United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/059 Dec. 1987
&ERA Project Summary
Simulation of Leachate
Generation from Municipal
Solid Waste
N. D. Williams, F. G. Pohland, K. C. McGowan, and F. M. Saunders
The modeling of leachate generation
from a municipal solid waste (MSW)
landfill or landfill simulation should
utilize a mechanistic approach which
properly accounts for the microbially
mediated processes of landfill stabili-
zation. Previous models have been
based on the solubility of waste con-
stituents in the water percolating
through a landfill. These models, called
washout models, provided a reasonable
approximation of leachate constituent
concentrations after the landfill or
landfill simulation had reached a period
of relative dormancy, called matura-
tion, but were deficient in predicting
leachate constituent concentrations in
the early stages of landfill stabilization,
and gas production and quality after
methane fermentation had been estab-
lished. These early stages in the life of
a landfill are extremely important,
because, in most cases, the highest
leachate strengths and the most
extreme conditions a liner or the
surrounding environment would be
subjected to occur during this period.
Similarly, the methane fermentation
stage is important in predicting the
potential for gas production, migration
and possible utilization.
A mechanistic three-step model,
GTLEACH-I, was developed to simu-
late the microbially mediated processes
of landfill stabilization in terms of
hydrolysis of substrate, acid formation
and methane fermentation. The model
was applied to two sets of experimental
data and provided a reasonable predic-
tion of volatile acids and gas
generation.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory, Cincin-
nati, OH, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The generation of leachate from a
landfill is a complex process depending
not only on the characteristics of the
landfilled wastes, but also on the: (1)
interaction of the waste with water
percolating through the landfill, (2)
operational variables such as waste
placement procedures, (3) climatic con-
ditions, (4) landfill design, and (5)
potential for interaction of the landfilled
waste with ground water. Leachate
characterization is further complicated
by the effects of microbial activity, which
mediates the conversion of both hazard-
ous and nonhazardous wastes and their
potential for transport and migration
from the landfill site.
Leachate characteristics and the rate
of leachate generation are dependent on
the time and stage of landfill stabilization.
Numerous investigations of the charac-
teristics of leachate generated from
municipal solid waste disposal have
indicated the highest concentrations of
a large number of leachate constituents
are generated during the early stages of
microbially mediated stabilization.
Therefore, if the purpose of a simulation
model is to predict changes in chemical
concentrations of leachate for assess-
ment of migration potential or liner/
leachate compatibility, it is necessary to
model the various phases of landfill
stabilization.
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Landfill Stabilization
The fate of waste constituents dis-
posed in a landfill can be envisioned as
a partitioning among the solid, vapor or
aqueous phases. Various microbial,
chemical, and physical transformations
alter the chemical and physical nature
of the waste and initiate transfer of
reaction products from one phase to
another. Likewise, waste constituents
are transported from the landfill in
aqueous solution and suspension by
washout and through the evolution of
gases.
Microbially mediated reactions control
the landfill environment for some time
after initial placement of waste and
strongly impact the outcome of other
chemical and physical transformations
leading to stabilization. Both aerobic and
anaerobic microbial processes occur in
a landfill; however, free oxygen is
typically available only in early stages of
landfill stabilization and is often
exhausted prior to the appearance of
leachate. For this reason, anaerobic
activity usually establishes and controls
leachate and gas quality during the active
life of a landfill.
In a steady-state anaerobic treatment
process, three steps occur simultane-
ously at a rate controlled by the slowest
step in the sequence so that there is little
accumulation of intermediate products
over time. However, in a non-homog-
eneous, batch-wise system such as a
landfill, the activity of acid-forming and
methane-forming bacteria may not be in
balance at any particular location in the
landfill at any one time. As a result,
landfills are often characterized by
temporal stages governed by the predom-
inance of changing microbial populations
in the life of the landfill.
A landfill has been described as
evolving through five stages as it
becomes stabilized—initial adjustment,
transition, acid formation, methane
fermentation, and final maturation.
Accordingly, microbially mediated reac-
tions first accomplish the transformation
and solubilization of waste materials into
the aqueous phase. Further transforma-
tions into intermediates such as the
volatile organic acids, followed by con-
version to methane, result in transfer of
conversion products to the gas phase.
