Clogging of Gravel Drainage Layers Permeated
with Landfill Leachate
Reagan McIsaac1 and R. Kerry Rowe, F.ASCE2
Abstract: Ten flow cells, called mesocosms, are used to investigate the effect of different gravel sizes 共38 and 19 mm兲 and operating
conditions on clogging of leachate collection systems. These mesocosms simulated in real time and real scale the two-dimensional
leachate flow conditions representative of a section of a continuous 300-mm-thick gravel drainage blanket adjacent to a leachate collection
pipe in a primary leachate collection system. The tests were terminated after 6 – 12 years of operation. In some mesocosms the full
300 mm of gravel was saturated. In others, the leachate level was initially set at 100 mm and the upper 200 mm were unsaturated.
Although the flow through all mesocosms was similar, the clogging in the fully saturated gravel was substantially more than in the
partially saturated gravel. After 6 years of operation, typically, less than 10% of the initial pore space was filled with clog material in the
unsaturated gravel. For the saturated zone, 45% of the initial pore space was filled with clog material in the fully saturated design as
compared to only 31% in the partially saturated design. The 38 mm gravel performed much better than the 19 mm gravel. For example,
it maintained a hydraulic conductivity that was higher than the 19 mm gravel even after operating for twice as long. Up to four mesocosms
were placed in series, with the effluent from one mesocosm being the influent for another. The reduction in mass loading within the first
mesocosm reduced the amount of clogging within the mesocosm later in series. There was a clear progression of decreasing amounts of
initial pore space filled with clog material in the last mesocosm in series, and most of the clogging was due to the vertically percolating
leachate.
DOI: 10.1061/共ASCE兲1090-0241共2007兲133:8共1026兲
CE Database subject headings: Municipal wastes; Landfills; Hydraulic conductivity; Service life; Biofilm; Calcium carbonate;
Drainage; Clogging.
Introduction
Leachate collection systems are a critical component of barrier
systems in today’s landfills. They control the leachate head acting
on the landfill base liner and collect and remove contaminants,
hence, minimizing contaminant impact on the environment. Modern leachate collection systems typically are comprised of a network of perforated high-density polyethylene 共HDPE兲 leachate
collection pipes embedded in a continuous drainage blanket of
uniformly graded granular material covering the landfill base
liner. Field evidence has shown that voids within the granular
leachate collection layer become filled with clog material as a
result of the growth of biomass, the bio-induced chemical precipitation of inorganic matter 共predominantly calcium carbonate兲 and
the accumulation of particulate matter 共Brune et al. 1994; Fleming et al. 1999; Maliva et al. 2000; Bouchez et al. 2003; Levine
et al. 2005兲. As the leachate collection layer clogs, the porosity
1
Ph.D. student, Dept. of Civil Engineering, Univ. of Western Ontario,
London, Ontario, Canada N6A 5B9
2
Professor, Geoengineering Centre at Queen’s—RMC, Queen’s Univ.,
Ellis Hall, Kingston, Ontario, Canada, K7L 3N6 共corresponding author兲.
E-mail: [email protected]
Note. Discussion open until January 1, 2008. Separate discussions
must be submitted for individual papers. To extend the closing date by
one month, a written request must be filed with the ASCE Managing
Editor. The manuscript for this paper was submitted for review and possible publication on September 11, 2006; approved on February 12, 2007.
This paper is part of the Journal of Geotechnical and Geoenvironmental
Engineering, Vol. 133, No. 8, August 1, 2007. ©ASCE, ISSN 10900241/2007/8-1026–1039/$25.00.
and the hydraulic conductivity can be reduced to the point where
the leachate head on the liner can no longer be controlled to the
design level 共typically, 0.3 m兲 共Rowe et al. 2004兲, thus shortening
the period of effective functioning of the leachate collection system. Since leachate collection systems may be required to collect
and remove leachate for extended periods of time, it is important
to be able to design them to minimize the clogging process and
prolong their long-term performance and service life.
The potential for clogging of many different design configurations used in practice is not fully understood. To investigate the
effect of different-sized drainage materials and operating conditions on clogging of the gravel drainage material, experiments
were initiated 共Fleming 1999; Fleming and Rowe 2004兲 to examine in real time and real scale the two-dimensional leachate flow
conditions representative of a section of a continuous granular
blanket adjacent to a leachate collection pipe in a primary
leachate collection system. Details regarding the design and early
operation of these cells 共called mesocosms兲 are given by Fleming
and Rowe 共2004兲. Relevant to this paper are four different design
configurations 共Table 1兲 involving a 300-mm-thick drainage
gravel layer. For all cases examined in this paper there was no
filter separator between the waste and the drainage gravel. The
effect of a filter has been reported by McIsaac and Rowe 共2006兲.
The objective of this paper is to examine the effect of grain
size, unsaturated versus saturated conditions, and mass loading on
the clogging of the gravel at the time of the termination of these
tests. Particular attention will be paid to the distribution of clog
material in the gravel 共especially over the last year of operations兲.
1026 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
Table 1. Summary of Mesocosm Design Variable and Duration of Operation. All Mesocosms Had a 300 mm Gravel Drainage Layer and a Waste Layer
Directly over the Gravel 共No Separator Filter兲
Test duration
Mesocosm
Gravel size
共mm兲
Leachate level
共mm兲
Leachate
influent
Leachate
effluent
C-03-38-PS-1
38
100
Fresh
To C-23
C-04-38-PS-1
38
100
Fresh
To C-26
C-19-19-PS-1
19
100
Fresh
Discharged
C-20-19-PS-1
19
100
Fresh
Discharged
C-23-38-PS-2
38
100
Second in series
To C-24
C-26-38-PS-2
38
100
Second in series
Discharged
C-24-38-PS-3
38
100
Third in series
To C-25
C-25-38-PS-4
38
100
Forth in series
Discharged
C-27-38-S-1
38
300
Fresh
Discharged
C-28-38-S-1
38
300
Fresh
Discharged
Note: C-28-38-S-1 was disassembled by Fleming 共1999兲 and details are reported by Fleming and Rowe 共2004兲.
Materials and Methods
Mesocosm Fabrication and Materials
The mesocosms 共Fig. 1 and Table 1兲 were fabricated 共Fleming
and Rowe 2004兲 from welded 9-mm-thick PVC sheeting and
were built at a large enough scale 共internal dimensions measuring
565 mm in length, 235 mm in width, and 574 mm in height兲 to
simulate 共at full scale, in real time, and with materials typically
used in practice兲 the last 0.5 m of a continuous granular blanket
adjacent to a leachate collection pipe in a primary leachate collection system. The collection system design generally consisted
of waste material overlying a 300-mm-thick gravel drainage layer
of crushed dolomitic limestone overlying a nonwoven geotextile/
sand cushion graded at 1.5% to a half section of PVC perforated
pipe.
The waste material was a mixture of refuse and cover soil
taken from auger boreholes in an area of the City of London
W12A Landfill Site and was 5 – 10 years in age at the time of
sampling. The 38 mm gravel had a D10 = 20 mm, D60 = 27 mm,
and D85 = 33 mm, an average initial porosity of 0.43, and an initial
hydraulic conductivity of 0.78 m / s. The 19 mm gravel had a
Days
Years
4,615
2,207
2,252
2,200
2,280
2,202
2,278
2,274
2,238
586
12.6
6.1
6.2
6.0
6.2
6.0
6.2
6.2
6.1
1.6
D10 = 10 mm, D60 = 16 mm, and D85 = 19 mm, and an average initial porosity of 0.37. The PVC perforated pipe had an internal
pipe diameter of 102 mm, two rows of perforations, perforation
diameter of 15.9 mm, and perforation spacing along the pipe of
127 mm.
