United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/141 September 1995
vvEPA Project Summary
Leachate Clogging Assessment
of Geotextile and Soil Landfill
Filters
Robert M. Koernerand George R. Koerner
This project focused on the perfor-
mance, design, testing, and recommen-
dations for filters used for leachate
collection drainage systems at the base
of landfills, waste piles, and other solid
waste facilities. The emphasis of the
project was on geotextiles because of
their manufactured uniformity, ease of
placement, and savings in landfill vol-
ume; natural sand soil filters were also
evaluated. Field exhuming of four sites
indicated that problems existed at three
of them. These three sites employed
"socked pipe," where a geotextile was
wrapped around perforated pipe. The
testing and subsequent design showed
that socked pipe designs should not
be used in landfills nor should permit-
ting agencies allow them this applica-
tion. At the fourth site where the
geotextile was moved away from the
pipe, in a trench-wrap configuration
performance was acceptable. Even fur-
ther, the laboratory testing portion of
the study indicated that an open
geotextile over the entire base of the
landfill (the footprint) is the proper de-
sign strategy and, thus, is recom-
mended for general use. The
introduction of a term called the "drain-
age correction factor" (DCF), in the
standard design equation was recom-
mended. This DCF was used to assess
the various design options, and the re-
sults corroborated findings at the ex-
humed field sites. Other related
investigations included the "no-filter"
design strategy (which can be used
only with extreme caution and when
accompanied by long-term testing) and
the use of biocides (which is not rec-
ommended).
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The proper collection, transmission, and
removal of leachate from the base of solid
waste landfills is at the heart of a proper
liquids management strategy. Although
many design issues are involved, exces-
sive system clogging is an often-raised
concern. Since most leachate collection
and removal systems consist of a filter, a
drainage material, and a perforated pipe
system, focusing on the material with the
smallest void spaces, i.e., the filter, is
logical.
Historically, leachate collection and re-
moval system filters have been granular
soils, primarily sands. These have recently
been replaced in large measure by
geotextiles because of the quality control
of manufactured geotextiles, their ease of
placement, and the subsequent savings
in landfill volume. This project focused
primarily on geotextile filters insofar as
the potential for excessive clogging by
leachate was concerned. Sand filters were
also evaluated for comparative purposes.
The project consisted of a number of sepa-
rate tasks brought together in a recom-
mended design methodology for
determining a factor-of-safety value for a
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specific candidate filter and a set of site
specific conditions.
Task 1 - Exhuming of Field
Sites
The first task was arguably the most
difficult and also the most rewarding of
the entire project. Field sites-of-opportu-
nity were solicited for the purpose of ex-
huming their respective leachate collection
and removal systems. Obviously, the over-
lying solid waste had to be removed be-
fore the collection system could be
investigated. Although only four sites were
obtained, they were very significant. Table
1 gives some of the physical details and
observations of the sites, and Table 2
gives the leachate characteristics at the
time of exhuming. Note that the leachate
removal system at Sites 1, 3, and 4 were
not functioning because their filters were
excessively clogged. Site 2 was still func-
tioning; however, flow rates were less than
the designer/operator had anticipated.
Comments and conclusions about these
exhumed sites include:
• All sites had relatively harsh leachates
high in total solids (TS) and/or bio-
chemical oxygen demand (BOD5).
• The exhumed sites that were exces-
sively clogged had geotextiles
wrapped directly around perforated
drainage pipes (socked pipes).
• Obviously, this practice of socked pipe
should not be used for leachate col-
lection systems.
• In the still-functioning site, a geotextile
was wrapped around gravel that in
turn, contained a perforated drainage
pipe.
• These observations led to the sug-
gested optimum design: using a filter
over the landfills's footprint and as far
away from the leachate removal pipe
network as possible.
• This suggested design had to be cor-
roborated by laboratory tests, ana-
lytic modeling, and appropriate design
modeling. The remainder of the project
focused on those specific tasks.
