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A landfill is a carefully designed structure built into or on top of the ground in which trash or waste is isolated from the surrounding environment. This is done with a bottom liner and a daily covering of soil.  There are many parts to a landfill. The bottom liner is only one part. The liner separates trash and subsequent leachate from ground water. The liner can be clay or synthetic (plastic). The liner effectively creates a bathtub in the ground. If the bottom liner fails, waste will migrate directly into the environment. This may be dangerous or cause certain hazards. Natural clay is often fractured and cracked. A mechanism called diffusion will move organic chemicals like benzene through a three-foot thick clay landfill liner in approximately five years. Some chemicals can degrade clay.

The very best landfill liners today are made of a tough plastic film called high density polyethylene (HDPE). A number of household chemicals will degrade HDPE, permeating it (passing through it), making it lose its strength, softening it, or making it become brittle and crack. Not only will household chemicals, such as moth balls, degrade HPDE, but much more benign things can cause it to develop stress cracks, such as, margarine, vinegar, ethyl alcohol (booze), shoe polish, and peppermint oil. A composite liner is a single liner made of two parts, a plastic liner and compacted soil, usually clay soil. Reports show that all plastic liners will have leaks. It is important to realize that all materials used as liners are at least slightly permeable to liquids or gases and a certain amount of permeation through liners should be expected.. Additional leakage results from defects such as cracks, holes, and faulty seams. Studies show that a 10-acre landfill will have a leak rate somewhere between 0.2 and 10 gallons per day.

Cells are another part of a landfill. Cells are old and new. This is where the trash is stored within the landfill. 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 a landfill. To do this, trash is compacted into cells. Cells contain only one day's trash. 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 that goes 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. In addition to compressing the trash into cells, space is conserved by excluding bulky materials, such as carpets, mattresses, foam and yard waste. 

The storm water drainage system is another part of a landfill. It collects rain water that falls on the landfill. It is important to keep the landfill as dry as possible to reduce the amount of leachate. This is done in two ways, excluding liquids from the solid waste or keeping the rainwater out of the landfill.

Another part is the leachate collection system. This collects water that has percolated through the landfill itself and contains contaminated substances. 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 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 collection pond. Leachate can be pumped to the collection pond or flow to it by gravity. 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. 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.

Another part of the landfill is the methane collection system. This collects methane gas that is formed during the breakdown of trash. 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 gas 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. The extraction system is a split system, meaning that methane gas can go to the boilers and/or methane flares that burn the gas. The reason foe the split system is that the landfill will increase its gas production over time 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. The last major part of a landfill is the cover or cap. This is an umbrella over the landfill to keep water out (to prevent leachate formation). It will generally consist of several sloped layers: clay or membrane liner (to prevent rain from intruding), overlaid with a very permeable layer of sandy or gravelly soil (to promote rain runoff), overlaid with topsoil in which vegetation can root (to stabilize the underlying layers of the cover). If the cover is not maintained, rain will enter the landfill, resulting in increasing leachate to the point where the bathtub overflows its sides and wastes enter the environment. Covers are vulnerable to attack from at least seven sources. One is erosion by natural weathering (rain, hail, snow, freeze-thaw cycles, and wind). Two is vegetation, such as shrubs and trees that continually compete with grasses for available space, sending down roots that will relentlessly seek to penetrate the cover. Three is burrowing or soil- dwelling mammals ( woodchucks, mice, moles, voles), reptiles ( snakes, tortoises), insects ( ants, beetles), and worms, which will present constant threats to the integrity of the cover. Four is sunlight. If any of these other natural agents should succeed in uncovering a portion of the umbrella it will dry out the clay permitting cracks to develop, or destroy membrane liners through the action of ultraviolet radiation.