Layers of Cells
At the most basic level, sanitary landfills are constructed in layers to collect and contain waste and its byproducts as it decomposes. Modern landfills are operated using a layering system intended to safely isolate waste and monitor byproducts, leaks and anything else that can harm the environment. Isolating the trash from environmental air, soil, and water is vital for preventing contamination.
Cells are the basic working structure in a landfill. Landfill layers are composed of cells built side by side until the layer is complete, at which time a new layer of cells is started.
Each day, trash is delivered, spread out and compacted so that the trash takes up a minimum amount of space in a single cell. The area where trash is deposited and worked is referred to as the daily workface. At day’s end, once the last waste is deposited and compaction is completed, a cover is placed over the workface to contain odors and deter unwanted pests.
Isolating Elements
There are several parts of a landfill system that are intended to isolate the waste and its byproducts from the environment. Liners of all types are the basic tools and are perhaps the most critical elements in a modern sanitary landfill.
The Liner System
The bottom layer of a sanitary landfill prevents liquids from passing from the landfill into the surrounding soil. This layer is often made up of densely compacted clay, but since clay is not always impermeable, engineers will usually cover this base with additional layer(s) of high-density plastic or geosynthetic material. These liners are typically used in municipal solid waste landfills.
The Gas Collection System
As organic waste decays, it naturally produces and releases methane, a greenhouse gas, into the atmosphere. Methane is also the primary component of natural gas, which can be a valuable resource for generating electricity. The third liner layer in a sanitary landfill is devoted to collecting the methane inevitably produced during decomposition. Gas extraction wells are connected to pipes which convey the methane to treatment areas where it is then vented, burned or converted into energy.
Capping
Once a landfill (or a section of cells) is full, it is permanently capped with an impermeable liner made of plastic; one of the more durable geotextiles. A minimum of two feet of soil covers the cap, then vegetation is planted to prevent soil erosion. Landfills are typically monitored for 30 years following capping to detect and correct any problems, at which point they’re considered safe for general re-use.
