A landfill is basically a big pit of garbage with soil on top, right? Think again. Today’s modern landfills are made up of much more than just the waste that goes into them. In fact, they are complex structural systems composed of many different parts, each one designed to address a particular landfill problem or issue.
If we imagine a cross-section of a typical landfill, then the different parts or layers we encounter working from the bottom up are as follows:
Prepared subgrade—These are the native soils located beneath the landfill location. Depending on the type of soil and size of the landfill, the soils may need special preparation in order to be ready for the weight and pressure of the landfill waste.
Compacted clay—Above the prepared subgrade is a layer, typically 24 inches thick, of compacted clay. This layer helps prevent landfill gas from escaping, and also acts as an additional barrier to stop leachate from seeping from the landfill into the soil and groundwater of the surrounding environment.
Geomembrane—Lying on top of the layer of compacted clay is a protective liner made of a special type of thick plastic known as high-density polyethylene (HDPE). HDPE is impermeable and highly resistant to degradation from leachate or other chemical compounds that might be found in the landfill. Just like a trash bag lining the interior of a trash can, the geomembrane helps keep leachate and landfill gas contained within the landfill.
Leachate collection system—The bottom of each landfill is designed as a leachate collection system: an area that collects and contains any liquids seeping from the waste, so that it can be easily removed from the landfill for proper disposal. After moving down through the landfill, the leachate reaches and trickles through the leachate collection layer, a kind of filter made up of a layer of gravel, sand, or sometimes a thick plastic mesh known as a geonet. Some landfills contain an additional filter underneath this collection layer, typically made of a geotextile fabric that helps separate solid particles from liquid.
After the leachate has passed through these filters, it runs into the leachate collection pipe system. Surrounded by a bed of gravel, these perforated pipes use gravity to carry the collected leachate to specially designed low points in the landfill that are called sumps. When leachate reaches the sumps, special pumps automatically remove the leachate and transport it to the landfill’s leachate management facilities.
Working landfill—Above the leachate collection system is the working landfill, where waste is deposited and contained as it arrives. But again, this part of the landfill is more complex than we might realize. Waste is not simply dumped into the open landfill—it’s carefully compacted in layers within smaller designated areas of the landfill called “cells.” Using cells rather than keeping the entire landfill open allows for more effective containment of the waste and reduces overall waste volume in the landfill. The practice also helps limit odors, prevents litter from scattering, and deters scavengers.
At the end of each working period (often a single day, depending on the capacity of the landfill and the area it serves), all the waste deposited during that period is covered with a six- to 12-inch layer of soil, or another approved material such as a spray-on foam, to isolate the waste from exposure to air and pests. Alternatively, some landfills use large panels of a tarpaulin-type material to cover the waste at the end of the working period. These panels are then removed the next day before new waste is deposited.
Composite cap system—When a landfill cell is filled to capacity and will no longer be receiving additional waste, it is permanently covered by what is known as a composite cap system, designed to seal off the top of that section of landfill. This composite cap system looks quite similar to the composite liner system found at the bottom of the landfill. It is composed of a layer of compacted clay placed directly on top of the waste; then the thick plastic layer of the geomembrane; then a drainage layer made of sand, gravel, or the plastic geonet mesh. These protective layers help prevent rain and other elements from infiltrating the landfill and contributing to leachate buildup.
Protective cover—As sections or cells of the landfill are completed, they receive additional protection in the form of a number of other protective surface layers. On top of the composite cap system, a layer of protective cover soil is laid down to help with moisture retention and support surface vegetation. This cover soil is topped with a further layer of nutrient-rich topsoil, which in turn is planted with cover vegetation like grasses and shrubs. When this cover of protective vegetation is fully established, the area can be maintained as a public open space.