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| Fig. 1: One of many landfills used for waste disposal in Malaysia, and a potential candidate for a landfill methane capture program. (Source: Wikimedia Commons) |
Methane released by decomposing waste in landfills presents a major contribution to greenhouse gasses emitted into the atmosphere. [1] In the United States, emissions from landfills comprise 72.5% of emissions from the waste sector. [2] With greater collective consciousness of global warming has come an increased interest in mitigating the carbon emissions caused by landfills. Methane is of particular concern as the IPPC estimates that methane possesses 23 times the global warming potential per unit volume of carbon dioxide (CO2). [3] One method of curbing methane emission is through capture and re-use of methane gas escaping from landfills. Such landfill methane recapture projects serve to both reduce the amount of carbon emitted into the atmosphere directly through capture of methane before it is released into the atmosphere as well as providing a potential source of power which can offset power generated through burning of fossil fuels.
Methane produced in landfills is typically the product of decomposition of organic wastes added to landfills, including food scraps, yard waste, and other organic wastes. [1,3,4] Shortly after such wastes are added to landfills, they undergo aerobic oxidation. Aerobic oxidation produces primarily water and carbon dioxide as byproducts. This process primarily occurs near the surface of landfills as it can only occur in the presence of atmospheric oxygen. [4] The majority of decomposition occurs after the waste has been covered over by additional layers of waste. Deprived of atmospheric oxygen, remaining organic waste is digested anaerobically in 3 stages. Bacteria first hydrolyze organic matter into soluble molecules. These are then converted into organic acids, which can then be broken down by methanogenic bacteria into CO2 and methane. [3]
In addition to methane, biodegredation of these materials produces carbon dioxide as well as trace amounts of so-called non-methane organic compounds (NMOCs). [4] While these gasses pose a risk of atmospheric and environmental pollution, reclamation efforts tend to focus on methane. Not only is methane a particularly concentrated source of pollution by volume, methane is also useful as a fuel source for generating electricity. While methods of electricity generation using methane as a fuel cell substrate on which to conduct reverse methanogenesis, the most common method of generating electricity from methane involves burning methane gas. [2,5,6] These methane-burning power sources utilize combustion of methane gas similarly to the use of fossil fuels in traditional power plants. [2]
Financial returns on landfill methane capture programs are twofold. The first is through carbon credit awarded when captured methane is re-used in a renewable energy generation strategy. The second is through sale of the electricity generated using methane recapture programs. [7] In particular, let us focus here on a particular case study of the efficacy of potential implementation of large-scale landfill methane capture programs in Malaysia. Malaysia is a case study of particular interest as it is a populous and developing country, with many developing urban areas. Recent growth in the nation's population has also led to a massive increase in waste generation, particularly municipal solid waste (MSW) generated in urban areas. From 1700 tons of MSW generated per day in 2002, the level of MSW generated per day has risen to 3100 tons per day in 2020. [8,9] First, we consider the total amount of methane which can be captured. Most of this waste is disposed of in landfills, such as the landfill pictured in figure 1, with most waste passing into unsanitary open dumping sites. [10] A total of six sanitary landfills were in operation in peninsular Malaysia in 2012. Publicly available estimates of the methane capture of these landfills has them accounting for 2,168 tons of recaptured methane, or 45,538 tons of CO2 equivalent. [7] The total amount of methane generated by unsanitary or unmanaged landfills is more difficult to quantify. Direct measurements are not conducted on the vast majority of landfills that do not already capture methane. [7] Johari et al. instead estimate this quantity using the IPCC methodology using the equation [11]
| CH4 Emissions (tonnes) | = | MSWT × MSWF × MCF × DOC × DOCF × F × | 16 12 |
MSWT is the total generated waste. This quantity is derived from existing estimates of the amount of MSW generated in Malaysia. MSWF is the fraction of municipal waste disposed of in landfills, estimated to be 80%. [10] MCF is the methane correction factor. This is typically dependent on landfilling practices and ranges between 0.4 and 1. As most of the landfills in Malaysia are unmanaged, a value of 0.6 is appropriate. [11] DOC is the fraction of degradable carbon, DOFF is the fraction of carbon that degrades in practice, and F is the fraction of methane in LFG. These latter 3 terms are estimated using standard IPCC parameters for landfills in developing nations to be DOC = 0.14, DOCF = 0.77, and F = 0.55. [7,11] This gives an estimated 310,220 tons of methane gas generated in peninsular Malaysia in 2010.
If captured, this gas can provide returns through both carbon credits and electricity generation. Using a standard carbon credit rate of US $13.20 ton-1 CO2 equivalent, capture of this much methane gas would yield a return of US $85 million, or 257 million Malaysian Ringgit in 2010. Using an estimate for return developed by Shin et al., we can estimate the total return of electricity from harnessing this much methane to be ~1.9 × 109 kWh of electricity, or about 1.5% of Malaysia's energy consumption in 2010. [7,10]While the exact details of how methane capture programs could be implemented in various landfills an Malaysia, it is clear that potential benefits could be gleaned from such programs. With contributions to MSW expected to rise over subsequent years, landfill methane capture and re-use could provide an alternative energy source to meet Malaysia's rising energy needs as well as curb carbon emissions. [7]
© Emiily thierstein. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. Spokas et al., "Methane Mass Balance at Three Landfill Sites: What Is the Efficiency of Capture by Gas Collection Systems?" Waste Manage. 26, 516 (2006).
[2] "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021," U.S. Environmental Protection Agency, EPA 430-R-23-002, 2023.
[3] N. J. Themelis and P. A. Ulloa, "Methane Generation in Landfills," Renew. Energy 32, 1243 (2007).
[4] M. F. M. Abushammala et al., "Regional Landfills Methane Emission Inventory in Malaysia," Waste Manage. Res. 29, 863 (2011).
[5] S. Kondaveeti et al., "Methane as a Substrate for Energy Generation Using Microbial Fuel Cells," Indian J. Microbiol. 59, 121 (2019).
[6] M. J. McAnulty et al., "Electricity From Methane By Reversing Methanogenesis," Nat Commun 8, 15419 (2017).
[7] A. Johari et al., "Economic and Environmental Benefits of Landfill Gas From Municipal Solid Waste in Malaysia," Renew. Sustain. Energy Rev. 16, 2907 (2012).
[8] S. Kathirvale et al., "Energy Potential From Municipal Solid Waste in Malaysia," Renew. Energy 29, 559 (2004).
[9] S. H. Fauziah, C. Simon, and P. , Agamuthu, "Municipal Solid Waste Management in Malaysia - Possibility of Improvement?" Malaysian J. Sci. 23, 61 (2005).
[10] U. N. Ngoc and H. Schnitzer, "Sustainable Solutions For Solid Waste Management in Southeast Asian Countries," Waste Management 29, 1982 (2009).
[11] W. T. Tsai, "Bioenergy From Landfill Gas (LFG) in Taiwan," Renew. Sustain. Energy Rev. 11, 331 (2007).
