Extraction wells and biogas recovery modeling in sanitary landfills


Rodríguez-Iglesias, J.; Vázquez, I.; Marañón, E.; Castrillón, L.; Sastre, H.

Journal of the Air & Waste Management Association 55(2): 173-180

2005


A general methodology is established that permits the characterization and evaluation of the optimum potential of biogas extraction at each vertical well in the sanitary landfill of Asturias, Spain. Twenty wells were chosen from a total of 225 for the study, and the maximum production flow of biogas, which is a result of the degradation of the municipal solid waste deposited within its area of influence, was determined for each well. It was found that this flow varied with time and is characteristic of each extraction well. The maximum extractable flow also was determined as a function of the composition of the biogas needed for its subsequent utilization. The biogas extraction yield in the wells under study varied between approximately 26 and 97%, with a mean recovery value of 82%. The low yields found in certain cases were generally caused by a sealing defect, which leads to excessive incorporation of air into the landfill gas through the surrounding soil or through the extraction shaft, and which make its subsequent utilization difficult.

TECHNICAL
PAPER
ISSN
1047-3289
J.
Air
&
Waste
Manage.
Assoc.
55:173-180
Copyright
2005
Air
&
Waste
Management
Association
Extraction
Wells
and
Biogas
Recovery
Modeling
in
Sanitary
Landfills
J.
Rodriguez
-Iglesias,
I.
Vazquez,
E.
Maranon,
L.
Castrillon,
and
H.
Sastre
Department
of
Chemical
and
Environmental
Engineering,
Higher
Technical
School
of
Engineering,
University
of
Oviedo,
Spain
ABSTRACT
A
general
methodology
is
established
that
permits
the
characterization
and
evaluation
of
the
optimum
potential
of
biogas
extraction
at
each
vertical
well
in
the
sanitary
landfill
of
Asturias,
Spain.
Twenty
wells
were
chosen
from
a
total
of
225
for
the
study,
and
the
maximum
production
fl
ow
of
biogas,
which
is
a
result
of
the
degradation
of
the
municipal
solid
waste
deposited
within
its
area
of
influ-
ence,
was
determined
for
each
well.
It
was
found
that
this
fl
ow
varied
with
time
and
is
characteristic
of
each
extrac-
tion
well.
The
maximum
extractable
fl
ow
also
was
deter-
mined
as
a
function
of
the
composition
of
the
biogas
needed
for
its
subsequent
utilization.
The
biogas
extrac-
tion
yield
in
the
wells
under
study
varied
between
—26
and
97%,
with
a
mean
recovery
value
of
82%.
The
low
yields
found
in
certain
cases
were
generally
caused
by
a
sealing
defect,
which
leads
to
excessive
incorporation
of
air
into
the
landfill
gas
through
the
surrounding
soil
or
through
the
extraction
shaft,
and
which
make
its
subse-
quent
utilization
difficult.
INTRODUCTION
The
present
study
forms
part
of
a
wider
project
studying
the
utilization
of
municipal
solid
waste
(MSW)
as
an
en-
ergy
source,
both
at
a
laboratory
pilot
-plant
scale
as
well
as
in
the
centralized
landfill
of
the
principality
of
Asturias,
Spain.'
MSW
landfilling,
one
of
the
most
widely
used
waste
management
techniques,
in
general
presents
two
IMPLICATIONS
In
this
paper,
a
general
model
is
described
that
allows
knowing
the
individual
biogas
extraction
yield
at
any
mu-
nicipal
solid
waste
landfill
in
a
quick
and
simple
way.
The
area
of
influence
of
each
well
is
determined,
and
the
biogas
leaks
are
reported
in
real
time.
This
model
also
permits
detection
of
well
floods,
improvement
in
the
extraction
regulation,
and
the
ability
to
assess
the
sailing
conditions
in
the
surrounding
area
of
each
well.
This
model
can
be
a
significant
tool
to
automate
the
biogas
extraction
system
and
to
substantially
reduce
the
atmospheric
emissions.
important
environmental
impacts:
the
production
of
pol-
luted
waters
(leachates)
and
the
generation
of
a
combus-
tible
gas
(biogas).
2
Biogas,
resulting
from
anaerobic
deg-
radation
of
the
organic
fraction
of
the
waste,
3
is
the
fundamental
subject
of
these
studies
at
the
landfill.
The
principality
of
Asturias
landfill
is
situated
in
the
Zoreda
Valley
and
started
functioning
in
January
1986;
a
total
of
6,344,000
t
of
MSW
had
been
deposited
there
by
January
2003.
The
landfill
has
a
usable
volume
of
10,500,000
m
3
,
a
capacity
that
will
allow
it
to
receive
refuse
from
the
Asturian
municipalities
for
—25
yr
from
its
opening
(i.e.,
until
around
the
year
2011).
As
can
be
seen
in
Table
1,
the
amount
of
eliminated
MSW
has
increased
with
time
until
the
present
day
with
an
approx-
imate
elimination
rate
of
570,000
t/yr.
