United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/6QQ/SR-97/071 September 1997
of
in
M. A. Barlaz, W. E. Eleazer, W. S. Odle, III, X. Qian, and Y-S. Wang
The objective of this research was to
characterize the anaerobic biodegrad-
ability of the major biodegradable com-
ponents of municipal solid waste
(MSW). Tests were conducted in qua-
druplicate in 2-L reactors operated to
obtain maximum yields. Measured
methane (CH4) yields for grass, leaves,
branches, food waste, coated paper,
old newsprint, old corrugated contain-
ers, and office paper were 144.4, 30.6,
62.6, 300.7, 84.4, 74.3, 152.3, and 217.3
mL CH4/dry g, respectively. While there
was a general trend of increasing CH4
yield with increasing cellulose plus
hemicellulose (carbohydrate) content,
many confounding factors precluded
establishment of a quantitative relation-
ship. Similarly, the degree of lignifica-
tion of a particular component was not
a good predictor of the extent of bio-
degradation.
In parallel with the decomposition ex-
periments, leachate from the decom-
position of each refuse constituent was
analyzed for toxicity using a modified
anaerobic toxicity assay. Leachate tox-
icity was not found in association with
the decomposition of any refuse com-
ponent other than food waste. How-
ever, substantial toxicity was measured
in leachate from the food waste reac-
tors. This toxicity was consistent with
the behavior of the reactors but could
not be simulated with high concentra-
tions of carboxylic acids and sodium.
The toxicity associated with food waste
leachate is not likely to inhibit anaero-
bic decomposition in U.S. landfills due
to the relatively low concentration of
food waste in MSW.
Most probable number (MPN) tests
were conducted to identify the compo-
nents of refuse that carry refuse-de-
composing microorganisms into
landfills and to evaluate the significance
of two typical cover soils as carriers of
refuse-decomposing microbes. Grass,
leaves, and branches were the major
identifiable contributors of refuse-de-
composing microbes to landfills, while
the cover soils tested did not typically
contain populations with the activities
required for refuse methanogenesis.
This Project Summary was developed
by EPA's Air Pollution Prevention and
Control Division of the National Risk
Management Research Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Approximately 62% of the municipal
solid waste (MSW) generated in the U.S.
is disposed of by burial in a sanitary land-
fill. The production of methane (CH4) from
sanitary landfills is well documented, and
there are about 119 landfill gas recovery
projects currently (January 1997) in op-
eration in the U.S. and Canada. While the
production of CH4 from landfills is well
established, there is large uncertainty in-
volved in estimating the amount and rate
of CH4 production. This uncertainty is in-
creasing as the composition of the MSW
buried changes due to increased recy-
cling.
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Development of integrated solid waste
management programs, which include re-
cycling and in some cases, combustion,
have led to a decrease in the use of
landfills. However, there is a limit to the
types of waste that can be recycled, and
combustion has not been the solid waste
management alternative of choice for many
communities. Thus, landfills will be a sig-
nificant part of MSW management for the
foreseeable future.
Both CH4 and carbon dioxide CO2 are
greenhouse gases that contribute to glo-
bal climate change. CH4 traps about 20
times more infrared energy than CO2 on a
volume basis. Consequently, although
landfill gas contains approximately equal
proportions of CH4 and CO2, CH4 is more
significant with respect to atmospheric cli-
mate change. Data on the amount of CH4
that can be expected from refuse already
buried, as well as CH4 that will result from
the decomposition of refuse buried in the
future, are needed to better assess the
impact of landfills on global climate change.
The overall objective of this research
was to develop information on the anaero-
bic decomposition of refuse that will im-
prove our ability to assess the impact of
sanitary landfills on global CH4 accumula-
tion. Three sets of experiments were con-
ducted to meet this objective: (1)
measurement of the CH4 potential of the
major biodegradable components of MSW;
(2) assessment of whether leachate toxic-
ity, associated with whole refuse or some
individual constituent, inhibits the onset or
rate of CH4 production; and (3) identifica-
tion of solid waste constituents that carry
the anaerobic bacteria required for refuse
methanogenesis. The results of each set
of experiments are summarized separately.
