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.

Landfills are not just mountains of trash - they use latest technology to keep the community as safe as possible from the dangers decomposing garbage can pose to the environment.

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. Let’s 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.

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

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.

Parts of a Landfill

The basic parts of a landfill, as shown below:

• 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
  
  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
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 a typical 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).

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.

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 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. 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.

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.

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. At the Cra;zy Horse Landfill in Salinas, a company collects the landfill gas, extracts the methane, and sells it to a nearby power company.

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!

Groundwater Monitoring
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.