This
is
because
of
an
increase
in
the
number
of
municipalities
that
elimi-
nate
their
MSW
via
this
landfill.
4,5
At
the
landfill,
the
waste
is
laid
out
in
duly
compacted
layers
of
—2.5
m
in
thickness,
with
densities
on
the
order
of
800-1000
kg/m
3
,
that
are
subsequently
covered
by
earth
from
the
landfill
area
itself.
In
this
way,
a
minimum
surface
area
is
exposed
to
rainfall,
and
the
smells
pro-
duced
by
fermentation
6,7
are
avoided
or
diminished.
The
leachates
produced
are
not
usually
recirculated
but
are
collected
at
the
base
of
the
landfill,
characterized,
and
subsequently
channeled
to
a
treatment
plant.
The
leachates
are
recirculated
only
during
dry
periods
in
the
summer.
Anaerobic
fermentation
of
organic
matter
generates
a
large
amount
of
biogas,
a
gaseous
mixture
composed
of
methane
(CI
-
1
4
)
and
carbon
dioxide,
with
small
quantities
of
hydrogen,
nitrogen,
hydrogen
sulfide
(H,S),
mercap-
tans,
and
other
minor
organics;
it
also
contains
a
large
amount
of
water
vapor.
8-10
The
emanations
of
biogas
produce
unpleasant
smells,
mainly
caused
by
I
-
1
2
5
and
mercaptans.
They
may
likewise
give
rise
to
hazardous
explosions
and
fi
res
as
a
result
of
their
high
inflammabil-
ity
if
they
are
not
released
in
a
controlled
way.
It
is,
therefore,
necessary
to
channel
the
emanations
of
gas
and
to
proceed
to
their
combustion,
both
for
environmental
Volume
55
February
2005
Journal
of
the
Air
&
Waste
Management
Association
173
Rodriguez
-Iglesias
et
al.
Table
1.
Amount
of
MSW
treated
at
La
Zoreda
landfill.
Tons
Treated
Year
122,000
1986
213,000
1987
238,000
1988
260,000
1989
300,000
1990
330,000
1991
350,000
1992
360,000
1993
360,000
1994
400,000
1995
400,000
1996
410,000
1997
420,000
1998
511,000
1999
538,000
2000
560,000
2001
572,000
2002
and
safety
reasons
as
well
as
because
of
the
possibilities
of
exploitation
that
they
present.0
The
system
adopted
at
the
landfill
for
the
recovery
of
biogas
has
led,
in
an
initial
phase,
to
the
elimination
of
unpleasant
smells
and
an
improvement
in
both
environ-
mental
and
safety
conditions,
because
the
gas
used
to
be
simply
burned
off
in
burners,
and
in
a
second
phase,
to
the
production
of
electrical
power
via
the
utilization
of
the
biogas
in
two
distinct
installations
with
different
uses.
Biogas
is
used
to
produce
electrical
power
for
self
-
consumption
by
means
of
a
450
-kW
motor
generator
fed
by
biogas.
This
produces
the
electricity
needed
to
cover
the
power
consumption
of
the
leachate
treatment
plant
and
the
industrial
waste
treatment
plant.
Biogas
also
is
employed
in
the
production
of
electrical
power
to
be
sold
to
the
national
power
grid,
the
gas
being
sent
to
eight
motors,
each
of
740
kW,
that
produce
electrical
power
at
380
V,
which,
after
being
transformed
to
20,000
V,
is
sent
to
the
national
power
grid.
This
allows
up
to
6.3
MW
to
be
supplied
for
its
subsequent
commercialization,
with
an
annual
production
estimation
of
more
than
33
million
kW
-hr.
Biogas
is
also
used
as
fuel,
because
it
is
sent
to
an
incinerator
of
hospital
waste,
where
it
is
used
to
achieve
the
appropriate
temperatures
in
the
combustion
chambers.
The
installation
for
biogas
recovery
is
made
up
of
225
vertical
wells
uniformly
distributed
over
the
landfill
sur-
face,
nine
regulating
stations,
two
extraction
units,
and
a
steel
burner
with
automatic
ignition.
The
relative
distance
between
wells
was
established
at
—40-45
m,
based
on
the
hypothesis
that
each
well
has
a
radius
of
influence
of
some
20
m.
Biogas
is
piped
from
each
well
to
one
of
the
nine
regulating
stations.
Of
the
nine
regulating
stations,
fi
ve
are
automatically
controlled,
three
are
semi
-automatically
controlled,
and
one
is
manual.
These
stations,
which
are
proportionally
distributed
among
the
wells,
eliminate
part
of
the
con-
densation
water
and
regulate
the
fl
ow
of
gas
sent
to
the
extraction
unit.
This
unit
in
turn
serves
to
aspire
the
gas
and
send
it
to
the
combustion
burner,
to
the
electrical
power
-generating
motors,
to
the
hospital
waste
incinera-
tor,
and
to
the
plant
for
treating
meat
byproducts.
