Technical review of methods to enhance biological degradation in sanitary landfills


Warith, M.A.; Sharma, R.

Water Quality Research Journal of Canada 33(3): 417-437

1998


Biological processes are known to reduce the organic fraction of municipal solid waste, but current landfilling practices have not been altered to reflect this knowledge. The advantages of enhancing degradation of solid waste are as follows: reduced period of leachate treatment, increased methane production, expedited landfill site reclamation through stabilized waste mining, and accelerated subsidence permitting recovery of valuable landfill air space. The techniques that can be used to enhance biological degradation include leachate recirculation, addition of nutrients, shredding, sludge addition, lift design, temperature and moisture content management. Manipulation of these variables promotes a more conducive environment for microbial activity. Research on landfill management strategies through laboratory and full-scale studies has shown the validity of applying the enhancement techniques with regards to reducing leachate strength and increasing methane production. These practices focus on the use of landfills as bioreactors, which enables long-term flexibility and assures compliance with future regulations and discharge standards.

Water
Qual.
Res.
J.
Canada,
1998
Volume
33,
No.
3,
417-437
Copyright
C
1998,
CAWQ
Technical
Review
of
Methods
to
Enhance
Biological
Degradation
in
Sanitary
Landfills
M.A.
WARITH
AND
R.
SHARMA
Ryerson
Polytechnic
University,
Department
of
Civil
Engineering,
350
Victoria
Street,
Toronto,
Ontario
M5B
2K3
Biological
processes
are
known
to
reduce
the
organic
fraction
of
municipal
solid
waste,
but
current
landfilling
practices
have
not
been
altered
to
reflect
this
knowledge.
The
advantages
of
enhancing
degradation
of
solid
waste
are
as
follows:
reduced
period
of
leachate
treatment,
increased
methane
production,
expedited
landfill
site
reclamation
through
stabilized
waste
mining,
and
accel-
erated
subsidence
permitting
recovery
of
valuable
landfill
air
space.
The
tech-
niques
that
can
be
used
to
enhance
biological
degradation
include
leachate
recirculation,
addition
of
nutrients,
shredding,
sludge
addition,
lift
design,
temperature
and
moisture
content
management.
Manipulation
of
these
vari-
ables
promotes
a
more
conducive
environment
for
microbial
activity.
Research
on
landfill
management
strategies
through
laboratory
and
full-scale
studies
has
shown
the
validity
of
applying
the
enhancement
techniques
with
regards
to
reducing
leachate
strength
and
increasing
methane
production.
These
practices
focus
on
the
use
of
landfills
as
bioreactors,
which
enables
long-term
fl
exibility
and
assures
compliance
with
future
regulations
and
discharge
standards.
Key
words:
landfill
leachate,
recirculation,
biological
degradation
of
solid
waste
Introduction
Major
concerns
regarding
the
impact
of
municipal
landfills
on
the
environment
are
related
to
leachate
quantity
and
quality,
gas
generation,
and
decomposition
processes
occurring
in
the
landfill.
The
main
processes
responsible
for
the
degradation
of
solid
waste
in
the
landfill
are
biological
processes.
It
is
desirable
to
minimize
the
time
period
in
which
degrada-
tion
occurs
in
order
to
reduce
gas
emissions
after
the
landfill
is
closed,
to
ease
the
requirements
of
leachate
treatment,
and
to
be
successful
in
reclaiming
the
landfill
site.
Leachate
that
has
a
high
concentration
of
organic
compounds
requires
a
greater
amount
of
oxygen
during
aerobic
treatment,
leading
to
higher
energy
costs.
By
enhancing
biological
degradation
processes,
the
period
in
which
the
leachate
is
highly
polluted
decreases.
The
peak
organ-
ic
concentration
of
the
leachate
is
also
lessened,
which
results
in
reducing
the
leachate
treatment
demand.
Landfill
gas
production
can
lead
to
effi-
cient
recovery
of
energy
from
the
landfill.
High
methane
concentrations
in
the
landfill
gas
are
important
for
efficient
energy
conversion.
When
gas
418
WARITH
AND
SHARMA
production
rates
are
accelerated,
energy
recovery
processes
can
begin
at
an
earlier
stage
and
can
be
decomposed
after
closure
of
the
landfill.
Several
possible
enhancement
techniques
can
be
implemented
to
increase
biological
activity
in
landfills.
These
techniques
include
leachate
recycling,
use
of
buffers
and/or
nutrients,
sludge
addition,
reduction
of
waste
particle
size,
waste
lift
design,
and
moisture
content
management.
Despite
knowledge
of
benefits
of
biological
enhancement
tech-
niques,
there
have
been
few
changes
in
recent
years
in
sanitary
landfill
construction
and
operation
techniques.
This
paper
reviews
the
current
state-of-the-art
in
landfill
leachate
management
strategies
involved
in
both
laboratory
and
full-scale
studies.
As
a
result,
it
reveals
the
validity
of
applying
various
enhancement
techniques
for
degradation
of
solid
waste
in
sanitary
landfills.
Biochemical
Process
in
Landfills
The
benefits
of
enhancing
biological
degradation
in
sanitary
landfills
are
only
possible
upon
understanding
the
basic
biochemical
processes
that
occur
in
such
environment.
Stabilization
of
waste
in
a
municipal
solid
waste
(MSW)
landfill
is
dominated.by
microbial
activity
which
governs
the
composition
of
leachate
and
the
generation
of
landfill
gas.
Concentration
of
organic
substances
in
a
landfill
and
the
age
of
the
land-
fill
are
the
other
two
factors
controlling
MSW
stabilization
(Chiampo
et
al.
1996;
Nozhevnikova
et
al.
1993).
The
most
important
stabilization
process
is
the
anaerobic
decomposition,
as
the
availability
of
oxygen
as
a
terminal
electron
acceptor
is
limited
in
the
sealed
landfill
(Qian
and
Barlaz
1996;
Watson-Craik
et
al.
1995;
Senior
1992).
Solid
Waste
Degradation
Sequence
The
sequence
of
waste
decomposition
begins
with
a
relatively
short
aerobic
degradation
phase, which
occurs
directly
after
the
waste
has
been
deposited
in
the
landfill;
it
is
followed
by
a
long
anaerobic
degradation
phase.
Methane
gas
and
carbon
dioxide
are
the
main
products
of
the
sta-
bilization
process.
Several
researchers
(Buckley
and
Lowery
1996;
Braber
1995;
Al-Yousifi
and
Pohland
1993;
Byrom,
1993;
Barlaz
et
al.
1989a;
Pohland
1980)
presented
an
idealized
sequence
of
the
processes
involved
in
the
decomposition
of
waste
and
the
consequence
on
the
landfill
gas
and
leachate
composition
during
the
various
phases
of
waste
decomposition.
Each
stage
of
waste
decomposition
is
characterized
by
its
own
set
of
phys-
ical,
chemical
and
microbial
activities
(Reinhart
and
Al-Yousifi
1996;
Christensen
and
Kjeldsen
1989;
Pohland
1980).
Major
bacterial
groups
involved
in
this
decomposition
process
include
fermentative
bacteria,
ace-
togenic
bacteria,
methanogenic
bacteria
and
sulphate
-reducing
bacteria.
Phase
I
involves
a
short
period
of
aerobic
decomposition
in
which
eas-
ily
degradable
organic
matter
is
consumed
and
carbon
dioxide
is
generated.
In
an
aerobic
environment,
a
large
number
of
interactions
among
species
are
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
419
responsible
for
biodegradation.
A
community
of
microorganisms
works
in
"syntrophy",
where
one
group
of
organisms
produces
metabolites
which
are
then
usable
by
other
members
of
the
group
(Senior
1992).
Phase
II
is
the
first
intermediate
anaerobic
decomposition
phase
and
occurs
immediately
after
the
aerobic
phase.
