The basic parts of a landfill, as shown in Figure 3, are:
- Bottom liner system - separates trash and subsequent leachate from groundwater
- Cells (old and new) - where the trash is stored within the landfill
- Storm water drainage system - collects rain water that falls on the landfill
- Leachate collection system - collects water that has percolated through the landfill itself and contains contaminating substances (leachate)
- Methane collection system - collects methane gas that is formed during the breakdown of trash
- Covering or cap - seals off the top of the landfill
Each of these parts is designed to address specific problems that are encountered in a landfill. So, as we discuss each part of the landfill, we'll explain what problem is solved.
Bottom Liner System
A landfill's major purpose and one of its biggest challenges is to contain the trash so that the trash doesn't cause problems in the environment. The bottom liner prevents the trash from coming in contact with the outside soil, particularly the groundwater. In MSW landfills, the liner is usually some type of durable, puncture-resistant synthetic plastic (polyethylene, high-density polyethylene, polyvinylchloride). It is usually 30-100 mils thick. The plastic liner may be also be combined with compacted clay soils as an additional liner. The plastic liner may also be surrounded on either side by a fabric mat (geotextile mat) that will help to keep the plastic liner from tearing or puncturing from the nearby rock and gravel layers.
Cells (Old and New)
Perhaps, the most precious commodity and overriding problem in a landfill is air space. The amount of space is directly related to the capacity and usable life of the landfill. If you can increase the air space, then you can extend the usable life of the landfill. To do this, trash is compacted into areas, called cells, that contain only one day's trash. In the North Wake County Landfill, a cell is approximately 50 feet long by 50 feet wide by 14 feet high (15.25m x 15.25m x 4.26m). The amount of trash within the cell is 2,500 tons and is compressed at 1,500 pounds per cubic yard! This compression is done by heavy equipment (tractors, bulldozers, rollers and graders) that go over the mound of trash several times). Once the cell is made, it is covered with six inches of soil and compacted further. Cells are arranged in rows and layers of adjoining cells (lifts).
In addition to compressing the trash into cells, space is conserved by excluding bulky materials, such as carpets, mattresses, foam and yard waste, from the landfill.
Storm Water Drainage
It is important to keep the landfill as dry as possible to reduce the amount of leachate. This can be done in two ways:
- Exclude liquids from the solid waste. Solid waste must be tested for liquids before entering the landfill. This is done by passing samples of the waste through standard paint filters. If no liquid comes through the sample after 10 minutes, then the trash is accepted into the landfill.
- Keep rainwater out of the landfill. To exclude rainwater, the landfill has a storm drainage system. Plastic drainage pipes and storm liners collect water from areas of the landfill and channel it to drainage ditches surrounding the landfill's base.
The ditches are either concrete or gravel-lined and carry water to collection ponds to the side of the landfill. In the collection ponds, suspended soil particles are allowed to settle and the water is tested for leachate chemicals. Once settling has occurred and the water has passed tests, it is then pumped or allowed to flow off-site.
Leachate Collection System
No system to exclude water from the landfill is perfect and water does get into the landfill. The water percolates through the cells and soil in the landfill much as water percolates through ground coffee in a drip coffee maker. As the water percolates through the trash, it picks up contaminants (organic and inorganic chemicals, metals, biological waste products of decomposition) just as water picks up coffee in the coffee maker. This water with the dissolved contaminants is called leachate and is typically acidic.
To collect leachate, perforated pipes run throughout the landfill (Figure 3). These pipes then drain into a leachate pipe, which carries leachate to a leachate collection pond. Leachate can be pumped to the collection pond or flow to it by gravity, as it does in the North Wake County Landfill.
The leachate in the pond is tested for acceptable levels of various chemicals (biological and chemical oxygen demands, organic chemicals, pH, calcium, magnesium, iron, sulfate and chloride) and allowed to settle. After testing, the leachate must be treated like any other sewage/wastewater; the treatment may occur on-site or off-site. At the North Wake County Landfill, leachate is released to the wastewater treatment plant in Raleigh, where it is treated and released into the Neuse River. Some landfills recirculate the leachate and later treat it. This method reduces the volume of leachate from the landfill, but increases the concentrations of contaminants in the leachate.
Methane Collection System
Bacteria in the landfill break down the trash in the absence of oxygen (anaerobic) because the landfill is airtight. A byproduct of this anaerobic breakdown is landfill gas, which contains approximately 50 percent methane and 50 percent carbon dioxide with small amounts of nitrogen and oxygen. This presents a hazard because the methane can explode and/or burn. So, the landfill gas must be removed. To do this, a series of pipes are embedded within the landfill to collect the gas. In some landfills, this gas is vented or burned.
More recently, it has been recognized that this landfill gas represents a usable energy source. The methane can be extracted from the gas and used as fuel. In the North Wake County Landfill, a company collects the landfill gas, extracts the methane, and sells it to a nearby chemical company to power its boilers. The extraction system is a split system, meaning that methane gas can go to the boilers and/or the methane flares that burn the gas. The reason for the split system is that the landfill will increase its gas production over time (from 300 cubic feet per minute to 1,250 cubic feet per minute) and exceed the capacity of the boilers at the chemical company. Therefore, the excess gas will have to be burned. It is not cost-effective to compress the excess gas to liquid and sell it.
Covering or Cap
As mentioned above, each cell is covered daily with six inches of compacted soil. This covering seals the compacted trash from the air and prevents pests (birds, rats, mice, flying insects, etc.) from getting into the trash. This soil takes up quite a bit of space. Because space is a precious commodity, many landfills are experimenting with tarps or spray coverings of paper or cement/paper emulsions. These emulsions can effectively cover the trash, but take up only a quarter of an inch instead of 6 inches!
When a section of the landfill is finished, it is covered permanently with a polyethylene cap (40 mil). The cap is then covered with a 2-foot layer of compacted soil. The soil is then planted with vegetation to prevent erosion of the soil by rainfall and wind. The vegetation consists of grass and kudzu. No trees, shrubs or plants with deep penetrating roots are used so that the plant roots do not contact the underlying trash and allow leachate out of the landfill.
Occasionally, leachate may seep through weak point in the covering and come out on to the surface. It appears black and bubbly. Later, it will stain the ground red. Leachate seepages are promptly repaired by excavating the area around the seepage and filling it with well-compacted soil to divert the flow of leachate back into the landfill.
At many points surrounding the landfill are groundwater monitoring stations. These are pipes that are sunk into the groundwater so water can be sampled and tested for the presence of leachate chemicals. The temperature of the groundwater is measured. Because the temperature rises when solid waste decomposes, an increase in groundwater temperature could indicate that leachate is seeping into the groundwater. Also, if the pH of the groundwater becomes acidic, that could indicate seeping leachate.