The
extraction
line
wells
are
formed
by
a
500
-mm
-
diameter
by
12
-m
-long
polyvinyl
chloride
(PVC)
pipe,
with
multiple
perforations.
These
holes
start
2.5
m
below
the
top,
have
a
diameter
of
10
cm,
and
are
situated
40
cm
apart.
These
pipes
are
introduced
into
the
mass
of
waste,
and
the
surrounding
space
is
fi
lled
with
materials
that
do
not
impede
the
movement
of
gas,
do
not
obstruct
the
perforations
in
the
tube,
and
are
both
easy
to
fi
nd
and
cheap
(Figure
1).
A
combination
of
gravel
and
used
tires
of
different
diameters
are
used
at
the
landfill.
A
metal
cylin-
der
is
placed
around
the
piping,
the
tires
are
placed
around
the
cylinder
and
gravel
inside,
care
being
taken
not
to
damage
the
PVC,
and,
fi
nally,
the
cylinder
is
re-
moved.
The
area
then
is
covered
and
very
gently
com-
pacted.
Next,
a
plastic
cover
is
placed
over
the
PVC
pipe
like
an
umbrella
and
is
fi
xed
to
the
pipe
with
adhesive
Butterfly
valve
HOPE
tube
;
.1•••-%
-
t3.
L...jo
4
1
-
"'
41
.
Preasu
re
and
sample
connection
O
PIC.
pipe
Frne
9
c•il
hockfll
Impervious
gaol
•-•
Craved
and
tyres
bo
okfil
I
.t
.
0
.
.
Figure
1.
Transversal
view
of
a
biogas
extraction
well.
Refuge
174
Journal
of
the
Air
&
Waste
Management
Association
Volume
55
February
2005
Rodriguez
-Iglesias
et
al.
tape.
Subsequently,
the
area
is
covered
with
earth
or
sand
in
such
a
way
that
the
piping
sticks
out
of
the
ground
so
that
it
may
be
connected
to
the
extraction
network.
This
surface
sealing
is
of
extreme
importance,
because
the
chimney
directs
the
fl
ow
of
biogas
into
itself
thanks
to
the
application
of
a
slight
depression.
This
depression
inevi-
tably
provokes
the
entrance
of
air.
To
minimize
this
en-
trance
of
air,
sealing
is
especially
rigorous
in
an
area
of
—3
m
around
the
well.
The
connection
of
the
PVC
piping
is
carried
out
by
means
of
a
metallic
head
to
which
a
90°
elbow
pipe
is
screwed.
This
elbow
pipe
is
connected
to
a
high
-density
polythylene
aspiration
line.
An
all
-or
-nothing
valve
must
be
installed
to
be
able
to
shut
off
the
well,
as
can
be
seen
in
Figure
1.
A
valve
measuring
—6
mm
in
diameter
must
be
placed
on
the
metallic
head,
enabling
the
taking
of
measurements
and
samples.
OPERATING
PROCEDURE
Several
assays,
which
are
elucidated
next,
were
carried
out
and
evaluated
with
the
aim
of
establishing
a
general
methodology
that
permitted
the
characterization
of
the
biogas
potential
extraction
of
each
extraction
well.
The
origin
of
this
modeling
is
based
on
the
fact
that
each
extraction
well
has
a
respective
area
of
influence
and,
therefore,
a
specific
volume
of
MSW.
This
waste
generates
a
specific
quantity
of
biogas,
depending
on
its
biogas
potential.
The
aim
was
to
develop
a
new
method
for
measuring
the
average
maximum
biogas
fl
ow
that
is
pro-
duced
per
waste
volume
equal
to
the
maximum
radius
of
influence
of
each
well.
The
airflow/gas
fl
ow
assay
was
carried
out
in
the
present
study.
This
consists
of
applying
different
extrac-
tion
pressures
to
the
well
and
registering
the
fl
ow
of
recovered
landfill
gas
(LFG),
its
composition,
and
the
de-
pression
at
the
head
of
the
well.
The
composition
of
the
gas
allows
one
to
discern
between
the
fl
ow
corresponding
to
the
biogas
and
the
fl
ow
corresponding
to
the
entrance
of
air
through
the
well.
12
The
sealing
around
the
extrac-
tion
well
is
not
perfect;
atmospheric
air
may
penetrate
into
the
landfill
because
of
the
negative
pressures
used.
This
air
will
mix
with
the
biogas
resulting
from
waste
degradation,
and
thus,
the
CH
4
concentration
in
the
bio-
gas
is
always
greater
than
that
in
the
extracted
gas
because
of
air
mixing.
The
real
CH
4
concentration
in
the
landfill
can
be
calculated
from
the
experimentally
measured
per-
centages
of
CH
4
and
oxygen
concentrations.