The
first
stage
consists
of
the
fermentative
bacteria,
which
are
a
large
heterogeneous
group
of
faculta-
tively
anaerobic
microorganisms.
The
fermenters
hydrolyze
and
ferment
solid
and
complex
dissolved
organic
compounds
into
primarily
volatile
acids,
alcohols,
hydrogen
and
carbon
dioxide.
The
acetogenic
bacteria
function
in
the second
decomposition
phase
and
are
also
a
large
hetero-
genic
group.
These
bacteria
convert
the
products
generated
by
the
fer-
menters
to
acetic
acid,
hydrogen
and
carbon
dioxide.
During
this
second
phase
the
leachate
is
acidic
with
a
pH
of
less
than
6.5,
and
may
contain
high
concentrations
of
fatty
acids,
calcium,
iron,
heavy
metals
and
ammonia
(Ejlertsson
et
al.
1996;
Doedens
and
Cord-
Landwehr
1989).
The
presence
of
ammonia
is
mainly
due
to
the
hydroly-
sis
and
fermentation
of
protein
compounds.
Nitrogen
in
the
gas
phase
is
reduced
as
a
result
of
carbon
dioxide
and
hydrogen
generation.
Furthermore,
the
initial
high
concentration
of
sulphate
in
the
leachate
is
slowly
reduced
as
the
redox
potential
drops.
The
generated
sulphide
may
also
precipitate
iron,
manganese
and
heavy
metals
that
were
dissolved
in
the
initial
stages
of
this
phase
(Christensen
and
Kjeldsen
1989).
Phase
III
is
the
second
intermediate
anaerobic
degradation
stage
in
which
methanogenic
bacteria
slowly
start
to
appear.
The
sulphate
-
reducing
bacteria
are
also
included
in
the
anaerobic
decomposition
process
since
this
group
of
bacteria
in
many
ways
resembles
the
methanogenic
group,
and
sulphate
is
a
major
compound
of
many
waste
types.
The
sulphate
-reducing
bacteria
are
obligate
anaerobes
and
may
convert
hydrogen,
acetic
acid
and
higher
volatile
fatty
acids
during
sul-
phate
reduction.
However,
this
group
is
more
likely
to
oxidize
organic
compounds
to
carbon
dioxide.
Therefore,
if
the
activity
of
sulphate
reduc-
ers
is
high,
less
organic
material
is
available
for
methane
production.
As
the
methane
gas
increases,
hydrogen,
carbon
dioxide
and
volatile
fatty
acid
concentrations
decrease.
The
conversion
of
fatty
acids
causes
the
pH
within
the
landfill
to
increase.
This
subsequently
reduces
the
sol-
ubility
of
calcium,
iron,
manganese
and
heavy
metals
in
the
leachate,
which
are
then
precipitated
as
sulphides.
Phase
IV
is
characterized
by
the
steady
production
of
methane
gas.
During
this
phase
;
methane
gas
constitutes
approximately
50
to
60%
(by
volume)
of
the
gas
composition.
The
high
rate
of
methane
gas
formation
maintains
low
concentrations
of
volatile
acids
and
hydrogen,
and
as
a
result
the
leachate
is
able
to
sustain
a
neutral
pH.
Phase
V
represents
the
stage
in
which
recalcitrant
wastes
is
decom-
posed.
This
phase
involves
low
methane
production,
constant
pH
levels
in
the
leachate
and
low
leachate
strength.
Nitrogen
starts
to
appear
again
at
this
stage
in
the
landfill
gas
due
to
diffusion
from
the
atmosphere
because
of
the
low
methane
gas
production
rate.
The
last
stage
is
domi-
420
WARITH
AND
SHARMA
nated
by
the
obligate
anaerobic
methanogenic
bacteria,
which
produce
methane
and
require
very
low
redox
potentials.
Two
groups
of
bacteria
comprise
the
methanogenic
group:
the
hydrogenophilic
and
the
ace-
tophilic
bacteria.
The
hydrogenophilic
bacteria
convert
primarily
acetic
acid
to
methane
and
carbon
dioxide.
This
idealized
waste
degradation
sequence
assumes
that
the
waste
is
homogeneous
and
of
constant
age.
A
realistic
landfill
occupying
waste
cells
with
highly
variable
age
and
composition
may
yield
a
somewhat
dif-
ferent
overall
picture
(Barlaz
et
al.
1989b).
Nozhevnikova
et
al.
(1993)
were
able
to
show
that
in
small
landfills
methane
that
is
produced
in
the
anaerobic
zone
can
be
oxidized
com-
pletely
in
the
upper
fill
layers.
This
ability
of
biogas
extraction
is
impor-
tant
not
only
as
an
additional
energy
source,
but
also
as
controlling
the
environmental
problems
associated
with
methane
and
its
release
to
the
atmosphere.
Through
the
usage
of
geophysical,
isotopic
and
microbiolog-
ical
techniques,
Nozhevnikova
et
al.
(1993)
were
able
to
provide
detailed
descriptions
and
confirmations
of
processes
occurring
in
landfill
sites.
Overall,
it
was
concluded
that
in
the
upper
fill
layer,
methane
became
heavier
and
carbon
dioxide
lighter
due
to
the
microbiological
oxidation
processes.
The
occurrence
of
methanogenesis
was
observed
in
the
upper
part
of
the
anaerobic
zone
where
the
organic
substance
concentration
was
relatively
high
(Nozhevnikova
et
al.
1993).
Governing
Abiotic
Factors
The
major
abiotic
factors
in
a
landfill
that
can
affect
methane
gas
pro-
duction
include
the
concentration
of
oxygen,
pH/alkalinity,
sulphate,
nutri-
ents
and
inhibitors
as
well
as
temperature
and
water
content
(Campbell
1993;
Christensen
and
Kjeldsen
1989;
Doedens
and
Cord-Landwehr
1989).
These
factors
alone
may
not
be
critical,
but
they
may,
however,
influence
other
parameters
which
control
MSW
degradation
process
rates
and
activ-
ities.
A
brief
description
of
the
abiotic
factors
follows.
Oxygen
Methanogenic
bacteria
are
particularly
sensitive
to
the
presence
of
oxygen.
Extensive
gas
recovery
pumping
may
create
a
substantial
vacu-
um
in
the
landfill,
forcing
air
into
the
landfill.
This
would
extend
the
aer-
obic
zone
in
the
landfilled
waste
and
eventually
prevent
formation
of
methane
in
these
layers.
However,
under
normal
conditions,
aerobic
bac-
teria
in
the
top
of
the
landfill
will
cause
solid
waste
to
readily
consume
the
oxygen
and
limit
the
aerobic
zone
to
less
than
1
m
of
compacted
waste
(Christensen
and
Kjeldsen
1989).
pH
Methanogenic
bacteria
exist
best
within
a
narrow
pH
range
of
6
to
8.
If
the
activity
of
methanogenic
bacteria
is
low,
for
some
reason,
their
con-
version
of
hydrogen
and
acetic
acid
decreases.
This
causes
the
hydrogen
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
421
pressure
to
build
up,
and
at
elevated
pressures,
acetogenic
bacteria
can-
not
convert
volatile
fatty
acids,
particularly
butyric
and
propionic
acid.
The
accumulation
of
these
acids
consequently
lowers
the
pH
within
the
landfill,
and
eventually
stops
methane
production.
The
methanogenic
system
in
the
landfill
is
rather
delicate,
and
balanced
relations
between
the
various
bacterial
groups
are
crucial
for
a
good
rate
of
methane
pro-
duction.
A
buffer
material,
such
as
demolition
waste
or
soil,
could
be
added
to
the
landfill
so
that
appropriate
pH
levels
are
maintained.
