Potential fugitive emissions from landfills are traditionally estimated by a method that begins by assuming that 100% of emissions will escape, and then deducts a default factor to represent the assumed efficiency of the collection system. This is problematic, because precise estimation of subsurface gas generation is often quite difficult, and because the concept of default collection efficiencies falsely assumes that all landfills are the same. This approach is flawed, because it is incorrect to assume that all gas that goes uncaptured by the collection system will be released as fugitive emissions.
The most commonly assumed collection efficiency is the EPA’s “default” of 75%. The EPA allows that site-specific collection efficiencies, where available, should be used instead of this default percentage. However, many regulators and operators are reluctant to adopt a different value, for fear that assuming a higher efficiency factor would be perceived as insufficiently conservative or as somehow flouting the EPA’s recommendation. This is particularly troubling, given that an exploration of the scientific literature reveals that the 75% figure has little grounding in scientific study.
Fugitive emissions are best considered in an analytical framework that considers both a landfill’s capture efficiency and its unique emission potential. Field monitoring studies, or comparative studies based on field monitoring of similar landfills, will yield more precise results than estimation or computation based on default values. Ideally, these will take the form of a “bracketed range” of potential emissions, with upper and lower bounds based upon worst-case and average-case scenarios, respectively.
This approach addresses the true concern: the actual emissions potential of the landfill. By setting aside the uncertainties of how much gas has been generated within the landfill and moving away from an overreliance on determining average capture efficiency percentage, the field monitoring approach directly addresses the question that any analysis should truly seek to answer: How much gas is escaping from the landfill?
LFG Generation
Waste undergoes a number of biological stages after being deposited in a landfill. When a section, or “cell,” of waste is first placed, oxygen is present in cell cavities as moisture in the cell bed and is lightly bonded to the waste. As a result, landfill waste decomposition is initially aerobic: It occurs in the presence of oxygen. Over time, this oxygen is consumed, the landfill waste degradation becomes anaerobic, and the decomposition of waste occurs in an oxygen-deficient environment.
While microorganisms that consume waste aerobically “exhale” water and carbon dioxide, as do humans, microorganisms that consume waste anaerobically “exhale” carbon dioxide and methane gas, and the relative composition depends on the subsurface environment. Therefore, landfill gas generally has little to no oxygen, and abundant carbon dioxide and methane. Carbon dioxide is abundant in landfill gas, because it is the simplest fully oxidized carbon compound. Methane is similarly abundant, because it is the simplest carbon compound that lacks oxygen. As landfill waste degrades, the methane content of the gas that is produced is typically around 50%.
Landfills have traditionally collected, or “captured,” landfill gas using buried pipes in order to prevent underground gas migration, the release of untreated emissions from the surface, and potentially explosive conditions. Today, traditional safety concerns regarding landfill gases such as methane and nonmethane organic hydrocarbons remain, but they have been joined by public concerns about emissions of odorants and greenhouse gases. According to the EPA, methane is about 21 times more powerful (by weight) than carbon dioxide at warming the atmosphere (United States Environmental Protection Agency: www.epa.gov/methane/scientific.html).
Future state and federal greenhouse gas regulations will undoubtedly address fugitive methane emissions from landfills. Consequently, it is more important than ever to address 21st century fugitive emission concerns with 21st century estimation methods.
Current Default Methodologies
Many of the traditional methodologies for calculating fugitive emissions from landfills share one fundamental commonality: They oversimplify the issue and assume that all landfills are essentially alike.
The most common example of this is the EPA’s “default collection efficiency” of 75%. This figure is often taken to represent the average, standard efficiency of landfill gas collection systems. The agency’s AP-42 document, which addresses emissions estimation, states that “collection efficiencies typically range from 60% to 85%, with a default efficiency of 75% recommended” (United States Environmental Protection Agency’s AP 42, fifth edition, Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Chapter 2: “Solid Waste Disposal,” November 1998). The EPA wisely recommends that default factors be replaced by site-specific data whenever possible. However, this recommendation is usually overlooked, and the 75% default value is often cited as a conservative guideline. This raises the question: Conservative when compared to what? What is the basis for this 75% capture efficiency?