The
assay
commences
with
the
valve
for
regulating
the
fl
ow
completely
closed
or
completely
open,
so
that
when
commencing
with
a
closed
valve,
the
fl
ow
of
biogas
will
be
null
and
the
pressure
in
the
well
will
be
positive.
During
the
assay,
the
regulating
valve
is
progressively
opened
or
closed
and
note
is
taken
of
the
different
Flare
Switch
board
B
ackfire
filter
C
ondens
ate
s
collector
0
0
Entranc
e
Control
valve
Fl
017/M
eter
Figure
2.
The
portable
biogas
extractor.
Pump
variables
under
study
(pressure,
position
of
the
valve,
fl
ow,
and
composition).
There
is
a
wait
of
30
min
between
measurements
to
allow
the
variables
in
the
well
to
stabi-
lize.
The
assay
ends
when
the
valve
is
completely
open
or
completely
closed.
To
carry
out
these
assays,
a
portable
biogas
extraction
unit
like
the
one
represented
in
Figure
2
was
used
along-
side
the
fi
xed
regulating
stations.
The
portable
biogas
extraction
unit
is
conceived
of
as
a
mini
-station
for
ex-
traction,
control,
and
incineration
of
biogas,
which,
once
connected
to
a
well,
creates
the
same
conditions
as
would
exist
if
the
well
were
connected
to
the
definitive
biogas
extraction
networks
The
portable
biogas
extraction
unit
is
made
up
of
four
basic
elements:
an
incineration
burner,
a
turbo
-blower
100
90
-
'41
80
-
70
60
-
50
-
40
-t
30
-
20
-
10-
0
0
F
landfill
gas
A
F
air
NIEFop
o
F
biogas
MEF
10
-
15
-
20
-
30
-
35
-
50
-
55
-10 -20 -30
-40
-50
Dp,
tnmwc
Figure
3.
Airflow/gas
flow
assay
for
well
Da.
60
-80
Volume
55
February
2005
Journal
of
the
Air
&
Waste
Management
Association
175
Rodriguez
-Iglesias
et
al.
80
70-
60-
E
50
40,
s.)
30-
20-
10
-
F
landfill
gas
F
air
M
EF
opt
o
F
biogas
MEF
6
-
-10 -20
-30
-40
D
p,
mmwc
Figure
4.
Airflow/gas
flow
assay
for
well
S7.
FPiZ
3CL,
a
gas
fl
ow
regulating
valve
Spriano
2C2,
and
a
condensates
separator.
The
operating
limits
for
the
equip-
ment
were
set
between
10
and
130
m
3
N/hr
of
biogas
at
55%
of
CH
4
in
volume.
The
extractor
is
equipped
with
an
ABB
I
-E21
-50-1-E
orifice
fl
ow
meter
furnished
with
a
Smart
LD
301
pressure
transmitter
electronic
display.
It
also
possesses
an
automatic
fl
ame
ignition
system
and
a
combustion
control
system.
The
composition
of
the
gas
and
the
depression
of
the
well
were
measured
using
a
portable
Geotechnical
Instruments
GA
94
IR
analyzer.
MODEL
DEVELOPMENT
Figures
3-9
present
the
complete
plots
of
the
Airflow/gas
fl
ow
assays
carried
out
at
seven
different
wells
belonging
to
two
different
areas
of
the
landfill.
Several
characteristic
100
90-
=
80-
E
4
70-
o
60-
30
-
20
-
10
-
0
F
landfill
gas
0
F
biogas
F
air
m
MEF
MEF
opt
10
0
-5
-10
Dp,
mmwc
Figure
5.
Airflow/gas
flow
assay
for
well
S4.
60
s
50
E
40-
3
-
c.)
a.)
E
20-
7s
s
10-
F
landfill
gas
F
air
M
EFopt
F
biogas
MEF
-10
-15
Dp,
mmwc
Figure
6.
Airflow/gas
flow
assay
for
well
S9.
-20
behaviors
can
be
observed
on
analyzing
the
graphs.
While
the
well
is
over
-pressurized,
the
fl
ow
of
air
entering
it
is
null,
and,
hence,
the
composition
of
the
LFG
extracted
will
co-
incide
with
that
of
the
biogas
that
is
produced
by
the
de-
composition
of
the
organic
matter
present
in
the
MSW,
which
contains
—60%
of
CH
4
.
As
the
depression
in
the
well
increases,
the
fl
ow
of
LFG
also
increases.
However,
the
con-
tent
of
CH
4
decreases
because
of
the
entrance
of
air.
The
increase
of
the
well
suction
pressure
leads
to
a
progressive,
albeit
nonlinear,
increase
in
the
biogas
fl
ow
until
an
asymp-
totic
value
is
reached,
characteristic
of
each
well.
This
value
is
the
maximum
production
fl
ow
of
biogas
(MPF)
and
rep-
resents
the
maximum
production
and
extraction
fl
ow
of
a
well
in
the
case
of
there
being
no
influx
or
outflow
of
air
into
140
120-
4
100-
40-
>
20-
F
landfill
gas
o
F
biogas
F
air
MEF
M
EFopt
-20
-40
Dp,
mmwc
Figure
7.