The mesocosm nomenclature 共Table 1兲 summarizes the case.
For example, C-24-38-PS-3 corresponds to mesocosm C-24 with
38 mm gravel, was partially saturated 共PS兲, and was the third
mesocosm 共3兲 in series. Eight mesocosms used 38 mm gravel,
two used 19 mm gravel, two operated with the 300 mm gravel
layer fully saturated 共S兲, and eight 共PS兲 with the bottom 100 mm
of the gravel saturated and the top 200 mm unsaturated. Some
mesocosms were placed in series with the effluent from one mesocosm being the influent for another with a maximum of four in
series 共Table 1兲.
The mesocosms in series were placed in line with a short
length of tubing connecting the effluent port from one mesocosm
to the influent port of the next in series. Four mesocosms 共C-0338-PS-1, C-23-38-PS-2, C-24-38-PS-3, C-25-38-PS-4兲 comprised
one series and two mesocosms 共C-04-38-PS-1, C-26-38-PS-2兲
comprised the other series.
Fig. 1. Schematic of experimental mesocosm cells and the prescribed interval spacing and location over which wet mass measurements were
made within the mesocosms at termination 共adapted from Fleming and Rowe 2004兲
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1027
Mesocosm Operation
The leachate used in the mesocosms was collected from the Keele
Valley Landfill 共KVL兲 共for details, see Rowe 2005兲. The leachate
was collected from a manhole on the main header line at the
downstream end of the leachate collection system.
Keele Valley Landfill leachate was introduced at rates representative of field conditions in two dimensions. The vertical rate
of approximately 73 mL/ day corresponded to an infiltration of
about 200 mm/ year. The horizontal flow of 3,456 mL/ day
共1.26 m3 / yr兲 was selected to simulate the average horizontal flow
in the drainage layer near the collection pipe corresponding to the
same vertical infiltration rate over a 25 m drainage path to the
collection pipe. Tests were conducted at 27± 2 ° C to simulate the
conditions anticipated in an active leachate collection system.
One mesocosm 共C-28-38-S-1兲 was terminated after 1.6 years
of operation and results were reported by Fleming and Rowe
共2004兲. Nine of the mesocosms were terminated after 6 years of
operation and one 共C-03-38-PS-1兲 after 12 years of operation.
Experimental Analysis
Operational Testing
A testing program was implemented to monitor and quantify the
amount of clogging and changes in leachate composition both
temporally and spatially. Water quality testing was performed on
leachate samples collected from before the influent valve and
after the effluent valve using test methodologies described by
McIsaac 共2007兲. These samples were tested immediately to obtain
chemical oxygen demand 共COD兲, calcium 共Ca2+兲 concentration,
and pH. Tests were performed to follow the change in drainable
porosity, and hence, the change in void volume, with time as
clogging developed. The measured drainable porosity is the ratio
of the volume of the leachate removed to the total volume of the
drained interval. The drainable porosity will be lower than the
actual porosity because of incomplete draining of the leachate
under gravity due to fluid adhering to the drainage medium and
clog material. Drainable porosities were measured over the discrete intervals shown in Fig. 2.
Termination Testing
Termination of the mesocosms allowed for the inspection of the
degree of clogging that occurred over their operational lifespan.
The clogged drainage gravel was removed and stripped of clog
material over the intervals shown in Fig. 1 and the total mass of
wet clog material was measured for each section. This mass includes biological matter, chemical precipitates, and fines that had
accumulated to form the clog material. The bulk density of the
clog material was measured using ASTM D 854 共ASTM 1998兲.
Based on the mass of clog removed from the disassembled mesocosms, the bulk density of the clog material, and the initial void
volume in each sample section, the volume of clog within each
section and the resulting void volume occupancy 共VVO兲 was calculated. The VVO is the ratio of the volume of pore space occupied by clog material to the initial void volume. Thus, a VVO of
100% would indicate that the initial void volume is completely
filled with clog material. Clog samples were sent for elemental
analysis. Hydraulic conductivity testing was performed on the
clogged gravel from C-03-38-PS-1 and C-20-19-PS-1. At termination, the lid of the mesocosm was removed from C-03-38-PS-1
and the unsaturated gravel layer was removed by hand. Saran
Wrap was placed over the surface of the saturated gravel and wax
was poured over the Saran Wrap to fill in the open void space at
Fig. 2. Vertical profiles through the mesocosms showing the interval
spacing and location over which drainable porosities were measured
within the mesocosms: 共a兲 38 mm gravel 共C-03-38-PS-1, C-04-38PS-1兲; 19 mm gravel 共C-19-19-PS-1, C-20-19-PS-1兲; second in series
共C-23-38-PS-2, C-26-38-PS-2兲; third in series 共C-24-38-PS-3兲; and
fourth in series 共C-25-38-PS-4兲; 共b兲 fully saturated 共C-27-38-S-1,
C-28-38-S-1兲
the gravel–lid contact and to build up a surface that was level on
which to place the lid. The lid was modified to fit inside the
mesocosm. The lid was placed into the mesocosm while the wax
was still warm so it would cure with the lid creating a tight seal
with the lid. The hydraulic conductivity was obtained knowing
the specified flow and the measured head difference between piezometers spaced at 100 mm intervals along the mesocosm. A
solid piece of clogged 19 mm gravel was removed from the first
120 mm of the saturated layer of C-20-19-PS-1 and placed into a
permeameter for hydraulic conductivity testing. Wax was used to
seal the sample to the inside of the permeameter and to prevent
short circuiting between the wall of the permeameter and the
clogged sample.
Influent and Effluent Leachate Characteristics
The leachate from Keele Valley was highly variable during the
period of testing. The change in the influent COD, Ca2+ concentration, and pH was monitored and is shown in Fig. 3, and reflects
the natural variability of leachate characteristics with time. The
strength of the leachate from samples collected from the constantly circulated storage tanks, the supply manifold, and from the
ends of the pump lines feeding the mesocosms are representative
of the influent strength entering the mesocosms. From
0 to 675 days laboratory feedstock data used to represent the mesocosm influent strength were obtained from Fleming 共1999兲.
From 675 to 1,500 days data were obtained from Armstrong
共1998兲. Subsequent data were obtained from McIsaac 共2007兲. The
leachate supplied to each operating mesocosm originated from the
same source.
