Clogging of unsaturated gravel permeated with landfill leachate

Transcription

1 1045 Clogging of unsaturated gravel permeated with landfill leachate Reagan McIsaac and R. Kerry Rowe Abstract: The results of an experimental investigation into the clogging of unsaturated, uniformly graded 50 mm gravel permeated with municipal solid waste landfill leachate are reported. The flow of leachate within the unsaturated gravel was heterogeneous and occurred in free-draining flow pathways. The leachate experienced reductions in the concentrations of both the organic and inorganic constituents after passing through the unsaturated gravel although there was very little clogging within the unsaturated gravel. The average drainable porosity was reduced by 8% after 8 years permeation. The biofilm was limited to areas on the gravel where leachate could be retained; predominantly on top of lateral gravel surfaces and near particle-to-particle contacts. As a result, only a small fraction of the total surface area of the unsaturated gravel was covered with biofilm. The short leachate retention time and the sporadic distribution of biofilm limited the degree of contact between the bacteria and the leachate and hence limited biologically induced clogging within the unsaturated gravel. The data suggest that leachate collection systems should be designed and operated such that the drainage material of the leachate collection system remains unsaturated for as long as possible. Key words: municipal wastes, landfills, leachate, clogging, hydraulic conductivity, service life, biofilm, calcium carbonate, drainage, unsaturated. Résumé : On présente les résultats d une étude expérimentale du colmatage d un gravier de 50 mm non saturé et avec une granulométrie uniforme infiltré par le lixiviant d un enfouissement municipal de matières solides. L écoulement du lixiviant à travers le gravier non saturé était hétérogène et se produisait librement le long de cheminements d écoulement de drainage. Le lixiviant a subi des réductions dans les concentrations des constituants tant organiques que non organiques après son passage à travers le gravier non saturé bien qu il y avait très peu de colmatage dans le gravier non saturé. La porosité moyenne de drainage a été réduite de 8 % après 8 années d infiltration. Le biofilm a été limité aux superficies sur le gravier où le lixiviat pouvait être retenu en prédominance sur le dessus des surfaces latérales de gravier et près des contacts particules-contre-particules. Ainsi, seulement une faible fraction de la superficie totale du gravier non saturé était couverte de biofilm. Les courts intervalles de rétention de lixiviat et la distribution sporadique de biofilm ont limité le de gré de contact entre les bactéries et le lixiviat, et ainsi, ont limité le colmatage induit biologiquement à l intérieur du gravier non saturé. Les données suggèrent que le système de collecte de lixiviat devrait être conçu et opéré de telle façon que le matériau de drainage du système de collecte du lixiviat demeure non saturé aussi longtemps que possible Mots-clés :déchets municipaux, remblai d enfouissement, lixiviat, colmatage, conductivité hydraulique, service de vie utile, biofilm, carbonnate de calcium, drainage, non saturé. [Traduit par la Rédaction] Introduction Modern landfills are designed and constructed with many different engineered systems to minimize the impact of the facility on the surrounding environment. The percolation of water through a landfill cover and subsequently through the waste generates a contaminated fluid (leachate) that, if allowed to escape to the environment, could contaminate both surface and groundwater. To minimize detrimental impacts caused by leachate, most modern landfills have a leachate collection system (LCS) that is intended to control leachate mounding on the underlying liner and to collect and remove Received 28 June Accepted 1 May Published on the NRC Research Press Web site at cgj.nrc.ca on 25 July R. McIsaac. Knight Piesold Ltd., 1650 Main Street West, North Bay, ON P1B 8G5, Canada. R.K. Rowe. 1 GeoEngineering Centre at Queen s RMC, Queen s University, Ellis Hall, Kingston, ON K7L 3N6, Canada. 1 Corresponding author ( kerry@civil.queensu.ca). contaminant from the landfill. These systems commonly comprise a network of regular spaced perforated leachate collection pipes embedded in a continuous blanket of coarse-uniform granular material. Unfortunately, the biological and chemical characteristics of leachate, combined with microbial activity, give rise to clogging of the granular material (Brune et al. 1994; Fleming et al. 1999; Maliva et al. 2000; Rowe et al. 2000; Bouchez et al. 2003). The clog material decreases the pore space available to transmit leachate, reduces the hydraulic conductivity of the granular layer, and consequently reduces the efficiency and the effective functioning of the LCS. Because these systems may be required to have a service life of decades to a century (Ontario regulation 232/98; Ontario Regulation 1998) or more depending on the contaminating life span of the facility, it is important to optimize their long-term performance. A properly functioning LCS will maintain the height of the leachate mound on the base of the landfill at a design level less than the thickness of the LCS drainage blanket. Under these circumstances, the lower zone of the LCS drainage Can. Geotech. J. 45: (2008) doi: /t08-053