Airflow/gas
flow
assay
for
well
S10.
-60
176
Journal
of
the
Air
&
Waste
Management
Association
Volume
55
February
2005
Rodriguez
-Iglesias
et
al.
30
2
5
M
c
E
z
1
5
-
E
10-
0
2
0
-
5
-
F
landfill
gas
o
F
biogas
F
air
M
EF
MEFopt
0
-10 -20 -30
-40
-50
-60 -70 -80
Dp,
mmwc
Figure
8.
Airflow/gas
flow
assay
for
well
S12.
or
from
the
well.
From
a
certain
degree
of
depression
on-
ward,
when
said
depression
is
increased,
no
substantial
in-
crease
in
the
fl
ow
of
biogas
extracted
is
achieved
that
justi-
fi
es
a
further
increase
in
the
radius
of
influence
of
the
well,
and
consequently,
the
depression
applied
at
its
head.
During
the
extraction
process,
atmospheric
air
perme-
ates
through
the
soil
according
to
the
characteristic
curve
of
each
well,
thus
diluting
the
biogas
concentration
and
in-
creasing
the
overall
extracted
LFG
fl
ow
rate,
as
can
be
seen
in
Figure
3.
Assuming
a
minimum
permitted
concentration
of
CH
4
in
the
extracted
LFG
suitable
for
successive
utilization
in
a
combustion
system
(52%),
it
is
then
possible
to
com-
pute
the
minimum
airflow
rate
(Fair
min
)
required
to
achieve
the
required
concentration.
Summing
this
Fair,
T
,
i
to
the
MPF
renders
the
maximum
theoretical
extractable
fl
ow
of
LFG
(MEF)
0p
,
of
the
well.
This
definition
supplies
information
about
the
amount
of
biogas
that
may
be
extracted
from
the
LFG
per
extraction
well
area.
MEF
opt
=
MPF
+
F
air
min
(1)
The
maximum
extractable
fl
ow
of
LFG
(MEF)
is
measured
once
the
composition
of
the
recover-
able
biogas
has
been
estab-
lished
(in
this
case,
52%
of
CH
4
)
for
its
adequate
utilization
in
combustion
motors.
The
fl
ow
that
each
well
can
independently
supply
and
the
80
F
landfill
gas
o
F
biogas
A
F
air
MEF
MEFopt
60
-
50
-
40
-
30
-
20
-
10-
0
0
-10
-20 -30 -40
0
-60
-70
-80
Dp,
mmwc
Figure
9.
Airflow/gas
flow
assay
for
well
S15.
depression
it
needs
to
do
so
may
be
determined
by
means
of
the
airflow/gas
fl
ow
assay.
The
optimum
situation
is
one
in
which
the
MEF
opt
coincides
with
the
MEF.
If
the
inlet
airflow
rate
is
higher
than
the
theoretical
airflow
rate
required
to
reach
the
target
CH
4
concentration
at
a
given
biogas
fl
ow
rate,
then
the
extracted
gas
is
overdiluted,
thus
forcing
a
reduction
in
the
applied
suction
pressure
to
limit
the
air
inflow.
The
biogas
extraction
yield
is
thus
lowered
below
the
MPF,
and
an
MEF
lower
than
the
ME-
F
opt
is
obtained.
The
greater
or
lesser
influx
of
air
is
because
of
the
nature
of
the
sealing
of
the
well;
the
better
the
sealing,
the
lower
the
amount
of
air
entering
the
well
and,
therefore,
the
greater
the
correspondence
between
the
MEF
opt
and
the
MEF.
It
is
thus
of
prime
importance
to
achieve
a
Table
2.
Results
obtained
in
the
study
of
several
extraction
wells
connected
to
regulation
station
D.
Well
MPF
(m
3
/hr)
MEF
oPt
(m
3
/hr)
MEF
(m
3
/hr)
SF
(m
3
air/mmwc)
k
(mmwc
-1
)
ER
(%)
r
Da
36.1
41.6
10.7
0.3486
0.091
25.7
0.9969
Di
28.1
34.4
28.1
0.0559
0.222
86.7
0.9955
D3
14.6 16.8 10.5
0.1621
0.185
62.3
0.9885
D4
17.7
20.4
18.4
0.014
0.369
90
0.9946
D5
8.7
10
8.9
0.0513
0.265
88.7
0.9932
D6
24.3
28
22.9
0.3702
0.305
81.6
0.9953
D7(1)
34
39.2
35.3
0.4933
0.19
89.9
0.997
D7(2)
20
23.1
18.4
0.035 0.256
79.5
0.9946
D8
20.2
23.3
22.7
0.071
0.342
97.3
0.9925
D9
24.84 28.7
25.2
0.0106
0.186
87.9
0.9986
D10
17.4
20.1
18.3
0.0581
0.215
91
0.9942
D11
29.5
34
33
0.0691
0.103
97
0.9951
D12
16.4
18.9
16.4
0.0174
0.095
86.7
0.9963
Mean
81.8
Volume
55
February
2005
Journal
of
the
Air
&
Waste
Management
Association
177
Rodriguez
-Iglesias
et
al.