During the operation of the majority of the mesocosms
共0 – 2,250 days兲, the leachate had a high organic strength 共COD
values兲 indicative of a young leachate. The organic strength was
mainly in the form of volatile fatty acids 共McIsaac 2007兲. After
2,775 days, when mesocosm C-03-38-PS-1 was the only one in
1028 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
Fig. 3. COD and Ca concentrations and pH values within influent leachate to the mesocosms 关data up to 675 days from Fleming 共1999兲; data
from 675 to 1,500 days from Armstrong 共1998兲兴
operation, the concentration of key raw leachate constituents from
the Keele Valley Landfill was lower than in previous years likely
due to treatment of the leachate in the KVL leachate collection
system before it reaches the collection sump 共Rowe and VanGulck 2003兲. Since the leachate collected had been subjected to
significant treatment it was not representative of leachate entering
the collection system. In order to have a composition 共especially
in key components such as COD and Ca2+兲 similar to earlier
Keele Valley Landfill leachate, the leachate feedstock for Mesocosm C-03-38-PS-1 was spiked between 2,826 and 3,950 days
共McIsaac 2007兲. After April 2004 共3,950 days兲 nonaugmented
“raw” Keele Valley Landfill leachate was delivered to Mesocosm
C-03-38-PS-1.
In general, the effluent pH values remained relatively constant
at about 7.5 despite variations in the influent pH 共Fig. 3兲. The
organics in the leachate degraded quickly as it passed through the
mesocosms. Only a short length of gravel 共565 mm兲 partially occluded with biofilm was required to cause a significant reduction
in the COD and Ca concentrations in the effluent leachate. In
Mesocosms C-03-38-PS-1 and C-04-38-PS-1, with 38 mm gravel,
the average COD and Ca concentrations in the effluent were 20
and 17% of that in the influent, respectively. The drop was higher
for the mesocosms filled with 19 mm gravel 共C-19-19-PS-1 and
C-20-19-PS-1兲 with the average COD and Ca concentrations in
the effluent being 17 and 9% of that in the influent, respectively.
After the first 100 days this reduction in COD and Ca concentration in the effluent leachate remained at these levels 共except as
noted below兲 for the remainder of the tests. An exception to this
generalization occurred at times when the partially saturated columns had much higher than normal influent COD concentrations
共Fig. 3兲. At these times there was less relative change in leachate
concentration with the effluent concentration for Mesocosms
C-04-38-PS-1 and C-03-38-PS-1 being 41–65% of the influent
concentration. This is likely the result of the inability of the mixed
population of bacteria to adapt to the increased concentrations
within a short period of time. In contrast, the fully saturated
gravel layer of C-27-38-S-1 was better able to cope with the large
fluctuations in influent COD concentrations, and the ratio of effluent to influent COD typically remained below about 30% during the period of high influent concentration. This is likely due to
the much greater amount of biomass per unit volume of leachate
and the longer retention times for leachate within a fully saturated
gravel layer that had the same flow rate as the partially saturated
mesocosm. This also corresponded to substantially more clog
mass being accumulated within the fully saturated gravel drainage
layer. Similarly, Mesocosms C-19-19-PS-1 and C-20-19-PS-1 that
were filled with the 19 mm gravel had normalized COD values
that were less variable than in the mesocosms filled with 38 mm
gravel 共C-03-38-PS-1 and C-04-38-PS-1兲 with average ratios of
effluent to influent concentration of 16% for COD during periods
of high influent concentration. The 19 mm gravel has a much
greater surface area per unit volume than the 38 mm gravel and
this provides a much greater surface area for biofilm growth. This
allows more exposure of the leachate to active biomass, and
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1029
Fig. 4. Distribution of wet solids within the mesocosms for partially saturated 19 mm gravel 共C-19-19-PS-1兲 after 6 years, saturated 38 mm
gravel 共C-27-38-S-1兲 after 6 years, and partially saturated 38 mm gravel after 6 years 共C-04-38-PS-1兲 and 12 years 共C-03-38-PS-1兲
hence more leachate treatment and greater clogging. An increase
in the variability of the normalized values was observed near the
end of the tests for the 19 mm gravel and this was hypothesized to
be because, at this time, the majority of the saturated void volume
was filled with dense inorganic clog and the shearing of biofilm
reduced the amount of active biomass in the voids. This hypothesis was confirmed at the time of disassembly where the clog
mass was found to be substantially drier and denser with substantially less soft biofilm filling the voids than observed for the mesocosms filled with 38 mm gravel.
The variations in the water quality of the leachate as it passed
through the mesocosms are consistent with the leachate chemistry
study by Rittmann et al. 共1996兲. An environment conducive to
clog development or the precipitation of CaCO3 is established
within the leachate as it passes through the drainage material.
Rittmann et al. 共1996兲 showed that the loss in COD was primarily
due to fermentation of acetic acid to carbonic acid. This resulted
in a shift in pH to higher values, which together with the increased carbonate concentration, promoted the development of
inorganic clog material.
Fully Saturated Conditions
Fig. 4 shows the mass of wet solids for four mesocosms and the
drainable porosities for all mesocosm are given in Figs. 5 and 6.
The VVO within the mesocosms is given in Table 2. A comparison of the results for Mesocosms C-04-38-PS-1 and C-27-38-S-1
shows that the fully saturated gravel yielded much greater clog
development and lower porosity than in the partially saturated
mesocosm. For example, for the 100–180 and 180– 260 mm intervals, 29 and 26% 共respectively兲 of the voids were filled with
clog material for the saturated gravel 共C-27-38-S-1兲 as compared
to only 7 and 9% for the unsaturated gravel 共C-04-38-PS-1兲.
Higher VVOs were also measured within the 0 – 100 mm interval
of the fully saturated gravel layer design than in the corresponding saturated zone of the mesocosms that were operated partially
saturated. The VVO was 87 and 55% within the 0–50 and
50– 100 mm intervals, respectively, for saturated Mesocosm
C-27-38-S-1 versus 76 and 40% within Mesocosm C-04-38-PS-1.
Fully saturated Mesocosms C-28-38-S-1 and C-27-38-S-1
were identical except that times of termination were 1.6 and
6.1 years, respectively. Within 1.6 years the gravel section from
0 to 70 mm was largely filled with clog material with a VVO of
92%. An additional 4.5 years of operation resulted in considerably higher amounts of clog throughout the remainder of the
gravel thickness 共see C-27-38-S-1 in Table 2兲. The VVO within
the middle of the mesocosms from 50 to 260 mm ranged from 12
to 17% after 1.6 years of operation to 26 to 55% after 6.1 years of
operation.
The total flow was the same through the 100 mm saturated
zone in C-04-38-PS-1 and the fully saturated 300 mm in C-2738-S-1 while the volume of pores through which the leachate
originally flowed was about three times larger for the fully saturated case. This appeared to result in less localized solid
共cemented兲 clogging but more soft clog in the fully saturated
mesocosm. For the fully saturated gravel the voids in the bottom
75 mm visibly appeared to be completely filled 共100% visual
VVO兲 with predominantly gelatin-like soft clog. However, this
soft clog had a porous structure that explains the calculated VVO
of 87% within the 0 – 50 mm interval. This was attributed to less
competition from the development of inorganic clog for void
space along with lower induced shear stresses on the active soft
biofilm from the flowing leachate than for the 100-mm-thick saturated mesocosm. Thus while the clog in the fully saturated gravel
was softer and porous, there was much more clogging than for the
mesocosms run with only 100 mm saturated.
