Environmental Engineering Reference
In-Depth Information
B. Monitoring of LFG
The monitoring program for landfill gas at waste landfill sites is recommended to
determine whether landfill gas is causing a hazard to human health or the environment.
Monitoring takes place throughout the operation of the plant and for many years during the
post-closure period, until emission levels of methane and carbon dioxide are at
environmentally insignificant levels, typically below 1.0% by volume of methane and 1.5%
by volume of carbon dioxide. Monitoring takes place within the landfill and outside the site
boundary. The monitoring program, including the frequency of monitoring, will be dependent
on the age of the site, the type of waste and the gas collection and control measures installed.
Frequency between measurements will vary from weekly, monthly or even quarterly,
depending on site-specific characteristics. More frequent monitoring may be required where
migration of gas is suspected. Monitoring techniques for landfill gas include, for example,
surface monitoring, sub-surface probes, gas monitoring wells, and boreholes. Surface
monitoring with portable instruments is mainly used to detect the presence of gas leaks
throughout the site. Sub-surface monitoring using gas probes is used to monitor gas
production and migration at depths of between 1 and 10m in the mass of waste and in the
surrounding environment. The probes may be left for long periods of time to monitor and map
the production of gas from the site throughout site operation and post-closure. The probes are
constructed of steel and plastic pipe, consisting of a porous lower section and a gas transfer
pipe which transfers the gas to the surface, where the gas sample is taken for analysis. Gas
monitoring well and boreholes consist of a porous plastic casing in direct contact with the
waste or geological strata. Probes or tubes may be permanently installed. They are installed
within the mass of waste and in the surrounding environment.
Gas sample analysis may take the form of portable instruments for gas analysis or
laboratory-based analysis. Portable analyzers may be simple devices, such as gas indicator
tubes, which produce a color change to indicate a concentration of a particular gas in a
sample. The gas is drawn through the tube on-site and an immediate indication of gas
concentration is obtained. The method is, however, subject to error. More sophisticated
instruments are available, such as infra-red gas analyzers and flame ionization detectors,
which would normally be housed in a portable laboratory or at the analytical laboratory. The
gas sample is piped directly to the analyzer or else a sample of the gas is taken in suitable
sealable containers, such as 'Teflon' bags or glass sample tubes and the sample is then
transferred to the instrument for analysis. The most accurate and reliable technique for gas
analysis is gas chromatography. A sample of the gas is taken in a suitable container to the
laboratory for analysis. The gas chromatograph can separate out individual gas components
and provide an accurate analysis, even at trace concentrations.
C. LFG Hazards
1. LFG explosion hazard
Landfill gas may form an explosive mixture when it combines with air in certain
proportions. The following conditions must be met for landfill gas to pose an explosion
hazard:
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