The relative rates of these transforma-
tions, combined with the rate of moisture
infiltration, determine the concentration
and mass flux of the various biochemical
intermediates and end-products. Super-
imposed upon the microbial transforma-
tions are solubility, speciation, oxidation-
reduction and solid-liquid gas equilibria
of both inorganic and organic chemicals.
These equilibria establish the partition-
ing of chemical components among solid,
liquid and gas phases.
One of the major difficulties in de-
scribing the fate of waste disposed in a
landfill stems from uncertainties in the
relative impacts of the various transfor-
mation and partitioning processes acting
on the waste, leachate and gas. Leachate
and gas composition and quantity data
reveal the net results of the various
transformation and partitioning pro-
cesses but do not necessarily indicate the
path taken. Furthermore, data are seldom
available and sufficient to describe the
composition of the solid phase. Thus, it
is difficult to assess the relative impacts
of adsorption, complexation and precip-
itation processes, since they all result in
transfer of a component from the aqeu-
ous to the solid phase or vice versa.
Likewise, assessment of the relative
effects of microbial degradation, chem-
ical conversion and sorption on the fate
of organic components disposed in a
landfill is also exceedingly challenging.
GTLEACH-I Model
Since the microbially mediated pro-
cesses of landfill stabilization control the
rate of generation and constituent
concentrations of leachate and gas
produced from an MSW landfill or landfill
simulation, it is necessary to use a
mechanistic model which incorporates
the effects of the stabilization process.
GTLEACH-I is a three-phase mechanistic
model capable of simultating changes in
leachate volatile acid concentrations and
methane gas emerging from the landfill
over time. The model is composed of
hydrogeologic and biologic modules and
is written for a single lift of a landfill or
a landfill simulation. Additional effort will
be required for application to full-scale
landfills.
Hydrogeologic Module
The rate and quantity of water flowing
through waste materials can be corre-
lated to the quantity and, to some extent,
the characteristics of leachate produced
in landfills. This is not to imply that a
simple washout model adequately pre-
dicts leachate characteristics. Stated
very simply, the characteristics of the
leachate depend primarily on the adsorp-
tion, solution, movement and microbially
mediated hydrolysis/conversion of
waste materials described by gravity flow
models and by biological waste conver- I
sion models.
The latter microbially mediated stabil-
ization processes are dependent on many
parameters, among which are the water
content, porosity, and distribution of
water in the waste, the contact time
between water percolating through the
landfill and the waste materials, and
changes in waste surface area, compo-
sition and porosity with time. These
parameters either affect or are depend-
ent upon the quantity and rate of water
flowing into and through the waste
materials.
The hydrogeologic module must
account for all flow into and out of the
landfill and its constituent cells. There-
fore, a quasi two-dimensional, determin-
istic model similar to the HELP model
could be incorporated into the hydrogeo-
logic module of GTLEACH-I. The HELP
model performs a sequential, time-based
water budget for a landfill cell. The water
budget is based on soil and waste
characteristics, climatological data and
landfill design parameters. The model
can be described as a component, semi-
empirical numerical model which eval-
uates and effects and interaction of
runoff, evapotranspiration, percolation
and lateral drainage.
To provide the data required to eval-
uate the time-dependent progression of
microbially mediated stabilization, sev-
eral modifications of the HELP model
were made that yielded unrealistically
high flow rates through the waste
materials. Therefore, it is anticipated that
the vertical flow model will be replaced
by a model that provides more realistic
flow rates and distribution of moisture
content as a function of depth and time
in the waste material. In addition, surface
run-on interaction with the site hydrol-
ogy and short-circuiting have been
considered necessary components.
Biologic Module
Based on observed landfill stabilization
trends it is evident that the appearance
and eventual disappearance of volatile
acids in leachate are primary indicators
of processes responsible for changes in
COD, pH, and oxidation-reduction poten-
tial within the landfill. Thus, a module
which is capable of simulating the
changes in volatile acid concentrations
in leachate over time could ultimately
form the foundation for simulation of
other indicator parameters such as pH
and oxidation-reduction potential (ORP)
along with corresponding changes in the
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mobility of hazardous constituents dis-
posed in a landfill.
