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.

-------
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

-------
 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.

-------
  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

-------