Methane, one of the main greenhouse gases (GHGs), has been assessed to have 28 times the global warming potential (GWP) of carbon dioxide over a 100-year time horizon in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). In municipal solid waste (MSW) landfills, methane is generated as a product of the anaerobic degradation of organic waste. United States Environmental Protection Agency (U.S. EPA) estimated that, in 2016, landfill methane emissions in the U.S. were approximately 107.7 million tons carbon dioxide equivalent (Mt CO2 e). And globally, it was estimated that methane emissions from landfilling of solid waste were 794.0 million tons of CO2 e in 2005. At both the U.S. and the global levels, landfilling was the third largest source of methane emissions, after enteric fermentation and natural gas & oil systems.
A broad range of topics about methane emissions from landfill are covered in this report, including the gas-generating processes in landfill, the theories about modeling landfill gas generation and emission, the developed models and the current estimates of landfill emissions, as well as the calculation and analysis on several aspects: 1) theoretical maximum methane generation per ton of MSW and actual methane emission per ton of MSW; 2) climate zone statistics about landfill gas generation model parameter, landfill methane generation, emission and recovery; 3) the time series of global landfill methane emissions with regional analysis and per capita analysis. The findings provide both theoretical information and empirical data on landfill methane emissions.
Currently, the most widely used model could be the 2006 IPCC Guidelines First-Order Decay (FOD) Method, which has been used by many countries to develop their national greenhouse gas inventories. In recent years, new methods based on direct measurements have been developed, such as the Back-Calculation Method used in the Greenhouse Gas Reporting Program (GHGRP).
The empirical formula of dry degradable organic waste in the U.S. is estimated as C6H9.21O3.73 when ignoring nitrogen (N) and sulfur (S). Methane generation per ton of MSW in the U.S. has been calculated to be 0.135 ton (or 189 Nm3) at maximum, which is 9% less than the previous estimation.
The actual landfill methane emissions per ton of MSW in the U.S. are much lower than this theoretical maximum generation value. The reason of the gap could be: 1) landfill gas collection systems, landfill gas destruction (flaring) and utilization projects reduce the methane emissions, 2) the intrusion of air at some parts of the landfill diverts the anaerobic degradation to aerobic degradation, 3) the biodegradable components in MSW cannot fully biodegrade due to their intrinsic properties and other limiting factors such as water content, temperature and pH.
Under dry basis, the degree of the biodegradation of the biodegradable components in U.S. MSW has been estimated to be 53.6%. At this degree, the expected methane generation would be 0.072 ton CH4 / ton MSW. Besides, the excessive underestimation of the quantity of landfilled MSW in the U.S. in EPA’s annual summary figures and tables of waste management has also been detected.
The GHGRP landfill facility-level data and the Köppen-Geiger climate classification GIS data are used to derive climate zone statistics.
For methane generation rate k, the order of main climate types, from in which the k value of bulk waste is high to in which that is low, would be warm temperate (C), equatorial (A), snow (D), and arid (B), or ACDB under another calculation option. This indicates precipitation/water may play a more important role than temperature in the generation of landfill gas.
For methane generation ratio (tons CH4 / ton MSW), those based on model estimation show the pattern that, in equatorial (A) climate, the generation ratio is the highest, followed by that in warm temperate (C) climate, snow climate (D) and arid (B) climate. While for those based on measurement, a different pattern has been shown that, the typical generation ratio in warm temperate (C) and snow (D) climate are very close and are higher than that in equatorial (A) climate, the lowest typical generation ratio is still in arid (B) climate. The lack of sufficient samples in equatorial (A) climate can be a possible reason, while this needs to be further analyzed.
Besides, the methane generation ratios based on measurement are all significantly less than the corresponding ratios based on model estimation, this implies there may exist systematical overestimation in the landfill gas generation model used, which is the 2006 IPCC Guidelines FOD Method.
The typical values of estimated collection efficiency are all relatively high (around 70%) and show small variations in different main climates. An interesting finding is that, the typical values of methane emission ratio show little difference in different main climates. To better understand this, more knowledge about how the landfill operators determine which emission value to report is needed.
The United Nations Framework Convention on Climate Change (UNFCCC) database and the Emissions Database for Global Atmospheric Research (EDGAR) are two separate sources of landfill methane emissions in different countries. Generally, there are varying degrees of difference between the two data sets in most countries because of the different methodologies used to develop them.
After comparison, the EDGAR data are selected as the basis to construct a complete time series of landfill methane emissions at the global level. It is estimated that the global methane emissions from landfills are 727.3 Mt CO2 e in 2012. If there is no significant implementation of landfill methane mitigation measures in the world, the rapid growth of landfill methane emissions in the near future should be expected.
The per capita landfill methane emissions have been calculated for almost all countries covering a time period from 1970 to 2017. It is estimated that, in 2012, every person on the planet emits 4.10 kg of landfill methane (102.50 kg CO2 e) on average annually.
By world region, the per capita landfill methane emissions in North America and in Europe & Central Asia are significantly higher than those in other regions, among which South Asia region has the lowest per capita emissions.
By income group, it has been shown that, for both total emissions and per capita emissions, higher income group emits more than lower income group.