United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/025 Aug. 1991
Project Summary
Landfill Leachate Clogging of
Geotextile (and Soil) Filters
Robert M. Koemer and George R. Koerner
The primary ieachate collection sys-
tem of most solid waste landfills con-
tains a filter layer which has historically
been a granular soil. Recently, however,
various types of geotextlle filters (both
woven and nonwoven) have been used
to replace the natural soil filters. A project
using six different landfill leachates and
aimed at Investigating the functioning of
different geotextlle filters was the focus
of this 36-mo long study.
In the Initial 12-mo, referred to as Phase
I, flow rates In various filters were inves-
tigated under aerobic conditions at six
different landfill sites with the use of the
site-specific leachates. The study Inad-
vertently found that the overlying granu-
lar soil clogged as much as the geotextlle
filter that was located downstream. The
effects of different types and styles of
geotextiles were generally masked by
the upstream soil clogging. A separate
anaerobic Incubation task under no-flow
conditions showed clogging to be
present but to a significantly lesser ex-
tent than occurred with the aerobic flow
tests. This clogging was believed to be
completely biological in nature rather
than a combination of sediment and bio-
logical processes. An Important finding
In this task was that blodegradatlon of
the geotextiles was not evidenced and
was concluded to be a non-Issue.
The subsequent 24-mo study, referred
to as Phase ll(a), led to the development
of a vastly Improved flow rate monitor-
ing device. With the use of these new
flow columns, which are made from PVC
fittings, locally available at hardware
stores and very inexpensive, a wide range
of variables were evaluated: I.e., four
different styles of geotextiles, geotextlle
alone versus sand/geotextile filters,
anaerobic versus aerobic conditions, and
six different landfill leachates. The re-
sulting 96 columns (4x2x2x6) were
evaluated for their flow rate behavior
over time and found to essentially repli-
cate the first year's aerobic test results.
Varying degrees of clogging by sedi-
ment or particulates and microorgan-
isms did occur for the various geotextlle
and natural soil filters that were evalu-
ated. After establishing this point, a se-
ries of backflush remediation attempts
were evaluated. In general, flow rates
were partially restored, but only tempo-
rarily.
In a separate task, referred to as Phase
ll(b) and conducted simultaneously with
Phase ll(a), bloclde-treated geosyn-
thetics were used at the two sites with
the most aggressive leachates. Although
the biocldes may have been effective In
killing microorganisms, the dead bacte-
ria were as troublesome as the viable
bacteria In creating subsequent clog-
ging.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
Is fully documented In a separate report
of the same title (see Project Report
ordering Information at back).
Introduction
The primary Ieachate collection and re-
moval systems at solid waste landfills are
generally overlain by a filter layer consisting
of a geotextile or a natural soil. Such filters
must serve the dual tasks of allowing the
Ieachate to pass into the underlying drain
while retaining the upstream particulate
matter without excessive clogging. This rep-
resents a severe challenge to the filter since
7v-> Printed on Recycled Paper
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many leachates contain large amounts of
suspended solids and/or microorganisms.
To investigate the behavior of several
geotextile filters and of an Ottawa-sand soil
filter, six landfill leachates were evaluated
under different experimental conditions. The
characteristics of the leachate are shown in
Table 1. Note that all of the sediment and
microorganisms contained in the six
leachates fall into a relatively tight particle
size distribution entirely within the silt-size
classification, i.e., they range from 0.074
mm down to 0.002 mm.
Phase I - Initial Flow Rate
Evaluations
This phase of the project, which lasted for
12 mo, used flow boxes for aerobic evalua-
tion and large containers for anaerobic in-
cubation with subsequent testing.
In the aerobic portion of the study, 12 in.
x 12 in. wooden flow boxes, 24 in. high were
used. The boxes were all constructed using
a base plate, a geonet drain, a geotextile
filter, and 6 in. of free draining sand. The
remaining 18 in. of the boxes were empty so
that falling head permeability tests could be
conducted. Leachate passed through the
sand and geotextile and then flowed within
the geonet, which was open at one end
only. The time of flight forgiven quantities of
leachate to pass through the system was
measured. Each of the six sites had at least
four boxes, the only difference being the
type of geotextile filter. Both woven and
nonwovengeotextileswere evaluated. They
consisted of various polymer types and
manufacturing styles.
The following findings are based on the
flow rate behavior over the 12-mo evalua-
tion period at each site.
(a) The flow rale measurements from the
original values all decreased but varied
considerably.