From 25 to 75 mm in the fully saturated gravel layer the clog
1030 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
Fig. 5. Drainable porosities in the mesocosms with a 38 mm drainage gravel: 共a兲 C-04-38-PS-1; 共b兲 C-03-38-PS-1; and 19 mm drainage gravel
共c兲 C-19-19-PS-1; and 共d兲 C-20-19-PS-1
material was predominately black, gelatin like, and soft. From
0 to 25 mm the clog material was a light tan color and was drier
and less viscous than the black biofilm 共Fig. 7兲. The light tan
color and change in texture of the clog material are likely a result
of the accumulation of material and siltation from the waste layer
due to the absence of an effective filter between the waste and the
gravel as indicated by the high silicon and aluminum content. The
amount of Si was 12.89%/dry and Al was 3.00%/dry in the
0 – 50 mm interval versus concentrations less than Si= 1.22%/dry
and Al= 0.25%/dry when a nonwoven geotextile filter was used
between the waste and the gravel 共McIsaac and Rowe 2006兲. The
Si and Al content of the fully saturated Mesocosm C-27-38-S-1
was also significantly higher than in Mesocosm C-04-38-PS-1
共Table 3兲. This is likely due to a periodic increase in the leachate
level during normal operation from a gas lock or periodic clogging of the effluent valve for example, that would cause leachate
to enter into the waste layer and likely facilitate the rinsing of
particulate matter from the waste material and the accumulation
within the base of the drainage gravel layer.
Greater amounts of biofilm growth occurred within the
100– 300 mm interval of the drainage gravel in the mesocosms
operating under fully saturated conditions than compared to the
same interval when maintained unsaturated. From 100 to 300 mm
a 3 – 5-mm-thick layer of biofilm growth localized on the top
lateral surface of all the gravel particles for the saturated mesocosm as shown in Fig. 8共a兲. This accounted for a large portion of
the clog occluding the voids 共VVO= 29% 兲. In contrast, for the
mesocosm where this zone was unsaturated, there was only a thin,
typically, less than 2 mm, film and a VVO of less than 9% with a
sporadic distribution of biofilm growth on the gravel 关Fig. 8共b兲兴.
This is considered to be because saturated conditions are more
conducive to biofilm growth and accumulation than unsaturated
conditions. When the gravel is saturated, the organic components
in the leachate are distributed throughout the 100– 300 mm intervals and there are sufficient nutrients to support the growth of
biofilms throughout the gravel and the contact time between the
bacteria and the leachate is far greater than when unsaturated.
These results highlight the improved performance of the
gravel drainage layer in terms of reduced clogging when the layer
is not allowed to saturate and hence the importance of pumping
leachate regularly rather than allow it to build up in the gravel
drainage layer.
Unsaturated Conditions
The amount and distribution of clog material within the unsaturated gravel in designs operating partially saturated was significantly different than in the saturated gravel layers discussed
above. Very little clogging occurred within the unsaturated gravel
layers of the mesocosms run for 6 years and the VVO, was typi-
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1031
Fig. 6. Drainable porosities in the mesocosm operating fully saturated: 共a兲 C-27-38-S-1, second in series; 共b兲 C-23-38-PS-2; 共c兲 C-26-38-PS-2,
third in series; 共d兲 C-24-38-PS-3, fourth in series; and 共e兲 C-25-38-PS-4
cally, less than 10% within the unsaturated layers from elevation
100 to 260 mm. Even after 12.6 years of operation, very little
clog had developed in the unsaturated gravel and the VVO was
12% in the unsaturated interval between 180 and 260 mm. The
VVO of 32% in the 100– 180 mm interval for Mesocosm C-0338-PS-1 arose because clogging of the layer from 0 to 100 mm
had caused the leachate level to rise and the interval
100– 180 mm was partially saturated at later times giving rise to
the substantially greater clogging of this zone in the test run for
12.6 years than for C-04-38-PS-1, which was run for 6.1 years.
Biologically induced clogging on the unsaturated gravel was not
uniform. Very little solid inorganic clog was observed on the un-
saturated gravel and the clog material was predominantly biofilm.
The black biofilm growth was localized to the top of relatively
horizontal surfaces of the unsaturated gravel while the bottom
surfaces remained relatively pristine. Clog did not develop on the
actual particle-to-particle contacts but black biofilm did grow
where the interface gap between two particles was between about
1 and 3 mm. This is attributed to the retention of leachate due to
a capillary fringe between the particles providing an environment
in which biological growth could occur. Once the void distance
between two particles was greater than 2 – 3 mm the capillary
action ceased. This highlights the benefit of using large uniform
particles such that the amount of retained moisture in the unsat-
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Table 2. Void Volume Occupancy within the Mesocosms
Mesocosm
Time
at termination
共years兲
Interval 共mm兲
260–300
180–260
100–180
50–100
0–50
260–270
150–260
150–210
75–150
0–70
C-19-19-PS-1 共19 mm gravel兲
6.0
69
18
17
39
62
—
—
—
—
—
C-27-38-S-1 共fully saturated兲
6.1
51
26
29
55
87
—
—
—
—
—
C-03-38-PS-1 共first in series兲
12.6
51
12
32
63
98
—
—
—
—
—
C-04-38-PS-1 共first in series兲
6.1
56
9
7
40
76
—
—
—
—
—
C-26-38-PS-2 共second in series兲
6.0
38
8
7
16
48
—
—
—
—
—
C-23-38-PS-2 共second in series兲
6.2
21
8
7
15
43
—
—
—
—
—
C-24-38-PS-3 共third in series兲
6.2
22
11
9
12
21
—
—
—
—
—
C-25-38-PS-4 共fourth in series兲
6.2
34
11
8
10
15
—
—
—
—
—
C-28-38-S-1 共fully saturated兲
1.5
—
—
—
—
—
17
—
12
16
92
Note: C-28-38-S-1 was disassembled after 1.6 years by Fleming 共1999兲 and details are reported by Fleming and Rowe 共2004兲. Unless otherwise noted, all
mesocosms had 300 mm of 38 mm gravel in the drainage layer and the design allowed the lowest 100 mm to be saturated and the remainder was
unsaturated.
Fig. 7. Accumulation of soft clog material surrounding a gravel
particle removed from the base of the fully saturated mesocosm
共0 – 50 mm sample interval兲. Light brown discoloration of the
typically black biofilm is due to particulate matter that was
transported in from the waste layer.
urated zone is minimized by minimizing the zones where a capillary fringe can develop.