Experiment 1: Measurement of
the CH4 Potential of the Major
Biodegradable Components of
MSW
The anaerobic biodegradability of the
major biodegradable components of MSW
was characterized by measurement of their
CH4 yield and the biodegradation of cellu-
lose and hemicellulose. The components
that were tested were grass, leaves,
branches, food waste, and four types of
paper—newsprint (ONP), old corrugated
containers (OCC), office paper (OFF), and
coated paper (CP). These are the most
common types of paper in MSW and also
represent the range of biodegradability that
could be expected from paper. At one
extreme, newsprint contains all of the lig-
nin of wood pulp. At the other extreme,
office paper has had almost all of the
lignin removed. The decomposition of
mixed residential refuse was also charac-
terized.
Tests were conducted in 2-L laboratory
reactors in quadruplicate. Each refuse
component was seeded with well-decom-
posed refuse to initiate refuse-decomposi-
tion. CH4 yield data have been corrected
for the background CH4 produced from
the seed. In the case of food waste, two
sets of reactors were tested. In the first
(F) series, there was insufficient seed, 30%
by volume, and the reactors remained in-
hibited. A second set of food waste reac-
tors (SF) was then initiated with 70% seed
by volume, and these reactors produced
measurable CH4.
The test conditions were designed to
measure the maximum CH4 production
potential of each component. This included
shredding, incubation at about 40°C, and
leachate recycle and neutralization. All re-
actors were monitored until they were no
longer producing measurable CH4, except
for the old corrugated container reactors
in which the CH4 yield increased by less
than 2% over the final 80 days of monitor-
ing. At the termination of the monitoring
period, reactors were destructively
sampled for analysis of the residual sol-
ids.
The CH4 yield, solids composition, and
extent of cellulose and hemicellulose de-
composition for each MSW component and
mixed MSW are presented in Table 1. As
summarized in Table 1, there was sub-
stantial variation in the range of CH4 yields
(30.6 to 300.7 ml/dry g) and the extent of
decomposition (28 to 94%) among the
components tested. In previous research
with mixed refuse, carbohydrates ac-
counted for 91% of the stoichiometric CH4
potential of MSW. Carbohydrates were the
major organic compounds analyzed in the
waste components tested here, and the
relationship between carbohydrate concen-
tration and CH4 yield is presented in Fig-
ure 1. While the data in Figure 1 show a
relationship, the relatively low correlation
coefficient (r2 = 0.49) and failure of the
regression line to pass through zero, sug-
gest that factors in addition to carbohy-
drate concentration influence CH4 yield.
Lignin decreases carbohydrate bioavail-
ability and is expected to confound the
relationship presented in Figure 1. The
components with the lowest yields are the
two sets of seed reactors and leaves.
These are also the components with the
lowest carbohydrate to lignin [(C+H)/Li)j
ratio. The (C+H)/Li ratio is a measure of
the degree of lignification. Values of 3 to 4
have been reported for fresh refuse, and
lower values are associated with decom-
posed refuse. There is a general trend of
more extensive cellulose biodegradation
(MC decreasing) in the less lignified sub-
strates [(C+H)/Li increasing] e.g., food and
office paper (r2 = 0.28). However, the quan-
titative relationship is weak because the
office paper (C + H)/Li is well above any
of the other components tested. The trend
towards increased cellulose loss with a
decreasing degree of lignification is most
definite among the four paper components.
There is not a linear relationship be-
tween (C+H)/Li and the extent of decom-
position (r2 = 0.02). However, it is
interesting to note that grass, which is
highly lignified, underwent nearly complete
decomposition as measured by either MC
or the extent of decomposition (Table 1).