(b) The relatively tightly woven geotextile
filter, with a 4% open area, performed
the poorest. For each of the four differ-
ent sites in which it was used, it clogged
beyond our detection limit. The time
periods were from 4-1/2 to 12 mo.
(c) Opening up the void space of the same
type of woven geotextile to a 10% open
area helped considerably. Flow rates
still decreased but were more in line
with the needle-punched nonwoven
geotextile types.
(d) The needle-punched, nonwoven
geotextiles performed equivalently.
They were similarly constructed but
were of different polymer types. The
results indicate that polypropylene,
polyester, and polyethylene fibers do
not appear to give significantly different
values in their flow rate response be-
havior.
(e) A heat-bonded, nonwoven geotextile
was used at two sites. Its response was
somewhat poorer than that of the
needle-punched nonwovens but better
than that of the 4% open-area woven
geotextile.
(f) The Phase I study indicates that use of
open-woven geotextiles and each of
the needle-punched, nonwoven
geotextiles resulted in equilibrium flow
conditions lasting between 6 and 12
mo. The flow rate was reduced from as
little as 20% of the original values (at
four sites) to as much as 80% (at two
sites). These reductions appeared to
be related to the strength of the leachate
insofar as their total solids (TS) and
microorganism content (BOD) were
concerned. In the worst cases, flow
rates were usually greater than 1.0 gal/
min-sq ft. This is equivalent to 6.2 x 107
gal/acre-day, which probably far ex-
ceeds most design requirements for
leachate collection system filters.
(g) The cause of the flow reductions cre-
ated somewhat of a dilemma. By cut-
ting a cross section of the boxes at the
end of the 12-mo period it was clearly
evident that the 6 in. of sand over the
geotextile was a major source of the
flow reduction. Clearly, the experiments
showed that soil clogging is every bit as
serious as geotextile clogging. Further-
more, the soil used was a very open-
graded, rounded sand (it was Ottawa
sand) having a permeability coefficient
of approximately 0.02 cm/sec (0.04 ft/
min). Thus, it actually meets, and even
exceeds, most regulatory criteria for a
drainage soil, let alone for a filter soil.
(h) Microscopic examination of the cross
sectioned soil/geotextile systems
showed heavy particulate clogging in
the upper half of the soil layer. Thereaf-
ter, the clogging was either fibrous or
consisted of very small clusters. Al-
though not conclusively proven, we
believed that the upper portion of the
soil column filtered the suspended sol-
ids out of the leachate and thereafter
biological activity spread throughout
the remaining portion of the soil column
and into the underlying geotextile. This
biological activity took numerous forms
including the deposition of precipitates
in the soil and in geotextile voids. Thus,
different geotextiles (all other things
being equal) responded differently to
the same site's leachate.
(i) The relative amounts of flow rate re-
duction between leachate sediment,
biological precipitates, and biological
growth could not be distinguished in
these particular tests.
The anaerobic portion of the study was
performed under completely submerged
conditions in 55 gal drums. Twelve samples
of each type of geotextile were suspended
on stainless steel racks and placed in the
various sites' leachate. One sample of each
type was removed for testing each month.
Four geotextile types were evaluated for
each of the six landfill leachates. After the
samples were removed, they were brought
to our laboratory and were tested for their
retained flow capability and possible strength
reduction. The general findings follow:
(a) Relatively minor flow reductions oc-
curred in all types of geotextiles evalu-
ated. The reduction values varied from
10% to 20%. Note that these amounts
are distinctly less than those that oc-
curred in most of the aerobic tests. We
believed that sediment clogging did not
form since flow was not occurring dur-
ing the incubation periods. Furthermore,
the absence of a soil column had a
dramatic (but quantitatively unknown)
effect on improving the flow rates.
(b) All of the exhumed geotextiles had
heavy biological growth that could be
easily seen and felt.
(c) Very informative scanning electron mi-
crographs taken at various times of
incubation were compared with the as-
received geotextiles. After 3 mo of in-
cubation, complete growth around the
individual fibers or growth in clusters
could generally be seen (Figure 1).
Although difficult to quantify, the amount
of growth was clearly related to the time
of immersion.
(d) The micrographs also revealed that the
biological growth was easily removed
from the fiber's surface. There appeared
to be no fixity or attachment to the
fibers.
(e) The above observation was corrobo-
rated by various strength tests per-
formed on the geotextiles after immer-
sion. Within the limits of our testing,
there was no strength reduction over
the 12-mo period. This suggests that
for these leachates, bbbgical degra-
dation of geotextiles is not a problem.
Phase II studies did not include the
polymer degradation issue.