Task 2 - Laboratory
Investigations
To determine the long-term allowable
permeability (kallow) of a particular filter
(geotextile or sand), an new test method
was proposed, carried through the neces-
sary committees, and eventually adopted
by the American Society of Testing and
Materials. Its designation is ASTM D1987,
and it is specifically intended to determine
the leachate permeability of geotextile and
soil landfill filters. In the course of this
project, 144 permeameters (Figure 1) were
constructed and used for periods of 120
to 300 days. The experimental variations
consisted of:
• 12 filters (10 geotextile and 2 sands)
• 4 permeants (water and 3 leachates)
• 3 flow rates (all significantly greater
than typical field flow rates)
The use of flow rates greater than field
flow rates constituted accelerated testing
with respect to the amount of leachate
passing through the filters. A typical re-
sponse curve for a single flow rate is
shown in Figure 2. When the equilibrium
value was determined, it was used with
the same type of filter at different flow
rates to establish a trend. Results of ac-
celerated tests at all three flow rates were
plotted and can be back-extrapolated to
field anticipated flow rates. These trends
for the 12 evaluated filters are given in
Figure 3. These curves represent a set of
Table 1. Overview of Exhumed Leachate Collection Systems
Site
No
1.
2.
3.
4.
Waste
Type
Domestic and
light industrial
Domestic and
light industrial
Industrial solids
and sludge
Domestic and rural
Age
Exhuming
10
6
0.5
6
Liquid
Management
Scheme
Leachate
recycling
Leachate
recycling
Leachate
withdrawal
Leachate recycling
Performance
Exhuming
Excessively
clogged
Marginally
clogged
Excessively
clogged
Excessively
clogged
Critical
Element in
Drainage System
Geotextile filter
Drain location
Geotextile filter
Geotextile filter
master curves of commercially available
filter materials for which ka||ow can be taken
at a particular site specific value of field
anticipated flow rate.
Task 3 - Analytic Modeling
To counterpoint the allowable perme-
ability of a given filter (as just described)
to a required permeability, a suitable ana-
lytic model is needed. This model must be
site specific for hydrology, waste type, ge-
ometry, material properties, etc. For this
purpose, the EPA-sponsored model en-
titled Hydrologic Evaluation of Landfill Per-
formance (HELP) is regularly used in the
United States and its use is becoming
common throughout the world. The HELP
model is a liquids balance model that
tracks the moisture in the waste and aug-
ments it with the site-specific rainfall and
snowmelt. This total amount is then parti-
tioned via a number of subroutines into
runoff, interception, transpiration, evapo-
ration, and infiltration. The infiltration is
then tracked through the various layers
until it meets the leachate collection and
removal system at the base of the landfill.
The value of required permeability (kreqd)
was obtained by sequentially varying a
series of trial permeabilities from 1.0 to 1
x 10'8 cm/sec while tracking the peak daily
discharge output of the model. A site spe-
cific value for kreqd was then defined as
the point at which the peak daily discharge
was negatively influenced by changes in
the trial permeability of the filter. In effect,
when the permeability of the trial filter
began to significantly decrease the amount
of leachate discharged, the value of kreqd
was reached. Version 3 of the HELP model
was used to develop the kreqd values of
Table 3, which were based on the charac-
teristics of the four sites.
Task 4 Design Method and
Substantiation
Having values of "kaNow" for a particular
filter and the HELP-generated "kreqd" value
for a particular landfill site allows for the
formulation of a factor-of-safety (FS)
against excessive filter clogging. A direct
comparison was not possible, however,
because of observations made at the field
exhumed sites. For a filter with only a
small drainage area directly beneath it, as
in the case of socked pipe, the classical
FS equation had to be modified. This was
done by using a "drainage correction fac-
tor (DCS) in the denominator of the con-
ventional FS equation. The DCF is defined
as the ratio of the landfill area divided by
the available drainage flow area immedi-
ately downstream of the filter. (In the case
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Table 2. Summary of the Leachate Characteristics of the Exhumed Field Sites
Site
No.
1.
