How Landfills Work
Modern landfills aren’t all identical in design, but most utilize similar technologies, though the exact sequence and type of materials used may different from site to site [source: WM.com]. Some basic parts of a landfill, as shown in the image above, include:
- plastic liners (C): separate trash and subsequent leachate from groundwater
- cells (I and J): where the trash is stored within the landfill
- stormwater drainage systems (G): collect rainwater that falls on the landfill
- leachate collection systems (D and K): collect water that has percolated through the landfill itself and contains contaminating substances (leachate)
- methane collection systems: collect methane gas that is formed during the breakdown of trash
- covering or caps: seal off the top of the landfill
Each of these parts is designed to address specific problems in a landfill.
So, as we discuss each part of the landfill, we’ll explain what problem is solved.
Bottom Liner System
A landfill’s major purpose and one of its biggest challenges is to contain the trash so that the trash doesn’t cause problems in the environment. The bottom liner, made of thick plastic, prevents the trash from coming in contact with the outside soil, particularly the groundwater [source: WM.com].
Trash is compacted by heavy equipment into areas, called cells, which typically contain a day’s worth of refuse to get the most use of the volume of space in the landfill. Once the cell is made, it is covered with 6 inches (15 centimeters) of soil and compacted further [source: Bolton].
To keep rainwater out, a landfill has a storm drainage system to route the runoff into drainage ditches and away from the buried trash. Concrete, plastic or metal culverts underneath nearby roads and stormwater basins, which can reduce the suspended sediment in the water to minimize soil loss from the landfill, are other parts of the system [source: Uteir].
Plastic drainage pipes and storm liners collect water from areas of the landfill and channel it to drainage ditches surrounding the landfill’s base. The ditches are either concrete or gravel-lined and carry water to collection ponds to the side of the landfill. In the collection ponds, suspended soil particles are allowed to settle and the water is tested for leachate chemicals. Once settling has occurred and the water has passed tests, it is then pumped or allowed to flow off-site.
Leachate Collection System
No system to exclude water from the landfill is perfect and water does get into the landfill. The water percolates through the cells and soil in the landfill similar to how water percolates through ground coffee in a drip coffee maker. As the water trickles through the trash, it picks up contaminants. This water with contaminants is called leachate and is typically acidic.
To collect leachate, perforated pipes run throughout the landfill. These pipes then drain into a leachate pipe, which carries leachate to a leachate collection pond [source: Austin Community Landfill].
Methane Collection System
Bacteria break down the trash in the absence of oxygen (anaerobic) because the landfill is airtight. A byproduct of this anaerobic breakdown is landfill gas, which contains approximately 50 percent methane and 50 percent carbon dioxide with small amounts of nitrogen and oxygen.
Methane is a serious issue for landfills because it’s a potent greenhouse gas, some 28 to 36 times more effective than carbon dioxide at trapping heat in the atmosphere. And landfills are the third-largest source of methane emissions in the U.S., accounting for about 15 percent of the gas that escaped into the atmosphere in 2019 [source: EPA]. Methane also is a potential safety hazard, since methane can explode and burn [source: NY Department of Health].
Covering or Cap
Putting down a covering of compacted soil seals the trash from the air and prevents pests (birds, rats, mice, flying insects, etc.) from getting into the trash. At New York’s Fresh Kills Landfill, trash is covered with at least 2 feet (0.61 meters) of soil, graded between 4 and 33 percent to help with stormwater drainage. That layer is topped by additional layers of synthetic fabric and plastic and a layer of soil to allow vegetation to grow atop the landfill [source: Freshkills Park Alliance].
At many points surrounding the landfill are groundwater monitoring stations. These are pipes that are sunk into the groundwater so water can be sampled and tested for the presence of leachate chemicals. The temperature of the groundwater is also measured. Because the temperature rises when solid waste decomposes, an increase in groundwater temperature could indicate that leachate is seeping into the groundwater. Also, if the pH of the groundwater becomes acidic, that could indicate seeping leachate [source: EPA].
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