2 1046 Can. Geotech. J. Vol. 45, 2008 Fig. 1. Experimental columns. blanket is saturated and the upper zone is unsaturated. The development of clog within the voids of the unsaturated zone of a LCS has not yet been the subject of long-term laboratory study. Fleming et al. (1999) reported observations from a field exhumation of a granular LCS at a large municipal landfill site. The drainage blanket was constructed from uniform, crushed dolomitic limestone of nominal 50 mm size. After about 4 to 5 years exposure to municipal landfill leachate, the amount of clog observed in the upper unsaturated portion of the drainage layer was considerably less than that in the lower saturated zone. The estimated void volume occupancy (VVO) (or reduction in free pore space between gravel particles) was visually estimated to be 50% 100% in the lower saturated zone of the drainage layer and 30% 60% in the upper unsaturated portion of the drainage layer where there was no geotextile and 0% 20% in the area where there was a geotextile between the waste and gravel. The difference can be largely attributed to the role of the woven (slit-film) geotextile as a separator, which prevented intrusion of waste material into the upper gravel. Thus, the amount of biologically induced clogging in unsaturated gravel appeared to be quite small in the 4 to 5 years of operation. The primary objective of this paper is to report the extent of clogging within the unsaturated zone of a gravel typically used in LCSs after up to 8 year permeation by leachate collected from the Keele Valley Landfill (KVL) in Toronto, Ontario. A secondary objective is to assess the effect of drainage length and leachate dose volume on the clogging of the gravel and treatment of the leachate. A tertiary objective is to obtain insight regarding flow through unsaturated gravel.

3 McIsaac and Rowe 1047 Fig. 2. Base drainage plate in column used to study flow behaviour, partitioned into 24 independent drainage basins of equal area, each of which collected only the drainage from the overlying gravel. Approach Three experimental columns (designated U1 U3) were fabricated from polyvinyl chloride (PVC) (schedule 80) plastic pipes and were built on a large enough scale (internal diameter of 300 mm) to simulate the unsaturated zone of a primary leachate collection drainage layer at full scale in real time and with materials typically used in practice. A conventional uniformly graded gravel (crushed dolomitic limestone) with a nominal size of 50 mm was used for the drainage layer. The thickness of the drainage layer (or flow length) in columns U1, U2, and U3 was 200, 400, and 600 mm, respectively. The columns were operated at a temperature of 27 ± 2 8C to simulate the conditions anticipated in an active LCS. Fresh leachate was collected from the KVL in Toronto, Ontario. With a footprint of 99 ha, the KVL has a design capacity of approximately m 3 and it received approximately t of municipal solid waste between 1984 and The leachate was collected from a manhole on the main header line at the downstream end of the LCS and hence had been subjected to treatment as it flowed through the LCS. Prior to use, the leachate was stored in a polyethylene tank at 7 ± 2 8C to discourage microbial growth during storage. The leachate was delivered by gravity to a gear pump through a manifold. The manifold allowed for temperature equilibration between the leachate and the columns (27 8C). The columns were outfitted with a gas delivery and venting system to ensure operation under anaerobic conditions. Cylinders filled with simulated landfill gas made up of 60% methane (CH 4 ) and 40% carbon dioxide (CO 2 ) were used along with a fermentation lock and a sealed system to produce an anaerobic environment in each column at a pressure of approximately 50 mm of water (approximately 0.5 kpa) above atmospheric pressure. The use of a fermentation lock ensured that a slightly positive gas pressure was maintained within the head space of the columns. The columns operated in an upright fashion with a downward flow of leachate (Fig. 1). To maintain typical leachate loading rates that would mimic those observed in a landfill in Ontario (about 0.2 m/year), a leachate loading rate of 39 ml/day would be required for each column. Such a low flow rate is very hard to distribute over the top crosssectional area of each column, and consequently, a leachate loading rate of 2.0 m/year (i.e., 10 times higher than typical rates) was administered to each column using an electronic leachate dosing system. The main components of the dosing system comprised a gear pump, dosing chamber with three solenoid valves, and an influent distribution system of tubing connecting each of the three solenoids from the dosing chamber to eight influent ports located on the top end cap of each column (Fig. 1). The dosing system was operated as follows: (1) Approximately 80 ml of leachate was pumped into the dosing chamber every 1.5 h. (2) One of the three solenoid valves would open to feed one of the three experimental columns. The first dose after the filling of the dosing chamber was considered the main dose. The volume of the main dose was typically 44 ml but varied from 22 to 66 ml of leachate. Leachate would gravity feed through the distribution of tubes into the eight influent ports. After this a solenoid valve controlling the gas supply was opened to blow the distribution lines free to ensure no residual leachate remained in the lines. (3) Because of the fittings used to connect the solenoid