This suggests that the lignin concentra-
tion does not always reflect the degree to
which lignin inhibits cellulose bioavailabil-
ity. Apparently, the lignins in grass are not
as restrictive to microorganisms as the
lignin in other components such as
branches. This result is consistent with a
report that stated, ". . .the chemistry of
grass lignocellulose varies considerably
from that of wood."
The solids decomposition (MC and MH)
and CH4 yield data document the biode-
gradability of even the most lignified sub-
strates, leaves and branches, as well as
all other components of MSW tested. The
absence of good linear relationships is
likely because a number of factors influ-
ence CH4 production and solids decompo-
sition. The biodegradation of newsprint
measured here is in contrast to reports on
the excavation of readable newsprint that
had been buried in landfills decades ear-
lier; however, these reported data did not
represent average values, but rather ob-
servations during an archaeological exca-
vation. The presence of readable newsprint
that had not undergone biodegradation
may be due to its isolation from other
factors required for biodegradation such
as bacteria, moisture, and nutrients. The
biodegradability of a newspaper buried in
a bag that did not break during waste
compaction would differ from the biode-
gradability of newsprint exposed to other
refuse components.
Based on the CH4 yields presented in
Table 1, a model was constructed to esti-
mate CH4 yields based on assumed com-
positions of buried refuse. These results
are summarized in Table 2. The actual
methane yield per wet kg of refuse buried
decreases by only 10% between the base
case (64.9 L CH4/wet kg) and the case
with the most recycling (58.6 L CH4/wet
kg). However, the appropriate way to
evaluate changes in methane yield is to
calculate the change in methane potential
-------
Table 1. CH4 Yield and Initial and Final Solids Composition Data Summary3
Reactor
Series
Seed
sd
Seed-2d
sd
Grass
sd
Raleigh
grass
sd
Leaves
sd
Branch
sd
Yield
mL CH4/
dryg
25.5
5.7
5.8
0.6
144.4
15.5
127.6
21.8
30.6e
8.6
62.6e
13.3
Cellulose
23.4
18.3
26.5
25.6
15.3
35.4
Hemi-
cellulose
4.7
3.7
10.2
14.8
10.5
18.4
Lignin
22.5
22.1
28.4
21.6
43.8
32.6
MCb
0.18
0.02
0.34
0.01
0.19
0.01
0.43
0.05
0.52
0.05
MHb
0.36
0.03
0.69
0.11
0.42
0.06
0.68
0.10
0.59
0.02
Extent of
Decomposition0
21.8
6.3
94.3
75.5
28.3
27.8
Food
46.1
6.2
8Q
.6
Second
Food
sd
ONP
sd
OCC
sd
OFF
sd
CP
sd
MSW
sd
300.7e
10.6
74.33
6.802
152.3
6.7
217.3
14.96
84.4
8.1
92.0e
4.1
55.4
48.5
57.3
87.4
42.3
28.8
7.2
9
9.9
8.4
9.4
9
11.4
23.9
20.8
2.3
15
23.1
0.24
0.02
0.73
0.05
0.36
0.01
0.02
0
0.54
0.01
0.25
0.03
0.58
0.04
0.46
0.06
0.38
0.01
0.09
0.01
0.58
0.06
0.22
0.05
84.1
31.1
54.4
54.6
39.2
58.4
" Data represent the average for each reactor set. Standard deviations (sd) are presented below the average where data are the average of all reactors in a set.
b The ratio of the cellulose (MC) or hemicellulose (MH) recovered from a reactor divided by the mass added initially.
c The measured CH4 yield divided by the yield calculated by assuming conversion of 100% of the cellulose and hemicellulose (and protein in the case of food waste) to CH4 and
CO2.
d Seed used for second set of food waste reactors.
