Retention in Landfill
Composition of Residential Materials Disposed of in Canada (2002). Source: Natural Resources Canada
Many people believe that plastics comprise the majority of all municipal solid waste. This is simply not so. Studies have shown that plastics make up approximately nine per cent of all municipal solid waste by weight and 14-20 per cent by volume.
Although waste management practices, such as recycling and reduction, will reduce the amount of waste going into landfill, there will always be some material that cannot be disposed of through these methods. Even energy recovery facilities end up with some waste, albeit very small quantities. More often than not, these residuals end up in a landfill environment.
Contrary to popular belief, very little degrades in a modern landfill. Modern landfills are sited and operated to avoid wet conditions and prevent as much biodegradation as possible. When organic matter does degrade in landfills, one of the emissions is methane gas (a greenhouse gas 21 times more potent than carbon dioxide). This means that biodegradable products in landfill are not necessarily a good thing and one of the reasons why Germany prohibited any form of degradable materials from entering their landfills in June 2005.
Plastics have numerous advantages over other materials when the are disposed of by landfilling due to the fact that they are easily compressed and have an inherent intertness meaning that they do not leach contaminents and can help stabilize materials in a landfill.
Today's modern landfills are composed of four main elements: a bottom liner (which can be made from clay, plastics or a composite material), a leachate collection system, a cover and an appropriate setting. The liner's role is to effectively prevent wastes from migrating directly into the environment, specifically the groundwater.
The better liners being used today are composed of a tough, high-density polyethylene (HDPE) plastic that is usually 30 to 100 mils thick. The plastic liner may be combined with compacted clay soils as an additional liner, and it may also be surrounded on either side by a fabric mat (geotextile mat) that will help to keep the plastic liner from being torn or punctured by the nearby rock and gravel layers. In any landfill environment, the use of a plastic liner is an important safeguard against groundwater contamination.