The study that provided the original reference and lone basis for the 75% capture efficiency figure cited in AP-42, focused on landfill gas generation, not capture efficiency. The 75% capture efficiency in the study did not refer to measured or estimated data from any landfill. Instead, it reflected an assumption made by the study’s authors and should not be misconstrued as a factor based on data. The study assumed a 75% collection efficiency to examine trends in methane generation rates using a study-specific generation model. The reference then concluded that a 75% collection efficiency assumption was a “quite good” fit for its landfill generation model with regard to the one particular landfill that was being studied (Reference 52 in United States Environmental Protection Agency’s AP 42, fifth edition, Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Chapter 2: “Solid Waste Disposal, Background Document,” November 1998). Although this may have been valid for the landfill in question, this should not be the basis for a model of every landfill in the entire country.
Many subsequent papers and studies have pointed out the deficiency of relying on the 75% default factor. However, they often go on to recommend another figure, be it 41% (Spokas, K., Bogner. J., Chanton, J. P., Morcet, M., Aran, C., Graff, C., Moreau-Le Golvan, Y., and Hebe, I. “Methane Mass Balance at Three Landfill Sites: What is the Efficiency of Capture by Gas Collection systems?” Waste Management, Vol. 26, Issue 5, 2005, pp. 516–525.), or 99% (Huitric, R. Kong, D., Scales, L., Maguin, S., and Sullivan, P. “Field Comparison of Landfill Gas Collection Efficiency Measurements”; SWANA 2007 Landfill Gas Symposium, Monterey, California, March 4–8, 2007), or another set percentage. Although those figures may have a firmer basis than the assumption relied on by the EPA in the AP-42 document, they ultimately represent a good fit for one specific landfill or even a particular type of landfill, not all landfills across the country. No single efficiency factor can accurately translate to all landfills, particularly those about which no site-specific data is available. Given the growing focus on preventing fugitive landfill gas emissions, the utility of a single default value for capture efficiency is dubious. While extrapolating data from like facilities allows for approximation in some cases, each landfill is unique relative to its emissions potential.
Landfill gas generation is directly affected by types and mixtures of landfill waste. Often, estimates of fugitive emissions rely on computer models to estimate generation of gas within the landfill, utilizing the EPA’s LandGEM or another model. Most of these models determine landfill gas generation based on default or project-specific waste constants. When information about a landfill’s waste composition and other factors (percent organics, percent initial moisture, etc.) is available, these models give more accurate predictions. Unfortunately, the waste data necessary for accurate modeling is not readily available for most existing landfills, rendering the modeling results more of a “best guess.” In general, landfill gas computer models can only predict gas formation within a factor of 2 (USEPA First-Order Kinetic Gas Generation Model Parameters for Wet Landfills, EPA-600/R-05/072, June 2005).
Thus far, we have seen the limitations of default efficiency factors, and of modeling gas generation based on an incomplete set of data. However, there is one fundamental misconception that reigns over the entire issue of capture efficiency. This misconception is the flawed and oversimplified calculation that
100% of Gas Generated
– % of Collected Gas Emissions
= Fugitive Gas Emissions
Capture efficiency and fugitive emissions are often discussed as if the amount of gas captured were directly proportional to fugitive emissions: “If capture goes down, emissions go up.” The mass balance equation may suggest that this is true, but this mindset relies too heavily on the mistaken assumption that gas loadings measured from deep within the landfill are the same as those that migrate to the surface. In reality, unique features of the landfill also play a role in mitigating landfill gas releases, as will be discussed subsequently. While accurately determining capture efficiency is an important part of the fugitive emissions equation, the percentage of gas captured does not inversely correlate with the percentage of gas that escapes.
Capture Analysis: Superior Methods
As we have seen, traditional estimation approaches are beset by methodological flaws and oversimplified assumptions. Figure 1 illustrates the variety of pathways by which we can determine fugitive emissions, including both traditional approaches and more site-specific approaches. What approaches make the most sense for a regulatory environment in which fugitive gas emissions from landfills face increasing scrutiny?
As the EPA notes in AP-42, a site-specific surface sampling program is preferable to the use of the default collection efficiency value of 75%. We will go further: A comprehensive sampling approach is the optimal method of analyzing fugitive emissions, because this approach best accounts for the individuality, the unique capture goals, and the operations of a landfill.