4 1048 Can. Geotech. J. Vol. 45, 2008 valves to the base of the dosing chamber, a small amount of leachate (average 15 ml; range 9 21 ml) was stored above the two remaining solenoids after the first main dose was completed. To prevent the dilution of the freshly supplied leachate at each dosing cycle with this remaining leachate and the eventual clogging of the solenoid valves, the two remaining solenoids not involved in the first main dose were flushed of this leachate. Hence, immediately after the first main dose, the remaining two solenoids were opened, one at a time, and the leachate (approximately 9 21 ml) was allowed to flow into the corresponding experimental columns. The gas supply line again was used to ensure the lines were purged of any leachate. (4) The dosing cycle was repeated every 1.5 h. A programmable logic controller was used to ensure that the first main dose would continuously cycle through the three columns with time. Experimental analysis Operational testing The leachate traveling down through the unsaturated gravel would drain out of the base of each column and collect in a reservoir. The reservoirs were drained on a regular basis to confirm the flow rate. Leachate quality testing was performed on leachate samples collected from before the influent valve and from the sample bottles that collected leachate over each 1.5 h dosing cycle. These samples were tested immediately to obtain chemical oxygen demand (COD), calcium concentration, ph, total and volatile suspended solids (TSS and VSS, respectively), and volatile fatty acid (VFA) concentrations (specifically acetic, proprionic, and butyric acid concentrations). The COD concentrations were measured using a Hach COD reactor, COD reagents, and a DR2000 spectrophotometer. The reagent leachate solution was heated at 150 8C for 2 h and then analyzed with the spectrophotometer. Calcium concentrations were obtained with a Philips PU9100X atomic absorption flame spectrophotometer. The ph was measured using a YSI water quality monitor (model 3400) equipped with the appropriate probes. The VFA concentrations were obtained by gas chromatography (GC) using a Varian 3400 instrument equipped with flame ionization detectors and 15 m 0.53 mm ID, 0.5 mm film NUKOL (Supelco, Bellfonte, Penn.) capillary columns. Injections to the Varian 3400 were automated (Varian 8200 auto sampler) and performed using solid-phase micro-phase extraction (SPME) with 10 min immersion 2 min desorption of a 75 mm carboxen polymethylsiloxane fiber (Supelco, Bellfonte, Penn.). All aqueous VFA samples were acidified in 1% phosphoric acid (H 3 PO 4 ) prior to analytical separation. Isovaleric acid was used as internal calibration standard. Tests were performed to assess the change in drainable porosity (and hence void volume) with time as clog developed. The drainable porosity was measured by periodically saturating the entire drainage layer with leachate and then measuring the volume of leachate drained over discrete intervals. The columns were drained slowly over the period of a day to avoid disruption of the biofilm. The measured drainable porosity is a ratio of the volume of drained leachate to the total volume of the drained interval. Termination testing Once testing was terminated, the columns were disassembled and the clogged drainage material was removed in approximately 75 mm intervals. The termination allowed for visual inspection of the degree of clogging that occurred over the 8 year operational life span. The drainage material was removed and stripped of clog material. Mass measurements were used to obtain clog properties such as the mass of total wet clog material, dry mass (after evaporation of water at 105 8C), and ash mass (after ignition in a muffle furnace at 550 8C) within different intervals. The dry (volatile and nonvolatile solids excluding water) and ash (nonvolatile solids excluding water) densities of the clog were calculated using ASTM D854 (ASTM 1998). Based on the mass of clog removed from the disassembled columns, the density of the clog material, and the initial void volume in each sample section, the volume of clog within each section and the resulting VVO was calculated, where 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 where they were digested with a lithium metaborate tetraborate (LiBO 2 ) fusion method and analyzed with inductively coupled plasma (ICP) spectrometry for major elements and coulometry for CO 2. Flow behaviour To gain insight regarding the distribution of flow in the unsaturated gravel and hence an appreciation of how this may affect clogging of unsaturated gravel, a column was constructed from transparent acrylic with a similar diameter (277 mm) as the experimental columns discussed previously and filled with the same 50 mm diameter gravel to a height of 600 mm in the exact same manner as that used for the other columns. The bottom of the column was partitioned into 24 independent drainage basins of equal area (25.3 cm 2 or 4.2% of the entire base area), each of which collected only the drainage from the overlying gravel (Fig. 2). The spatial and temporal variations in drainage volume were monitored to obtain information regarding flow in the column. Following the application of a dosed volume to the top of the gravel (50 ml), the drainage into each port was monitored over predetermined time intervals. The distribution of flow over the base area of the column was heterogeneous (Fig. 3). A dose event resulted in a rapid response (i.e., within 0 5 min, Fig. 3a) at the base of the column (note the flow rate scale in Fig. 3a is four times that for the other plots). The initial flow was greatest at port 7 and there was initially intermediate flow at five other locations (ports 1, 8, 11, 12, and 24), indicating that the majority of flow through the gravel localized in a few freedraining flow pathways immediately after dosing. Two of the pathways dominated (ports 7 and 24), conducting a majority of the flow with time. These two ports represent a small fraction (8.3%) of the total base drainage area, yet they received 35% of the total base flow after the first 5 min. As might be expected, the flow decreased with an increase in time. About 33% of the base area did not intercept any flow. The rapid transmission of fluid to the base of the

