The major hazards of landfill gas with respect to building development are summarized as follows:
(a) Flammabilily hazards of landfill gas
Landfill gas is flammable due to the presence of methane, which forms explosive mixtures with air in the range 5%-15% by volume. It should be noted, however, a further risk factor among the hazards of landfill gas is that these limits are not absolute and vary according to gas composition; for example as the proportion of carbon dioxide increases, the flammability of the mixture decreases.
(b) Asphyxiation and toxicity
Landfill gas is a potential asphyxiant and toxin to humans and animals, primarily due to the lack of oxygen in the composition of the gas. There is a risk of asphyxiation whenever the gas is allowed to accumulate in confined spaces with consequent oxygen depletion. The UK Health and Safety Executive recommend maximum short-term and long-term exposure limits for various gases and vapours including the common constituents of landfill gas. See table 1 for the main hazard concentrations.
Table 1 : Key Hazards of and Design Concentrations
(Card GB, Development On and Adjacent to Landfill, Journal of the Institution of Water and Environmental Management, Vol 6, No. 3, June 1992)
Landfill gas also gives rise to adverse effects on vegetation due to oxygen depletion. Migration of landfill gas displaces oxygen from the root zones and under certain conditions methane is oxidized to carbon dioxide and water which further depletes the available oxygen. Thus elevated levels of carbon dioxide in the near-surface zone can be indicative of the presence of landfill gas even if methane is not detected; and
From Landfill Gas to Energy: Technologies and Challenges Landfill gas produces characteristic strong odour which can constitute an unpleasant nuisance. The odours are produced by trace concentrations of the minor constituents, including hydrogen sulphide and complex organic compounds.
The degree of hazard from landfill gas and consequent risk to building development depend on:
- The composition;
- The rate of generation and emission;
- The rate of dispersion and diffusion; and
- The potential migration route.
Significant migration of landfill gas for considerable distances beyond landfill sites can occur when the geology and water table provide suitable conditions.