You have just finished your meal at a fast food restaurant and you throw your uneaten food, food wrappers, drink cup, utensils and napkins into the trash can. You don't think about that waste again. On trash pickup day in your neighborhood, you push your can out to the curb, and workers dump the contents into a big truck and haul it away. You don't have to think about that waste again, either. But maybe you have wondered, as you watch the trash truck pull away, just where that garbage ends up.
Americans generate trash at an astonishing rate of four pounds per day per person, which translates to 600,000 tons per day or 210 million tons per year! This is almost twice as much trash per person as most other major countries. What happens to this trash? Some gets recycled or recovered and some is burned, but the majority is buried in landfills. In this edition of How Stuff Works, we will examine how a landfill is made, what happens to the trash in landfills, what problems are associated with a landfill and how these problems are solved.
Private haulers drop off trash at a landfill.
How Much Trash is Generated?
Of the 210 million tons of trash, or solid waste, generated in the United States annually about 56 million tons, or 27 percent, is either recycled (glass, paper products, plastic, metals) or composted (yard waste). The remaining trash is discarded and is made up of mostly paper products, plastics, yard waste and wood (Figure 1).
How is Trash Disposed?
The trash production in the United States has almost tripled since 1960 (Figure 2). This trash is handled in various ways. About 27 percent of the trash is recycled or composted, 16 percent is burned and 57 percent is buried in landfills. The amount of trash buried in landfills has doubled since 1960. The United States ranks about in the middle of the major countries (United Kingdom, Canada, Germany, France and Japan) in landfill disposal. The United Kingdom ranks highest, burying about 90 percent of its solid waste in landfills.
What is a Landfill?
There are two ways to bury trash:
- Dump - an open hole in the ground where trash is buried and that has various animals (rats, mice, birds) swarming around. (This is most people's idea of a landfill!)
- Landfill - carefully designed structure built into or on top of the ground in which trash is isolated from the surrounding environment (groundwater, air, rain). This isolation is accomplished with a bottom liner and daily covering of soil.
- Sanitary landfill - landfill that uses a clay liner to isolate the trash from the environment
- Municipal solid waste (MSW) landfill - uses a synthetic (plastic) liner to isolate the trash from the environment
Trash is being loaded and compacted in a landfill.
The purpose of a landfill is to bury the trash in such a way that it will be isolated from groundwater, will be kept dry and will not be in contact with air. Under these conditions, trash will not decompose much. A landfill is not like a compost pile, where the purpose is to bury trash in such a way that it will decompose quickly.
Proposing the Landfill
For a landfill to be built, the operators have to make sure that they follow certain steps. In most parts of the world, there are regulations that govern where a landfill can be placed and how it can operate. The whole process begins with someone proposing the landfill.
In the United States, taking care of trash and building landfills are local government responsibilities. Before a city or other authority can build a landfill, an environmental impact study must be done on the proposed site to determine:
First, it must be determined if there is sufficient land for the landfill. To give you an idea how much land is needed for a landfill, we'll use the example of a site we visited, the North Wake County Landfill in Raleigh, North Carolina. This site has both a sanitary landfill, which was closed in 1997, and a working MSW landfill. The site takes up 230 acres of land, but only 70 acres is dedicated to the actual landfill. The remaining land is for the support areas (runoff collection ponds, leachate collection ponds, drop-off stations, areas for borrowing soil and 50- to 100-foot buffer areas).
- the area of land necessary for the landfill
- the composition of the underlying soil and bedrock
- the flow of surface water over the site
- the impact of the proposed landfill on the local environment and wildlife
- the historical or archaeological value of the proposed site
Second, the composition of the underlying soil and bedrock must be determined. The rocks should be as watertight as possible to prevent any leakage from reaching groundwater. The bedrock must not be fractured or you cannot predict where wastes might flow. You would not want the site near mines or quarries because these structures frequently contact the groundwater supply. At the same time, you must be able to sink wells at various points around the site to monitor the groundwater or to capture any escaping wastes.
Third, the flow of water over the area must be studied. You do not want excess water from the landfill draining on to neighboring property or vice versa. Similarly, you do not want the landfill to be close to rivers, streams or wetlands so that any potential leakage from the landfill will not enter the groundwater or watershed.
Fourth, you need to determine the potential effects of the landfill and possible contamination on local wildlife. For example, you would not want to locate it near nesting areas of local or migrating birds. You would want to avoid local fisheries, too.
Finally, if the site contains any historical or archaeological artifacts, you would not want to build a landfill there.
Once the environmental impact study has been completed, permits must be obtained from the local, state and federal governments. In addition, money will have to be raised from taxes or municipal bonds to build and operate the landfill. The North Wake County Landfill cost about $19 million to build and was paid for through municipal bonds. Because funding usually comes from some public source, public approval must be obtained through local governments or a referendum.