Sulphate
It
was
reported
that
in
both
batch
experiments
and
laboratory
land-
fill
simulations,
methane
generation
was
dramatically
reduced
in
the
presence
of
high
sulphate
concentration
within
the
landfill
environment
(Campbell
1993).
This
reduction
is
not
related
to
any
toxic
effects
of
sul-
phate
on
methanogenic
bacteria
but
rather
due
to
substrate
competition.
Nutrients
Microorganisms
that
participate
in
the
anaerobic
degradation
of
waste
require
nutrients
such
as
sulphur,
calcium,
magnesium,
potassium,
iron,
zinc,
copper,
cobalt,
molybdate,
selenium
and,
in
particular,
nitrogen
and
phosphorus.
These
nutrients
are
found
in
most
landfills.
However,
insufficient
homogenization
of
the
waste
may
result
in
nutrient
-limited
environment.
It
was
reported
that
the
optimal
ratios
between
organic
matter
(expressed
as
chemical
oxygen
demand),
nitrogen
and
phosphorus
are
listed
as
100:0.44:0.08
(Christensen
and
Kjeldsen
1989;
Stegmann
and
Spendlin
1989).
In
cases
where
there
is
a
limited
nutrient
for
anaerobic
degradation,
phosphorus
would
be
the
most
likely
element.
Inhibitors
In
addition
to
the
inhibitory
effects
of
oxygen,
hydrogen,
proton
activity,
and
sulphate,
it
has
been
suspected
that
carbon
dioxide,
salt
ions,
sulphide,
heavy
metals
and
specific
compounds
are
potential
inhibitors
of
methane
production.
It
was
reported
that
at
carbon
dioxide
partial
pres-
sures
between
0.2
to
1.0
atm,
the
conversion
of
acetic
acid
decreases
(Cossu
et
al.
1993;
Christensen
and
Kjeldsen
1989).
Cations
such
as
sodi-
um,
potassium,
calcium,
magnesium
and
ammonium
have
been
observed
to
stimulate
anaerobic
decomposition
at
low
concentration
while
inhibit
it
at
high
concentrations.
Temperature
It
was
documented
that
the
rate
of
methane
generation
increased
sig-
nificantly
(up
to
100
times)
when
the
temperature
was
raised
from
20
to
40°C
in
laboratory
simulations
(Christensen
and
Kjeldsen
1989).
Furthermore,
it
was
indicated that
in
a
deep
landfill
with
a
moderate
water
fl
ux,
landfill
temperatures
of
30
to
45°C
can
be
expected,
even
for
temperate
climates.
This
was
attributed
to
the
heat
fl
ux
from
the
landfill
422
WARITH
AND
SHARMA
to
the
surroundings
being
low
due
to
the
insulating
effect
of
the
waste;
the
heat
is
also
generated
by
the
anaerobic
decomposition
process,
which
may
result
in
a
temperature
rise
within
the
landfill
environment.
Moisture
content
The
availability
of
moisture
content
of
about
25
to
60%
has
shown
to
exponentially
increase
methane
gas
production
(Mata-Alvarez
and
Mertinez-Viturtia
1986;
Ham
and
Bookter
1982).
The
benefits
of
increased
water
content
in
a
landfill
include
limiting
oxygen
transport
from
the
atmosphere,
facilitating
exchange
of
substrate,
nutrients,
buffer,
dilution
of
inhibitors
and
spreading
of
microorganisms
within
the
landfill.
Methods
of
Enhancing
Degradation
Leachate
Recirculation
Leachate
recirculation
refers
to
the
collection
of
leachate
discharged
from
a
landfill,
and
redistributing
it
through
the
waste
to
enhance
biodegradation
and
reduce
the
contaminant
concentrations
in
the
leachate.
Several
studies
have
been
conducted
to
investigate
the
effects
of
leachate
recirculation
(McCreanor
and
Reinhart
1996;
Reinhart
1996;
Reinhart
and
AI-Yousifi
1996;
Townsend
1996;
Morelli
1992;
Pohland
1980)
on
MSW
biodegradation.
Pohland
(1980)
investigated
leachate
recycling
with
two
simulated
landfill
cells.
Each
cell
was
filled
with
about
3
m
of
shredded
MSW
and
allowed
to
reach
its
field
capacity
through
natural
rainfall.
One
cell
was
then
left
open
to
simulate
open
landfill
conditions
and
the
other
was
sealed
to
enable
landfill
gas
measurements
and
to
prohibit
water
evapo-
ration.
Tap
water
was
added
to
the
sealed
cell
in
an
amount
equivalent
to
that
of
rainwater
received
by
the
open
cell.
The
temperature
ranges
with-
in
the
two
cells
were
not
observed
to
differ
significantly
(Pohland
1980).
The
pilot
study
indicated
a
decline
in
leachate
contaminant
concentration
with
daily
recirculation
of
leachate.
This
observation
was
attributed
to
the
biological
stabilization
of
readily
available
organic
components
contained
within
the
cell.
It
was
noted
that
the
daily
recirculation
of
the
leachate
provided
the
microorganisms
with
sufficient
nutrients
and,
as
a
result,
overall
conversion
of
the
waste
was
enhanced.
Also,
it
was
observed
that
the
volatile
acid
concentrations
in
the
leachate
rose
sharply,
and
that
was
followed
by
fermentation
of
these
acids
to
produce
carbon
dioxide
and
methane.
Furthermore,
it
was
observed
that
a
reduction
of
biological
oxy-
gen
demand
(BOD),
chemical
oxygen
demand
(COD),
total
organic
car-
bon
(TOC)
and
volatile
acids
to
produce
carbon
dioxide
and
methane
had
occurred
at
a
faster
rate
in
the
sealed
cell
than
in
the
open
cell
(Pohland
1980).
This
observation
was
attributed
to
the
positive
exclusion
of
oxygen
which
is
detrimental
to
methanogenic
bacteria.
Doedens
and
Cord-Landwehr
(1989)
performed
a
more
detailed
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
423
study
of
leachate
recirculation.
The
investigation
was
conducted
on
three
scales:
test
cells
and
active
landfill
sections,
both
containing
leachate
recy-
cling
as
well
as
on
a
large-scale
landfill
equipped
with
recycling
tech-
niques.
The
purpose
of
the
test
cell
experiment
was
to
investigate
the
degree
of
stabilization
achieved
with
leachate
recirculation.
Four
test
cells
were
used,
each
consisting
of
an
air
tight,
temperature
-regulated
steel
cylinder
containing
compacted
shredded
waste
with
an
original
water
content
of
24
to
31%.
Test
Cell
1
was
supplied
with
660
mm/year
of
rainwater,
and
all
the
leachate
that
was
released
was
recycled
back
into
the
cell.
The
remaining
three
test
cells
originally
contained
50%
of
the
year's
precipitation
with
various
water
compositions.
No
leachate
recycling
was
carried
out
in
Test
Cell
2,
which
only
received
rainwater
as
well.
Test
Cell
3
was
irrigated
with
a
combination
of
rainwater
and
leachate.
Test
Cell
4
was
brought
to
a
saturation
point
with
the
leachate
from
a
stabilized
landfill
in
other
words,
to
the
point
where
the
daily
amount
of
water
entering
the
cell
equalled
approximately
the
amount
discharging
from
the
cell.
Test
Cell
1,
with
leachate
recirculation,
showed
the
highest
decrease
in
COD
concentration
in
the
leachate
during
the
examination
period
of
300
days.
Test
Cell
3
showed
the
lowest
loads
of
COD,
BOD,
Cl,
Zn
and
Pb
in
the
leachate.
In
Test
Cell
1,
with
leachate
recirculation
but
with
twice
the
amount
of
rainwater,
the
concentration
of
organics
decreased
faster,
but
the
leachate
contained
about
twice
the
amount
of
contamination
in
comparison
with
Test
Cell
3.