Phase ll(a) - Improved Flow
Rate Columns and Remediation
Attempts
This phase of the project, which lasted
24-mo, used vastly improved flow rate mea-
suring systems. This type of improved sys-
tem has been developed into a test method
and procedure adopted by the American
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Table 1. Details of Municipal Landfill Leachates Evaluated in this Study and Approximate
Leachate Characteristics
Site
Designation
PA-1
NY-2
DE-3
NJ-4
MD-5
PA-6
Approximate Leachate Characteristics at Start-Up
pH
8.0
5.5
5.8
7.4
6.8
6.5
COD*(mg/L)
15,000
20,000
40,000
45,000
1,000
10,000
TS(mg/L)
8,000
8,000
17,000
16,000
100
5,000
BOD, (mg/L)
2,000
5,000
24,000
25,000
150
2,500
'COD = chemical oxygen demand; TS - total solids content; and
BODS = biochemical oxygen demand at five days.
Figure 1. Scanning electron micrograph ofgeotextile fiber after 3 mo immersion in leachate
from Site NY-4 (400X).
Society for Testing and Materials as a Stan-
dard Test Method (ASTM D1987-91, Test
Method for Biological Clogging of Geotextile
or Soil/Geotextile Filters"). The flow rate
measuring column appears as shown in
Figure 2 for evaluations being performed in
a variable or falling head test mode.
Ninety-six of these devices were used
under the following set of conditions:
• four different geotextiles,
• without soil and with Ottawa sand above,
• aerobic and anaerobic conditions, and
• six landfill leachates.
Continuous flow rate testing was monitored
for 6-mo. Once trends were established, a
series of remediation procedures were at-
tempted.
The following comments apply to the first
6-mo of flow testing, i.e., before the first
remediation was attempted.
(a) The columns with sand above the
geotextiles clogged considerably more
than those with the geotextile alone,
i.e., 23% flow was retained for sand/
geotextiles versus 34% flow retained
for geotextiles alone. Note that if the
heat-bonded nonwoven fabrics are
eliminated from the geotextile group,
the flow rate retained by the geotextile
group would be 45%. This suggests
that geotextiles can clog less than natu-
ral soil filters.
(b) Of the four geotextiles evaluated, the
highest retained flow was achieved with
the lightweight needled nonwoven
(38%), with the heavyweight needled
nonwoven (34%), and woven mono-
filament (32%) slightly behind. The non-
woven heat-bonded fabric had the low-
est retained flow of only 10% after 6 mo
of evaluation.
(c) Of the various landfill leachate types,
the lowest retained flow rate resulted
from use of the NJ-4 (14%) and DE-3
(17%) leachates. Recall from Table 1
that these are the leachates with the
highest TS and BOD concentrations.
The other four landfill leachates and
their percentages of flow retained after
6 mo of testing were PA-6 (26%), MD-
5 (29%), PA-1 (38%), and NY-2 (41%).
After the initial 6 mo of flow rate testing
confirmed the results of the Phase I study,
remediation of flow columns was attempted.
The first remediation, a leachate backflush,
improved flow rate but to varying amounts
between the different columns. After 4 mo of
continued flow testing, the flow rates de-
creased and allowed for a second
remediation. This remediation used a water
backflush. Again flow rates increased, but
over the next 5 mo they again decreased.
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Clear
Plastic
Standpipe
Geotextile
Test
Specimen
How
Column
Flgun 2. Flow rate columns used in Phase ll(a)
and (b) studies.
The third remediation utilized a nitrogen gas
backflush, ft improved flow rates, but 3 mo
later they were once again reduced. The
fourth, and last, remediation was vacuum
extraction, which only nominally improved
flow rates when it was performed. Thereaf-
ter, the flow rate again decreased. The
overall average behavior of the 96 columns
is shown in Figured. It visually describes the
decreasing flow rate trends between
remediations and the rapid increase in flow
rates immediately following remediation. The
individual curves for each of the 96 columns
are given in the full report.
To quantitatively assess the overall per-
formance of the remediation attempts and
their relative performances in contrast to
one another, the data were analyzed with
respect to their percent of flow rate improve-
ment. Within each combination, however,
there were decided differences. For ex-
ample:
(a) Backflushing of geotextiles by them-
selves was more efficient than
backflushing of geotextile/sand sys-
tems. The average recovery efficien-
cies were 29% and 13%, respectively.
(b) With sand overlying a geotextile there
was no measurable difference from
one type of geotextile to another.