Building the Landfill
Once the environmental impact study is complete, the permits are granted and the funds have been raised, construction begins. First, access roads to the landfill site must be built if they do not already exist. These roads will be used by construction equipment, sanitation services and the general public. After roads have been built, then the landfill can be excavated. In the North Wake County Landfill, the landfill began 10 feet below the road surface.
Parts of a Landfill
Figure 3. This cross-section drawing shows the structure of a municipal solid waste landfill. The arrows indicate the flow of leachate.
The basic parts of a landfill, as shown in Figure 3, are:
Each of these parts is designed to address specific problems that are encountered in a landfill. So, as we discuss each part of the landfill, we'll explain what problem is solved.
- Bottom liner system - separates trash and subsequent leachate from groundwater
- Cells (old and new) - where the trash is stored within the landfill
- Storm water drainage system - collects rain water that falls on the landfill
- Leachate collection system - collects water that has percolated through the landfill itself and contains contaminating substances (leachate)
- Methane collection system - collects methane gas that is formed during the breakdown of trash
- Covering or cap - seals off the top of the landfill
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 prevents the trash from coming in contact with the outside soil, particularly the groundwater. In MSW landfills, the liner is usually some type of durable, puncture-resistant synthetic plastic (polyethylene, high-density polyethylene, polyvinylchloride). It is usually 30-100 mils thick. The plastic liner may be also be combined with compacted clay soils as an additional liner. The plastic liner may also be surrounded on either side by a fabric mat (geotextile mat) that will help to keep the plastic liner from tearing or puncturing from the nearby rock and gravel layers.
Cells (Old and New)
Perhaps, the most precious commodity and overriding problem in a landfill is air space. The amount of space is directly related to the capacity and usable life of the landfill. If you can increase the air space, then you can extend the usable life of the landfill. To do this, trash is compacted into areas, called cells, that contain only one day's trash. In the North Wake County Landfill, a cell is approximately 50 feet long by 50 feet wide by 14 feet high (15.25m x 15.25m x 4.26m). The amount of trash within the cell is 2,500 tons and is compressed at 1,500 pounds per cubic yard! This compression is done by heavy equipment (tractors, bulldozers, rollers and graders) that go over the mound of trash several times). Once the cell is made, it is covered with six inches of soil and compacted further. Cells are arranged in rows and layers of adjoining cells (lifts).
A bulldozer prepares a new cell in a landfill
In addition to compressing the trash into cells, space is conserved by excluding bulky materials, such as carpets, mattresses, foam and yard waste, from the landfill.
Storm Water Drainage
It is important to keep the landfill as dry as possible to reduce the amount of leachate. This can be done in two ways:
- Exclude liquids from the solid waste. Solid waste must be tested for liquids before entering the landfill. This is done by passing samples of the waste through standard paint filters. If no liquid comes through the sample after 10 minutes, then the trash is accepted into the landfill.
- Keep rainwater out of the landfill.
To exclude rainwater, the landfill has a storm drainage system. Plastic drainage pipes and storm liners collect water from areas of the landfill and channel it to drainage ditches surrounding the landfill's base.
This storm drainage pipe empties into a drainage ditch.
Drainage ditches run along the base of a landfill. The black pipe carries landfill gas to a pumping station.
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.
This collection pond is for catching storm water. The black liner helps channel the water and protect the underlying cells.
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 much as water percolates through ground coffee in a drip coffee maker. As the water percolates through the trash, it picks up contaminants (organic and inorganic chemicals, metals, biological waste products of decomposition) just as water picks up coffee in the coffee maker. This water with the dissolved contaminants is called leachate and is typically acidic.
To collect leachate, perforated pipes run throughout the landfill (Figure 3). These pipes then drain into a leachate pipe, which carries leachate to a leachate collection pond. Leachate can be pumped to the collection pond or flow to it by gravity, as it does in the North Wake County Landfill.
A leachate collection pond is designed to catch the contaminants that can get into water that goes through the trash in a landfill.
The leachate in the pond is tested for acceptable levels of various chemicals (biological and chemical oxygen demands, organic chemicals, pH, calcium, magnesium, iron, sulfate and chloride) and allowed to settle. After testing, the leachate must be treated like any other sewage/wastewater; the treatment may occur on-site or off-site. At the North Wake County Landfill, leachate is released to the wastewater treatment plant in Raleigh, where it is treated and released into the Neuse River. Some landfills recirculate the leachate and later treat it. This method reduces the volume of leachate from the landfill, but increases the concentrations of contaminants in the leachate.
Methane Collection System
Bacteria in the landfill 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. This presents a hazard because the methane can explode and/or burn. So, the landfill gas must be removed. To do this, a series of pipes are embedded within the landfill to collect the gas. In some landfills, this gas is vented or burned.