The
amount
of
contaminants
in
the
leachate
of
Test
Cell
2,
which
had
comparable
conditions
but
without
leachate
recirculation,
was
two
to
three
times
as
high
as
that
of
Test
Cell
3.
The
methane
gas
production
rate
observed
in
the
test
cells
was
used
as
a
measure
of
the
waste
stabilization.
It
is
evident
that
Test
Cell
2,
which
received
no
leachate
recirculation,
exhibited
the
highest
gas
production.
This
result
leads
to
the
conclusion
that
leachate
recirculation
did
not
enhance
the
stabilization
of
the
solid
waste.
Leachate
recirculation
did,
however,
stimulate
a
rapid
decrease
of
COD
and
BOD
concentrations
in
the
leachate.
Barlaz
et
al.
(1989b)
and
Leushner
(1989)
also
found
that
leachate
recirculation
without
nutrient
addition
is
ineffective
in
enhancing
methane
production
or
improving
leachate
quality
because
of
low
pH
levels.
They
noted,
however,
that
recycling
leachate
enhanced
microbial
activity
by
providing
better
contact
between
insoluble
substrates,
soluble
nutrients
and
the
microorganisms,
which
resulted
in
leachate
COD
and
total
volatile
acids
concentration
decrease,
once
methane
production
became
established.
In
various
laboratory
studies,
leachate
recycling
enhanced
the
decomposition
of
waste
and
improved
leachate
quality
when
a
variety
of
materials,
such
as
buffers,
nutrients
and
microbial
inoculum,
were
added
to
the
leachate
and
recirculated
through
the
solid
waste
(Bogner
1990;
Barlez
et
al.
1989b;
Leushner
1989;
Mata-Alvarez
and
Martinez-Viturtia
1986;
Stegmann
1983).
424
WARITH
AND
SHARMA
The
above
studies
all
agreed
that
buffering
the
leachate
being
recy-
cled
enhanced
decomposition
by
allowing
the
proper
pH
to
be
estab-
lished.
It
was
found
that
once
a
neutral
pH,
ranging
from
6.8
to
7.4,
was
reached,
rapid
methane
production
commenced.
Typical
buffers
include
caustic
and
calcium
carbonates.
It
was
observed
by
Stegmann
(1983)
and
Leushner
(1989)
that
the
addition
of
nutrients
such
as
nitrogen
and
phosphorus,
and
buffers
to
the
recycled
leachate
significantly
shortened
the
initial
phase
of
degradation,
and
methane
generation
commenced
earlier.
However,
the
continued
addition
of
nutrients
after
methane
production
had
started
did
not
improve
the
methane
production
rate
above
what
was
experienced
through
buffer
addition
alone.
Furthermore,
it
was
found
by
Barlaz
et
al.
(1989b)
and
Leushner
(1989)
that
anaerobically
digested
sludge
was
an
excellent
source
of
microbial
inoculum.
When
leachate
was
recycled
with
buffer,
nutrients
and
sludge,
the
rate
of
methane
production
was
sub-
stantially
higher
than
with
all
the
other
cases
and,
therefore,
achieved
the
fastest
rate
of
waste
stabilization.
The
effect
of
variable
rates
of
leachate
recirculation
on
solid
waste
stabilization
and
leachate
generation
rates
was
examined
by
Al-Yousifi
and
Pohland
(1993).
Recirculation
rates
of
25
to
100%
of
the
total
leachate
generated
were
employed
in
the
simulations.
They
found
that
the
higher
the
leachate
recycling
rate,
the
greater
the
quantity
of
methane
gas
pro-
duced.
This
observation
is
related
to
the
higher
quantities
of
organic
sub-
stance
made
available
during
high
leachate
recirculation.
However,
higher
leachate
recirculation
rates
resulted
in
longer
lag
times
before
methane
generation
began.
It
was
suggested
in
many
studies
that
the
leachate
recycle
rates
and
frequencies
during
the
early
acetogenic
phase
initially
be
low
and
then
gradually
increased
as
methanogenesis
becomes
established
(Pohland
and
Al-Yousifi
1994;
Cossu
et
al.
1993;
Farquhar
1989;
Pohland
1980;
Pohland
1975).
Also,
it
was
recommended
that
total
quantities
of
accu-
mulated
leachate
should
be
restricted
to
the
amount
needed
to
effectively
operate
the
landfill
system
as
a
bioreactor,
and
to
minimize
the
eventual
quantity
requiring
ultimate
disposal,
with
or
without
post
treatment
after
stabilization
of
the
landfill
had
been
achieved
(Pohland
and
Al-Yousifi
1994;
Ham
1993;
Bookter
and
Ham
1982;
Pohland
1975).
In
a
study
to
determine
the
reduction
potential
of
PCB
-contaminated
sediments
in
an
anaerobic
bioreactor
system,
Pagano
et
al.
(1995)
also
used
the
leachate
recycle
method.
Landfill
leachate
was
used
to
provide
a
carbon,
nutrient,
an
d/or
microbial
source.
There
was
significant
reduc-
tion
in
the
total
chlorine/biphenyl
of
the
original
Aroclor
sediments
in
the
laboratory
-scale
bioreactor
system.
In
terms
of
PCB
dechlorination
in
sed-
iments,
the
detoxification
is
attributed
to
methanogenic
conditions.
An
innovative
in
-line
sampler
was
validated
and
utilized
to
measure
the
cur-
rent
internal
status
of
the
bioreactor
system.
When
applying
this
tech-
nique
to
large-scale
landfill
sites,
it
was
found
by
the
researchers
that
degradation
of
PCB
-contaminated
sediments
was
inhibited
or
did
not
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
425
occur
because
of
engineering
designs,
which
create
dry
and
sterile
envi-
ronments.
In
order
to
effectively
promote
degradation
of
contaminants
in
landfills,
it
was
concluded
that
there
must
be
adequate
moisture
also
available
(Pagano
et
al.
1995).
Size
of
Waste
Particles
The
well
-mixed,
shredded
refuse
permits
greater
contact
between
the
key
constituents
required
for
methane
production:
moisture,
substrate
and
microorganisms.
Thus,
a
smaller
particle
size
could
increase
the
rate
of
the
hydrolysis
of
the
organic
waste.
However,
Barlaz
et
al.
(1990
and
1989b)
reported
that
refuse
with
250-
to
350
-mm
particle
sizes
produced
32%
more
methane
after
90
days
than
refuse
with
100-
to
150
-mm
particle
sizes.
The
reason
for
this
outcome
is
that
further
stimulation
by
a
reduced
particle
size
may
have
caused
too
rapid
a
rate
of
hydrolysis,
resulting
in
an
accumulation
of
acidic
end
products
and
a
lower
pH.
The
acidic
con-
ditions
then
limit
methane
production.
Sludge
Addition
Municipal
sewage
sludge
can
be
added
to
MSW
as
a
source
of
microorganisms
and
as
a
source
of
nitrogen
and
phosphorus
as
well
as
other
nutrients.
Typically,
th
ese
sludges
were
anaerobically
digested,
but
not
dewatered.
The
contents
of
septic
tank
bottoms
and
animal
manures
have
also
been
recommended
(Barlaz
et
al.
1990).
As
fresh
refuse
begins
to
decompose,
there
is
an
imbalance
between
the
fermentative
and
methanogenic
organisms,
leading
to
a
decrease
in
the
pH
of
the
system
(Townsend
et
al.
1996).
The
use
of
sludge
balances
activities
of
the
two
types
of
bacteria
and
prevents
the
initial
pH
decrease.
Rapid
refuse
decomposition
has
been
observed
in
test
lysimeters
when
municipal
sludges
were
added
(Barlaz
et
al.
1990).