(c) With only a geotextile, remediation was
most effective with the woven,
monofilament geotextiles (38% recov-
ery efficiency), slightly less effective
with the nonwoven, needled lightweight
(31%) and the heavyweight (30%)
geotextiles, and relatively ineffective
with the nonwoven, heat-bonded
geotextiles (16%).
(d) When sand is placed overthe geotextile,
there is no difference between anaero-
bic and aerobic remediation schemes.
(e) With only a geotextile, remediation was
slightly better under anaerobic condi-
tions than with aerobic conditions.
(f) When sand is placed over the geotextile,
the remediation recovery efficiency
rankings were: water > nitrogen >
leachate > vacuum
(g) With only a geotextile, the remediation
recovery efficiency rankings were: wa-
ter > leachate > nitrogen > vacuum
Phase ll(b) - Blocide Treated
Geosynthetlcs
Because of the relatively large flow rate
decreases observed in the course of this
study, an attempt at using biocides in the
flow system was undertaken. This was done
under the assumption that the biocide would
kill the microorganisms that come into con-
tact with it and that the nonviable (i.e., dead)
matter would pass through the system in
much the same way that fine particles or
sediment moves through any other filtra-
tion/drainage system. Because we believed
the biocide should be introduced on a long-
term basis rather than as one bulk dose, the
biocide was added to the polymer com-
pound during fabrication of the selected
geonets or geotextiles. The reasoning was
that the biocide would release over time (via
molecular diffusion) through the polymer
structure and migrate to the surface of the
ribs or fibers over a long period of time.
From the results of these biocide treated
geosynthetics, we believe the location of
the biocide vis-a-vis the initial formation of a
biofilm layer was critical. This was con-
firmed at the end of the tests after solidifying
the test columns with epoxy and cutting
them apart. Clearly, the biofilm layer was
occurring at the top of the sand column
some 2 to 3 in. above the biocide-treated
geosynthetics. Although there may have
been some flow rate improvement due to
high concentrations of biocide, it was very
subtle (at best) and was masked by the
inherent scatter in the test data. There was
essentially no difference between flow rates
in anaerobic versus aerobic conditions.
These findings led to additional tests with-
out sand above the biocide-treated
geosynthetic that forced the leachate to
interface directly with the biocide. Rather
than use a single type of geotextile, three
different types were utilized. The opening
sizes varied from 0.15 mm (nonwoven,
needle-punched style), to 0.21 mm (a wo-
ven monofilament), to 0.42 mm (another
woven monofilament). Quite clearly, the
flow rates through the largest opening size
geotextiles, i.e. the 0.42 mm, were the high-
est. This suggests that microorganisms
(dead or alive) must be able to pass through
the system. Whenever these microorgan-
isms reside on or within the small pores of
the filter, partial, or even complete, clogging
is possible.
Conclusions
A simulated field-oriented project con-
cerning biological clogging of landfill drain-
age systems was focused on geotextile
filter clogging. A number of domestic landfill
leachates were employed. The filter was
singled out (versus the geonet drain, drain-
age stone or perforated pipe) since it has
the smallest openings and is likely to be-
come clogged before other components.
Geotextiles were emphasized because they
are relatively new materials for this particu-
lar application.
Phase I results reoriented our initial goals
since the granular soils covering the filters
were clogging before the geotextiles were.
Furthermore, sediment and/or particulates
were a major factor in flow rate reductions,
which appeared to be synergistic with the
biological clogging. Clearly, partial fitter clog-
ging was occurring with a gradual reduction
of flow rate over time. These trends were
common to all six landfill leachates being
used. All of the landfills were domestic (Sub-
title "D") facilities; but their waste stream,
volume of waste deposited, and liquid man-
agement schemes differed. We recognized
early in this Phase I activity that remediation
attempts would be a necessary part of the
overall study, but the Phase I experimental
setup could not accommodate such activi-
ties. New and different test devices would
be needed if such attempts were to be
made. We did, however, draw the following
conclusions from Phase I activities.
• Filter clogging (as indicated by flow rate
reductions) over the 12-mo test period
varied widely, the range being between
10% and complete (i.e., to the limit of our
testing capability).
• A geotextile filter must be relatively open in
its pore structure if it is to limit the amount
of clogging, i.e., the geotextile must be
capable of passing the sediment or par-
ticulates along with the associated micro-
organisms into the down-gradient drain-
age system.
• The polymer type (polypropylene, polyes-
ter or polyethylene) comprising the
geotextile fibers appears to be a nonissue.
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100
Flow Rate
(% Retained)
0 2 4 6 8 10 12 14 16 18 20 22 24
Figure 3. Average response of 96 How rate columns from Phase II (a) activities.
