United States .„
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/166 November 1994
EPA Project Summary
Estimate of Methane
Emissions from U.S.
Landfills
Michiel R. J. Doom, Leonard A. Stefanski, and Morton A. Barlaz
Methane (GH4) flow rates from landfills
with landfill gas (LFG) recovery systems
can be used as surrogates for CH4 gen-
eration and successively for CH4 emis-
sions. The full report describes the devel-
opment of a statistical regression model
used for estimating CH4 emissions, which
relates LFG flow rates to waste-in-place
data from 105 landfills with LFG recovery
projects. The model has three linear seg-
ments, each of which applies to a distinct
landfill size class. Assumptions were re-
quired to account for the recovery effi-
ciency of LFG projects and for the prob-
able oxidation of CH in the top soil cover
of the landfill.
National GH4 emissions may be obtained
by applying the regression model to mu-
nicipal-waste-in-place data for U.S. land-
fills collected in 1986 by EPA's Office of
Solid Waste (OSW). This value is ad-
justed for CH4 emissions from industrial
landfills and CH4 that is currently recov-
ered or flared. For 1986, CH4 emissions
from U.S. landfills were estimated at 11 tg
(1012 g)/yr with lower- and upper-bound
values of 7 and 15 tg/yr, respectively. For
1992, estimates were between 9 and 18
tg/yr. The solid waste disposal rate was
estimated at 248 tg/yr.
The full report details uncertainties that
limit the quality of the above estimates.
Uncertainty arises from the difficulty in
performing quality assurance on the waste-
in-place data from a facility survey con-
ducted by EPA/OSW. The report con-
cludes with a discussion of trends that will
affect future LFG emissions, as well as
LFG utilization. Upcoming regulation for
controlling air emissions from landfills is
expected to be final by the fall of 1994
and to result in a reduction of 5-7 tq/yr of
CH4.
f/i/s Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
CH4 is a greenhouse, gas of particular
concern as its direct "and indirect effects
are estimated to be 20 times greater than
that of an equivalent mass of carbon diox-
ide. Landfills are known to be a significant
source of CH4, with global emissions esti-
mated between 20 and 70 tg/yr. However,
the existing estimates on emissions from
this source category were based on lim-
ited data using coarse assumptions. In an
effort to improve estimates of global CH4
emissions from landfills, the EPA's Air and
Energy Engineering Research Laboratory
began a research program aimed at iden-
tifying key variables that affect CH4 gen-
eration and at developing an empirical
model.
Landfills with gas recovery systems,
where LFG is collected and measured by
personnel on site, offer a unique opportu-
nity for studying CH4 emissions. LFG re-
covery rates can be used to estimate CH4
generation which in turn can be related to
CH, emissions. After the completion of a
pilot study, a large-scale program was
started where 21 landfills with LFG recov-
ery systems were surveyed. The objective
of this large-scale program was to de-
velop a statistical model of annual landfill
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CH4 emissions as a function of climate,
refuse mass and age, and other possible
parameters, as well as obtaining an emis-
sion factor that could be used to estimate
global CH4 emissions from landfills. The
report concluded that the mass of waste
in place showed a significant correlation
with CH4 generation.
Because a large amount of the variabil-
ity remained unexplained in the large-scale
field study, it was decided to refine the
correlation between LFG flow and waste
mass. A larger LFG recovery data base
was constructed, which included data from
most U.S. LFG recovery projects. With
the expanded and verified dataset con-
taining data on 105 U.S. LFG recovery
plants, a regression function was gener-
ated. This regression model, with three
linear segments, is described in the re-
port. In an appendix a simpler linear model
is detailed, which is in fact an emission
factor. It is suitable for use when only
total-waste-in-place data are available,
which is the case for most countries.
Regression Model
The LFG recovery data base was sub-
jected to statistical regression analysis.
The objective was to let statistical criteria
dictate the shape and position of the re-
gression curve with the constraint that the
curve needed to start in the origin. A re-
gression model with three different linear
segments was the result, where each seg-
ment applies to a distinct landfill size class.
