Methane emissions from landfills continue several decades (or sometimes even centuries) after waste disposal. Waste disposed in a given year thereby contributes to GHG emissions in that year and in subsequent years. Likewise, methane emissions from a landfill in any given year include emissions from waste disposed that year, as well as from waste disposed in prior years.
- Methane commitment (MC) assigns landfill emissions based on waste disposed in a given year. It takes a lifecycle approach and counts GHGs emitted by that waste, regardless of when the emissions occur, assigning lifetime emissions to the year in which the waste is disposed of.
- Waste-in-place (WIP) assigns landfill emissions based on emissions during that year. It counts GHGs actually emitted that year, resulting from the historic (pre-inventory year) disposal of waste.
Some annual inventories use WIP, since it provides annual direct emissions. However, the ICLEI U.S. Community Protocol for Accounting and Reporting of Greenhouse Gas Emissions, while allowing the use of a WIP approach, requires an estimate of emissions resulting from disposal of community-generated waste, using a methane commitment approach – essentially an estimate of the current and future greenhouse gas burden imposed as a result of disposal of waste during the inventory year. EPA’s WARM decision making tool also uses a methane commitment approach.
Overview of advantages and disadvantages: The table below highlights pros and cons associated with methane commitment (MC) and waste-in-place (WIP) methods.
| Consideration | Methane commitment (MC) | Waste-in-place (WIP) |
| Simplicity of implementation, data requirements | Pro: Based on quantity of waste disposed during inventory year, requiring no knowledge of prior disposal. | Con: Based on quantity of waste disposed during inventory year as well as existing waste in landfill(s). Requires historic waste disposal information. |
| Consistency with annualized emissions inventories | Con: Does not represent GHG emissions during inventory year. Rolls together current and future emissions and treats them as equal. Inconsistent with other emissions in the inventory. | Pro: Represents GHG emissions during the inventory year, consistent with other emissions in the inventory. |
| Decision-making for future waste management practices | Pro: Sensitive to long-term GHG reductions. | Con: Not as immediately sensitive to long-term GHG reductions. |
| Credit for source reduction/ recycling | Pro: Accounts for emissions affected by source reduction, reuse, and recycling. | Con: For materials with significant landfill impacts, not as immediately sensitive to source reduction, reuse, and recycling efforts. |
| Credit for engineering controls, heat/power generation | Con: Doesn’t count current emissions from historic waste in landfills, thus downplaying opportunities to reduce those emissions via engineering controls. | Pro: Suitable for approximating amount of landfill gas available for flaring, heat recovery, or power generation projects. |
| Credit for avoided landfill disposal | Con: Overstates short-term benefits of avoided landfill disposal. | Con: Spreads benefits of avoided landfill disposal over upcoming years. |
| Accuracy | Con: Requires predicting future gas collection efficiency and modeling parameters over the life of future emissions. |
Example: To illustrate the extent to which MC and WIP can differ, the following simple example assumes a landfill open from 2000, with equal quantities of waste disposed of in each year up until 2014, no changes in waste composition, and no changes in landfill operating conditions (gas collection, etc.). The table illustrates how the lifetime (total) emissions are the same (25 g methane per kilogram waste disposed), for each individual year (2008 – 2014). In this example, 2010 MC emissions are 25.1 g methane, versus 10.6 for 2010 WIP, per kilogram of waste disposed.
| Year that emissions occur | ||||||||||
| Year waste is disposed | 2000-2008 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 | 2014 | Post-2014 | Total emissions |
| 2000-2008 | 32.7 | 8.8 | 8.5 | 8.2 | 7.9 | 7.6 | 7.3 | 7.0 | 180.8 | 268.8 |
| 2008 | 0 | 0 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 | 1.0 | 18.3 | 25.1 |
| 2009 | 0 | 0 | 0 | 1.3 | 1.2 | 1.2. | 1.1 | 1.1 | 19.3 | 25.1 |
| 2010 | 0 | 0 | 0 | 0 | 1.3 | 1.2 | 1.2 | 1.1 | 20.4 | 25.1 |
| 2011 | 0 | 0 | 0 | 0 | 0 | 1.3 | 1.2 | 1.2 | 21.5 | 25.1 |
| 2012 | 0 | 0 | 0 | 0 | 0 | 0 | 1.3 | 1.2 | 22.7 | 25.1 |
| 2013 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1.3 | 23.9 | 25.1 |
| 2014 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 25.1 | 25.1 |
| Total emissions | 32.7 | 8.8 | 9.8 | 10.6 | 11.5 | 12.3 | 13.1 | 13.9 | 331.9 | 444.7 |
This hypothetical example was modeled using first-order-decay and parameters from EPA (1995-2005). It assumes a half-life of 18.2 years.
Models: In the absence of direct measurements of landfill methane emissions, inventories and GHG calculators use models to estimate emissions. There are several models to convert a given quantity and composition of waste disposed into landfill GHG emissions. IPPC and EPA use a first-order decay model to estimate WIP methane emissions.
The models have similar parameters to account for the factors that affect methane emission, summarized in the text and table below. The parameter representing how much methane can be generated from a given quantity of deposited waste (Lo in the table) has input from influencing factors, including:
- Waste composition (types and quantities of materials as reflected in DOC, DOCf)
- The delay before the deposited waste begins to undergo methane-forming reactions (MCF)
- The fate of carbon in the methane-forming reactions (anaerobic decay can also produce carbon dioxide, F)
Not all the generated methane escapes the landfill. Some is recovered through collection processes and some is oxidized to carbon dioxide prior to escape into the atmosphere. Modeling the quantity of methane emitted from the landfill requires parameters to account for:
- How much methane is recovered through collection processes (R)
- How much methane is oxidized prior to escape into the atmosphere (Ox)
WIP models that determine emissions within a given period of time take into account factors that affect the rate of methane formation. The rate and extent of methane production from waste depend on local factors, including waste material type, moisture, and temperature. Faster decay is reflected by use of a larger value for the rate constant k (or a smaller half-life).
| Parameter | IPCC | EPA (1990-2005) | ||
| DOC, degradable organic carbon | How much of the deposited waste is organic carbon, by mass | Depends on waste type | 20.3% | |
| DOCf, decomposable organic carbon | How much of the waste’s organic carbon can decompose | 50% | 50% | |
| MCF, methane correction factor | How much carbon remains after an initial delay (before methane-forming reactions start) | 100% | 100% | |
| F, fraction of methane generated in landfill gas | To what extent the decomposition reactions convert carbon to methane | 50% | 50% | |
| Lo, methane generation potential | How much methane can form from the deposited waste | Depends on waste-specific parameters DOC | 99 m3 CH4/Mg | |
| R, recovery rate (or quantity methane recovered) | How much of the generated methane is recovered for flaring or electricity/heat generation | 0, unless actual quantity known | 44.8% average for MSW and industrial landfills | |
| Ox, oxidation rate | How much of the generated methane is oxidized | 10% | 10% | |
|
k, first-order decay rate constant (often expressed in terms of t1/2, half-life) |
How fast the degradable organic carbon decomposes | Value specific to material type and temperature/ moisture |
0.02, 0.038, 0.057 /yr depending on rainfall (half life 34.7, 18.2, 12.2 years) |