5 McIsaac and Rowe 1049 Fig. 3. Flow rates at the base of the column during different time intervals. column would suggest relatively short leachate retention times. Short leachate retention times would minimize the contact time between the bilofilm on the gravel and the leachate and hence limit the biological activity to areas on the gravel where leachate is retained (on the top horizontal surfaces of the gravel and at gravel-to-gravel contacts) after the majority of the flow had passed through the gravel. Some fluid was seen to cascade rapidly through the gravel and some moved slower and formed water droplets on gravel particles. When a water droplet had grown to a critical size, it would drop under the influence of gravity. After they began to fall, these droplets were seen to contact other gravel particles and displace of water from intermittent zones of saturation (i.e., larger horizontal surfaces on the gravel and fluid in capillary zones at gravel-to-gravel contacts). Thus, although there was a rapid response at the base of the column to a dose event, there was mixing and dilution of the influent fluid with the fluid retained on the gravel at each dosing. Since the length of the three columns was different, but the dose was the same for each column, the dose volumes represent a different proportion of the fluid attached to the gravel particles in each column. There was also a range related to the variable doses used in the dosing sequence, as described previously. The volume of fluid in a dose was about 2% 13% of the fluid that was retained in the gravel for column U1, 1% 6% for U2, and 1% 4% for U3. Thus, there was considerable scope for dilution of contaminants in the leachate in each case, as it mixed with fluid retained on the gravel particles while it passed through the gravel. Influent leachate characteristics Because the collected leachate had been subjected to treatment within the LCS in the field, VFAs and inorganic contaminants such as calcium were already removed (Fleming et al. 1999). Therefore, to make the leachate injected into the columns more representative of a leachate exiting the waste and entering a LCS, the leachate feedstock for the columns was spiked. Calcium chloride dihydrate (CaCl 2 2H 2 O) was added to the raw KVL leachate to increase the calcium concentration. A mixture of acids that contained acetic, propionic, and butyric acids in a ratio of 20:10:1 was added to the raw leachate to raise the COD concentration. These organic components were selected because they represent the degradable organic load in the leachate, and the relative proportions were based on meas-

6 1050 Can. Geotech. J. Vol. 45, 2008 Fig. 4. Variations in COD, calcium concentrations, and ph in influent and effluent leachate with time.

