Methane Generation and Capture of U.S. Landfills
Analysis of the U.S. EPA (Environmental Protection Agency) database of 2,549 MSW (Municipal Solid Waste) landfills showed that there were 1,164 operating landfills in which 348 million short tons (316 million metric tons) of waste were landfilled in 2017.
by Nickolas J. Themelis and A.C. (Thanos) Bourtsalas
Analysis of the U.S. EPA (Environmental Protection Agency) database of 2,549 MSW (Municipal Solid Waste) landfills showed that there were 1,164 operating landfills in which 348 million short tons (316 million metric tons) of waste were landfilled in 2017. In total, these landfills occupy about 370 million square meters of land so it is not possible to monitor the generation of LFG (Landfill Gas) generation accurately, or collect most of the LFG generated. This study was based on the hypothesis that, on the average, methane generation is proportional to the tonnage of wastes landfilled annually. The Landfill Methane Outreach Program of the EPA (EPA-LMOP) compiles annual operating data of all methane-capturing landfills. Our analysis of the 2018 data for 396 LMOP operating landfills showed that 210 million short tons of wastes were deposited and 5.06 million short tons of methane were captured, i.e., an average capture of 0.024-ton CH4/ton waste. On the basis of the anaerobic reaction of the DOC (Degradable Organic Carbon) in landfilled wastes, the average rate of methane generation from all operating U.S. landfills was estimated to be 0.05 ton of CH4 per ton of annual capacity; this number corresponds to bioreaction of about one half of the total organic carbon in MSW. On this basis, the average rate of CH4 emission from the 396 LMOP landfills was estimated to be 0.026-ton CH4 per annual ton of deposition and the average efficiency of LFG capture, 48%. Adding up all 1,164 operating landfills, their total emission of methane was estimated at 11.9 million metric tons of CH4. At CH4/CO2 equivalence of 25, this number corresponds to CO2-eq emissions of 270 million metric tons, i.e., 5.1% of the U.S. energy related carbon dioxide emissions.
Key words: Wastes, LFG, methane generation, methane capture, methane emissions, carbon dioxide emissions, greenhouse gas
Landfilling of post-recycling urban wastes results in significant emissions of methane, a potent GHG (Greenhouse Gas). The U.S. EPA (Environmental Protection Agency) has estimated that landfilling of wastes is the third largest source of methane emission to the atmosphere . Therefore, it is necessary to provide an accurate estimate of the tonnage of wastes landfilled and the associated methane emissions, in order to deploy efficient measures for GHG mitigation. The objectives of this study were to determine:(a) the tonnage of solid wastes disposed at all operating MSW (Municipal Solid Waste) landfills in the U.S., (b) the tons of methane captured at U.S. landfills, as reported by the Landfill Outreach Methane Program) of EPA (LMOP-EPA; ); and(c) use this data to estimate the total methane emissions from all U.S. operating landfills.
Corresponding author: Nickolas J. Themelis, Ph.D., past Chair of Department of Earth and Environmental Engineering, Columbia University; current research area: sustainable waste management.
2. Magnitude of Landfilling in the U.S.
The EPA "Facts and Figures”  on MSW generation are based on estimates of annual consumption of various materials and their respective lifetimes. From this amount are subtracted the reported tonnages of recycling, composting, and combustion with energy recovery (commonly called "waste to energy” or WTE). This methodology resulted in the EPA estimate of 146 million short tons landfilled in 2018 . The Columbia-Biocycle surveys (2003-2012) were based on questionnaires sent to the waste management agencies of the fifty states and compilation of the landfilling and other data they provided; the latest Columbia-Biocycle survey (2012; ) showed that 250 million short tons (227 million metric tons) of various wastes were deposited in U.S. MSW landfills. Later studies by Powell et al. , for 2012, and by EREF (Environmental Research &Education
Foundation) , for 2013, reported total landfilling of 262 million tonnes and 220 million tonnes, respectively; all these numbers were much higher than the EPA Facts and Figures estimate .
The present study relied on the 2018 EPA database of 2,549 municipal solid waste (MSW) landfills. Our analysis of this database (7) showed that there were 1,164 operating landfills in which 348 million short tons (316 million metric tons) of waste were landfilled. It is believed that this number is reliable because all U.S. landfills are required by law to report the tons of solids deposited annually in each landfill. The large difference from the "146 million tons” of the EPA "Facts and Figures”  is due to the fact that U.S. landfills, in addition to what EPA considers as "MSW landfills”, receive industrial and agricultural wastes, as well as roadkill, wastewater sludge, and other residues of human activity. The magnitude of landfilling in the U.S. is illustrated by the fact that, in total, 7.9 billion tonnes of wastes have already been deposited in U.S. operating landfills ; 931 of these landfills, containing 7.4 billion tons of wastes, reported using 370 million square meters of land. The total design capacity of 970 US reporting landfills is 20.8 billion metric tons of wastes; assuming current rates of landfilling, the U.S. operating landfills have remaining capacity of at least 40 years.
3. Estimate of Maximum Methane Generation per Ton of Solids Landfilled
Table 1  shows estimates of the atomic composition of various biogenic components of the urban wastes. Sulfur and nitrogen are relatively minor components and occur principally in food wastes.
Excluding the minor elements, the average C,H,O molecular structure in U.S. MSW can be approximated by the molecular composition C6H10O4, as recommended by Themelis and Ulloain a widely cited publication. This composition corresponds to the C,H,O structural formula of at least ten organic compounds, such as ethyl butanedioic acid, succinic acid, adipic acid, ethylene glycol diacetate, and others.