The three-step processes of hydroly-
sis/solubilization, acid formation and
methane fermentation were logical
choices on which to base the develop-
ment of GTLEACH-I. Since it has been
applied successfully in the design and
operation of other anaerobic processes,
it was reasonable to assume that the
three-step process would also be suc-
cessful in modeling anaerobic stabiliza-
tion in landfill systems. Furthermore,
such a mechanistic model could predict
the concentration of volatile acid inter-
mediates as a function of time as well
as the rate of conversion of leachate COD
to methane.
Hydrolysis/solubilization is an impor-
tant process in the degradation of organic
matter, and it is often considered to be
the rate-limiting step in the acid phase
of anaerobic digestion. The rate of
hydrolysis is affected by many factors
including pH, temperature, microbial
biomass and associated substrate, the
remaining concentration of paniculate
substrate, and hydrolysis product con-
centration. It is evident that any one or
all of these factors may change with time
in a batch-wise operation such as a
landfill.
At constant temperature and pH,
hydrolysis may be approximated as first-
order with respect to particulate or solid
organic substrate concentration. There-
fore, this approach was used to model
hydrolysis/solubilization in GTLEACH-I
as expressed by the equation:
dM _ v >•
- -K.HM
dt
where.
M = solid substrate concentration
expressed as if it were suspended
in landfill moisture, and
KH = hydrolysis/solubilization rate
constant.
While the approximation of first-order
kinetics for hydrolysis is reasonable
when considering the acid formation and
methane fermentation phases of landfill
stabilization separately, difficulty is
expected in modeling the transition
between the two phases when pH and
other leachate characteristics change
dramatically. Additionally, as relatively
easily hydrolyzed substrates are
exhausted and more refractory sub-
strates begin to be utilized, a correspond-
ing change in the rate of hydrolysis
should be manifested. Production of
volatile acids was represented in
GTLEACH-I by a single group of acid-
forming bacteria rather than a more
complicated set of equations for multiple
populations of bacteria.
The major reason for selecting a single
equation to model behavior of a group
of microorganisms is that the relative
activities of the various populations of
microorganisms were assumed constant
with respect to each other, so that the
net reaction follows Monod kinetics.
While the validity of this assumption may
be questioned when applied to landfills,
the single-equation Monod model of
acidogenesis was selected to allow
application of the model to the database
currently available and to minimize the
number of input parameters to the model.
One other aspect of methanogenesis
to be considered is that methanogen
growth and concurrent methane produc-
tion can be suppressed at pH levels below
the range of 6.8 to 7.5. This explains the
virtual absence of methane production
observed during the acid phase of landfill
stabilization when the pH of landfill
moisture can be well below pH 6. That
methanogenesis is established at all is
attributed to the heterogeneous nature
of landfills which allows methanogens
to begin to grow in small pockets of the
landfill not completely saturated with
volatile acids. Methanogen growth grad-
ually spreads out from these pockets until
a methanogen population is established
sufficient to reduce the concentrations
of volatile acids in the landfill moisture
and increase the pH, thereby allowing
unsuppressed methanogen growth
throughout.
As a first attempt at modeling landfill
processes, it was decided to provide for
suppression of methanogenesis during
the acid phase by simply delaying initi-
ation of methanogenesis for a specified
period of time in GTLEACH-l. An exten-
sion of this approach will require devel-
opment of a pH-inhibition term for
methanogenesis which allows for
methanogen growth at a suppressed rate
during periods of low leachate pH.
Diffusional resistance to reactions in
a dense matrix such as a microbial film
or aggregate may have significant influ-
ence on reaction kinetics by reducing the
efficiency of the microbial mass. Due to
difficulties encountered in evaluating the
total microbial mass, the thickness of
biofilms or aggregates, and due to
heterogeneities in landfill environments,
diffusional resistances were not explic-
itly treated in GTLEACH-I. Instead, they
were subsumed in the fitting of the
Monod rate constant to experimental
data.
Evaluation of Model
The initial values of the kinetic growth
constants for acidogenic and methano-
genic bacteria were selected based on
compiled data. The parametric analysis
indicated that the GTLEACH-I model
provided a reasonable approximation of
the quantities of volatile acids and
methane gas with time. Using the values
of the parameters which provided the
best fit of the data in the parametric
analysis, the model was used to predict
the quantities of volatile acids and
methane gas for the boundary conditions
of selected landfill simulation studies.
