United States
Environmental Protection
Agency
Research and Development
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-60o7S2-84-141 ~Sept7?984
i /
&EPA Project Summary
Landfill Gas Production from
Large Landfill Simulators
i
Larry W. Jones, Robert J. Larson, and Philip G. Malone
A study was conducted to investigate
gas production rates and composition
in municipal solid waste (MSW). Im-
proved monitoring methods were used
to corroborate and add to previous
studies. A completely automated gas-
monitoring system was used on four
sanitary landfill simulators (lysimeters
or test cells) of two different sizes.
Gas was produced in four phases: an
aerobic phase, a nonmethanogenic
anaerobic phase, an unstable methano-
genic phase, and a stable methanogenic
phase. The last stage was just being
reached as the experiment was termi-
nated.
The automated gas-measuring system
and the gas-chromatograph-based,
gas-analysis system used in the study
both functioned satisfactorily. Gas
samples were collected in an all-metal
collection system, as plastic and glass
vessels proved unsatisfactory.
The two sizes of test cells produced
very similar volumes and compositions
of total gas, but the small cells produced
more methane and less hydrogen than
the large cells. Relatively high, consis-
tent levels of nitrogen were found in the
gas from this study. This factor could
pose serious problems regarding the
use of this gas for energy.
The study demonstrated that the
conditions present in the average MSW
landfill are not ideal for maximum
production of methane. Further studies
are needed on the effects of environ-
mental and nutritional factors in me-
thane production.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory, Cincinnati, OH, 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
Many investigators have reported the
production of combustible gas from mu-
nicipal solid wastes (MSW) placed in
anaerobic environments such as landfills.
But to exploit MSW as a possible fuel
source, gas produced during its decompo-
sition must be accurately analyzed and
measured. This study uses improved
monitoring methods to corroborate and
add to previous investigations of MSW
gas production rates and composition.
The study also examines problems related
to scaling of large laboratory simulation
tanks. Two sizes of test cells were in-
cluded to allow broad comparisons of the
data obtained here with those of other
studies. The project uses a completely
automated gas monitoring system on four
sanitary landfill simulators (lysimeters or
test cells).
Methods and Materials
Test Cells
All gases were collected from four (two
each of two sizes) carefully sealed MSW
test cells. The cells were cylindrical steel
(7.25-mm rolled plate) tanks. Each of the
two smaller test cells had an inside
diameter (ID) of 0.91 m and was 1.83 m
high; each of the two larger cells had an
ID of 1.83 m and was 3.66 m high (see
Figure 1). All interior surfaces were coated
with coal-tar epoxy to protect the walls
from corrosion and the contents from
contamination. Cells were loaded with a
30.5-cm layer of clayey-sandy soil, a 3-
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Deionized
Water
Deionized
Water
I
Washed^
Pea
Gravel
Thermistor •—-
61 cm
1
Polypropylene^
Beads ~~
91.4 cmOD
f|&feife?-
So/l'Lihe'r-
PVC
Valve
122cm
Washed
Pea
Gravel
.22.9 cm 61 cm
7.6 cm *_».
Thermistors T
61 cm
183 cm
,30.5 cm ~
Polypropylene
61 cm Beads
10.8 cm
Figure 1. Schematic diagram of the municipal solid waste test cells.
cm layer of polypropylene beads (6.25-
mm diameter), a layer of MSW (0.79 m3
for the small cells and 6.42 m3 for the
large cells) from noncommercial collec-
tion routes in Warren County, Mississippi,
and a 7.6-cm layer of washed pea gravel
to help disperse the leaching fluid'over
the surface. A perforated diffusion pan
was bolted to the inside of the test cell top
as an additional aid, and a headspace (23
cm for the small cells and 84 cm for the
large cells) was left below the test cell top
plate. After the test cells were loaded and
all monitoring devices were tested, the
cells were sealed by welding the steel lids
to the tops and pumping metal sealant
into a machined groove on the undersides
of the lids.
The test cell profiles were designed and
constructed to simulate 1 - and2-m-thick
cores of MSW taken from a municipal
landfill. Thetest cells resembled medium-
density cells in a sanitary landfill contain-
ing unprocessed wastes in the humid
eastern United States.
Gas-Flow Measuring and
Sampling Systems
The gas-flow measuring and sampling
systems consisted of gas probes in each
tank that collected the gas produced by
the decaying waste, a gas-flow measuring
and logging device, and a system that
allowed the extraction of an uncontami-
nated gas sample for analysis.
Three gas probes were installed in each
test cell. Each consisted of a perforated
0.6-cm copper tube coated inside and out
with coal-tar epoxy to prevent corrosion
by leachate. The copper tubing was fitted
into a perforated polyvinyl chloride pipe to
protect the tubing during waste compac-
tion.
The gas-flow measuring system oper-
ates using a pressure-controlled flow.
Components of the system include a
differential pressure switch/gauge, three
solenoid valves (normally closed), one
linear mass gas f lowmeter, one controller,
seven shut-off valves, and one pressure
gauge. A data logger was used to record
gas production data from all four test
cells.
Two sampling methods were tested
during the first 18 days of test cell
operation—one using flexible plastic bags
and the other using stainless steel cyl-
inders. The gas-bag system had to be
abandoned because of oxygen diffusion
into the bags. The system with stainless
steel cylinders was used through the
remainder of the experiment; it used grab
samples collected from the test cells in
glass gas bulbs through the gas-measur-
ing system with the same pumping
procedure used for the gas bags. Shortly
after the change to grab samples, the O-
rings on the gas bulbs began to leak,
requiring the use of an all-metal system
using a compressor and steel storage
tank. After this all-metal system was
installed, the oxygen levels were at o
below detection limits.