2.
3.
4.
Landfill
Type
Municipal
Municipal
Municipal
Municipal
PH
10
6
0.5
6
COD
(mg/l)
31,000
10,000
3,000
24,000
TS
(mg/l)
28,000
3,000
12,000
9,000
BOD5
(mg/l)
27,000
7,500
1,000
11,000
Inflow
Upper
End Cap
100 mm
1 1
1 1
100mm
r
Soil
(Optional)
X
Support
Gravel
Lower _/ ^n 1 r*
End Cap 1
-Geotextile Specimen
Containment
Ring
Outflow
100mm
Figure 1. ASTM D1987 type permeameters.
Water
Leachate "L"
Leachate "P"
Leachate "D"
0 20 40 60 80 100 120 140 160 180 200
Time (days)
Figure 2. Typical permeability test results for a particular geotextile filter.
of socked pipe, its value is very large).
The resulting formulation was as fol-
lows:
FS = A3//™/
kreqdxDCF
where:
FS = factor-of-safety (against
excessive filter clogging)
kaiimv = allowable filter permeability
kreqd = required filter permeability
DCF = drainage correction factor
With the use of k^^, value for the
geotextile exhumed at each of the four
field sites, the kreqd value for each of
the field sites from the HELP model,
and the calculated site specific DCF,
we obtained the data of Table 4. Here
it can be seen that the three sites with
excessively clogged geotextiles could
easily have been predicted as failures
based on their extremely low FS val-
ues.
Possible Less Expensive
Alternative
Because the suggested laboratory
work and design modeling are both time
consuming and expensive, we explored
conditions in which a "default" geotextile
could be used as the filter. We con-
cluded that if the leachate was rela-
tively mild, i.e., TS < 2500 mg/L and
BOD5 < 2500 mg/L, geotextiles with the
properties shown in Table 6 could be
used with a reasonable degree of con-
fidence. The proviso, however, is that
the geotextile must cover the full foot-
print of the landfill or cell under consid-
eration. In the context of this study, this
type of design is defined as an aerial
filter with a drainage correction factor
of one, i.e., DCF = 1.0.
Additional aspects of the study in-
vestigated the use of biocides (which
were not particularly encouraging) and
the "no filter" design scenario (which
places emphasis on potential clogging
of the downstream drainage stone).
Both of these design strategies can be
evaluated by the laboratory test meth-
ods and design formulation developed
in this study.
If the leachate has higher values than
2500 mg/L for TS and for BOD5, the
procedure and details given for Tasks
1 through 4 should be followed. The
laboratory test data and the requisite
design may permit less conservative
filters than those described in Table 6.
Properly designed they are acceptable.
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The values of strength listed in the
above table are required Class 2 and Class 1
values per the proposed AASHTO M288
specification for transportation facilities in
the high and very high survivability rat-
ings, respectively [15].
Conclusions
This project, which focused on the fil-
ters of landfill leachate collection and re-
moval systems, resulted in a design meth-
odology to predict the anticipated FS
against excessive filter clogging. It evalu-
ated laboratory and analytic models, along
with making observations from field-ex-
humed sites. The use of the design model
nicely substantiated the field findings. Use
of the modified FS equation is recom-
mended for design of leachate collection
filters to assess the possibility of exces-
sive clogging at the base of solid waste
landfills, waste piles, and other solid waste
facilities.
The full report was submitted in fulfill-
ment of CR-819371 by Drexel University
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
u
0)
u
I
0)
Ottawa Sand
Concrete Sand
N7W
N14W
N32W
A10W
H4NPNW
H8NPNW
H16NPNW
T4HBNW
P6NPNW
N22NW/W
10° 10
Flow Rate (1/ha-day)
10'
Figure 3. Master curve of 12 filters for "k „ " determination at a Site specific flow rate.
Table 3. Input Data of Exhumed Sites for Use in HELP Model to Obtain Required Filter Permeability
S/fe
Wo.
1.
2.
3.
4.