A major accomplishment of the model
was the application of kinetic constants
for hydrolysis/solubilization and acido-
genesis in the simulation of two sets of
experimental data having substantially
different moisture application rates. The
model was also a useful diagnostic tool
for examining differences in microbial
populations resulting from different
leachate management options.
Conclusions
A useful interpretation of leachate
generation and leachate characteristics
is a highly complex and difficult under-
taking, primarily because landfill disposal
of waste is site specific. The local
hydrogeology and landfill design and
operation all influence the potential for
infiltration and percolation and the rate
of saturation of the waste materials.
Therefore, leachate characteristics and
opportunity for migration will vary in
accordance with these factors.
Numerous evaluations and simula-
tions of landfill performance have been
conducted to characterize leachate
generation patterns. Some of the data
from these studies have been used to
develop numerical predictive models
descriptive of leachate generation as a
function of time. A review and further
development of these models led to the
following conclusions.
1. Data descriptive of microbially
mediated processes of landfill
stabilization are necessary compo-
nents of an effective numerical
landfill simulation model and are
influenced by waste type, availa-
-------
bility of nutrients, moisture con-
tent, and opportunity for biological
or physical/chemical conversion.
2. Comprehensive analysis of waste
characteristics and waste constit-
uent distribution must be available
as input data for predicting leach-
ate constituent concentration
changes as a function of time, to
indicate substrates susceptible to
conversion, to help assess any
delays in the progress of landfill
stabilization, and to appropriately
distinguish microbial mediation
from simple washout.
3. There are no established tech-
niques to yield data sufficient to
accurately predict hydraulic con-
ductivity at landfills. Data from
simulation studies performed to
date are presumptive of flow con-
ditions and generally ignore non-
homogeneity of the leaching
matrix.
4. The GTLEACH-I model has been
developed to simulate the micro-
bially mediated process of landfill
stabilization in terms of hydrolysis
of waste substrate, acid formation
and methane formation.
5. GTLEACH-I provides reasonable
predictions of volatile acid and gas
generation during landfill stabiliza-
tion and may be expanded to
predict variations of pH, pE, and
ORP as functions of time. These
latter relationships again require a
comprehensive database which
must include acid-base and
oxidation-reduction equilibria not
usually present in existing compi-
lations of landfill analyses.
6. Calibration of the GTLEACH-I
model has been limited by a
general lack of substrate specific-
ity, the uncertainty of flow distri-
bution and short-circuiting, partic-
ularly during the early stages of
landfill stabilization, the possibility
of retardation of microbial media-
tion, the influences of population
dynamics as substrate conversion
proceeds and the waste mass is
engulfed, and the potential for
containment or release of the liquid
and gas transport phases.
Recommendations
Based on the insights obtained from
the review and utilization of available
data, the following recommendations for
future research are proposed:
1. Develop implementation strategies
for any future research on landfill
stabilization to assure that suffi-
cient data are generated to support
the further development and veri-
fication of predictive models cap-
able of more accurately modeling
both short-term and long-term
variations in leachate quality and
quantity.
2. Initiate a field study that provides
sufficient data to support the
refinement and verification of the
numerical model.
3. Initiate complementary studies to
measure hydraulic conductivity of
landfilled wastes to variable phys-
ical and chemical characteristics in
a temporally and spatially distrib-
uted fashion. The study should
provide data and information
regarding spatial variation of
hydraulic conductivity and time
related changes in hydraulic
conductivity.
4. Refine and expand the GTLEACH-
I model to predict pH, pE, and ORP
as a function of time, to incorporate
more realistic descriptions of flow
retardation or inhibition, and to
allow prediction of selected inor-
ganic and organic constituent
concentrations as population
dynamics affect the overall pro-
cesses of landfill stabilization.
N. D. Williams, F. G. Pohland, K. C. McGowan, and F. M. Saunders are with
Georgia Institue of Technology. Atlanta. GA 30332.
Jonathan G. Herrmann is the EPA Project Officer (see below).
The complete report, entitled "Simulation of Leachate Generation from
Municipal Solid Waste," (Order No. PB 87-227 005/A S; Cost: $ 18.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal ftoad
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati OH 45268
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PA
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-87/059
0001820 HHER
NORMAN NIEDERGANG
USEPA REGION V
230 SO. DEARBORN ST.
CHICAGO IL 60604
-------
United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-87/060 Dec. 1987
SEPA Project Summary
Novel Vapor-Deposited Lubricants
for Metal-Form ing Processes
J. J. Mills
This report gives results of a labora-
tory study of the feasibility of using
vapor-phase lubrication to lubricate
industrial metal forging dies. It gives
results of six tasks conducted during
the study and discusses the potential
production and environmental impact
of the process. If a vapor lubrication
system can be developed for general
industrial use it can significantly reduce
the volume of forging lubricants required
by present industrial forging operations.