8 Yield data for the leaf reactors exclude L2, data for the branch reactors exclude B4,and data for the second food and MSW reactors were corrected for leakage.
based on the yield multiplied by the mass
landfilled. Using this calculation, the po-
tential reduction in methane production is
25.5% and 38% for the recycling cases
based on national averages and local re-
cycling rates, respectively. Thus, these
data suggest that recycling can have a
substantial impact on the volume of meth-
ane available for recovery over the de-
composition period.
Where CH4 is released to the atmo-
sphere, recycling clearly reduces the
amount of CH4 released from landfills.
However, at landfills where there is an
active program to compare the relative
benefits of recycling and energy recovery.
Given the CH4 potential data for individual
constituents measured here, this analysis
could be done on a component-specific
basis because the results may be differ-
ent for two different types of paper or
between yard waste and paper.
The calculated composite CH4 yields in
Table 2 range from 58.6 - 64.9 L CH4/wet
kg of MSW. These values are low relative
to landfill gas models that generally as-
sume a yield of 62.3 to 112.2 L/kg. This is
surprising in that the CH4 yields measured
here were measured under optimal condi-
tions and should be considerably higher
than values assumed for field conditions.
There are two potential explanations for
this discrepancy. The first explanation is
that the assumed waste composition is in
error. The data presented in an EPA re-
port represent an estimate of MSW gen-
eration and exclude a number of wastes
that are buried in landfills. Some of these
other wastes have high CH4 yields (waste-
water treatment plant sludge and agricul-
tural and food preparation wastes), while
others have little or no CH4 potential (wa-
ter treatment plant sludge and construc-
tion and demolition debris). A second
explanation for the discrepancy in yield
calculations pertains to the assumptions
used by the landfill gas models. The range
of values used, 62.3 to 112.2 L/kg, is
based on field measurements and an esti-
mate of the mass of waste buried in a
landfill. While this mass is accurately
known in newer landfills where all waste
received is weighed, this mass represents
only an estimate at older facilities and
errors of 20 to 30% would not appear to
be unreasonable. Thus, the values as-
sumed in practise may be inaccurate.
-------
350 -i
Second Jk
food ^
r2 = 0.493
100
Cellulose + Hemicellulose (%)
Figure 1. CH4 yield vs. carbohydrate concentration.
Table 2. Calculated CH4 Yield Based on Measured Yields and Assumed MSW Composition
Case
Base Case-No Recycling
Recycling at National Average
Recycling-Typical Local Program
Yield
(L CH4/
wet kg)
64.9
59.9
58.6
Methane
Reduction3
(%)
na
25.5
38.0
Recycle
Rate
(%)
na
19.4
30.9
1 Calculated from the CH4 yield multiplied by the mass buried after recycling relative to the CH4 yield and mass buried
in the base case.
Experiment 2: Measurement of
the Anaerobic Toxicity of
Leachate Associated with the
Decomposition of Individual
Refuse Components
The anaerobic toxicity of leachate asso-
ciated with the decomposition of each
refuse component tested above was mea-
sured in parallel with the decomposition
experiments. Leachate was sampled three
times from each reactor. For food waste,
four samples were collected from the F
reactors, but no samples were collected
from the SF reactors. Six samples were
collected from the MSW reactors. The ini-
tial strategy was to sample each reactor
twice during the acid phase and twice
during the decelerated CH4 production
phase. However, except for the first set of
food reactors (F), the acid phase was
very brief. As a result, only one sample
was collected from most reactor sets dur-
ing the acid phase.
Leachate toxicity was evaluated using a
modified anaerobic toxicity assay (ATA).
The ATA included anaerobic medium,
ground refuse as a carbon source, and an
inoculum. The inoculum was a methano-
genic consortium enriched from decom-
posed refuse with ground refuse as a
carbon source. CH4 production from the
ground refuse was measured in triplicate
in the presence and absence of leachate.
Leachate was tested at final concentra-
tions in the ATA of 25 and 80% of its
original strength. Two sets of controls were
also inoculated. Controls containing inocu-
lum and medium but no refuse were used
to measure background CH4 production
from the inoculum. Controls containing in-
oculum, medium, and ground refuse were
used to compare CH4 production in the
presence and absence of leachate.