However, even some site-specific analyses have an overreliance on determining the amount of gas generated. They begin by obtaining the amount of landfill gas being captured by the collection system, use computer models to estimate the total amount of gas within the landfill, and ultimately subtract the former value from the latter value to obtain an amount of fugitive emissions (This method was used for a comparative study in: Michels, M.S., and Hamblin, G. M. “LFG Collection Efficiency is Improving in Wisconsin,” SWANA 2006 Landfill Gas Symposium, St. Petersburg, Florida, March 2006). As discussed earlier, this premise is far from certain, and generation models have an inherent margin of error.
By contrast, field monitoring of fugitive emissions from a landfill’s surface does not rely directly on a generation model to determine capture efficiency. Instead, this approach sidesteps the ill-conceived notion that capture efficiency is the key to determining fugitive emissions, and addresses fugitive emissions head-on. There are many methods for measuring emission flux at landfills. These methods have been outlined in a number of recent papers in which field comparisons have been conducted (Babilotte, A., Lagier, T., Fianni, E., and Taramini, V. “Fugitive methane emissions from landfills: A field comparison of five methods on a French landfill”, Global Waste Management Symposium, Colorado, September 2008.) (Tregoures, A., Beneito, A., Berne, P., Gonze, M.A., Sabroux, J.C., Savanne, D., Pokryszka, Z., Tauziede, C., Cellier, P., Laville, P., Milward, R., Arnaud, A., Levy, F. and Burkhalter, R. “Comparison of seven methods for measuring methane flux at a municipal solid waste landfill site,” Waste Management Research, 1999; 17; 453–458.).
The results of studies conducted at three French landfills offer a good example of collection efficiencies calculated using this approach. These studies present landfill methane emissions measurements using approaches that “draw a box around the landfill” and measure actual emissions rather than estimating total landfill gas in the system (Tregoures, A., Beneito, A., Berne, P., Gonze, M.A., Sabroux, J.C., Savanne, D., Pokryszka, Z., Tauziede, C., Cellier, P., Laville, P., Milward, R., Arnaud, A., Levy, F. and Burkhalter, R. “Comparison of seven methods for measuring methane flux at a municipal solid waste landfill site,” Waste Management Research, 1999; 17; 453–458) and (Morcet, M., Aran, C., Bogner, J., Chanton, Spokas, K., and Hebe, I. “Methane Mass Balance: A Review of Field Results From Three French Landfill Case Studies,” Proceedings Sardinia 2003, Ninth International Waste Management and Landfill Symposium, published by CISA [Environmental Sanitary Engineering Centre], Italy).
There are many factors that must be considered when using this approach to ensure that the data collected are representative. An effective method is to “bracket” the emissions. This is achieved by collecting some samples that will over-predict overall emissions, taken from areas likely to have above-average fugitive emissions, and some samples that will closely predict overall emissions, taken from areas likely to have average fugitive emissions. These samples will, respectively, present upper and lower bounds for potential fugitive emissions from the landfill. The fugitive emissions value is then conservatively “bracketed” within these two values. This approach, even when considering the error associated with sampling, as well as the diurnal, seasonal, and life cycle fluctuations present at the time of sampling, can paint a more realistic picture of potential fugitive emissions than other methods. By setting aside the uncertainties of how much gas has been generated within the landfill, the field monitoring approach addresses the true issue: how much gas is escaping from the landfill?
As the field monitoring approach may prove impracticable for certain facilities, a comparative approach may be preferred. In these cases, it is reasonable to assume that the landfill in question has similarities to other landfills that have been carefully studied. While we have repeatedly stressed that not all landfills are alike, within the set of all landfills there are certainly subsets with very strong similarities, and each subset contains landfills that have undergone extensive examination. The key to this approach is to identify those landfills for which data is available and that are most similar to the landfill in question, and then to compile that data, cell by cell, for a specific landfill in question.