Occasionally,
there
is
an
increase
in
carboxylic
acid
production
due
to
the
sludge
addition.
Thus,
often
a
buffer
is
added
in
conjunction
with
the
sludge.
The
buffer
maintains
the
pH
of
the
refuse
ecosystem
near
neu-
tral,
allowing
the
methanogenic
bacteria
in
the
sludge
to
acclimatize
to
the
refuse
system
more
quickly
than
in
the
absence
of
a
buffer.
The
addi-
tion
of
sludge
and
buffer
has
been
successful
in
promoting
the
onset
of
methane
production
from
fresh
refuse.
Another
inoculum
for
fresh
refuse
is
old
refuse.
Methanogenesis
can
be
stimulated
by
adding
old,
anaerobically
degraded
refuse,
as
the
waste
acts
as
a
dilutant
against
the
accumulation
of
toxic
compounds.
The
addi-
tion
of
old
refuse
has
been
proven
to
be
a
very
effective
enhancement
technique
(Barlaz
et
al.
1990).
To
improve
the
conditions
for
methane
production,
sludge
and
composted
MSW
were
added
to
shredded
fresh
MSW
(Doedens
and
Cord-Landwehr
1989).
The
sludge
was
anaerobically
digested
but
not
dewatered,
and
the
old
MSW
was
composted
for
1
to
2
months.
The
ratio
of
MSW
solids
to
sludge
solids
was
7:1
on
a
dry
weight
basis.
A
positive
426
WARITH
AND
SHARMA
effect
due
to
the
use
of
composted
MSW
was
noted
by
Doedens
and
Cord-
Landwehr
(1989).
Methane
gas
production
was
noted
to
be
highest
when
MSW
was
mixed
with
composted
MSW
(Stegmann
and
Spendlin
1989).
Also,
it
was
noted
that
the
BOD
concentration
rapidly
decreased
when
composted
MSW
was
added;
additions
of
inorganic
material
and
sawdust
to
shredded
MSW
enhanced
gas
production;
and
addition
of
food
waste
resulted
in
an
inhibition
of
methane
gas
production
(Stegmann
1983).
From
the
above
studies,
it
can
be
concluded
that
high
concentration
of
organic
acids
due
to
the
introduction
of
food
waste
inhibit
methane
production.
In
other
words,
the
acidic
environment
prevents
growth
of
methanogenic
populations.
The
addition
of
inert
material
dilutes
the
organic
acids,
thereby
reducing
the
organic
acid
to
solids
ratio.
It
was
also
found
that
the
BOD
concentration
of
the
leachate
decreased
dramatically
when
stable
methane
production
took
place.
However,
the
organic
con-
tent
of
the
leachate
may
still
be
high
even
when
gas
production
is
at
a
maximum
because
the
gas
production
is
not
stable.
It
comes
into
question
whether
or
not
organic
leachate
concentration
can
be
used
as
an
indicator
for
gas
production
and
composition.
Lift
Design
The
usual
practice
in
modern
sanitary
landfill
operation
is
to
place
waste
in
highly
compacted
2
to
3
m
lifts,
with
or
without
daily
cover.
It
has
been
found
that
MSW
compacted
at
a
low
density
in
thin
layers
with-
out
daily
cover
produces
high
strength
leachate
over
short
periods
of
time
(Stegmann
1983).
Also
it
was
reported
that
refuse
built
up
in
2-m
lifts
degrades
slower
than
in
1.3-m
lifts.
The
COD
concentration
of
leachate
was
found
to
be
a
function
of
solid
waste
lift
thickness
(Ham
and
Bookter
1982).
In
addition,
the
period
of
the
elevated
COD
levels
was
longer
in
deeper
waste
lifts.
It
was
concluded
that
the
lift
thickness
is
an
important
factor
which
affects
the
leachate
composition
(Ham
and
Bookter
1982).
In
landfill
cells,
where
a
second
lift
was
added
to
a
lift
that
had
been
in
place
for
5
years,
the
leachate
data
showed
no
major
changes
in
organic
com-
position.
It
was
determined that
the
lower
lift
was
able
to
attenuate
the
leachate
from
the
higher
lift
(Stegmann
1983).
Daily
cover
has
an
adverse
effect
on
enhancement
of
biological
degradation.
Experiments
by
Ham
and
Bookter
(1982)
showed
that
the
application
of
soil
cover
dramatically
increased
the
period
during
which
COD
concentrations
were
high.
The
leachate
had
a
high
organic
content
for
the
first
3
to
4
years
after
placement.
Cells
that
were
not
covered
reached
high
COD
concentrations
quickly,
followed
by
a
rapid
stabiliza-
tion
of
waste
in
which
COD
values
were
maintained
at
low
levels.
The
time
required
to
reach
these
low
levels
was
less
than
a
year.
Covering
the
refuse
prolonged
the
period
of
elevated
leachate
COD
values.
Methane
concentration
is
increased
and
methane
production
is
delayed
by
the
presence
of
soil
cover
(Ham
and
Bookter
1982).
The
land-
fill
cover
not
only
reduces
the
fl
ow
of
decomposition
gases
out
of
the
landfill
and
prevents
oxygen
from
entering,
but
it
also
affects
the
decom-
428
WARITH
AND
SHARMA
trolling
gas
production
(Barlaz
et
al.
1990).
It
was
reported
by
Christensen
and
Kjeldsen(1989)
that
there is
an
exponential
increase
in
gas
production
rates
between
25
and
60%
moisture
content.
In
a
landfill,
the
lack
of
opportunity
for
contact
between
microor-
ganisms,
substrate
and
growth
factors
limits
biodegradation.
As
the
mois-
ture
content
increases,
opportunity
for
contact increases.
Therefore,
adjusting
the
water
content
to
field
capacity
initially
or
providing
a
con-
tinuous
fl
ow
of
water
through
the
refuse
accelerates
decomposition.
High
moisture
contents
stimulate
the
hydrolysis,
but
an
accelerated
rate
of
hydrolysis
can
be
inhibitory.
At
high
waste
densities,
each
particle
is
in
closer
contact.
Thus,
for
the
same
moisture
content,
there
is
more
water
in
contact
with
the
particles.
As
solid
waste
density
increases,
there-
fore,
the
optimum
moisture
content
decreases.
A
study
conducted
by
Stegmann
(1983)
used
two
laboratory
lysime-
ters,
each
filled
with
shredded
MSW
mixed
with
industrial
wastes.
There
was
no
water
addition
in
one
lysimeter,
and
the
other
was
maintained
at
a
65%
moisture
content.
There
was
no
leachate
recirculation
in
both
lysimeters.
Methane
was
not
produced
in
the
first
lysimeter
until
after
a
year
due
to
high
organic
acid
concentrations,
which
resulted
in
low
pH
levels.
The
acidic
conditions
inhibited
growth
of
methane
-producing
bac-
teria,
and
as
a
consequence
waste
did
not
decompose.
It
was
found
that
by
adding
water,
a
portion
of
the
organic
acid
can
be
removed
by
the
leachate
and
methane
production
can
occur
(Stegmann
1983).
Temperature
According
to
Barlaz
et
al.
(1990),
greater
than
90%
of
the
methane
potential
of
municipal
refuse
can
be
attributed
to
the
cellulose/hemicel-
lulose
action.
Therefore
one
must
understand
the
parameters
limiting
the
degradation
of
these
polymers
to
CO
2
and
CH
4
.
Through
the
interactions
of
various
microbial
populations
such
as
sulphate
-reducing
bacteria
(SRB)
and/or
methanogens,
cellulose
catabolism
is
mediated.
The
sulphate
-
reducing
bacteria
activity
allows
a
reduction
in
the
redox
potential,
which
creates
a
suitable
reducing
environment
for
methanogens
to
function.
