Landfills

Landfills used to be called "dumps" and modern landfills evolved from ancient practices of putting a community's waste in one area. Landfills require a good management, tight engineering design, and careful maintenance even after they stop receiving waste. The EPA reports that there are 1,269 active landfills in the US (as of 2017). This doesn't include closed landfills that no longer receive waste. About half of MSW goes to landfills. The rest is mostly recycles, composted, or incinerated.

Putting untreated waste into an uncontrolled, non-engineered landfill is bad for the environment even though it is done in many places around the world. In the second half of the 20th Century “sanitary landfills” were developed to isolate waste. These are engineered to protect aquifers and have layers of soil between waste. While active, waste is regularly compacted at the landfill.

Sanitary landfills have a set features:

• Hydrogeological isolation
• A waste disposal plan and final restoration plan for when the landfill stops accepting waste
• Employees on site during the work week/ Trained operators of earth-moving equipment
• Regular compacting of newly-arived waste Covering layers of waste with soil

The American Society of Civil Engineers calls sanitary landfills "a method of disposing of refuse on land without creating nuisances or hazards to public health or safety, by utilizing the principles of engineering to confine the refuse to the smallest practical area, to reduce it to the smallest practical volume, and to cover it with a layer of earth at the conclusion of each day's operation, or at such more frequent intervals as may be necessary."

The US Department of Agriculture names three methods creating sanitary landfills. The area method is where wastes are spread, followed by compaction, and then followed by soil covering. A mound builds up over the natural land contour which started as relatively flat. The trench method involves digging a trench in the existing land and burying the waste. There is also the slope method where waste is deposited on a natural slope, essentially evening out the surface contour.

When compacted, the waste in landfills can reach a density of 600 to 800 kg per cubic meter. By contrast water weighs 1000 kg per cubic meter and soil is typically 1800 kg per cubic meter. Each waste cell is typically 3 to 5 meters high. The dimensions can vary but the rule of thumb among civil engineers is that the ratio of length to height should be 10:1.

The top of the pile is called the "working face". When the landfill is active and receiving waste, approximately 2 feet of waste is placed on the face and a vehicle overruns the waste to compact it. At the end of the working day, the cell is covered by 15 cm (6 inches) of daily cover, which is soil and sometimes compost or chipped wood. Vendors sell mixes that can be sprayed on the waste to control odor. Cover ends up being 20 percent of the material in the landfill, but this method has found widespread use because of the advantages of cover. It reduces moisture entering the waste, reduces odors and risk of fire, discourages infiltration of the waste by rodents, and allows vehicles to drive over the waste.

At the bottom of the landfill is a liner, designed to retard leakage of liquid to the groundwater. These geomembranes are made from polymers, fillers, plasticizers, carbon black, and other additives. In some engineered facilities, a layer of clay is also used to line the bottom. Combinations of multiple plastic lines and clay lawyers may be employed.

Costs for businesses to dispose of waste in landfills varies widely by area of country. The US EPA says the average cost in 2019 was $55.36 per ton.