Methane Oxidation: a Comprehensive Study
It is important to improve knowledge about landfill emissions and to determine better ways to measure them, as opposed to relying on default approaches.
Tuesday, May 31, 2011
By Jeff Chanton
Landfill gas (LFG) that is not collected or vented passes through landfill cover soils before it escapes to the atmosphere as “fugitive” methane emissions. Bacteria that consume methane live within the soil over a landfill. One of the greatest uncertainties in estimating methane emissions from landfills is how much methane these bacteria consume. Landfill soil cover methane oxidation is carried out by a kind of bacteria called a methanotroph. The methano part of the term denotes methane, and troph means food. The bacteria essentially eat the methane, combining it with oxygen to produce carbon dioxide, water, and, of course, more bacteria. They function only in the upper layer of the landfill cover where methane and oxygen overlap, as depicted in Figure 1. This layer can vary in thickness from 10 to 100 cm (5 to 40 inches).
Methanotrophic bacteria are ubiquitous and are found in almost every type of soil. They are particularly abundant in landfill covers and in wetlands. However, they even are found in forest soils, where they consume methane directly from the atmosphere.
An added benefit of these bacteria is that in addition to consuming methane, they also breakdown a wide range of other volatile organic compounds that can be present in landfill gas. These compounds include the hazardous air pollutants (HAPs) benzene, toluene, and vinyl chloride. This process is known as co-metabolism.
The EPA recognizes the activity of landfill soil methanotrophs and uniformly credits them with reducing fugitive methane emissions from landfills by 10%. The 10% is derived from the very first comprehensive study of landfill methane oxidation, which took place in New Hampshire in the early 1990s. Even though a number of subsequent studies have indicated that the value may be greater than 10%, the EPA has not yet revised this 10% default value. Two literature reviews published in 2009 in the scientific literature indicated that the default value might be too low by a factor of three (see References 1 and 2).
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| Figure 2. There are two stable isotopes of carbon, one of mass 12, and one with a mass of 13. Each has six protons, which determine the element, but one has six neutrons while the other has seven. |
Therefore, a comprehensive study to determine methane oxidation at a number of our facilities across the US was undertaken. Studies were conducted seasonally at 20 different landfill covers on 37 occasions. The study covered a range of climatic types and included five climate zones: Site locales ranged from the California desert to Florida.
To determine methane oxidation in our field study, we used a stable isotope technique developed at Florida State University. There are two stable isotopes of carbon: 13C, which is about 1% abundant; and 12C, which comprises 99% of carbon atoms (Fig. 2).
Stable isotopes are useful for determining methane oxidation, because as the bacteria consume methane, they eat the methane containing the lighter 12C at a faster rate than they eat the methane containing 13C. This is because the bonds formed by the 13C contain compounds stronger and harder to break (Figure 3).
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Figure 3. As a result of its slightly larger mass, 13C atoms form stronger chemical bonds. When methane is oxidized in the cover soil, the 12C methane reacts just a bit faster than the 13C methane. The methane not oxidized is left behind or residual, and bears greater amounts of 13C-methane, called 13C enrichment. The shift in the amount of 13C methane across the landfill cover is directly propositional to the
amount of methane consumed by the bacteria. |
Thus, methane that has been exposed to oxidation (residual methane) is enriched in the heavy isotope, 13C, relative to methane that has not been exposed. The degree of the enrichment, or the shift in the 13C/12C ratio following exposure, is proportional to the fraction of methane oxidized (Figure 4).
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| Figure 4. This figure depicts methane passing from the anoxic waste layer, where it is generated, across an oxidized soil layer, and then shows its release into the air. The size of the arrow depicts the amount of methane transported. The arrow gets smaller by 50% in this example, indicating that the methanotrophic bacteria reduce the fugitive methane emissions by half in the soil cover (50% oxidation). With new laser-based technology, we can determine the quantity of the red arrow. The only way to determinethe quantity of the black arrow, the flux of methane to the soil cover and its subsequent reduction there (by microbial oxidation), is with the isotope technique. |
National Comprehensive Oxidation Study
- Studies were conducted seasonally
- Twenty different landfill covers
- 37 Sampling events
Climatic types
- Humid subtropical (Texas, Florida)
- Humid continental—warm summer (lower Midwest and Atlantic states)
- Humid continental—cold summer (upper Midwest)
- Mediterranean (San Francisco Bay Area)
- Arid (eastern Colorado and southern California)
Results
Mean methane oxidation within the soil covers was 38% ± 4%. The results compared extremely well with a recent literature review (Reference 2), which compiled some 30 recent publications and reported a value of 35% ± 5% oxidation. Both values are considerably greater than the default value (Figure 5).