These
two
microorganisms
also
compete
for
the
same
substrate
(acetate
and
H
2
)
(Watson-Craik
et
al.
1994).
In
an
experiment
conducted
by
Watson-Craik
et
al.
(1994),
a
multi-
stage
continuous
culture
model
system
was
used
to
separate
the
physio-
logical
groups
of
the
isolated
association
without
the
loss
of
overlap
of
their
association.
Thus,
indiyidual
species
were
studied
without
disturb-
ing
the
association.
The
multistage
continuous
culture
systems
were
inoc-
ulated
with
cellobiose
and
butyrate
-degrading
methanogenic
microbial
associations
enriched
from
fresh
(one
month
old)
shredded
refuse
obtained
from
Wilderness
Landfill
Site.
After
carbon
sources
were
select-
ed
(cellobiose
and
butyrate),
association
establishment
and
stabilization,
the
different
temperature
parameters
were
studied
(mesophilic
and
ther-
mophilic
ranges).
Temperature
is
a
very
important
factor
because
of
the
varying
ranges
found
in
such
a
heterogeneous
landfill.
The
direct
effect
of
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
429
temperature
on
microbial
activity
could
be
manipulated
to
optimize
methane
production.
Thus,
it
is
necessary
to
realize
the
temperature
con-
straints
on
individual
microorganism
in
order
to
control
the
methano-
genic
fermentation.
The
optimum
temperature
range
for
methanogenesis
was
generally
found
to
be
from
30
to
35°C
(Watson-Craik
et
al.
1994).
Due
to
further
increase
in
temperature
(45
to
55°C),
methanogenic
populations
were
inhibited.
Furthermore,
it
was
concluded
that
prolonged
exposure
to
these
temperatures
may
result
in
an
unbalanced
fermentation
or
the
redirection
of
electron
fl
ow
in
the
presence
of
exogenous
sulphate
to
thermophilic
propionate
-utilizing
SRB.
It
was
suggested
that
a
refuse
emplacement
strategy
be
employed
to
control
the
temperature.
Enzyme
Addition
Lagerkvist
and
Chen
(1993)
have
experimented
with
the
addition
of
cellulolytic
enzymes
under
methanogenic
and
acidogenic
conditions
using
0.1
m
3
landfill
models.
Enhanced
degradation
was
observed
fol-
lowing
enzyme
addition.
The
conversion
of
volatile
solids
was
approxi-
mately
40
to
50%
for
both
methanogenic
and
acidogenic
conditions.
Cellulolytic
enzymes
were
used
because
the
major
component
of
the
degradable
municipal
solid
waste
is
cellulose,
and
by
manipulating
the
enzyme
activity,
it
is
possible
to
control
and
enhance
the
hydrolysis
of
cel-
lulose.
Lagerkvist
and
Chen
(1993)
examined
enzyme
effects
in
acido-
genic,
methanogenic
and
the
semi
-aerobic
environments.
The
study,
how-
ever,
was
only
concerned
with
the
acid
and
methanogenic
conditions
of
the
landfill.
In
the
experiment,
four
models
were
used:
two
for
methanogenic
models
(1,
2)
and
the
other
two
(3,
4)
for
acidogenic
mod-
els.
Leachate
recycle
and
temperature
maintenance
at
30°C
was
main-
tained
throughout
all
four
models.
Lagerkvist
and
Chen
(1993)
concluded
that
the
increased
acid
production
generated
by
the
enzyme
addition
dur-
ing
intense
methane
production
was
not
sufficient
to
suppress
the
subse-
quent
methanogenesis
that
occurred.
This
was
explained
by
the
nature
and
content
of
the
waste
and
its
non
-homogeneous
distribution.
Enzyme
addition
during
the
decline
of
gas
production
did
not
cause
an
increase
in
the
production
rate
of
gas
(Lagerkvist
and
Chen
1993).
Landfill
Model
Application
As
mentioned
before,
it
is
beneficial
to
minimize
the
time
period
in
which
degradation
occurs
in
order
to
reduce
gas
emissions
after
the
land-
fill
is
closed,
to
ease
the
requirements
of
leachate
treatment,
and
to
be
suc-
cessful
in
reclaiming
the
landfill
site.
The
main
objective
of
a
study
done
by
Wall
and
Zeiss
(1995)
was
to
test
the
ability
of
biological
enhancement
to
reduce
the
time
to
reach
stabilization
and
to
determine
the
effects
of
biodegradation
on
settlement.
In
order
to
conduct
this
study,
six
landfill
test
cells
were
constructed
to
model
both
settlement
and
decomposition
430
WARITH
AND
SHARMA
over
extended
periods
of
time.
Of
the
six,
three
acted
as
model
bioreactor
landfills
and
the
others
as
secure
vaults.
All
six
were
monitored
for
gas
composition
(CO
2
and
CH
4
)
and
volume,
leachate
pH
and
TOC
as
well
as
refuse
settlement.
The
following
conditions
were
exposed
to
cells
1
and
3
to
enhance
biodegradation:
temperature
maintained
at
25°C,
refuse
initially
saturated
with
distilled
water,
approximately
50
litres,
leachate
recycle
on
a
weekly
basis,
and
buffer
(Na
2
CO
3
and
K
2
CO
3
)
and
anaerobically
digest-
ed
sewage
sludge
addition.
For
the
dry
-vault
system,
temperature
was
maintained
at
4°C
and
no
additional
moisture
or
microbial
seed
was
added
to
inhibit
waste
degradation.
Results
of
this
study
suggested
that
test
cells
could
effectively
model
actual
landfill
behaviour.
Wall
and
Zeiss
(1995)
indicated
that
biodegradation
and
settlement
occur
in
three
distinct
stages:
initial
compression
(settlement
that
occurs
directly
when
an
exter-
nal
load
is
applied
to
a
landfill),
primary
compression
(compaction
due
to
the
dissipation
of
pore
water
and
gas
from
the
void
spaces),
and
secondary
compression
(due
to
creep
of
the refuse
skeleton
and
biological
decay).
When
studying
the
relationship
of
microbial
mass
and
activity
in
the
biodegradation
of
waste,
Murphy
et
al.
(1995)
were
able
to
draw
interest-
ing
conclusions
under
aerobic
conditions.
The
municipal
solid
waste
was
incubated
in
lysimeters
with
moisture
content
controlled
with
recycled
leachate.
Both
anaerobic
and
aerobic
conditions
were
studied.
The
data
revealed
that
aeration
resulted
in
increased
biomass
production
and
greater
cellulolytic
activity.
The
aerobic
inground
digester
included
leachate
recirculation
in
order
to
provide
optimum
moisture.
Various
parameters
were
assessed,
including
bacterial
biomass
and
number
counts
by
adenosine
triphosphate
analysis,
acridine
orange
counts,
viability,
adenylate
energy
charge
and
cellulose
activity.
The
results
determined
the
following
advantages
(Murphy
et
al.
1995):
(1)
increased
degradation
speed
and
completeness,
(2)
elimination
of
costly
leachate
buffering
systems
employed
in
anaerobic
operations,
(3)
no
need
for
gas
collection
as
methane
and
hydrogen
sulphide
production
would
be
elim-
inated,
and
(4)
aeration
that
allows
reduced
odour
problems
and
acceler-
ate
degradation
of
cellulose
in
landfills.
Field
Applications
Leachate
recirculation
is
the
most
common
enhancement
mechanism
used
in
the
field.
The
U.S.
SPA
reported
that
more
than
200
landfills
use
leachate
recirculation
as
a
means
of
leachate
management
(Reinhart
1993).
Some
of
the
problems
that
limit
acceptance
of
this
method
in
the
field
include
excessive
head
on
the
bottom
liner,
clogging
of
liner
drainage
sys-
tems,
and
leachate
breakouts.
Full-scale
research
and
development
will
help
identify
proper
operating
procedures
to
avoid
some
of
these
problems.
