Clogging of Gravel Drainage Layers Permeated with Landfill Leachate

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


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


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-


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-


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


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


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.


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


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


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.

References Armstrong, M. D. 共1998兲. “Laboratory program to study clogging in a leachate collection system.” MS thesis, Dept. of Civil and Environmental Engineering, Univ. of Western Ontario, London, Ont., Canada. ASTM. 共1998兲. “Standard test methods for specific gravity of soil solids by water pycnometer 共D854-98兲.” Annual book of ASTM standards,

Vol. 04.08, West Conshohocken, Pa. Bouchez, T., Munoz, M.-L., Vessigaud, S., Bordier, C., Aran, C., and Duquennoi, C. 共2003兲. “Clogging of MSW landfill leachate collection systems: Prediction methods and in situ diagnosis.” Proc., 9th Int. Sardinia Symp. on Waste Management and Landfill 共CD-ROM兲, CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy. Brune, M., Ramke, H. G., Collins, H. J., and Hanert, H. H. 共1994兲. “Incrustation problems in landfill drainage systems.” Landfilling of waste: Barriers, E & FN Spon, London, 569–605. Fleming, I. R. 共1999兲. “Biological processes and clogging of landfill leachate collection systems.” Ph.D. thesis, Dept. of Civil and Environmental Engineering, The Univ. of Western Ontario, London, Ont., Canada. Fleming, I. R., and Rowe, R. K. 共2004兲. “Laboratory studies of clogging of landfill leachate collection and drainage systems.” Can. Geotech. J., 41共1兲, 134–153. Fleming, I. R., Rowe, R. K., and Cullimore, D. R. 共1999兲. “Field observations of clogging in a landfill leachate collection system.” Can. Geotech. J., 36共4兲, 685–707. Levine, A. D., et al. 共2005兲. “Assessment of biogeochemical deposits in landfill leachate drainage systems.” Final Rep., Florida Center for Solid and Hazardous Waste Management, Univ. of Florida, Gainesville, Fla., 具http://www.hinkleycenter.com/publications/bysubject. htm典. Maliva, R. G., et al. 共2000兲. “Unusual calcite stromatolites and pisoids from a landfill leachate collection system.” Geology, 28共10兲, 931– 934. McIsaac, R., and Rowe, R. K. 共2006兲. “Effect of filter separators on the clogging of leachate collection systems.” Can. Geotech. J., 43共7兲, 674–693. McIsaac, R. S. 共2007兲. “An experimental investigation of clogging in landfill leachate collection systems.” Ph.D. thesis, Dept. of Civil and Environmental Engineering, The Univ. of Western Ontario, London, Ont., Canada. Rittmann, B. E., Fleming, I. R., and Rowe, R. K. 共1996兲. “Leachate chemistry: Its implications for clogging.” Proc., North American Water and Environment Congress 共CD-ROM兲, ASCE. Rowe, R. K. 共2005兲. “Long-term performance of contaminant barrier systems.” Geotechnique, 55共9兲, 631–678. Rowe, R. K., Armstrong, M. D., and Cullimore, D. R. 共2000兲. “Mass loading and the rate of clogging due to municipal solid waste leachate.” Can. Geotech. J., 37共2兲, 355–370. Rowe, R. K., Quigley, R. M., Brachman, R. W. I., and Booker, J. R. 共2004兲. Barrier systems for waste disposal facilities, E & FN Spon, London. Rowe, R. K., and VanGulck, J. F. 共2003兲. “Experimental and modeling study into clogging of leachate collection systems.” Proc., 9th Int. Sardinia Symp. on Waste Management and Landfill 共CD-ROM兲, CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy. VanGulck, J. F., and Rowe, R. K. 共2004兲. “Influence of landfill leachate suspended solids on clog 共biorock兲 formation.” Waste Manage., 24共7兲, 723–738.