Only a fraction of the total surface area of the unsaturated
gravel layer was available for leachate retention and biofilm
growth. The percentage of the surface area of the unsaturated
gravel covered with biofilm increased from approximately
30± 10% coverage within the upper 共180– 260 mm兲 to 50± 10%
coverage in the lower 共100– 180 mm兲 sections of the unsaturated
gravel layer. This is much lower than the 100% biofilm coverage
within the saturated gravel layer. The increase in the amount of
biofilm on the unsaturated gravel with depth coincides with a
greater distribution of leachate 共and nutrients兲 observed on the
gravel with depth. The sporadic distribution of active biofilm in
the unsaturated gravel limits the degree of contact between the
bacteria and the leachate as the leachate flows through the unsaturated gravel and thus limits biologically induced clogging under
unsaturated conditions. The amount of calcium in the clog material in the upper unsaturated gravel 共13.1%/dry兲 was substantially
less than in the lower unsaturated gravel 共27.8%/dry兲, indicating
Table 3. Composition of Clog Removed from the Mesocosms with 19 and 38 mm Gravel after 6 Years
Parameter
38 mm gravel 19 mm gravel 19 mm gravel Fully saturated Fully saturated 2nd in series 3rd in series
4th in series
C-04-38-PS-1 C-19-19-PS-1 C-19-19-PS-1 C-27-38-S-1
C-27-38-S-1 C-23-38-PS-2 C-24-38-PS-3 C-25-38-PS-4
lower
upper
lower
lower
upper
lower
lower
lower
midsaturated
saturated
saturated
saturated
saturated
saturated
saturated
saturated
gravel layer
gravel layer
gravel layer
gravel layer
gravel layer
gravel layer
gravel layer
gravel layer
Water content 共%/wet兲
74.5
Organic matter 共TVS;%/dry兲
14.9
45.75
Carbonate as CO3 共%/dry兲
Calcium, Ca 共%/dry兲
25.63
Magnesium, Mg 共%/dry兲
2.24
Silicon, Si 共%/dry兲
0.79
Iron, Fe 共%/dry兲
5.10
Sodium, Na 共%/dry兲
0.85
Aluminum, Al 共%/dry兲
0.19
Potassium, K 共%/dry兲
0.42
Phosphorus, P 共%/dry兲
0.11
Titanium, Ti 共%/dry兲
0.01
Manganese, Mn 共%/dry兲
0.02
Strontium, Sr 共mg/kg兲
1,000
Barium, Ba 共mg/kg兲
120
0.560
Ca/ CO3
Note: TVS total volatile solids.
73.46
6.06
45.42
22.87
4.60
2.25
3.36
0.56
0.49
0.37
0.10
0.02
0.07
1,230
130
0.504
69.18
5.35
49.51
24.94
4.84
1.15
3.15
0.61
0.30
0.33
0.09
0.02
0.10
1,050
120
0.504
43.9
8.2
32.67
16.19
3.84
12.89
2.73
0.89
3.00
1.22
0.08
0.19
0.24
313
262
0.496
62.5
13.0
36.38
18.07
3.71
8.88
3.64
0.93
2.22
1.05
0.09
0.14
0.18
496
245
0.497
80.62
19.63
45.15
21.30
5.16
1.77
4.15
1.05
0.38
0.53
0.11
0.02
0.12
470
100
0.472
71.92
10.64
43.10
20.94
4.90
6.59
2.28
0.77
1.24
0.63
0.07
0.06
0.08
360
140
0.486
78.89
14.89
41.19
20.01
4.84
6.78
3.11
0.88
1.49
0.73
0.08
0.08
0.09
470
200
0.486
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1033
Fig. 8. Gravel after 6 years operation from 100 to 180 mm interval for: 共a兲 fully saturated; 共b兲 unsaturated conditions. Note that the fully
saturated conditions created an environment conducive to biological growth on the gravel and the development of thick biofilms evident in 共a兲.
also that very little biologically induced clogging was likely occurring in the upper unsaturated gravel layers. The clog material
removed from the upper unsaturated gravel layer of C-03 had
substantially more Si 共17.1%/dry兲 and Al 共3.5%/dry兲 than in the
lower unsaturated layer 共Si= 4.32%/dry and Al= 1%/dry兲, which
is likely from the accumulation of fines rinsed from the waste
material on the top lateral surfaces of the gravel. The composition
of the clog removed from the lower unsaturated gravel layer of
C-03-38-PS-1 共Table 4兲 does indicate that the clog that has developed is similar in composition as that in the saturated gravel
indicating that similar biologically induced clogging is occurring
in the lower unsaturated gravel as is in the saturated gravel. It was
also observed at termination that some of the gravel particles
were covered with a thin layer of leachate but not a layer of
biofilm. This suggests that the leachate flow pattern through the
unsaturated gravel layer is not constant with time. The low flow
rate combined with a transient flow pattern does not provide a
constant supply of leachate 共nutrients兲 to sustain active biological
growth over the entire surface area of the particles in the unsaturated gravel layer.
As identified in column studies by Rowe et al. 共2000兲, leachate
contaminant mass loading has a significant impact on the rate and
extent of clogging. Reducing the mass of nutrients and inorganic
material for biological activity and precipitation reduced clogging. Compared to the saturated gravel layer the unsaturated
gravel layer of a leachate collection system operates under lower
flow rates and as a result the mass loading in terms of flow rate
are lower than in the saturated gravel layers adjacent to a leachate
collection pipe. Thus, the relative lack of clog material within the
unsaturated gravel of the mesocosms versus the large amount of
Table 4. Composition of Clog Removed from the Mesocosms 共C-03-38-PS-1兲 with 38 mm Gravel after 12.6 years
Parameter
Water content 共%/wet兲
Organic matter 共TVS;%/dry兲
Carbonate as CO3 共%/dry兲
Calcium, Ca 共%/dry兲
Magnesium, Mg 共%/dry兲
Silicon, Si 共%/dry兲
Iron, Fe 共%/dry兲
Sodium, Na 共%/dry兲
Aluminum, Al 共%/dry兲
Potassium, K 共%/dry兲
Phosphorus, P 共%/dry兲
Titanium, Ti 共%/dry兲
Manganese, Mn 共%/dry兲
Strontium, Sr 共mg/kg兲
Barium, Ba 共mg/kg兲
Ca/ CO3
Note: TVS total volatile solids.
Upper
unsaturated
gravel layer
Lower
unsaturated
gravel layer
Upper
middle
saturated
gravel layer
Lower
influent
saturated
gravel layer
Lower
middle
saturated
gravel layer
Lower
effluent
saturated
gravel layer
Pipe
left-hand
corner
Pipe
right-hand
corner
65.77
8.50
24.82
13.08
2.99
17.06
2.86
1.27
3.47
1.40
0.09
0.20
0.05
300
340
0.527
57.46
8.50
45.42
27.80
1.71
4.32
1.47
0.69
0.99
0.49
0.18
0.06
0.02
380
190
0.612
50.73
8.82
44.47
27.23
1.47
4.55
3.08
0.65
0.92
0.46
0.11
0.05
0.10
1,200
220
0.612
33.30
6.90
52.38
30.52
1.83
0.86
4.13
0.34
0.15
0.16
0.13
0.01
0.20
1,030
150
0.583
57.94
10.04
48.29
27.52
1.65
1.44
4.41
0.66
0.38
0.34
0.10
0.02
0.18
1,330
180
0.570
59.90
9.95
44.47
27.02
1.68
4.23
3.37
0.74
0.76
0.51
0.16
0.04
0.09
860
200
0.608
77.39
3.79
52.51
34.45
1.15
1.39
1.00
0.47
0.42
0.25
0.30
0.02
0.02
450
190
0.656
77.39
15.00
34.51
21.08
1.12
3.69
6.99
1.22
0.85
0.81
0.12
0.04
0.09
1,200
210
0.611
1034 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
Table 5. Clog Densities 共Mg/ m3兲 within the 19 and 38 mm Gravel
19 mm gravel
38 mm gravel
Interval
共mm兲
Influent
Middle
Effluent
Influent
Middle
Effluent
50–100
0–50
1,709
1,483
1,799
1,468
1,483
1,438
1,300
1,574
1,281
1,164
1,428
1,235
clogging found within the saturated gravel is attributed to the
lower leachate mass loading to the unsaturated gravel layer. Also,
McIsaac and Rowe 共2006兲 observed different degrees of clogging,
with the use of different separator-filters and this will also result
in different degrees of leachate treatment before the leachate enters the unsaturated gravel layer. Therefore, the mass loading in
terms of leachate strength 共organic and inorganic concentration兲
entering the unsaturated gravel layers will differ for different
filter–separator designs.
