Landfill Methanogenesis: Changing the Rate of Landfill Methane Production

Landfill gas consists largely of methane, a gas that is formed within landfills as a result of methanogenesis that occurs during the decomposition of organic waste. Methanogenesis can be defined as either: 1) "Bacterial conversion of methanogenic substrates [acetate, formats, hydrogen, carbon dioxide] into methane and carbon dioxide" (Leverenz et al, 2002); or 2) "The production of methane by bacteria. Because they thrive without oxygen, methanogenic bacteria have an important role in the subsurface, where oxygen is often absent" (National Research Council, 1994).

What Can Cause Changes in Landfill Methane Production

Generally, when an existing landfill makes an operational change without compensating by changing the horizontal or vertical dimensions of the landfill, it can change the methanogenesis -- the rate of landfill methane production. Some examples of operational changes that can affect landfill methanogenesis include:

1. If a landfill operator were to increase the moisture content of the waste stream added to the landfill
2. Change the way in which waste is bailed and compacted
3. Change the material used to cover the waste, or change the thickness of the material used to cover the waste
4. Increase the surface compaction of the landfill

Monitoring Landfill Methane Gas Production

US Environmental Protection Agency (EPA) rules require landfill operators to take regular methane measurements within 5-10 cm (2-4 inches) from the landfill surface using a suitable portable methane detector such as an organic vapor analyzer (OVA, flame ionization detector (FID), or similar device that is capable of measuring methane concentrations [ § 60.755(d)]. Methane concentrations are measured following the procedures outlined in EPA Method 21 for VOC leaks, except that "methane" replaces all references to "volatile organic compounds" (VOC) and the calibration gas is 500 ppm methane in air [ § 60.755(d)]. Methane surface concentrations need to be monitored around the perimeter of the collection area and along a pattern that traverses the landfill at 30 meter intervals, and at areas where visual observations indicate elevated concentrations of landfill gas (for example distressed vegetation, and/or cracks or seeps that may have formed in the landfill cover).

Spikes in Surface Methane Concentrations

Surface monitoring of methane will point out any areas where methane concentrations may be higher than expected. In the event that methane is being emitted from the landfill due to increased methanogenesis, my recommendation is to analyze what has changed in terms of the waste streams coming in, as well as other operational changes that may have affected the rate of landfill methane production on your site.

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