Anatomy of a Landfill
Modern day landfill design is considerably more sophisticated than the simplistic dumps from years ago. Dumps were nothing more than holes in the ground filled with garbage. Landfills, however, are engineered structures designed to prevent toxins from polluting nearby land and subsurface water supplies. Landfills consist of five primary systems: 1) composite bottom liner system, 2) leachate collection system, 3) working landfill, 4) composite cap system and 5) a protective cover. Each of these shall be discussed below.
1) Composite Liner System
After the site for the landfill has been excavated the subgrade is prepared by creating a compacted clay bottom layer. This layer is then covered with a thick membrane typically fabricated from a special type of plastic called high-density polyethylene (HDPE). HDPE is extremely durable and can resist degradation from potentially hazardous liquids filtering through the landfill above. This liquid, called leachate, consists primarily of rainwater but also contain chemicals contained within the waste. The membrane and the compacted clay layer work together to prevent the leachate from leaching into the environment. They also help prevent the escape of landfill gases which consist primarily of methane and carbon dioxide.
2) Leachate Collection System
The landfill leachate collection system is used to collect and remove leachate so it can be properly reated and disposed of. A layer of sand or gravel is installed directly on top of the HDPE membrane. Sometimes a thick plastic mesh called geonet may be used as an alternative. This layer allows the leachate to drain by gravity to a pipe collection system. This system consisting of perforated pipes moves the leachate to low stops called sumps. Pumps located within the sumps transport the leachate to leachate treatment facilities or some other method of disposal. The entire piping system is covered with a geotextile fabric that helps separate solid particles from the liquid and prevents clogging of the pipes.
3) Working Landfill
The area directly above the leachate collection system is the working landfill. This constitutes the largest majority, by volume, of the landfill. As waste arrives it is compacted in relatively small areas called cells. At the end of each day the waste is covered by 6 to 8 inches of soil.This daily cover helps reduce odors, deters scavengers and prevents litter from scattering. A mature landfill that has reached its capacity will have several layers of waste consisting of dozens and dozens of cells.
4) Composite Cap System
When the landfill reaches its permitted height a layer of compacted clay is placed over the waste. A thick plastic membrane of HDPE is then laid over the clay. The plastic and the clay layer work together to form a cap that helps reduce the amount of rain water entering the landfill and helps reduce odors from escaping the landfill. A layer of sand or gravel is installed directly on top of the HDPE membrane. As with the leachate collection system sometimes geonet is used as an alternative. This layer drains excess rainwater from a protective cover soil layer above it to enhance stability and prevent water from infiltrating the landfill cap system. A geotextile fabric similar to felt may be installed on top of the drainage layer to separate solids from the liquid and prevent clogging of the drainage layer.
5) Protective Cover
As portions of the landfill are completed a protective cover is installed. This cover consists of a layer of protective cover soil which protects the landfill cap system. Above this is a final layer of top soil to support the growth of vegetation through water retention and by providing nutrients. Lastly, cover vegetation consisting of native grasses and shrubs are planted to prevent erosion and provide a visually pleasing environment.