Leachate toxicity was not measured in
association with the decomposition of any
refuse component other than food waste.
However, leachate associated with the
food waste reactors containing 30% seed
and 70% food waste (F) exhibited sub-
stantial toxicity, and this toxicity was gen-
erally consistent with the behavior of the
reactors.
The toxicity of the food waste leachate
could not be simulated with synthetic
leachate containing high concentrations of
carboxylic acids and sodium. ATAs with
20, 5, 15, and 12 g/L of acetate, propi-
onate, butyrate, and sodium, respectively,
suggested that high concentrations of bu-
tyric acid and sodium inhibited the onset
of CH4 production, but that refuse micro-
organisms could acclimate to these con-
centrations within 5 to 10 days under the
conditions of the ATA. The corresponding
concentrations of undissociated acetic, pro-
pionic, and butyric acids were 113, 27,
and 96.8 mg/L, respectively. Comparison
of carboxylic acid concentration data
from the S and SF reactors series indi-
cated that the refuse ecosystem was
able to tolerate and recover from 142,
35, 24, and 305 mg/L of undissociated
acetic, propionic, i-butyric, and butyric
acids, respectively. These concentra-
tions of undissociated, carboxylic acids
are higher than concentrations reported
to be inhibitory in previous research with
anaerobic digesters.
Experiment 3: Identification of
Solid Waste Constituents that
Carry the Anaerobic Bacteria
Required for Refuse
Methanogenesis
The objective of part of this study was
to identify the components of refuse that
carry refuse-decomposing microorganisms
into landfills. A second objective was to
evaluate the significance of two typical
cover soils as carriers of refuse-decom-
posing microbes. Refuse buried in a sani-
tary landfill is typically covered with 15 cm
of soil daily. Recently, geotextile sheets
and foams have been proposed as alter-
natives to soil to minimize the volume of
soil in a landfill. While soil may contribute
refuse-decomposing microorganisms to
landfills, the proposed alternatives almost
certainly do not.
The total anaerobic population and the
subpopulations of cellulolytic, hemicellu-
lolytic, hydrogen-producing acetogenic
(based on butyrate catabolism) bacteria
-------
and acetate- and hydrogen (H2)/CO2-uti-
lizing methanogenic bacteria were enu-
merated by the most probable number
(MPN) technique on several waste com-
ponents: grass, branches, leaves, food
waste, whole refuse, and landfill cover
soil. For each component, the objective
was to enumerate microbial populations
on a representative sample in the form in
which it would typically enter a landfill.
Although paper represents 37.6% of
refuse, it was not tested because it is
likely populated with bacteria originating
in wet components of refuse.
Microbial enumerations were performed
by MPN tests. Thus, it was necessary to
form a liquid inoculum from solid samples.
The technique used here was similar to a
technique developed previously to process
smaller samples. In the laboratory, refuse
samples were placed in a 113-L plastic
garbage can which had been wiped with
ethanol and purged with sterile argon. A
measured volume of filter sterilized anaero-
bic phosphate buffer (23.7 mM, pH 7.2)
was then added to a sample to form a
slurry. The sample was then stirred by
hand (covered with arm length gloves).
Next, four samples were removed using a
1-L beaker, and the liquid was poured into
a sterile, 4-L, argon-purged, Erlenmeyer
flask. The liquid in this flask served as the
inoculum for MPN enumerations. Inocula
were serially diluted in phosphate buffer
(23.7 mM, pH 7.2). For soil, 250 to 300 g
of each sample was added directly to a
nitrogen-purged flask, 2.5 L of sodium py-
rophosphate (0.1%, pH 7) was added, and
the slurry was shaken for 2 minutes. The
slurry was then allowed to settle for 1
minute after which a liquid sample was
removed for use as an inoculum.