The
effectiveness
of
leachate
recirculation
is
highly
dependent
on
the
technique
chosen
to
distribute
the
leachate.
Some
of
these
methods
include
prewetting
of
waste,
spraying,
surface
ponds,
vertical
injection
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
431
wells,
and
horizontal
infiltration
devices.
Experimental
study
was
conducted
on
full-scale
test
cells
at
the
Mountain
View
Controlled
Landfill
project,
CA,
to
investigate
the
effect
of
various
enhancement
techniques
(Pacey
1989).
One
cell
contained
addi-
tional
water,
a
calcium
carbonate
buffer,
recycled
leachate,
and
sewage
sludge.
Another
cell
was
identical
to
the
first
cell,
but
did
not
use
leachate
recirculation.
Another
cell
contained
buffer
and
sludge,
with
recircula-
tion,
but
no
additional
water.
A
fourth
cell
had
a
buffer
and
the
fifth
had
added
sludge,
both
of
them
with
additional
water.
There
was
also
a
con-
trol
cell.
After
more
than
4
years,
the
remaining
methane
producing
potential
of
each
cell
was
measured.
The
cell
with
the
greatest
remaining
methane
producing
potential
(57%)
was
the
cell
with
only
buffer
and
additional
water,
and
the
cell
with
the
least
potential
(17%)
was
the
first
cell
which
incorporated
all
the
enhancement
techniques,
including
addi-
tional
water,
buffer,
sludge
and
recirculation.
This
proves
the
effectiveness
of
the
techniques
in
stimulating
gas
production,
as
the
cell
with
the
high-
est
remaining
methane
producing
potential
is
the
cell
which
exhibited
the
least
degradation.
Results
from
the
Seamer
Carr
Landfill
in
the
UK
indicated
that
a
40%
reduction
in
COD
concentrations
occurred
within
20
months
of
the
initia-
tion
of
leachate
spraying,
compared
to
leachate
from
an
area
where
leachate
recirculation
was
not
employed
(Reinhart
and
Carson
1993).
Leachate
recirculation
was
also
found
to
be
beneficial
in
other
full-scale
landfills:
the
Owens-Corning
Landfill
in
Ohio,
the
Central
Solid
Waste
Management
Centre
Landfill
in
Delaware,
Central
Facility
Landfill,
Worcester
County,
Maryland,
and
the
Southwest
Landfill
in
Alachua
County,
FL
(Reinhart
and
Carson
1993;
Reinhart
1996).
Large-scale
experiments
were
performed
at
the
Bornhausen
Landfill
in
Goslar,
Germany
(Doedens
and
Cord-Landwehr
1989).
Three
test
cells
were
constructed
in
thin
layers
with
and
without
covering
and
recircula-
tion.
The
results
found
no
increase
in
the
organic
content
of
the
leachate
after
350
to
450
days
of
leachate
recirculation.
The
thin
layer
construction
technique
proved
to
be
more
effective
than
the
leachate
recirculation
process.
The
cover,
with
an
additional
water
dosage
under
the
lining
sys-
tem,
did
not
improve
leachate
quality,
but
did
minimize
the
volume
of
leachate
that
required
treatment.
Another
two
test
sites
at
Bornhausen
were
constructed,
one
with
recirculation,
and
the
other
without.
The
time
period
for
stabilization
was
twice
as
long
for
the
,site
without
recirculation
than
the
site
which
used
leachate
recirculation.
This
study
also
found
that
recirculating
leachate
over
old
landfill
reactors
in
which
stabilized
leachate
was
already
pro-
duced
resulted
in
BOD
reductions
of
90
to
99%
(Doedens
and
Cord-
Landwehr
1989).
The
effects
of
leachate
recycling
on
landfill
stabilization
were
inves-
tigated
by
Townsend
et
al.
(1996)
on
a
full-scale
landfill
in
north
-central
Florida.
The
leachate
recycling
system
was
constructed
and
operated
on
a
section
of
the
composite
lined
landfill.
An
infiltration
pond
leachate
recy-
432
WARITH
AND
SHARMA
cling
system
was
used
to
recirculate
the
leachate
to
the
landfill.
Samples
of
leachate,
landfill
gas
and
landfilled
solid
waste
were
collected
and
analysed
throughout
a
4
-year
period
(1988-1992),
before
and
after
the
start
of
leachate
recycling
(Townsend
et
al.
1996).
The
subsidence
of
the
landfilled
waste
was
also
measured
in
wetted
and
dry
areas
of
the
land-
fill.
During
the
4
-year
period,
the
lined
section
of
the
landfill
received
approximately
300
tons
of
waste
per
day.
This
study
indicated
that,
in
general,
leachate
recycling
did
not
improve
the
quality
of
the
leachate.
Measurements
of
pH,
total
dissolved
solids,
COD,
BOD
and
ammonia
of
the
leachate
were
performed
over
the
4
-year
period.
Leachate
recycling
reduced
the
pH
of
the
leachate
from
levels
above
neutral
to
levels
below
neutral.
However,
the
drop
in
pH
was
not
low
enough
to
inhibit
microbial
activity.
COD
and
BOD
concentrations
in
the
leachate
reached
levels
sim-
ilar
to
those
before
leachate
recycling
was
implemented.
Leachate
TDS
and
ammonia
increased
somewhat
over
the
period
before
and
after
leachate
recycling
began
(Townsend
et
al.
1996).
The
study
by
Townsend
et
al.
(1996)
also
reported
the
conditions
for
waste
stabilization
present
in
areas
with
and
without
leachate
recycle.
This
was
proven
through
measurements
of
methane
gas
in
the
landfill,
waste
temperature
and
pH
of
the
leachate.
In
areas
where
leachate
was
recirculated,
the
moisture
content
of
the
landfill
waste
was
significantly
increased.
Furthermore,
measurements
of
landfill
subsidence
and
bio-
chemical
methane
potential
of
the
waste
indicated
that
the
degree
of
sta-
bilization
was
greater
in
areas
where
the
leachate
recycling
occurred.
The
issue
of
daily
cover
was
addressed
in
the
Seamer
Carr
Landfill
study
(Reinhart
and
Carson
1993).
The
low
permeability
cover
that
was
located
between
higher
permeability
waste
lifts
caused
horizontal
move-
ment
and
leachate
ponding.
This
problem
was
compounded
when
large
volumes
of
leachate
were
recirculated.
Leachate
accumulations
of
1.2-m
depths
were
reported
due
to
the
use
of
daily
cover.
This
same
problem
was
also
discovered
at
the
Lycoming
County
Landfill,
PA,
where
clays
and
silty
soils
were
used
for
daily
cover
(Reinhart
and
Carson
1993).
To
alleviate
these
problems,
daily
cover
should
be
minimized
or
avoided,
or
soils
of
high
permeability
should
be
used.
Aspects
of
lift
design
were
tested
in
a
full-scale
landfill
at
Lingen,
West
Germany
(Stegmann
1983).
In
one
cell,
refuse
was
compacted
in
sev-
eral
0.5
to
1
m
lifts.
The
second
cell
contained
waste
1
m
high
without
compaction.
After
6
months,
another
lift,
1
m
in
height,
was
placed
on
top
of
this
first
lift
in
the
same
manner.
Above
this,
waste
was
continually
placed
in
compacted
2-m
lifts.
Both
cells
employed
leachate
recirculation.
The
BOD
leachate
concentration
was
significantly
lower
in
the
cell
with
the
uncompacted
lift.
In
addition,
the
COD
concentration
was
2000
mg/L
in
the
cells
with
the
uncompacted
initial
lifts,
while the
COD
was
ten
times
higher
in
the
cell
which
contained
strictly
the
compacted
lifts.
It
was
therefore
advantageous
to
provide
uncompacted
layers
in
this
case.
