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WM Responses to Covanta & Peter Anderson Comments on

Methane Oxidation Provision

Covanta Comment:

The Rule’s current defaults allow landfills to assume collection efficiencies of up to 95% and then 

apply a soil oxidation factor of 10%. The EPA is now proposing to increase the soil oxidation factor 

up to 35% in certain circumstances. In stark contrast, the U.S. EPA’s own Office of Research and 

Development, after a multi-site two-year study of measured methane emissions from landfills 

found that “the data collected does not support the use of collection efficiency values of 90% or 

greater as has been published in other studies.” Instead, the recent EPA Report found total 

abatement efficiencies of 38 – 88%, including the effects of soil oxidation of methane in landfill 

cover soils. The effects of soil oxidation are inherently covered, because the EPA study looked at 

methane concentrations above the landfill surface. This technique cannot distinguish between 

methane not emitted and methane oxidized in cover soils.

Response:

The Covanta comments cite the January 2012 report, Quantifying Methane Abatement Efficiency at 

Three Municipal Solid Waste (MSW) Landfills, EPA/600/R-11/033 calling it a multi-site, two-year 

study. The study did not, nor was it designed to measure methane oxidation at the three subject 

landfills. Instead, the report purports to be a comprehensive study of the use of optical remote 

sensing using OTM-10 to measure landfill-wide methane emissions and evaluate methane abatement 

efficiency. In fact, this report summarizes the results of very limited field research designed to 

measure methane from a portion of three landfills, all of which were located in a single climate zone. 

The field research comprised only five methane measurement campaigns performed at the three 

landfills during the Spring and late Fall for Sites A and C, and a single, two-week period during 

Summer for Site B. Not only is the scope of the field research extremely limited, but also the report 

provides neither a robust survey of the scientific literature, nor a survey of the extensive field 

research that has been conducted in varying climate zones and seasons. In fact, the report’s list of 

references is limited to four papers by one scientist (Hashmonay) and eleven EPA reports, seven of 

which were authored by the project manager of this subject report.

The commenter is chastising the Agency for not incorporating the findings of a single, three landfill 

study that did not even attempt to measure methane oxidation, while the Agency instead used a far 

more robust and pertinent data set to underpin its rulemaking. The EPA based its proposed revision 

of the methane oxidation factor on the findings of more than 90 peer-reviewed field and laboratory 

studies of methane oxidation in various types of landfill covers and in a wide variety of climate zones.

The commenter cites a report that makes no conclusions about methane oxidation. Rather, from five 

measurement campaigns at three southeastern landfills, the report develops a range of methane 

abatement efficiencies (38% - 88%), and concludes, based on this limited field work that “the data 

collected does not support the use of collection efficiency values of 90 percent or greater as has 

been published in other studies.” The data set is so limited and so lacking in representativeness, that 
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it really supports no conclusions about methane abatement at all. Furthermore, the study ignores 

the results of OTM-10 research conducted at a far larger universe of landfills (20 landfills located in all 

U.S. climate zones) and which has undergone peer review prior to publication in the scientific 

literature. 

Covanta Comment:

Since the abatement efficiencies published in the EPA Report consider the effects of soil oxidation, higher 

assumed soil oxidation rates, as the EPA has proposed, would require lower collection efficiency defaults in 

order to remain consistent with the EPA’s own work.

Response:

The EPA report in no way attempted to evaluate or measure soil methane oxidation. The study 

merely assumed the default 10% methane oxidation factor. Furthermore, the measurements of 

surface methane above the landfill surfaces were fraught with technical errors that biased the 

emission measurements. The EPA study contractor placed the open path tunable diode laser and 

reflectors either adjacent to or downwind of the landfill gas collection wells (documented in the 

report’s photographs). This improper placement of the methane measurement plane could lead to 

inaccurate surface readings because the measurement plane was placed too close to the most 

probable sources of methane, thus biasing the readings. The researchers also failed to quantify

properly methane emissions from nearby adjacent sources (e.g., the hog farm adjacent to Landfill B), 

which may have contributed to the methane emissions attributed to the landfill. Without resolving 

these serious technical issues, EPA can neither validate the data from this study, nor use the study to 

make conclusions about methane abatement efficiencies at the subject landfills or any other landfills. 

The report cannot be used to draw inferences regarding methane oxidation values. 

Peter Anderson Comment:

“...oxidation only occurs in landfill covers that are comprised only of soil (with the necessary depth,

porosity, temperature and microbes), and not through composite covers that include a lowpermeable geomembrane.”