60
50-
F
landfill
gas
*
F
biogas
F
air
MEF
M
EFopt
o
20-
0
I I
10-
0
20
10
0
-10 -20 -30 -40 -50 -60 -70
-80
Dp,
mmwc
Figure
10.
Airflow/gas
flow
assay
for
well
B5(1).
sealing
in
which
the
airflow
introduced
into
the
MPF
of
the
depression
(Dp
m
„)
is
equal
to
or
less
than
the
theo-
retical
Fair
min
.
The
contribution
of
air
to
the
total
fl
ow
presents
a
clear,
linear
tendency
with
respect
to
the
depression
ap-
plied
at
the
well
head.
The
absolute
value
of
the
slope
of
this
straight
line
is
called
the
sealing
factor
(SF),
which
represents
the
fl
ow
of
air
entering
the
well
per
mmwc
of
the
applied
depression.
The
greater
the
SF,
the
greater
the
degree
of
sealing
of
the
well,
and
vice
versa.
It
is
always
of
interest
in
the
adequate
exploitation
of
an
extraction
system
to
employ
all
means
available
to
improve
the
sealing,
thus
causing
the
MEF
to
correspond
more
to
the
MPF,
improving
the
degree
of
extraction.
60
a-,
.o
50-
E
z
.4.)
30-
E
a)
E
20-
o
4
10-
F
landfill
gas
F
air
MEFopt
o
F
biogas
MEF
20
10
0
-10 -20 -30
-40
-50
-60
-70
-80
Dp,
mmwc
Figure
11.
Airflow/gas
flow
assay
for
well
B5(2).
178
Journal
of
the
Air
&
Waste
Management
Association
150
a-,
125-
E
100-
.
4
.
0
75
-
L
E
50-
-
25-
F
landfill
gas
F
air
o
F
biogas
MEF
20
0
-20
-40
-60
-80
-100
Dp,
mmwc
Figure
12.
Airflow/gas
flow
assay
for
well
A2(1).
If
this
is
expressed
mathematically,
the
airflow
rate
behavior
corresponds
to
a
zero
order
kinetic
equation,
where
the
air
influx
rate
to
the
total
fl
ow
per
millimeter
of
water
gauge
of
depression
applied
to
the
well
head
is
constant
and
equal
to
the
SF.
dF
airair
=
SF
dD
p
(2)
Integrating
this
equation
between
0
and
F
air
and
between
D
po
and
D
p
,
the
following
equation
is
obtained:
Fair=
—SF
D
p
+
D
p0
SF
(
3
)
The
biogas
fl
ow
rate
behavior
corresponds
to
a
fi
rst
-order
kinetic
equation,
in
which
a
decrease
in
the
biogas
extrac-
tion
capacity
per
millimeter
of
water
gauge
applied
to
the
well
head
leads
to
an
increase
in
the
nonextracted
biogas
fl
ow
rate.
dF
biogas
dD
k
(MPF
F
biogas
)
p
(4)
Integrating
this
equation
between
0
and
F
iaiogas
and
be-
tween
D
po
and
D
p
,
the
following
equation
is
obtained:
F
biogas
=
MPF
(1
(p
p
D
p0
)
)
(
5
)
The
LFG
fl
ow
rate
is
obtained
by
adding
together
the
biogas
fl
ow
rate
and
the
fl
ow
rate
of
the
air
incorporated
during
the
extraction
process.
Volume
55
February
2005
Rodriguez
-Iglesias
et
al.
140
-
120
-
E
100-
z
O
80
-
w
60
-
>
40
-
20
-
F
landfill
gas
F
air
F
biogas
MEF
0.
20
0
-20
-10 -60
-80
-100
D
p,
mmwc
Figure
13.
Airflow/gas
flow
assay
for
well
A2(2).
=
MFP
(1
-
e
k.
(D
P
D
Po
)
)
SF
D
p
+
D
p0
SF
F
landf
ill
gas
(6)
D
v:
,
corresponds
to
the
relative
pressure
in
which
the
extracted
fl
ow
rate
is
0.
If
D
po
is
positive,
this
means
that
when
the
pressure
is
0
there
is
a
substantial
biogas
fl
ow
rate
generated
by
the
well
itself,
thus
showing
a
high
degree
of
biological
activity,
as
occurs
in
wells
S4
(Figure
5),
S15,
and
S9.
On
the
other
hand,
if
D
po
is
negative,
this
means
that
a
certain
depression
in
the
well
head
is
needed
to
reach
the
necessary
radius
of
influence
to
begin
extract-
ing
biogas,
as
occurs
in
wells
S9,
S10,
Da,
S7,
and
S12
(Figures
3,
4,
6,
7,
8).
