Document ID: EPA-HQ-OAR-2012-0934-0119
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2013-11-13T05:00Z

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 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 low-permeable 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 low-permeable 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.

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 OTM-10 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 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 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 5.3x 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).  These data are also presented and evaluated in paper by 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.

   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.