Gas samples were analyzed on a
Perkin-Elmer Sigma 3* gas chromato-
graph(GC) for oxygen, hydrogen, nitrogen,
carbon dioxide, methane, and water
vapor.
Data Reduction and
Presentation
The raw data tapes from the data logger
were read by a Martek Model 421 -DRS
magnetic tape reader and printed on a
paper tape printer. Another magnetic tape
reader transferred the data to a Hewlett-
Packard Model 9830 computer through a
Martex Model 421-12-DRS computer
interfacing model. The data were then
verified, reorganized, and stored in raw
form on magnetic tapes in an array that
allowed easy access and manipulation by
the computer. The raw data were reduced
by engineering units and corrected for the
nonlinear output from the thermistors
and barometric pressure sensors and for
variations in gas composition. Programs
have been developed to present the final
data in any of several forms.
Results
The relative amounts and kinds of gases
found in this study correspond with those
reported for the theoretical stages of
bacterial succession in common anaer-
obic digesters: (1) an aerobic phase, (2) a
nonmethanogenic anaerobic phase, (3)
an unstable methanogenic phase, and(4)
a stable methanogenic phase. Gases pro-
duced in the last phase are the most
desirable since they are usually 45% to
60% methane, with the balance i"-vng
easily removed carbon dioxide.
The aerobic or first phase consisted of
the rapid uptake of any residual oxygen in
the MSW and the release of nearly equal
amounts of carbon dioxide. This phase
apparently took place before the first gas"
sampling, 24 to 48 hours after the cells
were sealed. Only very low levels of
oxygen were found in these first samples.
This result could be due to the high initial
temperature of the waste, which would
result in high metabolic rates for the
microbiological flora.
The anaerobic second phase was indi-
cated by the production of large volumes
of carbon dioxide. This phase was also
well under way in the first 2 or 3 days
after cell sealing. The total gas output of
the cells (mainly carbon dioxide and
'Mention of trade names or commercial products
does not constitute endorsement or recommend
tion for use '
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nitrogen) decreased dramatically over the
next 100 to 150 days as the MSW
approached water saturation and leachate
production. Hydrogen production was
noted in all of the test cells 60 to 90 days
before methane was detected in any of
them.
The third phase (unstable methano-
genic) began for three of the four test cells
at about the time that leachate began to
be produced, between days 250 and 300.
Methane evolved slowly and somewhat
variably over the remainder of the experi-
ment, with the gas output attaining 10%
to 25% methane only after nearly 4 years
of fermentation.
The fourth phase (stable methane
production), with its constant 40% to 60%
methane concentrations, was never
reached by any of the cells over the 1500
days of the study. But all of the cells were
increasing in methane production when
the experiment was terminated. In only
one of the small cells did the percentage
of methane in the released gas increase
continuously, reaching 26% by the end of
the experiment and still increasing.
The absence of a stable methane
production phase may have been caused
by less-than-optimum conditions in the
leachate such as low nitrogen-to-carbon
ratios and low total volatile acids as
substrates for the methanogenic bacteria.
Other factors may have been the lower-
than-optimum temperatures in the un-
insulated test cells, high levels of toxic
metals or organics, or the acidic pH's of
the leachates. Stable methane production
is not uncommon after only a few weeks
under ideal circumstances; but develop-
ment times of 100 to 300 days are more
common, and longer times are not un-
usual.
The total volume of gas produced varied
only about 12% between the small and
large cells (17.75 and 20.1 ml/kg dry
MSW per day, respectively). Compositions
of the total gases evolved were also quite
consistent for the major gases from all
test cells (70.5% and 69.6% for carbon
dioxide and 19% and 21 % for nitrogen in
the small and large cells, respectively).
These values represent the first 630 days
of the experiment. During the same
period, fermentative activity in the test
cells was quite consistent. The same
consistency is shown for the first 100
days when about half the total gas was
produced.
The relatively high, consistent levels of
nitrogen found in the gas from this study
could pose serious problems regarding its
use for energy. These levels could have
resulted from denitrif ication, or they could
have been due to contaminating air drawn
into the test cell. Further study of these
causes and careful monitoring of nitrogen
levels is needed in future studies of
methane production.
The amount of flammable gases pro-
duced varied considerably within both
sizes of test cells. Hydrogen was the only
gas that had consistent variations be-
tween the small and large test cells, and
this result may be accounted for by
chance. The smaller cells consistently
produced much less hydrogen than the
larger cells over the first 630 days of the
experiment, possibly because of the larger
methane production in the smaller cells.
Conclusions
The use of an automated, gas-measur-
ing system effectively monitored the
volume of gases released from four landfill
simulation test cells. Results show that
conditions present in the average MSW
may not be ideal for maximum methane
production, but that very appreciable
amounts of methane(and carbon dioxide)
can be expected from the average landfill
over extended periods. Further studies of
the effects of environmental and nutri-
tional factors on the time required to
develop stable methanogenic conditions
and bacterial populations are needed so
that conditions in the landfill can be
modified to maximize or minimize me-
thane production, depending on the use
to be made of the site.
The full report was submitted in fulfill-
ment of I nteragency Agreement No. EPA-
IAG-D4-0569 by the USAE Waterways
Experiment Station under the sponsorship
of the U.S. Environmental Protection
Agency.
Larry W. Jones. Robert J. Larson, and Philip G. Ma/one are with USAE Waterways
Experiment Station. Vicksburg. MS 3918O.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Landfill Gas Production from Large Landfill
Simulators. "(Order No. PB 84-235 779; Cost: $ 13.00. 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:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•tl U S GOVERNMENT PRINTING OFFICE, 1984 - 759-015/7822
3
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