Guidelines on remediation of contaminated sites
Appendix 4.5
Introduction to landfill gas
This Appendix contains a brief introduction to the issue of landfill gas. For further information, see Report No. 69 from the Environmental Protection Agency /1/.
When organic waste decomposes through chemical processes, gas develops. Decomposition mainly develops methane and carbon dioxide. In addition to these, various other inorganic gases and organic vapours are developed. All the gases that develop are collectively called landfill gas. For details about processes of decomposition, see Miljøprojekt (‘Environmental Project’) No. 84 /2/. Methane production will depend on the actual conditions; e.g. decomposition in a 10 m deep landfill will be largely finished after a period of 15-30 years, given optimum conditions for degradation. Under unfavourable conditions, decomposition may last more than a 100 years.
Naturally, the rate of gas generation depends on how degradable the waste is. A landfill will typically have areas of waste with no organic content or containing largely easily decomposed waste, where gas production has already stopped. Appendix 5.1 features a simple empirical model for an estimated assessment of the rate of gas generation.
Landfill gas consists mainly of the gases methane and carbon dioxide, both of which are odourless. In addition to these, landfill gas may in some cases contain lesser amounts of other substances that have an odour.
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Degradation reduces the volume of waste, causing the ground to settle. This may cause
cracks and fissures in the foundations and drainage systems of any buildings on the site.
The development of gases may cause pressure above that of the atmosphere, forcing the landfill gas upwards and out of the landfill to be diluted with outdoor air. Emission will vary, among other things according to meteorological conditions, especially changes in barometric pressure. Gas transport may occur both through diffusion and through convection.
Where the area producing the gas is covered by asphalt and buildings, or where the surface is sealed off through other causes (e.g. a long period of precipitation or severe frost) the gas will be prevented from leaking up. As a consequence, pockets of gas may develop, from which gas can leak into buildings through fissures and cracks in floors and foundations, or leak sideways through natural strata of sand and gravel or through ducts and conduits.
Methane is flammable in concentrations of between 5 and 15 vol. % of outdoor air. This is called the lower and upper explosion limit, respectively. Gas mixtures with methane content above the lower explosion limit will comprise an explosion hazard when mixed with outdoor air. If methane collects in sewage systems, hollow spaces below houses, etc., there may be danger of explosion.
Carbon dioxide may constitute a hazard, as concentrations between 4 and 7 vol. % may cause unconsciousness, and higher concentrations can be lethal. Landfill gas can also cause abnormal plant growth in gardens and open lawns; either because of an oxygen deficiency in the root zone or due to a direct toxic effect.
References
| /1/ | Lossepladsgas (‘Landfill Gas’). Report No.
69. The Environmental Protection Agency, 1993 [Tilbage] |
| /2/ | Alternativ lossepladsteknologi. En litteraturgennemgang (‘Alternative
landfill technology. A review of literature on the subject’). Environmental Project
No. 84. The Environmental Protection Agency, 1987. [Tilbage] |