The size classes and equations for the
three segments of the curve are:
I x< 1.128 y= 19.822 x
II 1.128 sSx< 4.082 y = 1.652 x + 20.495
III x & 4.082 y = 9.195 x-10.294
where:
X:
y
•• welled waste, tg and
: LFG flow rate, rrvVmin.
Welled waste is defined as the quantity
of waste from which LFG is extracted
through the recovery wells. In order to
convert y in cubic meters per minute to
the actual mass flow of CH4 released to
the environment in grams per minute, a
factor needs to be introduced. This factor
adjusts for the efficiency of the gas recov-
ery system and for the percentage of CH4
that is oxidized on its way out of the
landfill. Recovery efficiency is estimated
at 75%, and the oxidation is assumed to
be 10%. The factor also adjusts for the
amount of CH4 in LFG, which is approxi-
mately 50%.
Estimate of Total Waste in
Place
In 1986, OSW conducted a survey in
which detailed information on 1,175 U.S.
landfill facilities was compiled in a data
base (EPA/OSW-Westat Database). This
population was designed to be a stratified
random sample of all U.S. landfills; there-
fore, its data could be extrapolated by
means of scaling factors, to obtain total
waste in place for the U.S. This data base
contains data that make it possible to esti-
mate waste in place by two different meth-
ods that are described and compared in
the full report. The total amount of waste
landfilled in the U.S. up to and including
1986 was estimated at 4.7x1015 g (5.2x109
tons).
Estimate of U.S. Landfill
Methane Emissions
Application of the regression model to
the waste mass data from the EPA/OSW-
Westat Database and multiplication of the
results with the conversion factor yield
national CH4 emissions from landfills. This
estimate is then adjusted for the amount
of CH4 currently being recovered or flared.
In 1992, 1.2 tg of landfill CH4 was recov-
ered and approximately 0.5-tg/yr of-CH4
was flared. It is estimated that an addi-
tional 15 tg of industrial waste is landfilled
annually Jn the U.S. For 1986 CH4 emis-
sions from U.S. landfills were estimated at
11 tg/yr with lower- and upper-bound val-
ues of 7 and 15 tg/yr, respectively. Meth-
ane emissions from U.S. landfills in 1992
were estimated at 13 tg/yr with lower- and
upper-bound values of 9 and 18 tg/yr.
Uncertainties and Future
Trends
The full report details uncertainties that
limit the quality of the emissions estimates.
The main uncertainty arises from the in-
ability to quality assure the waste-in-place
data from the EPA/OSW-Westat Database.
There are several indications that, since
its publication, the \ data base has been
subject to alterations: the scaling factors
seem too high, waste-in-place data do not
match up, and a density conversion has
taken place. No documentation has been
retrieved for any of these issues. The pos-
sible uncertainties are inflated by the up-
date from 1986 to 1992. Other uncertain-
ties arise from the fact that only few data
are available for CH generation at small
landfills, because LFG recovery plants can
usually be found at larger landfills.
Due to legislative and economic pres-
sure, there will be a tendency toward larger
and fewer landfills. Landfill gas recovery
projects become more feasible as the land-
fill size increases, which should lead to a
reduction in CH4 emissions. As a result of
source reduction, increased recycling,
composting, and combustion, the yearly
amount of landfilled waste will continue to
decrease, which will also lead to a reduc-
tion in annual CH4 emissions from land-
fills. The influence of changes in waste
management will likely be overshadowed
by the effect of the Clean Air Act rule as it
is implemented over the next several
years. This rule, requiring a gas collection
system and add-on control device at af-
, fected landfills,-is expected to result in the
control of 500 to 700 sites, reducing CH4
emissions by 5-7 tg/yr.
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Michiel R. J. Doom is with £. H. Pechan and Associates, Inc., Durham, NC 27707
and Leonard A. Stefanski and Morton A. Barlaz are with North Carolina State
University, Raleigh, NC 27695.
Susan A. Thomeloe is the EPA Project Officer (see below).
The complete report, entitled "Estimate of Methane Emissions from U.S. Landfills,"
(Order No. PB94-213 519; Cost: $19.50, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Tn'angle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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