Landfill was defined in Study Session 1 as an area of land set aside for the final disposal of solid waste. Ideally the site is managed to prevent people and animals from entering and the deposited waste is covered with soil to isolate it from the environment. However many informal sites do not cover the waste or have any other control measures. We are using ‘landfill’ here as a general term that applies to any site where solid waste is deposited for final disposal.
There are many different types of landfill, some with greater environmental impact than others. In all of them the waste gradually decomposes by a combination of biological, chemical and physical processes. During these decomposition processes, two major emissions are of primary concern – leachate and landfill gas:
- Leachate is the polluted water that emerges at the base of the landfilled waste. It is formed in two ways. Rainwater landing on the waste slowly flows over and through the waste and soluble substances are dissolved in the water. Also, some of the decomposition reactions taking place in the waste produce liquid that can be acidic. Some substances, such as toxic metals, tend to dissolve more easily in acids, making the final leachate more harmful. If leachate enters a watercourse used to provide human or animal drinking water or for irrigation, people can be exposed to these pollutants.
- Landfill gas is formed in large landfills through degradation of the waste in anaerobic conditions. Landfill gas consists of a mixture of carbon dioxide and methane, which are both greenhouse gases that contribute to global climate change. It is also flammable and will burn if exposed to a flame or other source of ignition. In extreme cases, the gas can build up in a landfill and explode, with the risk of injury and death. Managed landfill sites have vent pipes that allow the gas to get out of the waste and be released to the air or burned in a controlled way.
The different types of landfill can be ranked according to their potential to cause environmental pollution. Starting with the worst, they are:
- Indiscriminate waste disposal
- Communal open dumping
- Burial in pits
- Controlled landfill
- Sanitary landfill.
These types of landfill are described in the following sections.