Microbial populations on each waste
component and whole refuse are reported
in Table 3. Total anaerobic and
hemicellulolytic populations were present
on all components tested, while the pres-
ence of cellulolytic, acetogenic, and
methanogenic bacteria was more limited.
Thus, identification of the waste compo-
nents that are the major contributors of
cellulolytic, acetogenic, and methanogenic
bacteria is evaluated here. Yard waste
(grass, leaves, and branches) most con-
sistently carried the microorganisms re-
quired for refuse methanogenesis.
Surprisingly, food waste did not carry ei-
ther cellulolytic or methanogenic bacteria,
and one of two food waste samples con-
tained only one acetogen per gram. Popu-
lations of cellulolytic, acetogenic, and
methanogenic bacteria were generally
lower in the mixed refuse samples com-
pared to the grass, leaves, and branch
samples.
Table 3. Anaerobic Microbial Populations on Refuse Components (Most Probable Number—Iog10
cells/dry g)a
Trophic Total Hemicellu- Methanogen Methanogen
Group Anaerobes lolytic Cellulolytic Acetogen Acetate H2/CO2
Grass
(April 92)
Grass
(July 92)
Branches
9.8
9.8
6.5
7.9
9.5
4.2
1.4
t>
2.5
0.7
1.8
1.3
1.6
t>
1.1
1.8
1.3
0.8
Leaves
(Nov. 91)
Leaves
(Nov. 92)
Food
(Mar. 92)
Food
(Aug. 92)
Refuse
(July 92)
Refuse
(Sept. 92)
5.8
6.9
>8.0C
9.4
9.3
8.4
4.1
4.4
5.3
6.2
6.6
6.3
1.0 <0.4 1.0
1.8 4.4 3.0
<-0.4 <-0.1 <-0.1
<-0.4 0 <0
0.4 0.2 3.6
<-0.2 <-0.1 <-0.1
0.7
3.8
<-0.1
<0
5.0
0.8
Grass, leaves, and branches were the
major identifiable contributors of refuse-
decomposing microbes to landfills. About
9% of the refuse stream is characterized
as "miscellaneous" and contains many dif-
ferent items. In addition to diapers and
pet wastes, there may be other compo-
nents in the miscellaneous fraction that
carry refuse-decomposing microbes. How-
ever, their presence is small, and they are
likely to be poorly distributed. The impor-
tance of yard waste should be considered
as solid waste management programs are
implemented. Where there is interest in
CH4 recovery from landfills, banning yard
waste from landfills may be self-defeating.
Unless, of course, the landfill is receiving
substantial volumes of other wastes known
to carry refuse-decomposing microbes.
The cover soils tested did not typically
contain populations with the activities re-
quired for refuse methanogenesis. Thus,
efforts to develop lower volume alterna-
tives to cover soil will not adversely im-
pact the input of refuse-decomposing
microbes to landfills.
Financial support for this research was
provided by the Climate Change Research
Program of the U.S. Environmental Pro-
tection Agency, Waste Management In-
corporated, the National Science
Foundation, and S. C. Johnson Wax &
Son. This support is gratefully acknowl-
edged. We are also grateful for the assis-
tance of Kathi McBlief in editing, typing,
and figure preparation.
" Data reported as less than a number indicate that no positive tubes were detected. The number reported assumes
one positive tube in the first dilution.
b MPN results code was anomalous and not reported.
0 All tubes were positive at the highest dilution tested.
-------
M. A. Barlaz, W. E. Eleazer, W. S. Odle, III, X. Qian, and Y-S. Wang are with North
Carolina State University, Raleigh, NC 27695-7908.
Susan A. Thorneloe is the EPA Project Officer (see below).
The complete report, entitled "Biodegradative Analysis of Municipal Solid Waste in
Laboratory-Scale Landfills," (Order No. PB97-189674; Cost: $35.00, 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 Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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