Landfill gas monitoring is the process by which gases that are released from landfills are electronically monitored.

Techniques for the monitoring of landfill gas

Surface monitoring is used to check the integrity of caps on waste and check on borehole monitoring. It may give preliminay indications of the migration of gas off-site. The typical regulatory limit of methane is 500 parts per million (ppm) by volume (in California, AB 32 may push this limit down to 200 ppm). In the UK the limit for a final landfill cap is 1*10-3 milligrams per square metre per second, and for a temporary cap it is 1*10-1 mg/m2/s ( as measured using the Environment Agency's " Guidance on Monitoring landfill gas surface emissions " LFTGN 07, EA 2004 ). Surface monitoring can be broken down into Instantaneous and Integrated. Instantaneous monitoring consists of walking over the surface of the landfill, while carrying a flame ionization detector (FID), like the Photovac MicroFID. Integrated consists of walking over the surface of the landfill, while pumping a sample into a bag. The sample is then read with a FID or sent to a lab for full analysis. Integrated regulatory limits tend to be 50 ppm or less.

Gas probes, also known as perimeter or migration probes, are used for Subsurface monitoring and detect gas concentrations in the local environment around the probe. Sometimes multiple probes are used at different depths at a single point. Probes typically form a ring around a landfill. The distance between probes varies but rarely exceedes 300 metres. The typical regulatory limit of methane here is 50,000 parts per million (ppm) by volume, or 1% methane and 1.5% carbon dioxide above geological background levels in the UK ( see " Guidance on the monitoring of Landfill Gas " LFTGN03, EA 2004).

Ambient air samplers are used to monitor the air around a landfill for excessive amounts of methane and other gases. The principal odoriferous compounds are hydrogen sulfide ( which is also toxic ) and the majority of a population exposed to more than 5 parts per billion will complain ( World Health Organisation : WHO (2000) as well as volatile organic acids. Air quality guidelines for Europe, 2nd ed. Copenhagen, World Health Organization Regional Publications, European Series).

Monitoring of the landfill gas itself can be used diagnostically. When there is concern regarding the possibility of an ongoing subsurface oxidation event, or landfill fire, the presence in the landfill gas of compounds that are more stable at the high temperatures of such an event ( above 500 deg C ) can be evidence for such a process occurring. The presence of propene, which can be formed from propane at temperatures above several hundred degrees C, supports high temperatures. The presence of elevated concentrations of dihydrogen (H2) in the landfill gas is also consistent with elevated temperatures at remote locations some distance from the gas-extraction well. The presence of H2 is consistent with thermal inactivation of CO2-reducing microbes, which normally combine all H2 produced by fermentation of organic acids with CO2 to form methane (CH4). H2-producing microbes are less temperature-sensitive than CO2-reducing microbes so that elevated temperatures can inactivate them and their recovery can be delayed over the H2-producers. This can result in H2 production without the (usually )corresponding consumption, resulting in elevated concentrations of H2 in the landfill gas (up to >25%[v:v] at some sites). Thermal deactivation of CO2-reducing microbes has been used to produce CO2 (rather than methane) from municipal solid waste (Yu, et al., 2002).

Types of landfill gas monitoring

A monitor may be either a

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