Effect of Particle Size
The unsaturated 19 mm gravel clogged more rapidly than the
38 mm gravel. The 19 mm gravel resulted in more clog mass
共Fig. 4兲, higher VVO values 共Table 4兲, and lower drainable porosities 共Fig. 5兲 within the upper and lower sections of the unsaturated gravel layer than the 38 mm gravel. Due to the smaller size
of the gravel there were more particle to particle contacts where
capillary action could retain the leachate and there were more flat
lateral surfaces within the layer where leachate could pool on the
surface of the gravel. The result was a higher volume of retained
leachate in the unsaturated layer of the 19 mm gravel that allowed
for the formation of clog material, predominantly the growth of
biofilm.
A similar amount of clog mass was removed from the
50 to 100 mm interval of the saturated layer 共Fig. 4兲 and the
drainable porosities were similar 共Fig. 5兲 for both the 19 and
38 mm gravel. However, the density of the clog was, typically,
higher in the 19 mm gravel 共Table 5兲 due to the greater abundance
of cementatious clog material. As a result the VVO value for the
19 mm gravel was marginally less than for the 38 mm gravel
although the mass of clog per unit initial clean void volume was
higher.
Within the 0 – 50 mm interval of the saturated gravel, significantly less clog mass was removed for the 19 mm gravel than the
38 mm 共Fig. 4兲 and the corresponding VVO values were 62 and
76% for the 19 and 38 mm gravel, respectively. However, at disassembly of the mesocosms the clog material from the 19 mm
gravel appeared to be more mature than that from the 38 mm
gravel. Column studies performed by VanGulck and Rowe
共2004兲, which were terminated at different elapsed times showed
clog progressed through different stages. As observed in their
study, initially, biofilm developed quickly on the drainage material, then changed with time to a soft slime then to a slime with
hard particles 共sand-size solid material in a soft matrix兲, and then
to a solid porous concretion of coral like “biorock” structure. At
disassembly of the mesocosms filled with 19 mm gravel the clog
material appeared and felt harder and was absent of a thick layer
of soft active biofilm. The entire surface area of the particles
throughout the saturated gravel 共for both the upper and lower
saturated layers and influent, middle, and effluent sections兲 were
completely covered with hard inorganic clog and the entire saturated gravel layer was cemented together like concrete. Most of
the constrictions within the 19 mm gravel were filled with hard
chemical clog and the voids between constrictions were partially
filled. Some of the voids were filled with hard clog material having its own pore structure and hence secondary porosity. The
19 mm gravel had smaller openings to its remaining voids than
the 38 mm gravel. The accumulation of soft clog was abundant in
the larger voids of the 38 mm gravel. The lower seepage velocities and corresponding lower induced shear stresses due to the
larger constriction openings in the 38 mm gravel allowed for the
accumulation of softer clog than in the 19 mm gravel layer. At the
effluent end of the 38 mm gravel mesocosms, some voids were
visually filled with a thick viscous clog however, unlike the
19 mm gravel, the particles were only lightly cemented together.
The clog material within the first 100 mm of the gravel at the
influent end of the 38 mm gravel mesocosms was more cemented
than at the effluent end but had not reached the same level of solid
cementation as observed in the 19 mm gravel. Not all of the
gravel particles in the influent end of the 38 mm gravel were
completely covered with hard inorganic clog after 6 years. Some
of the voids were filled with 0.5– 2 mm clog material. Thick layers of soft biofilm were present. The cemented 38 mm gravel was
separated by gentle prying with a screwdriver with more effort
required in the influent end. Excessive force was not required to
breakup the clog for the 38 mm gravel, in distinct contrast to the
19 mm gravel as discussed below.
For the mesocosm tests using 19 mm diameter gravel, excessive force applied by a hammer to a chisel was required to break
up the entire cemented gravel layer for sampling 关Fig. 9共a兲兴. Even
after 25 min exposure to mechanical agitation in a sieve shaker,
chunks of concreted gravel remained intact. Cementation within
the unsaturated 19 mm gravel was observed whereas none was
observed for the unsaturated 38 mm gravel 关Fig. 9共c兲兴. Also, for
the 19 mm gravel, less clog mass was required to give rise to
substantial cementation than for the larger 38 mm gravel. Thus
the high surface area, abundant particle-to-particle contact points,
smaller void openings, and shorter distances between individual
particles resulted in a highly cemented gravel with the same or
even less clog mass for the 19 mm gravel than for the 38 mm
gravel within a 6 year period.
Within the saturated gravel, lower VVO 共Table 2兲 were measured within the 19 mm gravel than for the 38 mm gravel yet the
average measured hydraulic conductivity through the first
120 mm of clogged 19 mm gravel was 2.7⫻ 10−5 m / s after only
6 years of operation. This was more than 47% lower than the
5.2⫻ 10−5 m / s measured for the 38 mm gravel after 12.6 years of
operation 共Fig. 10兲. Thus, due to the pore structure and the preferential development of clog at void openings, less clog was required to cause substantial reductions in hydraulic conductivity
for the 19 mm gravel than the 38 mm.
Composition of the clog in the 19 mm gravel was similar to
that in the 38 mm gravel except that more magnesium was measured in the 19 mm gravel clog material 共Table 3兲. The amount of
organic matter in the saturated 19 mm clog material was the lowest of any mesocosm and is due to the relatively uniform intense
mature clog formation throughout the 19 mm gravel discussed
previously.
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1035
Fig. 9. Cemented mass of gravel removed adjacent to the influent port for: 共a兲 19 mm gravel 共C-19-19-PS-1兲; 共b兲 38 mm gravel 共C-04-38-PS-1兲
after 6 years. Note the larger size of the cemented clump under otherwise similar conditions for the 19 mm over than 38 mm gravel. Cementation
of the unsaturated 19 mm gravel was also observed 共c兲.
Based on the foregoing discussion, it can be concluded that the
38 mm gravel performed much better over a 12.6 year period
than the 19 mm gravel did over a 6 year period and, hence,
should be preferred for use as a drainage material for leachate
collections systems that require a long service life.
Mesocosms in Series
Fig. 10. Measured hydraulic conductivities of the clogged gravel: 共a兲
38 mm gravel 共C-03-38-PS-1兲 after 12 years; 共b兲 19 mm gravel 共C20-19-PS-1兲 after 6 years
Fig. 11 shows the distribution of wet mass with distance from the
inlet of the mesocosm for the mesocosms that were in series.
