessible open space is becoming scarce and many communities are unwilling to accept the siting of a landfill within their boundaries. Many major cities have already exhausted their landfill capacity and must export their trash, at significant expense, to other communities or even to other states and countries.
A landfill has three stages of decomposition. The first one is an aerobic phase. The solid wastes that are biodegradable react with the oxygen in the landfill and begin to form carbon dioxide and water.Temperature during this stage of decomposition in the landfill rises about 30°F (16.7°C) higher than the surrounding air. A weak acid forms within the water and some of the minerals are then dissolved. The next stage is the anaerobic one in which microorganisms that do not need oxygen break down the wastes into hydrogen, ammonia, carbon dioxide, and inorganic acids.
In the third stage of decomposition in a landfill, methane gas is produced. Sufficient amounts of water and warm temperatures have to be present in the landfill for the microorganisms to form the gas. About half of the gas produced during this stage will be carbon dioxide, but the other half will be methane. Systems of controlling the production of methane gas are either passive or active. In a passive system, the gas is naturally vented into the atmosphere, and may include venting trenches, cutoff walls, or gas vents to direct the gas. An active system employs a mechanical method to remove the methane gas and can include recovery wells, gas collection lines, a gas burner, or a burner stack. Both active and passive systems have monitoring devices to prevent explosions or fires.
While landfills may outwardly appear simple, they need to operate carefully and follow specific guidelines that include where to start filling, wind direction, the type of equipment used, method of filling, roadways to and within the landfill, the angle of slope of each daily cell, controlling contact of the waste with groundwater, and the handling of equipment at the landfill site.
Considerations have to be made regarding the soil that is used as a daily cover, which is usually 6 in (15.2 cm) thick, an intermediate cover of 1 ft (30.5 cm), and a final cover of 2 ft (61 cm). The compacting of the solid waste and soil has to be considered as well, so that the biological processes of decomposition can take place properly.
Shredding of solid wastes is one method of saving space at landfills. Another method is baling of wastes. The advantages to shredding are twofold. The material can be compacted to a greater density, thereby extending the life of the landfill, and it can be compacted more quickly as well. Less cover is required and there is also less danger of spontaneous fire. Landfills using shredded materials produce more organic decomposition than those disposing of unshredded solid wastes. The advantages of baling are an increase in landfill life because of an increase in waste density. Hauling times are reduced, as are litter, dust, odor, fires, traffic, noise, earth moving, and land settling. Less heavy equipment is needed for the cover operation and the amount of time it takes for the land to stabilize is reduced.
When the secure landfill reaches capacity, it is capped by a cover of clay, plastic, and soil, much like the bottom layers. Vegetation is planted to stabilized the surface and make the site more attractive. Sump pumps collect any fluids that filter through the landfill either from rain water or from waste leakage. This liquid is purified before it is released. Monitoring wells around the site ensure that the groundwater does not become contaminated. In some areas where the water table is particularly high, above-ground storage may be constructed using similar techniques. Although such facilities are more conspicuous, they have the advantage of being easier to monitor for leakage.
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