• Both anaerobic and aerobic conditions
promote clogging; the relative amounts,
however, were not capable of being iden-
tified because of differing test setups.
• The strength of the geotextiles was not
adversely affected by the 12-mo expo-
sure to the various leachates. This finding,
coupled with numerous micrographs which
showed no chemical attachment of bacte-
ria to the fibers, led us to conclude that
biological degradation of polymeric based
geotextiles does not occur.
Phase ll(a) of the study saw the develop-
ment of a new and vastly improved test
device for flow rate evaluation. The 4-in.-
diameter flow columns developed during
this project have the following capabilities.
• All types of cross sections can be evalu-
ated: geotextiles by themselves, soil/
geotextile systems, soil/geotextile/geonet
systems, or soil/geotextile/gravel systems.
• Anaerobic or aerobic conditions can be
maintained.
• Flow rates can be evaluated using falling
head or constant head measurements.
• The devices are relatively small and quite
portable. Therefore, they can be stored
indoors and taken to a site for evaluation,
or stored at the site, or even stored within
the leachate storage tank or sump,
• Various methods for remediation of dogged
systems can be evaluated.
• The test devices and their measurement
protocol have recently been adopted as
an ASTM Test Method underthe designa-
tion of D1987-91.
• The test devices and their contents can be
solidified by epoxy and cut in half to visu-
ally observe the conditions existing within
the cross section.
• Since all parts of the device consist of PVC
plumbing and swimming pool accesso-
ries, they are readily available, easily
sealed by chemical wipes, and inexpen-
sive.
The following conclusions were reached
from this Phase ll(a) study.
• Flow rate reductions were similar to the
results of Phase I, and the conclusions
drawn earlier have been substantiated.
• If geotextile and/or soil filters are to be
used in leachate collection systems, they
should have sufficiently open voids to
pass the sediment or particulates along
with the microorganisms contained in the
leachate into the downstream drainage
system.
• The limiting or equilibrium flow rate re-
tained must exceed the site specific de-
sign requirement. If flow rates over time
are not adequate, remediation is neces-
sary. It was found that the water backf lush
technique gave the best results (35% im-
provement), nitrogen gas backf lush (23%),
and leachate backf lush (17%) methods
were next. The vacuum extraction was the
least effective; it provided only nominal
improvement (2%).
• The periodicity of backflushing to open up
a clogged or semi-dogged filter system
appears to be approximately 6 mo.
Incorporating biocides into the geotextile
(or geonet) polymer structure to keep the
flow system open was Phase ll(b) of the
study. The concept was to add various
amounts of a time-released biocide into the
polymer compound as the product was
manufactured — biocide that would essen-
tially diffuse to the surface of the fibers
during its service life. On contact, it would
kill the viable m icroorganisms in the leachate.
In the tests that were conducted on 16
separately built flow columns, some experi-
mental evidence indicated that 2% and 4%
biocide was partially effective. The remains
of the dead bacteria must, however, be
permitted to pass through the system, and
this apparently could not happen for our
particular tests setups. Thus, the idea of a
very open filter system was further rein-
forced.
Recommendations
Based on the major findings of th is project,
namely,
• under continuous flow of landfill leachate
a gradually decreasing flow rate will occur
for all types of filters (soil or geotextile)
and eventually reach an equilibrium value,
• the equilibrium value of flow rate will vary
according to the type of filter, the type of
leachate, and the hydraulic gradient, and
• the equilibrium flow rate for any given
filter system must be compared with the
design required fbw rate to ultimately
assess the adequacy of the filter's de-
sign,
we feel that the following recommendations
should be considered regarding geotextile
and soil filters placed over different types of
leachate collection drains.
(a) Design criteria should be developed
that considers the amount, size, and
type of microorganisms and sediment
present in the leachate along with con-
ventional issues such as hydraulic gra-
dient and type of filter.
(b) Leachate collection systems at land-
fills that are decommissioned or ex-
humed for other reasons should be
investigated in light of the results of this
study.
(c) This particular project should be fol-
lowed by another effort aimed at a
larger variety of geotextile filters along
with design guidance and field perfor-
mance of existing systems.
Thefull report was submitted in fulfillment
of Assistance ID No. CR-814965 by the
Geosynthetic Research Institute of Drexel
University under the sponsorship of the
U.S. Environmental Protection Agency.
•&U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40059
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Robert M. Koemer and George R. Koemer are with Drexel University, Philadelphia, PA
19104.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Landfill Leachate Clogging of Geotextile (and Soil)
Filters," (Order No. PB91- 213 660/AS; Cost: $23.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
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