The
most
common
situation
found
was
that
D
po
was
practically
0,
meaning
that
there
was
neither
a
high
degree
of
biological
activity
generating
large
amounts
of
biogas
nor
the
need
for
high
depressions
to
start
extracting
biogas.
Once
these
concepts
have
been
defined,
an
exploita-
tion
rate
(ER)
may
be
easily
established
for
the
biogas
extraction
potential
of
a
well,
which
shall
be
defined
as
the
relationship
that
exists
between
the
MEF
and
the
MEF
opt
:
MEF
ER
=
MEF
100
(7)
opt
MODEL
VERIFICATION
Data
obtained
in
1997
by
Mar-
tin
et
al.'
2
were
employed
with
the
aim
of
checking
the
sound-
ness
of
the
suggested
model.
In
said
study,
F
air
/F
gas
assays
were
carried
out
to
model
LFG
migration
in
different
extraction
wells,
also
at
the
same
landfill,
with
the
goal
of
controlling
the
manual
extrac-
tion
of
biogas.
The
kinetic
constant
values
obtained
when
applying
the
model
described
in
the
present
study
are
shown
in
Table
2,
as
are
the
correlation
coefficients
for
the
theoretical
biogas
production.
The
graphs
resulting
from
these
assays
can
be
seen
in
Figures
10-13.
A
very
high
degree
of
correlation
was
observed
for
the
obtained
data
in
the
four
wells
tested,
because
the
worst
value
of
r
was
0.9962
in
well
A2(2).
Wells
A2(1)
and
A2(2)
presented
a
very
high
value
of
MEF
opt
,
as
can
be
seen
in
Table
2.
It
is
impossible
to
reach
this
degree
of
extraction
in
the
landfill
because
of
the
characteristics
of
the
extraction
system
itself,
as
it
has
a
maximum
capacity
of
extraction
of
between
100
and
120
m
3
/hr.
5
In
this
case,
the
area
of
influ-
ence
of
these
two
wells
should
be
divided
up
between
one
or
more
newly
created
wells
in
the
vicinity
with
the
aim
of
dividing
out
this
potential
between
different
wells
so
as
to
exploit
them
more
efficiently.
The
SF
and
k
values
obtained
by
Martin
et
al.
were
in
accordance
with
the
values
described
previously
and
ranged
between
0.20
and
0.33
m
3
air/mmwc
for
SF
and
between
0.004
and
0.026
mmwc
-1
for
k.
RESULTS
AND
DISCUSSION
Table
3
shows
the
results
of
the
assays
carried
out
in
13
randomly
selected
wells
connected
to
extraction
station
D
at
the
landfill.
This
station
was
made
up
of
12
lines
and
24
wells.
The
extraction
area
of
this
station
was
the
youngest
when
these
assays
were
conducted.
Considerable
varia-
tions
between
the
different
extraction
wells
were
ob-
served.
The
minimum
ER,
25.7%,
was
reached
in
well
Da.
This
well
was
not
connected
to
any
extraction
line
be-
cause
of
its
low
efficiency.
The
highest
ER
value,
97.3%,
was
reached
in
well
D8.
The
average
yield
for
the
13
selected
wells
was
82%.
As
can
be
appreciated,
the
values
of
the
SF
vary
os-
tensibly,
as
do
those
of
the
kinetic
constant
employed
in
the
mathematical
model,
and
are
characteristic
of
each
extraction
well,
because
they
depend
on
factors
such
as
Table
3.
Results
obtained
in
the
study
of
several
extraction
wells
connected
to
regulation
station
S.
Well
MPF
(m
3
/hr)
MEF
opt
(m
3
/hr)
MEF
(m
3
/hr)
SF
(m
3
air/mmwc)
k
(mmwc
-1
)
ER
(%)
r
S2
36.6
42.2
10.4
0.2522
0.064
24.6
0.9988
S4
45.4
52.3
45.9
0.8686
0.306
87.7
0.9996
S7
33.7
38.9
35.3
0.4722
0.2
90.6
0.9905
S9
22.5
25.9
9.2
1.152
0.694
35.5
0.998
S10
77.2
89.1
78.8
0.6011
0.684
88.4
0.9964
S12
15.7
18.1
5
0.0575
0.035
27.5
0.9746
S15
32.3
37.2
18.2
0.4741
0.066
48.9
0.9964
Mean
57.6
Volume
55
February
2005
Journal
of
the
Air
&
Waste
Management
Association
179
Rodriguez
-Iglesias
et
al.
Table
4.
Values
of
the
model
kinetic
constants using
the
0
a
,
r
/O
gas
assays
described
by
Martin
et
al.