Most of the mass was accumulated within the first 200 mm from
where the raw leachate entered the system. For the 50– 100 mm
saturated interval, the amount of clog mass steadily decreased
until it reached a relatively constant value at about 900 mm from
the inlet. For the 0 – 50 mm saturated layer most of the mass was
contained in the first 1,000 mm from the inlet of the “fresh”
leachate, but there was generally a gradual decline in mass along
the entire 2,200 mm flow path. This was expected given the findings from Rowe et al. 共2000兲 that showed that a reduction in mass
loading reduces clogging. Due to the reduction in mass loading
with distance from the initial inlet, there was a clear progression
of decreasing VVO values in the saturated 100 mm from 58 to
29–32 to 16 to 12% moving from the first to last mesocosm in
series. Very little inorganic hard clog or cementation of the gravel
Fig. 11. Distribution of wet solids in the mesocosms in series 共C-04-38-PS-1, C-23-38-PS-2, C-24-38-PS-3, and C-25-38-PS-4兲
1036 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
was observed after the first mesocosm indicating very little inorganic mass was available to precipitate on the granular medium
for the later series mesocosms. Moving from the first to last mesocosm in series, the percentage of calcium in the clog material
decreased, with percentages ranging from 25.6 to 21.3 to 20.9 to
20.0%/dry for C-04-38-PS-1, C-23-38-PS-2, C-24-38-PS-3, and
C-25-38-PS-4, respectively, but had a higher percentage magnesium 共5.2, 4.9, and 4.8%/dry for C-23-28-PS-2, C-24-38-PS-3,
and C-25-38-PS-4, respectively兲 compared to the mesocosm first
in series 共Ca= 25.6%/dry, Mg= 2.2%/dry in C-04-38-PS-1兲.
The biofilm was less viscous and thinner in the saturated layer
as the distance from the inlet increased. This is likely due to a
decrease in the concentration of organic constituents in the
leachate with distance due to microbial activity in the first
570 mm from the inlet. As discussed earlier, at exit from the first
mesocosm, the organic concentrations in the leachate had dropped
to 18% of the initial influent values. Drainable porosities in the
saturated gravel layer increased from 0.2 in the first mesocosm in
series, to approximately 0.35 in the second mesocosm and greater
than 0.40 in the third and fourth in series 共Fig. 6兲. The leachate
organic load in the mesocosms in series was sufficient to maintain
some growth of biofilm that generally resulted in more clog mass
than in the unsaturated gravel. In the last mesocosm most of the
clogging was due to the vertically percolating leachate. The third
and fourth mesocosms in series had a thin 共approximately 9 mm
thick兲 layer of gritty ooze 共an accumulation of fines and biofilm兲
on the top surface of the base geotextile and is likely due to the
washing of fines from the waste layer in the absence of a
separator-filter between the waste layer and the drainage gravel.
As would be expected, the clogging of the unsaturated zone
was the same in all mesocosms since being in series had no effect
on the leachate reaching the unsaturated zone. This illustrates the
repeatability of the experiments, as does the similarity of results
for duplicated Mesocosms C-26-38-PS-2 and C-23-38-PS-2
共Table 2兲.
Biofilm development within the saturated layers of the mesocosm in series occurred predominantly on the top of the gravel
and at particle-to-particle contacts. This indicates that clog development within the saturated layer is not initially uniform over the
entire surface area of the gravel particles and that the accumulation of fines on the top lateral surfaces of the gravel may promote
accelerated clog development initially. However, eventually clogging extended fully around the gravel in the saturated zone.
It is noted that the clogging observed in these mesocosm tests
over 6 years was less than observed in the field after 4 – 5 years
by Fleming et al. 共1999兲. This suggests that the Keele Valley
leachate used in the mesocosm tests was considerably lower in its
concentration of fatty acids and calcium than the leachate that
must have been flowing into the Keele Valley collection system
from the waste. This hypothesis is consistent with the findings
from the mesocosms in series, which show that there is considerable depletion of fatty acids and calcium with distance as leachate
passes through the drainage layer. This indicates that the use of
the end-of-pipe leachate concentration for predicting collection
system performance may not be conservative and more research
is required to characterize leachate as it enters leachate collection
systems in the field.
Effect of Time
Mesocosm C-03-38-PS-1 operated for twice as long as its duplicate C-04-38-PS-1 and experienced significantly more clog devel-
Fig. 12. Clog in pipe after: 共a兲 6 years 共C-04-38-PS-1兲; 共b兲
12.6 years 共C-03-38-PS-1兲 of operation
opment within certain areas of the drainage gravel. As might be
expected, the additional 6 years of operation had no real affect on
the amount of intruded waste material at the waste/gravel interface in the interval 260– 300 mm 共Fig. 4兲 and similar VVO values
of 56 and 51% were measured in this interval for C-04-38-PS-1
and C-03-38-PS-1, respectively.
Fig. 5 shows a general slow decreasing trend in the drainable
porosity values with time due to the relatively slow continuous
clog development in the interval of 60– 100 mm of gravel that
remained unsaturated for the entire 12 years of operation. For this
unsaturated zone there was a 33% increase in VVO, from 9%
after 6 years to 12% after 12 years, however the clogging in the
unsaturated zone remained well below that in the saturated gravel
where the VVO was 98 and 63% in the lower and upper saturated
gravel of C-03-38-PS-1. Thus, the additional 6 years of operation
resulted in the 0 – 50 mm saturated interval becoming essentially
totally occluded with clog with a VVO of 98% after 12 years
compared to 75% after 6 years. An additional 23% of the void
volume was filled with clog in the 50– 100 mm saturated interval
and an additional 25% in the initially unsaturated 100– 180 mm
layer between 6 and 12 years. The additional 6 years of operation
also resulted in significantly more clog mass within the pipe as
shown in Fig. 12. After 12 years, 75% of the pipe volume was
filled with clog material. Clog material did not develop on the
portion of the pipe above the top perforations 共Fig. 12兲 and it did
not occlude the perforations. Between the top perforation and the
deposit of clog material in the pipe, the pipe wall was coated with
an approximately 4-mm-thick layer of very hard cemented clog
material. The majority of the clog material in the pipe was calcium and carbonate 共Table 4兲. Although the top half of the accumulated clog material in the pipe appeared to be somewhat
homogeneous, the clog material in the bottom half of the pipe was
not. In the left-hand corner of the pipe 共Fig. 12兲 the clog material
was sand sized material in a soft matrix and was denser, drier, and
flat black compared to the other corner 共right-hand side of Fig.
12兲, which was a soft, gelatin, and very glossy black slime.
Hydraulic conductivity measurements were made along the
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007 / 1037
length of C-03-38-PS-1 in the saturated gravel layer just prior to
termination. The average measured hydraulic conductivities from
Section 1 to 6 were 5.2⫻ 10−5, 9.6⫻ 10−5, 1.8⫻ 10−4, 1.4⫻ 10−3,
4.5⫻ 10−4, and 2.8⫻ 10−4 m / s 共Fig. 10兲. The lowest hydraulic
conductivity in the 38 mm gravel was measured at the influent
共Section 1兲 and increased through to Section 4. Sections 5 and 6
had values less than Section 4. The high value obtained in Section
4 coincides with the lower amount of mass measured within the
middle of the mesocosm 共Fig. 10兲 within the 0 – 50 mm saturated
interval. The clog that was deposited in the pipe was sufficient to
cause a reduction in hydraulic conductivity to values similar to
that measured in the clogged gravel directly adjacent to the pipe.
The hydraulic conductivity for the clog material in the pipe 共Section 6兲 was 2.8⫻ 10−4 m / s compared to 4.5⫻ 10−4 m / s in the
clogged gravel adjacent to the pipe 共Section 5兲.