A methane collection pipe helps capture the hazardous gas.
A methane "flare" is used for burning landfill gas.
More recently, it has been recognized that this landfill gas represents a usable energy source. The methane can be extracted from the gas and used as fuel. In the North Wake County Landfill, a company collects the landfill gas, extracts the methane, and sells it to a nearby chemical company to power its boilers. The extraction system is a split system, meaning that methane gas can go to the boilers and/or the methane flares that burn the gas. The reason for the split system is that the landfill will increase its gas production over time (from 300 cubic feet per minute to 1,250 cubic feet per minute) and exceed the capacity of the boilers at the chemical company. Therefore, the excess gas will have to be burned. It is not cost-effective to compress the excess gas to liquid and sell it.
Covering or Cap
As mentioned above, each cell is covered daily with six inches of compacted soil. This covering seals the compacted trash from the air and prevents pests (birds, rats, mice, flying insects, etc.) from getting into the trash. This soil takes up quite a bit of space. Because space is a precious commodity, many landfills are experimenting with tarps or spray coverings of paper or cement/paper emulsions. These emulsions can effectively cover the trash, but take up only a quarter of an inch instead of 6 inches!
An experimental tarp provides daily cover of the landfill cells.
When a section of the landfill is finished, it is covered permanently with a polyethylene cap (40 mil). The cap is then covered with a 2-foot layer of compacted soil. The soil is then planted with vegetation to prevent erosion of the soil by rainfall and wind. The vegetation consists of grass and kudzu. No trees, shrubs or plants with deep penetrating roots are used so that the plant roots do not contact the underlying trash and allow leachate out of the landfill.
Grass and other plants cover the municipal solid waste landfill.
Occasionally, leachate may seep through weak point in the covering and come out on to the surface. It appears black and bubbly. Later, it will stain the ground red. Leachate seepages are promptly repaired by excavating the area around the seepage and filling it with well-compacted soil to divert the flow of leachate back into the landfill.
Seepage of leachate (black) can be seen through a weak spot in the cover.
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 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.
A groundwater monitoring pipe stands in the center. The two yellow markers on either side make it more visible so that equipment operators will not run into the monitoring station.
What Happens to Trash in a Landfill?
Trash put in a landfill will stay there for a very long time. Inside a landfill, there is little oxygen and little moisture. Under these conditions, trash does not break down very rapidly. In fact, when old landfills have been excavated or sampled, 40-year-old newspapers have been found with easily readable print. Landfills are not designed to break down trash, merely to bury it. When a landfill closes, the site, especially the groundwater, must be monitored and maintained for up to 30 years!
Even after a landfill is closed, the trash buried there will remain.
How is a Landfill Operated?
A landfill, such as the North Wake County Landfill, must be open and available every day. Customers are typically municipalities and construction/demolition companies, although residents may also use the landfill. A layout of a typical landfill with supporting structures is shown in Figure 4.
Figure 4. This overview shows the supporting stations and structures of a landfill. The drawing is based on the structure of the North Wake County Landfill in Raleigh, North Carolina.
Near the entrance of the the site is a recycling center where residents can drop off recyclable materials (aluminum cans, glass bottles, newspapers, blend paper, corrugated cardboard). This helps to reduce the amount of material in the landfill. Some of these materials are banned from landfills by law because they can be recycled.
Residents drop off waste at the landfill.
Large trucks transfer municipal solid wastes from all parts of the county to the landfill.
As customers enter the site, their trucks are weighed at the scale house. Customers are charged tipping fees for using the site. The tipping fees vary from $10 to $40 per ton. These fees are used to pay for bonds or operation costs. The North Wake County Landfill has an operating budget of approximately $4.5 million, and part of that comes from tipping fees.
Along the site, there are drop-off stations for materials that are not wanted or legally banned by the landfill. A multi-material drop-off station is used for tires, motor oil, lead-acid batteries and drywall. Some of these materials can be recycled.
Here is a multi-material drop-off station, where residents can dispose of hazardous household wastes.
In addition, there is a household hazardous waste drop-off station for chemicals (paints, pesticides, other chemicals) that are banned from the landfill. These chemicals are disposed by private companies. Some paints can be recycled and some organic chemicals can be burned in incinerators or power plants.
Other structures alongside the landfill are the borrowed area that supplies the soil for the landfill, the runoff collection pond, leachate collection ponds, and methane station.
Landfills are complicated structures that, when properly designed and managed, serve an important purpose. In the future, new technologies called bioreactors will be used to speed the breakdown of trash in landfills and produce more methane.
Lots More Information!
Our thanks to Marshall Parrish, education specialist, and Ivan Dickey, solid waste inspector, of Wake County Solid Waste Management Division, Raleigh, N.C. for the tour of the landfill and their advice in the preparation of this article.