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| Figure 5. Comparison is shown of the percent of methane oxidation results from the national comprehensive study of 20 landfills with a recent literature review and with the default value of 10%. |
The study also found that the fraction of methane oxidized in arid sites was significantly greater than oxidation in Mediterranean sites, or in cool and warm continental sites. Subtropical sites had significantly lower CH4 oxidation than the other types of sites (Figure 6). Our data indicate that the fraction oxidized is inversely related to methane loading of the soil cover. Arid sites have less methane production within the waste, as they are limited by water. Thus, the cover is better able to consume the more limited input of methane. At more humid sites, methane production is greater and more methane impinges on the cover soil, reducing the efficiency of the methane-oxidizing bacteria.
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Figure 6. Landfill cover methane oxidation (percent oxidized) is depicted as a function of climate type across the US. Sampling covered 20 different landfill covers 37 times in a seasonal study of methane oxidation in differing climate types. The fraction of CH4 oxidized refers to the percentage of CH4 delivered to the base of the cover that was oxidized to CO2 and
partitioned to microbial biomass instead of being emitted to the atmosphere as CH4. The overall average was 38% ± 4%. |
This finding indicates that an effective way to increase the fraction of methane oxidized by a landfill cover is to reduce the quantity of methane entering it. In the early life of a landfill, when methane production rates are high, this can be achieved with an effective gas-collection system, which reduces the pressure and concentration of methane at the base of the cover and thereby reduces the methane entering the cover. Reducing the flow of methane through covers will allow more oxygen penetration downward into the cover to aid oxidation.
It is important to note that these values represent the fraction of CH4 oxidized upon passing through the soil. Comparisons of chamber-derived CH4 emissions to CH4 emissions measured with OTM-10, a laser-based approach that measures CH4 emissions from soil plus infrastructure leakage, have indicated that chamber fluxes were 66% of those determined with the laser-based approach. If we assume that 66% of the CH4 emissions were via the soil and oxidized to 38%, and 34% of the emissions were via pipes and wells and not oxidized at all, then we derive a value of 0.66 × 38% = 25%, using the mean value.
Why Is This Important?
- Current default value regulations set the amount of fugitive landfill emissions at 25% of collected methane reduced by 10% oxidation. That is, then, emissions are set at 23% of collected methane.
- This study showed that the 10% oxidation factor is too low by a factor of almost 4, and that fugitive emissions should be set at 25% of collected methane factored by 25 to 38%
- With a 10% oxidation factor, fugitive emissions are figured at 23% of collected methane.
- With the 38% oxidation factor, fugitive emissions are figured at 15.5% of collected methane.
- With a 25% oxidation factor, fugitive emissions are figured 19% of collected methane.
- This whole approach toward calculating fugitive methane emissions discourages innovation and improvement in the operation of gas-collection systems. Consider a landfill with a faulty gas collection system that collects 100 units of methane. Under the current default regulations, its fugitive emissions are defined as 23 units. If the operators of this landfill were to install more gas wells and tune them better, so that they doubled their gas collection to 200 units of methane, they are rewarded by having their fugitive emissions increase by a factor of 2 as well, to 46 units—even though this can obviously not be the case! If the landfill is collecting more methane, how can more be escaping? That is why we are working to improve knowledge about landfill emissions and to determine better ways to determine them, as opposed to relying on default approaches.
- Methane oxidation is particularly important at older, smaller landfills where there is not a gas-collection system. In these situations, methane emissions are assumed to be 100% of gas production, calculated with a model. The only emission reduction is due to methane oxidation.
Author's Bio: Jeff Chanton is the John Widmer Winchester professor of oceanography with Florida State University's Department of Earth, Ocean, and Atmospheric Science. |
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