Let’s start by saying, there are 3 different types of physical trash disposal systems when people think “landfill.”
- A dump
- A landfill
- A sanitary landfill .
A dump, simply put, is a big mountain of trash. It’s an antiquated system that is generally no longer allowed in modern countries.
The trash is trucked in, and dumped on the ground where a bulldozer pushes it into a great big pile while compacting it.
They’re health and environmental disasters. Animals go into them and get sick, then spread their diseases to local populations, possibly contaminating food and water sources. Rain and runoff go through the trash, dissolving whatever can be dissolved and carrying away any particles that are small enough to “wash away.” This now-dirty water is called leachate. It is highly contaminated, toxic, and all around bad news. It goes into the ground and contaminates the soil and the groundwater (if present). Wind and other weather events can carry pollutants and trash away causing further environmental problems.
Dumps are disgusting and stupid and a remnant of a stupider time, but they cheap.
Landfills are a step up from dumps. They are big holes dug in the land that we fill with trash. They’re slightly better than dumps because at the end of each day, the trash is covered with a layer of clean soil (but not always).
This prevents a small amount of direct environmental damage because animals have a hard time getting to the trash and getting sick, and wind won’t blow the trash around as easily. Water getting in there and reaching the soil and groundwater, however, still occurs.
The fact that it’s covered with soil and compacted also causes a new problem: methane. The trash has some organic component (usually food and garden scraps/grass clippings/paper/cardboard/etc) that will biodegrade (break down).
Aerobic biodegradation needs three things: moisture, oxygen, bacteria. The byproduct of this is carbon dioxide and heat (and I guess food for bacteria growth).
When you remove the oxygen, a different type of biodegradation occurs: anaerobic biodegradation. This doesn’t need oxygen, but the byproduct is now methane, which is more polluting and easily flammable and/or explosive.
It gets released directly into the air, it smells, and it can possibly explode. The landfill smells (not specifically the methane. Methane is odorless) Not good.
A sanitary landfill is the current standard and mostly the only physical trash storing process that is allowed in most advanced countries. This is similar to a regular landfill but it has several upgrades.
First, the location goes through a very thorough selection process. It should be quite high above the groundwater level to prevent contamination, it should be built away from natural waterways (streams, rivers, lakes, seas, oceans), it should be built in an area that is not prone to landslides nor earthquakes, and it should be relatively easily accessible. Not all of these criteria are met all the time.
Second, the “hole” for the landfill is dug out entirely at the start. It’s designed with a slope so that liquids will follow in one direction. The bottom of the hole is lined with a layer of impermeable material (usually a thick layer of clay that doesn’t allow water to seep through, sometimes rubber).
Above the impermeable layer, a layer of sand is placed to protect the impermeable layer, then a layer of coarse gravel. Within the gravel later, a drainage system is placed to eventually collect the leachate and bring it to a processing area.
At the same time, a bunch of wells are built around the landfill area. Usually at least one “uphill” the landfill, and a bunch all around, and several downhill from it. These are used to monitor the groundwater to see if there’s contamination while the landfill is being used (and after it’s capped).
Next, the landfill starts getting filled in. The trash comes in and is placed on the bottom. It’s spread out and compacted. At the end of every day, the daily trash is covered with a layer of fresh soil (usually kept from when they dug out the landfill at the start). The soil is compacted and this forms what is called a cell. A trash bubble surrounded by soil containing one day’s worth of trash. This keeps animals away and prevents wind from caring stuff away.
A cell is generally never more than 1m in height. Only once a layer of cells covers the entire bottom of the landfill do they start placing trash on top of them. A layer of cells is called a lift. There can be several lifts in a landfill.
Eventually (usually the landfill is designed to have a useful life of about 35 years), the landfill is full. When that happens, the landfill must be capped and closed off. This is done by covering the topmost lift with layer of compacted soil. This top soil layer is made is such a way that there is a high point, a peak if you will, and we’ll see why in a moment.
After soil, a layer of crushed stone is placed, similarly with a “peak.” Within this stone layer, a pipe with holes is placed near and around the peak. Remember the methane I mentioned earlier, from anaerobic degradation? This pipe will collect and eventually vent or capture it. The crushed stone is there to allow it to follow upwards easily and direct it toward the pipes– the same reason there’s a peak.
After the venting/collection system is installed, the gravel is covered with a layer of geotextile (basically cloth), then a layer of fine sand, and then another layer of impermeable clay (or rubber). This impermeable layer connects to the bottom layer, essentially forming an envelope around all the 35 years worth of trash. On top of this later, more geotextile, sand, then soil. Now you’ve got a huge open field where cities often build parks.
Over the next 35-50 years, the amount of leachate and methane produced are monitored and the groundwater wells are continuously sampled and tested. The amount of leachate and methane produced are a good indicator of whether or not the trash had been “stabilized.” That means it’s relatively safe and a much smaller health hazard.
This whole time the leachate is also collected and processed to prevent it from contaminating the surrounding areas. The methane is either vented directly to the air or, more often, collected and used (read burned) to produce energy to power the landfill monitoring stations, or sold to energy companies, or used as a starter for anaerobic digestion of wastewater sludge (that’s a whole different story for water treatment). Eventually, once the landfill is deemed stable, it can be mined for resources.
So that’s how a landfill works. Wall-E shouldn’t be possibly since the trash isn’t on the ground and open to the air/nature. But that’s only true for countries with modern economies that can afford them.