Total clogging of the base of the drainage gravel in C-03-38PS-1 and the bottom portion of the pipe prevented significant flow
of leachate through the bottom perforations in the pipe and resulted in an increase in the leachate level such that it could flow
into the pipe through the top perforations. This change in flow
with the consequent increase in mass loading resulted in the previously unsaturated gravel intervals from 0 to 20 and
20 to 60 mm 共Fig. 5兲 to become saturated and led to an increase
in the clog formation in this zone compared to that at 6 years. The
drainable porosity results for C-03-38-PS-1 共Fig. 5共b兲兲 show that
by approximately 2,500 days 共6.8 years兲 the saturated gravel intervals 共−20 to 0 mm and −40 to − 20 mm兲 had reached drainable
porosity values of less than 0.1. Beginning around 2,100 days
共5.7 years兲 the measured drainable porosity in the initially unsaturated zone from 0 to 20 mm, just above the initially saturated
zone, decreased from about 0.35 to 0.20 in roughly 500 days
共1.4 years兲 as the zone became saturated and mass loading increased due to the preferential flow of leachate through this more
permeable gravel. As further clogging developed in the saturated
zone the leachate level continues to rise and so the initially unsaturated zone from 20 to 60 mm experience a rapid decrease in
drainable porosity over a period of 2.6 years from a relatively
unclogged value of approximately 0.40 at 2,550 days 共7 years兲 to
0.20 at 3,500 days 共9.6 years兲. Very little clog developed within
the drainage gravel after approximately 10.3 years 共3,750 days in
Fig. 5兲 due to the low strength of the leachate supplied to C-0338-PS-1 after this time.
Clog samples were retrieved from many locations within
Mesocosm C-03-38-PS-1 to allow an identification of the spatial
distribution of clog composition with the mesocosm. The upper
saturated layer had percentages of Si and Al of 4.5 and 0.9%/dry,
respectively, that were higher than the corresponding values of
1.4 and 0.4%/dry in the lower saturated layer. This indicates that,
in the absence of a filter separator, the fines transported from the
waste material above the drainage gravel could potentially be
trapped in the soft biofilm in the upper saturated layers. It was
also found that the percentages of Si and Al of 17.0 and 3.5%/dry,
respectively, in the upper unsaturated gravel was much higher
than the lower unsaturated gravel where the corresponding values
were 4.3 and 1%/dry, respectively. At the influent end of the saturated gravel of C-03-38-PS-1, where the cementation of the
gravel was more intense and the clog was more mature than in the
middle or effluent sections of the mesocosm, calcium and carbonate 30.5 and 52.4%/dry represented almost 83% of the clog while
the biofilm only represented 6.9%/dry. This clog was also had a
moisture content of 33.3%/wet, which was much lower than 58–
60%/wet in the lower middle and effluent sections of the saturated
gravel. These sections also had lower calcium carbonate with Ca
representing 27.5–27.0%/dry and carbonate 48.3–44.5%/dry for a
total of 75.8 and 71.5% of the clog. In contrast there was more
biofilm that at the inlet, with biofilm representing 10.0% of the
dry mass.
Conclusions
Mesocosm experiments which simulate the last 500 mm of the
drainage layer closest to the leachate collection pipe in a landfill
under field conditions were terminated after 1.6, 6, and 12 years.
The mesocosms discussed herein examine the effect of saturated
versus unsaturated conditions, grain size, and mass loading on the
clogging process and the extent of clog development. This work
has shown that:
1. Maintaining the 300 mm drainage systems fully saturated resulted in greater overall clogging, with a VVO of 45%, than
was observed for mesocosms where the saturated zone was
confined to only 100 mm and the VVO was 31% after
6 years. Operating the full height of the gravel drainage layer
submerged increased the retention time of the leachate within
the gravel and created an environment more conducive to
microbial growth on the gravel throughout. The development
of thick biofilms and accumulation of soft clog was the
dominant clog mechanism over the entire drainage layer
thickness for the fully saturated layers. Periodic increases in
the leachate level into the waste layer during the operation of
the fully saturated designed mesocosm resulted in more clogging due to siltation and the rinsing of particulate matter into
the base of the drainage layer. In a field case, leachate collection systems operating fully saturated would offer even
less control of leachate levels and this phenomena may be
expected to further increase clogging within the gravel layer,
especially in designs with no filter separators between the
waste and the gravel. Designing leachate collection systems
that maintain low saturated leachate heights would reduce
the potential for clogging due to fines rinsing from the waste.
2. Very little clogging occurred within the unsaturated gravel
layers. VVO were, typically, less than 10% over 6 years and
12% over 12 years in the unsaturated gravel. The clog material that did develop within the unsaturated gravel was predominantly biological and was limited to areas on the gravel
where leachate could be retained for instance on the top
lateral surfaces of the gravel and near particle-to-particle
contacts. Short leachate retention times, low leachate contaminant mass loading in terms of flow rate and concentration, and a sporadic distribution of biofilm limit biologically
induced clogging within the unsaturated gravel. Leachate
drainage systems should be designed to operate with a minimum saturated drainage height, for example, by keeping
them pumped and not allowing them to remain in a saturated
state, to reduce the impact of clogging and to extend the
service life of the leachate collection system.
3. The 38 mm gravel performed much better over a 12.6 year
period than the 19 mm gravel did over a 6 year period. The
saturated 38 mm gravel layers were not as severely cemented
with dense precipitated clog as the 19 mm gravel and there
was considerably less soft biofilm within the unsaturated
38 mm gravel layers than the 19 mm gravel. A hydraulic
conductivity of 5.2⫻ 10−5 m / s for the 38 mm gravel after
12.6 years was higher than the 2.7⫻ 10−5 m / s measured for
the 19 mm gravel after 6 years. Less clog was required to
cause the reduction in hydraulic conductivity for the 19 mm
1038 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2007
4.
5.
gravel than the 38 mm. Thus, as large a diameter gravel as
possible should be used in the granular drainage layer of a
leachate collection system. The use of large diameter, relatively uniform gravel minimizes the surface area per unit
volume and increases the void volume and distance between
voids required to fill with clog before affecting the flow of
leachate through a granular drainage layer.
For mesocosms in series there was less clogging as one
moved away for the initial influent port. This was consistent
with the lower organic and inorganic loading for the later
series mesocosms. In the last mesocosm most of the clogging
was due to the vertically percolating leachate. These results
confirm the importance of minimizing mass loading 共e.g., by
closer spacing of leachate collection pipes兲 in terms of extending the service life of leachate collections systems.
The clogging observed in these mesocosm tests over 6 years
was less than observed in the field at the Keele Valley Landfill after 4 – 5 years by Fleming et al. 共1999兲. This implies
that the Keele Valley leachate used in the mesocosm tests
was considerably lower in its concentration of fatty acids and
calcium than the leachate that must have been flowing into
the collection system from the waste at Keele Valley. Thus
the use of the end-of-pipe leachate concentration for predicting collection system performance may not be conservative
and more research is required to characterize leachate as it
enters leachate collection systems in the field.
Acknowledgments
Funding for the research was provided by the Natural Sciences
and Engineering Research Council of Canada. The writers are
grateful to Eugene Benda and Bernie Chau from the City of Toronto for their valuable support and assistance. Assistance by Dr. J.
F. VanGulck and Andrew Cooke with the termination of the mesocosms is also gratefully acknowledged. These tests were initiated by Dr. I. R. Fleming and were maintained for several years
by Mr. Mark Armstrong; their contributions are very gratefully
acknowledged.
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