Cell Area
(ha)
2.8
2.8
2.9
5.6
Base Slope
(acre)
7
7
2.9
13.8
Pipe Spacing
(0%)
1.5
1.5
2.0
1.5
I/
drainage Stone
(m)
61
61
61
31
(ftj"
200
200
200
100
(cm/sec)
0.01
0.3
0.3
0.3
(cm/sec)
1x10-5
1 x 10-5
5x 1O5
1x105
Table 4. Corroboration of the Modified Factor-of-Safety Equation as Applied to Four Exhumed Field Sites
Site
1.
2.
3.
4.
Observed
Performance
Terrible
Good
Terrible
Poor
kallow
(cm/s)
6x 10-4
1 x 1O2
9x103
9x103
kreqd
(cm/s)
1 x 10-5
1 x 1O5
1 x105
1 x105
Value of
DCF
24,000
140
990
1,700
Calculated
FS Value
0.0003
7.1
0.18
0.53
Predicted
Performance
Failure
Acceptable
Failure
Failure
The variable term that greatly decreased the FS values was the DCF (Table 4). As seen in Table 5, for a number of design scenarios, the value of DCF
can be enormous.
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Table 5. Selected Values of Drainage Correction Factors for Use in Calculating the Factor-of-Safety of a Leachate Collection Filter*
Drain
Configuration
/Area/ coverage
Geotextile
wrapped around
gravel (i.e., socked
trench wrap)
Geotextile around
corrugated pipe (i.e.,
socked
piped)
Geotextile around
smooth wass
pipe (i.e., socked
pipe)
Drain
Spacing
(m)
n/a+
15
30
45
60
15
30
45
60
15
30
45
60
(ft)
n/a
50
100
150
200
50
100
150
200
50
100
150
200
Drain
Size
(mm)
n/a
450x300
450x300
450x300
450x300
150
150
150
150
150
150
150
150
(in.)
n/a
18x12
18x12
18x12
18x12
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
Hole
Size
(mm)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
12
12
12
12
(in.)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
0.5
0.5
0.5
0.5
Number
of Holes
(per m)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
1.8
1.8
1.8
1.8
(per ft)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
6
6
6
6
Drain
Correction
Factor
1
10
20
30
40
60
130
190
260
7,500
12,000
18,000
24,000
+n/a = Not applicable.
"All calculations are based on a 0.4 (1 acre) cell.
Table 6. Recommended Geotextile Filters for Use with Relatively Mild Landfill Leachates (Those Having TSS and BOD5 Values < 2500 mg/L)
Type of
Geotextile
Sand Protection
Layer Over Filter
Select Waste
Placed Directly on Filter
Woven Monofilament
Mass per unit area,
g/sq. M (oz/sq yd)
Percent open area, %
Grab tensile strength, N (Ib)*
Trapezoidal tear strength, N (Ib)
Puncture strength, N (Ib)
Burst strength, kPa (Ib/sq in.)
Nonwoven Needle Punched
Mass per unit area, g/sq. M (oz/sq yd)
Apparent opening size, mm (sieve size)
Grab tensile strength, N (Ib)
Trapezoidal tear strength, N (Ib)
Puncture strength, N (Ib)
Burst strength, kPa (Ib/sq in.)
170
10
1100
400
400
1800
270
0.212
1100
400
400
1800
(5.0
(250)
(90)
(90)
(400)
(8.0)
(#70)
(250)
(90)
(90)
(400)
200
10
1400
490
490
2200
400
0.212
1400
490
490
2200
(6.0)
0000
(300)
(110)
(110)
(500)
(12.0)
(#70)
(310)
(110)
(110)
(500)
*N=Newton
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Robert N. Koerner and George R. Koerner are with Drexel University,
Geosynthetic Research Institute, Philadelphia, PA 19104.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Leachate Clogging Assessment of Geotextile
and Soil Landfill Filters," (Order No. PB95-265542; Cost: $27.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
National Risk Management Research Laboratory
Cincinnati, OH 45268
Official Business
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