Response:

The commenter’s statement is factually wrong. The data underpinning the EPA’s rulemaking relied 

upon evaluations of methane oxidation at 90 landfills, looking at oxidation in sandy, clay and mixed 

soil covers, as well as in organic covers composed of composted yard waste. Compost covers have 

been shown to be highly effective in oxidizing methane. Furthermore, composite covers with a lowpermeable membrane in addition to a soil cover are highly effective in mitigating methane emissions 

and they are being applied in the U.S. as final cover and in several countries in Europe for that 

purpose. The geomembrane significantly retards diffusion of methane from the waste layers up to 

the soil layer. As a result, there is no or negligible methane to be oxidized in the soil layer.The 

methane is well contained in the waste layer, where it can be collected by the landfill gas collection 

system. Landfills with composite final covers have been shown to have very low methane emissions 

as measured using the OTM-10 method.
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Peter Anderson Comment:

“...in order to have an effective oxidizing cover, the landfill must sacrifice significant collection 

capacity in the active gas system. How significant? The available field data using vertical radial 

plume mapping suggests almost a 300% difference. “

The commenter’s statement is incorrect. Effective oxidation requires a lower flow velocity of 

methane up through the cover soil to ensure that the oxidative capacity of the cover is not 

overwhelmed by too much methane. Lower methane flow can occur due to effective landfill gas 

capture by the gas collection system, or because the landfill is not producing a high volume of landfill 

gas because it is an older site or is located in a dry climate, which inhibits methane formation. Thus, 

effective landfill gas collection results in low methane loading to the cover which in turn results in 

higher oxidation percentages, approaching 100 percent, . The best way to insure a high percentage

of methane oxidation is with a good gas collection system (Chanton et al., 2011a,b). 

EPA’s technical background document describing the Agency’s review of the scientific data on landfill 

methane oxidation reiterates this conclusion and states, “As methane generated in the landfill flows 

up through a landfill’s cover soil, a fraction of that methane will be oxidized or metabolized by 

methanotrophic bacteria near the soil surface. The relative amount of methane removed from the 

landfill gas stream as the gas passes through the soil surface layer is termed the fraction oxidized or 

oxidation fraction. The fraction oxidized depends on a number of factors including:

 The flow velocity of landfill gas, or the methane flux, through the soil surface (higher

flux rates result in lower residence times in the aerobic soil layer and less oxidation);

 The porosity of the soil layer (greater porosity leads to more air ingress and a thicker aerobic 

soil layer, which leads to more oxidation);

 The number and types of microorganisms in the surface soil layer; and

 The soil surface temperature and moisture, which may increase or inhibit microbial activity.”

The study by Waste Management cited by the commenter summarized the results of several years of 

field research conducted at 20 landfills across the U.S. measuring landfill emissions using EPA’s OTM10 method. The study did not measure or address landfill gas collection efficiency, but measured 

methane flux above the surface of the landfills. The study involved landfills with gas collection, but 

energy generation was not a consistent feature of all the sites studied. 

Peter Anderson Comment:

Of paramount concern, of course, continues to be the controlling fact that the entire oxidation case 

ignores the reality that the minor gains claimed are dwarfed by the enormous increase in gas that 
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escapes since gas collection is ineffectual without a geomembrane, as shown by Waste 

Management’s own study referred to earlier. The gains are relatively minor because even 35% 

oxidation only applies to the 5% or 25% of the gas generation that is not captured. The single most 

effective way to reduce fugitive emissions has nothing to do with oxidation, but with the 

installation of a composite cover, including a geomembrane, which defeats oxidation.

Response:

The field and laboratory data for 90 landfills, which were evaluated by EPA in developing this 

rulemaking, simply do not support Mr. Anderson’s assertion. Successful methane emissions control 

at landfills is a combination of effective gas collection systems, which mitigate methane flow from the 

waste upwards to the cover, as well as properly maintained covers. The covers need not be final 

covers, nor do they need to be composite covers with a geomembrane component to achieve 

oxidation rates well above the EPA default of 10 percent. Final covers, particularly composite covers, 

are not used except at inactive, closed landfills or in closed, inactive landfill cells. While composite 

final covers with geomembrane layers are very effective at mitigating methane emissions, it is very 

clear from the 90 studies of landfill cover oxidation in the scientific literature that intermediate soil 

and organic covers achieve high rates of methane oxidation. The results of these peer-reviewed 

studies of landfill methane oxidation are as follows: 

1. Clay cover: The scientific literature evaluated by EPA included 31 studies of oxidation in clay 

covers. The mean fraction of methane oxidized was 30 percent, while the median fraction 

oxidized was 29 percent.

2. Sandy soils cover: The literature included 16 studies of methane oxidation in sandy soil covers 

with the mean oxidation value being 54 percent, while the median value was 50 percent 

methane oxidized. 