12
Well
MPF
(m
3
/hr)
MEF„,,
(m
3
/hr)
MEF
(m
3
/hr)
SF
(m
3
air/mmwc)
K
(mmwc
-1
)
ER
(%)
B5
(1)
30.7 35.4
19
0.2023
0.018
53.6
0.9988
B5
(2)
23
36.6
17
0.2228
0.026
63.9
0.9987
A2
(1)
194.7
224.6
69
0.2589
0.004
30.7
0.9973
A2
(2)
194.5
224.4
55
0.3319
0.004 24.5
0.9962
the
characteristics
of
the
waste,
the
evolution
of
the
bio-
logical
process
within
the
landfill,
meteorological
condi-
tions,
and
so
on.
The
values
of
the
SF
in
the
extraction
wells
connected
to
station
D
ranged
between
0.0106
and
0.4933
m
3
air/mmwc
and
the
values
of
the
constant
k
varied
between
0.091
and
0.369
mmwc
-1
.
Table
4
shows
the
results
obtained
in
the
assays
car-
ried
out
in
seven
randomly
selected
wells
connected
to
regulation
station
S.
In
this
case,
the
extraction
area
was
the
oldest
dumping
area
and,
therefore,
the
lowest
yields
in
the
whole
landfill
are
to
be
expected.
There
were
14
connected
lines
in
this
extraction
grid,
each
with
its
ex-
traction
well.
The
lowest
exploitation
rate
obtained
was
in
well
S2,
with
a
value
of
24.6%
of
extracted
gas
over
its
total
potentiality;
the
highest
took
place
at
well
S7,
with
a
value
of
90.6%
and
a
recovered
fl
ow
rate
of
35.3
m
3
/hr.
The
average
ER
obtained
was
57.6%.
The
SF
ranged
considerably
between
0.05
m
3
air/
mmwc
in
well
S12
and
1.15
m
3
air/mmwc
in
well
S9,
whereas
the
k
values
remained
between
0.035
mmwc
-1
in
well
S12
and
0.699
mmwc
-1
in
well
S9.
As
in
the
previous
experiments,
the
SF,
MPF,
and
MEF
values
are
character-
istic
of
each
well
and
no
apparent
relationship
because
of
geographical
proximity
is
observed
between
them.
These
values
vary
with
time
and
are
only
valid
for
each
well
and
for
a
certain
period
of
time,
as
a
continuously
developing
biological
process
exists
in
the
landfill.
CONCLUSIONS
The
model
described
fi
tted
the
F
air
/1'
ga
,
assays
carried
out
and
those
described
in
the
bibliography
perfectly.
The
worst
correlation
coefficient
achieved
was
0.9746
in
well
S12
and
the
best
was
0.9996,
in
wells
B5(1)
and
S4.
This
model
makes
it
easier
to
understand
the
landfill
biogas
extraction
process
without
using
a
complex
mathematical
algorithm.
The
model
supplies
significant
information
for
the
correct
management
of
LFG,
such
as
the
SF,
MPF,
MEF,
and
the
optimum
MEF
opt
.
These
parameters
are
charac-
teristic
of
each
extraction
well
and
evolve
with
time,
mainly
because
of
the
variation
in
biodegradation
rates.
The
mean
exploitation
rate
found
in
the
experiments
carried
out
in
the
13
randomly
chosen
wells
connected
to
the
lines
of
regulating
station
D
at
the
principality
of
Astu-
rias
landfill
was
82%,
the
highest
yield
being
97.3%
and
the
lowest
27%,
because
of
excessive
incorporation
of
air.
For
the
other
group
of
seven
wells
stud-
ied,
the
mean
exploitation
rate
found
was
57.6%,
and
the
values
ranged
between
24.6
and
90.6%.
ACKNOWLEDGMENTS
This
research
was
funded
by
COGERSA,
S.A.
(Consortium
for
Solid
Waste
Manage-
ment
in
Asturias)
and
the
II
Research
Program
of
Asturias-
FICYT
(Foundation
for
the
Support
of
Research
in
Applied
Science
and
Technology,
Asturias,
Spain),
contract
PA-
AMB97-02.
The
authors
also
gratefully
acknowledge
the
assistance
of
Santiago
Fernandez,
manager
of
COGERSA.
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About
the
Authors
Jesus
Rodriguez
-Iglesias,
I.
Vazquez,
E.
Maranon,
L.
Castrillon,
and
H.
Sastre
are
with
the
Department
of
Chemical
and Environmental
Engineering,
Higher
Tech-
nical
School
of
Engineering,
University
of
Oviedo,
33204
Spain.
Address
correspondence
to:
Jesus
Rodriguez
-
Iglesias,
Universidad
de
Oviedo,
Area
de
Tecnologia
del
Medio
Ambiente,
Edificio
Este
s/n,
Campus
de
Viesques,
Gijon,
Asturias,
Spain,
33204;
phone:
+1-3
49-8518-
238
4;
e-mail:
jesusr@epsig.uniovi.es.
180
Journal
of
the
Air
&
Waste
Management
Association
Volume
55
February
2005