For example, the user’s manual for the Mexico landfill gas model includes a table that lists landfill characteristics related to gas collection systems (User’s Manual, Mexico Landfill Gas Model, Version 1.0, Prepared by SCS Engineers on behalf of the USEPA’s Landfill Methane Outreach Program, November 2003). Once the user determines specific similar characteristics, landfill collection efficiency can be estimated. Cell-by-cell assumptions based on a point in time in the cell’s life cycle are also cited in an update on current state-of-the-practice prepared for the Solid Waste Industry for Climate Solutions (Current MSW Industry Position and State-of-the-Practice on LFG Collection Efficiency, Methane Oxidation, and Carbon Sequestration in Landfills, prepared for Solid Waste Industry for Climate Solutions by SCS Engineers, July 2007 and July 2008). This analysis suggests generic factors for cells based on whether the cells are active, have intermediate cover, or are closed with final cover.
Site-specific analysis—whether it be based on field work or a comparative analysis of similar, closely studied landfills—offers more precise and realistic estimations of fugitive emissions. In moving away from assuming default factors, modeling based on incomplete data, or inversely correlating collected emissions to fugitive emissions, we must embrace two fundamental analytical principles. First, the use of site-specific data is always preferable; and second, the consideration of unique landfill factors, which have a significant impact on fugitive gas emissions, must be taken into account.
Unique Landfill Factors
Traditional approaches to landfill emissions estimation often overlook the influence of unique factors that can potentially mitigate fugitive emissions. These factors ultimately play a significant role in a landfill’s fugitive gas emissions, mitigating or increasing fugitive emissions beyond the effects of the collection system. The presence of these unique factors means that some potential emissions are neither captured nor emitted as fugitive. A methodology of accurately measuring fugitive emissions must take into account unique landfill factors, including biofiltration, climate, cover, and balancing.
A unique landfill feature that influences fugitive emissions is biofiltration: the biochemical transformation of methane and other landfill gas components as they filter through landfill cells and cover materials. For example, methane is both formed as waste degrades and consumed by aerobic bacteria. As uncollected gases migrate toward the surface, they travel through layers of the landfill. Gas can be absorbed, adsorbed, or digested in the process. This is especially true as migrating gas nears the surface and infiltrates oxygenated soils, where it can be converted to carbon dioxide and water by aerobic digestion. The breakdown of methane in the soil reduces overall fugitive emissions, and should be considered in any analysis of a landfill’s fugitive gas releases. The effect of methane oxidation has been studied in many landfills (Oonk, H. and Boom, T. “Landfill Gas Emission Measurements using a Mass-Balance Method.”) and (Bogner and Spokas, K., “Landfill CH4: Rates, Fates, and Role in Global Carbon Cycle”, Chemosphere, Vol. 26, Nos. 1–4, pp 369–386, 1993.) and (Abichou, T., Mahieu, K., Chanton, J., Romdhane, M., Mansouri, I., Green, R., Johnson, T., Hater, G., Barlaz, M. and Goldsmith, D. Using a Modeling Approach to Estimate Methane Emissions and Oxidation from Landfills within AP-42 Guidelines, Global Waste Management Symposium, Colorado, September 2008.) and is particularly prevalent during the warmer months (Borjesson, G., Chanton, J., and Svensson, B. “Methane Oxidation in Two Swedish Landfill Covers Measured with Carbon-13 to Carbon-12 Isotope Ratios”, Journal of Environmental Quality 30: 369-376, 2001) and (Diot, M., Bogner, J.E., Chanton, J., Guerbois, M., Hebe, I., Moreau-le-Golvan, Y., Spokas, K., and Tregoures, A., LFG Mass Balance: A Key to Optimize Landfill Gas Recovery, Proceedings Sardinia 2001 International Solid and Hazardous Waste Symposium, published by CISA, University of Cagliari, Sardinia, 2001).