The laboratory results indicate that it
may be possible to reduce potential air
pollution emissions from forging using
vapor lubrication by as much as 85%.
This would be accomplished by using
85% less lubricant volume during metal
forging.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
The forging and shaping of metal parts
is one of many metal fabricating pro-
cesses that may generate emissions of
volatile organic compounds (VOCs) and
hydrocarbons. In typical metal forming
operations hot metal is squeezed in dies
to produce metal shapes in the form of
the die cavity. This process may require
many intermediate forming and shaping
steps using successively more accurate
dies to reach the finished product. A key
aspect of these shaping steps is the
lubrication of the dies and metal parts to
allow easy release of the part from the
die. The used lubricants frequently result
in emissions containing VOCs and poten-
tially toxic metal to the atmosphere.
This report presents the results of a
Phase I study that investigated the feasi-
bility of using vapor-phase lubrication for
industrial metal forging dies. It presents
the results of six tasks conducted during
the study and discusses the potential
production and environmental impact of
the effectiveness of the process. A vapor
lubrication system developed for general
industrial use could significantly reduce
the volume of forging lubricants required
by present industrial forging operations.
The project proposes to use a vapor-
phase polymer film to lubricate forging
dies in their closed position. An injection
device allows lubricant vapor to be applied
automatically through passages in the
flange areas of the die. This eliminates
large volumes of liquid die lubricants and
the resulting emissions typically gen-
erated during this operation.
Project Plan
Six tasks were performed during the
project. Each was designed to produce
vital elements and data for a future pilot
scale unit. The six tasks were to:
• Establish a fully operational, labora-
tory scale vapor-phase lubricant
delivery system (LDS).
• Formulate lubricants and evaluate
the lubricity of the vapor-deposited
polymers using the ring compression
test.
• Forge parts using conventional
lubricants to provide baseline data.
• Modify the forging die to permit
vapor-phase lubrication.
• Forge five parts using vapor-phase
lubrication in a modified die.
• Quantitatively compare the emis-
sions from vapor-deposited lubrica-
tion with those from the convention-
al oil-based lubrication system.
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Emission Results
The volume of lubricant used during
each experiment was determined qualita-
tively by the metal flow and release pro-
perties exhibited by each technique. Metal
flow is defined by the interface friction
factor (m value) which is a measure of
metal flow within the die. Release pro-
perties are defined by the relative ease
with which the part can be removed from
the die. It was assumed that all lubricant
used during each experiment was vola-
tilized to the atmosphere. This represents
the worst case scenario for the process.
Table 1 summarizes the results.
Conclusion
Although this project included a limited
number of experiments, it did show that
vapor-phase lubrication is feasible for
metal forging. It can also result in a
significant reduction of potential emis-
sions to the atmosphere. The process
could reduce emissions from forging and
casting operations by as much as 85%.
The project represents only the first
step, laboratory feasibility, of the develop-
ment program for vapor-phase lubrication.
Significant research and development still
remains, including die lubrication system
design and lubricant formulation
development.
Table 1. Average Emissions and Forging Parameters
Average block and finish per part, ml
Average forging force, ton (kN)
Average forging time, sec
Conventional
Lubrication
76
75(667)
60
Vapor-Phase
Lubrication
11.3
75 1 '667)
60
J. J. Mills is with Martin Marietta Laboratories, Baltimore, MD 21227.
Charles H. Darvin is the EPA Project Officer (see below).
The complete report, entitled "Novel Vapor-Deposited Lubricants for Metal-
Forming Processes," (Order No. PB 87-227 351/AS; Cost: $11.95, 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 Officer can be contacted at:
Air and Energy Engineering 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
j JAN-4'88 f'.^j; !
V •• //""•?! = 0.2
Official Business
Penalty for Private Use $300
EPA/600/S2-87/060
. 0000329 PS
*GENCT
230 S OESR80RN STREET
CWICAGO IL
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