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horizontal or vertical patterns. Where direct measures to control LFG emissions exist at a landfill, it is
normally assumed that it is the control of methane that is of concern, although the USEPA’s New Source
Performance Standards (NSPS) and Emission Guidelines (EG) for landfills (USEPA, 1996) are
specifically targeted at control of non-methane organic compounds (NMOC) and hazardous air
pollutants (HAPs). Collected LFG can be vented, flared, oxidized, or used as part of an LFGTE
scheme. Collection of LFG for flaring results in the conversion (thermal oxidation) of potential CH4
emissions to biogenic carbon dioxide (CO2) emissions, and is the most important example of an
engineered control on the level of greenhouse gas (GHG) emissions from a landfill. Important natural
controls of methane emissions include microbial oxidation (i.e. consumption of methane by
methanotrophic microorganisms in aerated landfill cover soils) and carbon sequestration (i.e.,
permanent removal of biogenic carbon from the atmosphere).
Landfill Gas Emission Estimates and Assumptions: Section 2.4 (i.e., Municipal Solid Waste
Landfills) of the USEPA document “AP-42, Fifth Edition, Compilation of Air Pollutant Emission Factors,
Volume 1: Stationary Point and Area Sources” (USEPA, January 1995, most recently updated under
Supplement E in November 1998, draft update in review October 2008) deals with gas emissions
from landfills. The current AP-42 version states: “…the USEPA method of estimating emissions could
result in conservative (i.e. high) estimates of emissions, since it provides estimates of LFG generation and
not LFG release to the atmosphere. Some capture and subsequent microbial degradation of organic
LFG constituents within the landfill surface layer is likely to occur. However, no data was identified to
adequately quantify this process.” This quote illustrates that the only way to rigorously quantify the
extent of methane control achieved is to have a measure of collected methane and fugitive
methane emissions from the same area at the same time. While measures of collected methane
are readily available, measures of fugitive emissions are considerably more difficult to obtain
and have only been reported for a few landfills; a comprehensive summary is provided in SWICS
(2009). In one study reviewed (Spokas, et al, 2006) the following equation was used to estimate
net emissions:
CH4 generated = CH4 emitted + CH4 oxidized + CH4 recovered + CH4 migrated + ∆ CH4 storage
Ignoring subsurface gas migration, calculation of net air emissions from landfills is a therefore a
function of three main factors – methane recovered through gas controls, cover system design (in
terms of control of gas flux emissions and optimization of methane oxidation potentials), and
carbon sequestration. In brief, LFG collection efficiency is the amount of LFG that is collected
relative to the amount generated by the landfill. Methane oxidation is consumption of methane
by methanotrophic microorganisms in aerated landfill cover soils. Carbon sequestration (as
applied to landfills) refers to the portion of biogenic carbon in waste that does not degrade
completely after disposal, but rather is permanently stored in the landfill in a stable form.
The USEPA, along with state and local regulators, often use assumed gas collection efficiencies to
calculate landfill emissions for regulatory purposes. A default value of 75 percent (as a
representative mean of a reported range of 60 to 85 percent) is frequently assumed as set forth
in the most current (i.e., 1998) version of the AP-42 document. However, the collection efficiency
presented therein was based on engineering judgment and professional opinion, compiled by
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