Climate has a direct effect on landfill gas emissions, with both daily and seasonal weather changes exerting a large influence. Regional weather conditions also play a large role in the formation of landfill gas in certain climates in the United States, with rainfall and temperature changes influencing fugitive landfill gas generation and releases alike. The changes in gas dynamics brought on by rainwater are utilized by bioreactor technologies towards a similar purpose (Gardner, B. State-of-the-Practice for Energy Recovery from Bioreactor Landfills, 11th Annual LMOP Conference and Expo, January 2008). Variations in the levels of methane oxidation under different types of weather have been noted (Borjesson, G., Chanton, J., and Svensson, B. “Methane Oxidation in Two Swedish Landfill Covers Measured with Carbon-13 to Carbon-12 Isotope Ratios”, Journal of Environmental Quality 30: 369-376, 2001) and (Diot, M., Bogner, J.E., Chanton, J., Guerbois, M., Hebe, I., Moreau-le-Golvan, Y., Spokas, K., and Tregoures, A., LFG Mass Balance: A Key to Optimize Landfill Gas Recovery, Proceedings Sardinia 2001 International Solid and Hazardous Waste Symposium, published by CISA, University of Cagliari, Sardinia, 2001). The climate effect on emissions rates was examined in a recent study in France, which demonstrated that barometric pressure fluctuations could result in changes in landfill gas storage capacity that temporarily exceeded anticipated fugitive emissions (Diot-Morcet, M., Aran, C., Bogner, J., Evaluation of the seasonal variation of the methane mass balance at a French landfill, proceedings, SWANA Landfill Gas Symposium, Monterey, CA, March 25–28, 2002).
The choice of cover material and the stage of landfill cover present at any given time have a large impact on the mitigation of fugitive emissions. Landfills can be divided into areas based on their degree of cover: These generally include active areas, areas with intermediate cover, and areas of final cover. Emissions from areas with intermediate or final cover will vary based on the type of cover material chosen and the amount of rainwater that is displaced by installing more permanent covers that shed water. Each cover type produces a corresponding mitigation in the release of fugitive emissions (Chanton, J.P., Powelson, D.K., Abichou, T., Hater, G., and Bogner, J, Improving Stable Isotope Estimates of the Amount of Methane Oxidized in Landfill Covers, Global Waste Management Symposium, Colorado, September 2008.). A recent study showed a significant difference in fugitive emissions between clay and geosynthetic covers (Mosher, B.W., Czepiel, P. M., Harriss, R. C., Shorter, J. H. Kolb, C. E., McManus, J. B., Allwine, E., and Lamb, B. K. 1999. “Methane Emissions at Nine Landfill Sites in the Northeastern United States,” Environmental Science and Technology, 33 (12), 2088–2094, 1999.).
Some landfills have little to no monitoring requirements and are not adjusted on a daily, weekly, or monthly basis. By contrast, other landfills, such as highly regulated NSPS facilities or facilities that have experienced odor problems, have very strict monitoring requirements aimed at limiting fugitive emissions and odor. The impact of well balancing, the presence of landfill operators, and the degree to which emissions are (or could be) monitored will all affect fugitive emissions (Augenstein, D., Yazdani, R., Imhoff, P., Improving Landfill Methane Recovery—Recent Evaluations and Large Scale Tests, presentation at Methane to Markets Partnership Expo, China, 2007). A recent odor screening study using NSPS monitoring methods concluded that fugitive emissions from a closed cell are extremely low (Huitric, R. and Kong, D. Measuring Landfill Gas Collection Efficiency Using Surface Methane Concentrations, SWANA 2006 Landfill Gas Symposium, St. Petersburg, Florida, March 27–30, 2006).
In summation, an examination of fugitive emissions that is not site-specific, or does not take into account unique landfill factors, cannot be considered accurate or comprehensive.
Conclusions
Many of the traditional methodologies for determining fugitive emissions from landfills exhibit an overreliance on capture efficiencies and landfill gas generation estimates. Ultimately, the effectiveness of a landfill’s efforts to control fugitive emissions ought to be based upon the actual emissions that are released from the landfill, not estimations of what is being created underground.
Estimation of fugitive emissions should draw upon site-specific data obtained by field monitoring of the landfill or comparative analysis of field monitoring data from similar landfills.
An analysis should look beyond capture efficiency to consider unique landfill factors such as climate, cover, biofiltration, and balancing when determining a landfill’s actual fugitive emissions potential.
Fugitive emissions are best considered in an analytical framework that considers a landfill’s percentage of fugitive emissions within a bracketed range, taking into account both capture efficiency and unique factors. The bracketed range of potential emissions considers upper and lower bounds based upon worst-case and average-case scenarios, respectively.
Ultimately, capture efficiency is not important in and of itself. Determining capture efficiency may not even be necessary, depending on the emissions-potential estimation approach used. Examining actual fugitive emission potential is far more important.