The complete anaerobic bioreaction of the compound C6H10O4 is as follows:
According to this equation, the concentration of methane in LFG should be 3.25/6 = 54%. However, our analysis of the 2018 LMOP database  showed an average concentration of 47% methane in LFG, probably due to partial oxidation of methane in the upper layer of landfills. This concentration was assumed in converting the volumes of LFG, in the LMOP database, to tons of methane.
One metric ton of MSW, excluding moisture, plastics and inorganic materials, contains about 370 kg of C6H10O4; if completely reacted anaerobically, it would generate 0.135 tons CH4 per ton of MSW. However, Cossu et al.  estimated that the readily biodegradable fraction of MSW is about one half of the total organic carbon (Table 2).
4. Methane Capture at 396 U.S. Operating Landfills
Biodegradation occurs over several years after deposition but since the rate of deposition in a particular landfill does not change much from year to year, it was assumed that the rate of methane generation during any year is proportional to the annual rate of deposition. In the LMOP spreadsheet , the biogas volume captured was reported in million standard cubic feet per day ("mmscfd”). These numbers were converted to maximum tons of methane captured per year in each landfill, by assuming 365 days/year operation and 47% methane in the LFG. i.e., the average CH4 concentration in LFG reported in the LMOP database . On the basis of these two assumptions, the methane generation corresponding to 1 million standard cubic feet of captured LFG/day was calculated to be 3,818 short tons of methane per year.
The authors’ analysis of the EPA-LMOP database  included 396 MSW landfills which received 210 million short tons of waste in 2018. The spreadsheet of the analysis is too large to include in this report but it is available on the web ; as an example, Table 3 presents the reported and calculated data for th largest thirty California landfills. Fig. 1 is a plot by the authors of the reported tonnages of methane collected at all EPA-LMOP landfills, as a function of the tonnage landfilled annually in each landfill. As expected, there is substantial variation among these landfills, especially for those of very large capacity (and land area), where collection is more difficult. The average rate of capture was found to be 0.024 ton of methane per ton of solids deposited. In comparison, the average capture of California landfills (Table 3) was 0.027ton methane/ton of annual capacity and of New York landfills , 0.023 ton/ton annual capacity.
The results of the authors’ analysis  of the EPA-LMOP spreadsheet are shown in Table 4.
5. Comparison of Results with Other Estimates of Landfill Methane Emissions
As noted earlier, the complexities of monitoring the enormous surface areas of landfills make difficult the measurement of methane fluxes from landfills. A notable exception is the aerial survey of four Indiana landfills by Cambaliza, Shepson et al. . Table 5 compares the measured emission rates of these landfills with the emission data calculated on the basis that, on the average, landfills generate 0.05 tons CH4per ton of MSW landfilled.
The estimated methane emissions are based on the assumption that the Indiana landfills generated 0.05 ton CH4 per ton of annual capacity; from this amount was subtracted the reported amount of CH4 captured at these four landfills, as per the LMOP database . The measured numbers were obtained by multiplying the reported fluxes , in mol/s, by the number of seconds in a year. For two Indiana landfills, there is fair agreement between calculated and measured emissions; for the other two, the measured fluxes are about one half of the estimated emissions.
The U.S. Environmental Protection Agency  has estimated that in 2018 the landfill GHG emissions were 110.7 million metric tons of carbon dioxide. This estimate is 41% of our estimate of 270 million metric tons (Table 4) for two reasons:
Despite many technical papers on sanitary landfilling, there is no published information as to the estimated generation of methane per ton of waste landfilled. Yet, in the light of recent catastrophic events attributed to climate change, this information is essential to governments concerned with climate change in deciding wheter post-recycling wastes should be disposed in landfills or used as fuel in WTE power plants. Climatic conditions vary widely across the continent and organic carbon can take years to fully decay. Also, it is not possible to monitor or simulate accurately methane generation in landfills spread over hectares of land. For the above reasons the GHG emissions of ladfills have been underestimated. On the basis of the numbers presented in this paper, aconservative estimate of the average generation of methane at U.S. landfills is 0.05-ton CH4/ton of waste landfilled annueally, which is less than one half of the calculated number (0.116-ton CH4/ton waste) for complete bioreaction of the bioderadable organic carbon contained in the landfilled materials. The average capture and utilization of methane at the 396 operating landfills under the EPA-LMOP program was found to be at 0.024-ton CH4/ton of waste landfilled, corresponding to an average landfill gas capture efficiency of 48%. It is interesting to note that, on the average, California landfills captured 0.027-ton CH4/annual ton landfilled and New York landfills, 0.023-ton CH4/ton, despite the differences in climate and geography.
According to the above data, a landfill of annual capacity of one million tons and assumed life of 40 years is projected to generate, in its lifetime, a total of two million tons of methane (=0.05 × 1,000,000 × 40). When the landfill is equipped to capture and utilize some of this methane, as in the case of the 396 EPA-LMOP landfills, the methane emissions to the atmosphere can be reduced by at least 48%. In total, in 2018, the U.S. landfills emitted 10.8 million metric tons (11.9 million short tons) of methane. On a 100-year horizon, this was equivalent to 10.8 x 25 = 270 million metric tons of carbon dioxide, i.e., 5.1% of the U.S. energy related emissions (5.27 billion metric tons; ).
This study was sponsored by the Earth Engineering Center of Columbia University in the City of New York.
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published: David Publishing, 7|2021
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