3. “Other” covers: The literature included 30 studies in “other” cover soils with the mean 

oxidation at 36 percent and the median fraction oxidized at 30 percent. 

4. Organic covers: The literature included 12 studies of methane oxidation in organic (compost) 

covers with the mean oxidation at 38 percent and the median fraction oxidized at 37 percent. 

5. The overall mean oxidation value across all of the studies was 38 percent while the overall 

median oxidation fraction was 33 percent. 

EPA also examined methane oxidation as a function of methane loading to the cover layer of the 

landfill. Recent studies show that the percent oxidation is an inverse function of the rate of emission 

(Stern et al.,2007; Rachor et al., 2011; Chanton et al., 2011a,b). At lower emission rates, the 

methanotrophs in the soil cover can consume a larger portion of the methane delivered to them, 

potentially oxidizing 95 to 100 percent (Humer and Lechner, 1999, 2001a, Huber-Humer 2008; 

Powelson et al., 2006, 2007; Kjeldsen et al., 1997). As flux rates increase, the percent oxidation 

decreases and the methanotrophs can become saturated with methane. Thus, as methane emission 

increases, percent oxidation decreases (Powelson et al., 2006, 2007). These studies underscore that 
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effective landfill gas collection works in concert with landfill cover to mitigate fugitive emissions. The 

success of geomembrane covers relates to their ability to trap landfill emissions, but other types of 

soil covers have been shown to oxidize methane at percentages far greater than EPA’s default factor 

of 10 percent.

Peter Anderson Comment:

The decay rate that the rule provides for recirculating landfills is k=0.057. There are two fatal 

problems with using a k value that low. First, the rule indicates that k=0.057 is also to be used for 

dry tomb landfills in areas with 40 inches or more of annual precipitation, but was not calculated to 

be, and was never intended to reflect, the significantly greater moisture levels inside landfills that 

recirculate leachate, and, in consequence, experience twice the subsidence and presumably twice 

the gas generation. Second, the agency’s own hornbook for input values to the first order decay 

model, AP-42, specifically states that the k value for landfills that recirculate leachate, which it 

abbreviates as “wet landfills,” is 0.3, or 5.3× greater.

Response:

The AP-42 chapter for MSW landfills to which Mr. Anderson refers is in draft form, and has not yet 

been finalized by the EPA. Furthermore, the Agency recommends against using the draft chapter, as 

it will undergo significant revision before it is finalized due to a large volume of new data obtained 

since the draft chapter was written. The definition of “wet” landfills on Page 2.4-6 of the draft 

chapter states, “wet landfills are defined as landfills which add large amounts of water to the waste.” 

This definition has no practical application because it is unclear what volume constitutes a “large” 

amount of water. It appears that the imprecision in this definition reflects the imprecision in the 

definition of “wet” used in the Reinhart study upon which the new k value is based. It is widely 

accepted that the first order decay constant k used in the LandGEM model should vary as a function 

of the moisture content of the waste because it has been observed that increasing the waste 

moisture content leads to an enhanced rate of methanogenesis. One problem in relying on the 

Reinhart report to support a new k factor, is that no analysis of the influence of the waste moisture 

content, the amount of water added, or annual precipitation on the modeled k was performed. The 

study classified sites simply as wet or dry without any documented criteria. Without knowledge of 

how wet or dry the sites were, it is not possible to determine under what conditions an enhanced k 

value would apply. 

There are additional sources of data on gas production from full-scale operating bioreactor landfills 

that the EPA should consider in developing a new k factor:

1. A report detailing research on the CRADA project at the Outer Loop landfill uses longitudinal 

landfill gas studies ranging over several years to develop k values based on site-specific 

determination of Lo (USEPA, 2006)1. These data are also presented and evaluated in paper by 

 

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United States Environmental Protection Agency, Landfill Bioreactor Performance, Second Interim Report; EPA/600/R07/060 Office of Research and Development, National Risk Management Laboratory: Cincinnati, OH, 2006.
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Tolaymat, et al. 2010 which indicated a k for bioreactor test cells of 0.11 yr-1 using a site 

specific Lo of 54.8 m3 Mg-1

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.

2. A report on the performance of bioreactor landfills in North America, funded by the EPA’s 

Office of Research and Development contains information on the development of k values 

from several full-scale bioreactor landfills and shows lower k values than the 0.3 yr-1 value 

proposed in the draft AP-42 chapter.

 

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Tolaymat, T.M., Green, R.B, Hater, G.R., Barlaz, M.A., Black, P., Bronston, D., and J. Powell, 2010, “Evaluation of Landfill 

Gas Decay Constant for Municipal Solid Waste Landfills Operated as Bioreactors,” Journal of the Air and Waste 

Management Association, 60:91-7