Efforts
can
be
made
to
reduce
the
time
for
aerobic
decomposition
in
the
first
layer.
When
the
aerobic
Rottedeponie
Landfill
in
Germany
was
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
433
converted
to
a
highly
compacted
anaerobic
operation,
the
BOD
concentra-
tions
of
the
leachate
remained
at
the
low
levels
which
were
present
during
aerobic
conditions
(Stegmann
and
Spendlin
1989).
Refuse
should
be
placed
in
thin
layers
(0.2-0.3
m)
over
large
areas.
This
results
in
higher
leachate
production
rates,
but
leachate
concentrations
are
expected
to
decrease
ear-
lier.
Since
large
areas
of
the
waste
are
in
contact
with
the
atmosphere,
aer-
obic
processes
take
place
to
a
certain
degree
and
thus
reduce
the
readily
degradable
organic
fractions.
Also,
moisture
distribution
is
much
more
even.
The
study
also
examined
the
effect
of
the
lift
thickness
on
the
leachate
organic
content.
It
was
noted
that
the
BOD
and
COD
concentra-
tions
are
significantly
higher
for
the
deeper
solid
waste
lifts.
One
unique
landfill
design
in
the
UK
involved
using
air
injection
through
vertical
gas
wells into
completed
landfills
to
promote
aerobic
degradation.
The
aerobic
metabolism
processes
that
were
stimulated
raised
the
temperature
of
the
landfill
and
promoted
anaerobic
activity,
as
evi-
denced
by
the
increase
in
methane
production
(Reinhart
and
Carson
1993).
Many
researchers
have
found
the
importance
of
studying
microbio-
logical
processes
within
landfills.
There
was
also
a
need
to
establish
a
method
to
assess
the
degree
of
decomposition
of
waste
within
a
site
or
the
energetic
potential
of
any
landfill
site.
The
work
of
Attal
et
al.
(1992)
and
Iza
et
al.
(1992)
developed
a
procedure
to
accurately
estimate
the
degree
of
decomposition
of
waste
in
a
site
which
would
cover
the
complete
mass
of
the
waste.
The
final
objective
was
to
obtain
sufficient
data
to
define
a
procedure
capable
of
determining
the
energetic
potential
of
any
landfill
site
(Attal
et
al.
1992).
The
degree
of
decomposition
was
based
on
a
biochemical
methane
potential
test
and
various
gases
were
monitored,
including
CO
2
,
H
2
and
N2.
The
quantity
of
biogas
was
calculated
from
the
volume
and
composi-
tion
of
the
gaseous
phase.
Waste
was
sorted
and
stored
at
4°C,
until
calci-
nation
at
550°C.
Organic
matter
was
estimated
by
the
volatile
suspended
solid
percentage
(%
VSS).
The
samples
were
then
diluted
and
purged
with
N2,
and
finally
sealed
and
incubated
at
55°C
(Attal
et
al.
1992).
Many
conclusions
were
drawn
from
the
the
results
of
the
study
con-
ducted
by
Attal
et
al.
(1992).
The
size
of
the
particles
of
waste
was
an
important
factor.
This
study
chose
25
to
30
kg
having
an
acceptable
20%
error;
however,
other
studies
vary
from
50
kg
to
several
tons.
Another
fac-
tor
was
location
within
the
landfill
the
heterogeneous
nature
of
the
municipal
solid
waste
had
to
be
accounted
for.
This
study
relied
on
the
fact
that
materials
of
the
same
age
are
in
horizontal
layers
and
waste
is
disposed
homogeneously
in
each
layer.
Temperature
(50°C)
at
10
m
and
below
indicated
active
anaerobic
biological
decomposition
under
ther-
mophilic
conditions.
Methane
levels
detected
indicated
that
it
increased
with
depth.
The
low
proportion
of
methane
observed
in
the
upper
layer
suggested
that
it
must
be
in
the
hydrolysis
acidification
phase.
Biological
parameters
such
as
%
VSS
and
methane
potential
were
also
observed.
Percent
VSS
decreased
as
depth
increased.
It
was
found
that
the
longer
the
waste
burned
the
greater
the
mineralization.
For
example,
at
25m
the
434
WARITH
AND
SHARMA
waste
was
completely
mineralized.
Also
the older
the
waste,
the
lower
methane
potential.
In
conclusion,
the
biodegradable
potential
of
the
waste
increased
as
organic
matter
content
increased
(Attal
et
al.
1992).
The
overall
advantage
of
the
sampling
technique
and
technical
procedure
developed
by
Attal
et
al.
(1992)
showed
the
validity
of
studying
anaerobic
degradation
of
MSW
in
landfills.
With
the
possibility
of
changing
sample
sizes,
it
allowed
for
the
heterogeneous
nature
of
various
landfill
sites.
It
also
revealed
that
within
a
landfill
there
is
a
regular
structure
a
number
of
layers
with
its
own
homogeneous
waste
of
different
ages
(Attal
et
al.
1992).
Summary
and
Conclusion
Stabilization
of
solid
waste
in
sanitary
landfills
is
dominated
by
bio-
logical
processes.
Several
techniques
to
enhance
biological
degradation
and
to
reduce
leachate
production
have
been
presented.
Laboratory
-scale
studies
indicate
that
recycling
leachate
alone
reduces
the
organic
content
in
the
leachate,
but
does
not
enhance
waste
stabilization.
However,
by
adding
buffer,
nutrients
and
municipal
sludge
to
the
leachate
being
recir-
culated,
not
only
was
waste
degradation
enhanced,
but
leachate
quality
was
also
improved.
In
terms
of
enhancement
by
reducing
the
size
of
waste
particles,
stud-
ies
have
found
that
the
smaller
size
particles
increase
the
rate
of
hydroly-
sis
up
to
a
minimum
waste
size.
Degradation
of
waste
particles
smaller
than
the
minimum
size
resulted
in
a
decreased
methane
production.
High
moisture
contents
stimulate
methane
gas
production.
By
increasing
the
moisture
content
above
the
field
capacity
of
the
waste,
the
rate
of
gas
production
is
increased.
Temperature
also
has
an
important
role
in
biodegradation.
The
opti-
mum
temperature
for
methanogenesis
based
on
model
studies
show
30
to
35°C
to
be
advantageous.
By
adding
anaerobically
digested
sewage
sludge
to
MSW
as
a
micro-
bial
and
nutrient
source,
refuse
decomposition
can
be
enhanced.
The
com-
bination
of
sludge
and
buffer
also
promotes
the
onset
of
methane
pro-
duction
when
added
to
fresh
MSW.
Methanogenesis
is
stimulated
by
the
addition
of
old,
partially
degraded
MSW
to
fresh
refuse.
When
leachate
percolates
through
layers
of
degraded
MSW,
the
concentration
of
organics
reduces
signifi-
cantly.
The
design
of
waste
lifts
in
landfills
was
found
to
have
an
effect
on
the
degradation
rates
of
organic
materials.
Deep
lifts
resulted
in
high
COD
concentrations
in
the
leachate.
Daily
cover
had
a
negative
effect
on
gas
production
and
leachate
quality.
It
was
suggested
that
landfills
constructed
with
thin
lifts
and
cover
be
placed
after
a
short
stabilization
period.
The
placement
of
uncompacted
layers
of
old
MSW
at
the
bot-
tom
of
a
new
landfill
should
be
considered
as
means
of
leachate
treatment.
ENHANCING
BIOLOGICAL
DEGRADATION
OF
SOLID
WASTE
435
Acknowledgment
Support
for
this
research
was
provided
by
Ryerson
Polytechnic
University
and
NSERC.
We
thank
K.
Nguyen
and
T.
Wise
of
the
University
of
Ottawa
who
helped
in
the
first
stages
of
this
project.
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