7 McIsaac and Rowe 1051 Fig. 5. Variation in normalized COD and calcium for dose volumes for column U1: (a) less than 25 ml, (b) greater than 25 ml. urements of the concentrations in younger KVL leachate (McIsaac 2007). The leachate supplied to each column was from the same source. The changes in the influent COD, calcium concentrations, and ph were monitored and are shown in Fig. 4 (solid symbols). The data reflect the variability of leachate from KVL. Before 1720 days, the average (and standard deviation) influent COD concentration, calcium concentration, and ph were 8500 mg/l (3310 mg/l), 940 mg/l (710 mg/l), and 7.0 (0.5), respectively. From 1720 days onward, the three VFAs and calcium were added so that the influent COD and calcium concentrations were in-

8 1052 Can. Geotech. J. Vol. 45, 2008 Fig. 6. Variation in normalized COD and calcium for dose volumes for column U2: (a) less than 25 ml, (b) greater than 25 ml. creased to give average (and standard deviation) influent COD and calcium concentrations of mg/l (2570 mg/l) and mg/l (540 mg/l), respectively, and the average ph was 4.6 (0.2). Acetic, propionic, and butyric acid concentrations were measured for the influent and effluent leachate with time. Leachate quality results The effluent COD and calcium concentrations along with the ph shift between the influent and effluent leachate are shown in Fig. 4. For each date that testing was conducted, three data points are shown for each column (where they

9 McIsaac and Rowe 1053 Fig. 7. Variation in normalized COD and calcium for dose volumes for column U3: (a) less than 25 ml, (b) greater than 25 ml. are very close they overlap and the individual data points cannot be seen). These are the results from nine separate doses (each involving a different dosed volume) collected from the three columns (each with a different drainage layer thickness: 200, 400, and 600 mm, respectively). These doses were applied every 1.5 h (as explained in the Operations section) over a 4.5 h sample collection period. Reductions (sometimes large) in the organic (indicated by the COD concentrations) and inorganic (indicated by the calcium concentrations) strength of the leachate were measured after the leachate had traveled through the unsaturated gravel. Normalized values of COD and calcium were obtained by divid-

10 1054 Can. Geotech. J. Vol. 45, 2008 Fig. 8. Variation in removed COD and calcium for dose volumes for column U1: (a) less than 25 ml, (b) greater than 25 ml. ing the effluent concentrations by the influent concentrations. Normalized values were used to allow a better understanding of trends in the data, given the variability in the influent leachate. The average normalized effluent COD values before 1720 days were 0.43 for U1 (200 mm), 0.31 for U2 (400 mm), and 0.31 for U3 (600 mm). After 1720 days, the average values were 0.59, 0.27, and 0.27, respectively. The average normalized effluent calcium values before 1720 days for U1, U2, and U3 were 0.47, 0.35, and 0.32, respectively. After 1720 days, the average values were 0.84, 0.30, and 0.35, respectively. The ph of the leachate increased as it passed through the unsaturated gravel from an

11 McIsaac and Rowe 1055 Fig. 9. Variation in removed COD and calcium for dose volumes for column U2: (a) less than 25 ml, (b) greater than 25 ml. average influent ph of 7.0 before 1720 days to an average effluent ph of 7.6, 7.7, and 7.7 for U1, U2, and U3, respectively (i.e., a shift of about 0.7 ph units). After 1720 days, the average ph increased from 4.6 (influent) to an average effluent ph of 6.3, 7.4, and 7.4 for U1, U2, and U3, respectively (i.e., a shift of about ph units) after passing through the unsaturated gravel. In general, relatively constant effluent ph values were measured throughout the operation of the columns despite variations in the influent ph measurements. Likewise, relatively steady state effluent COD and calcium concentrations were measured except at times when there was a large change in the influent organic

12 1056 Can. Geotech. J. Vol. 45, 2008 Fig. 10. Variation in removed COD and calcium for dose volumes for column U3: (a) less than 25 ml, (b) greater than 25 ml. loading. At these times ( days and at the time when the influent leachate strength was increased at 1720 days and shortly after), the effluent concentrations were elevated (i.e., less relative treatment) for all columns as is apparent from the high normalized COD and calcium values for column U1 (0.59 and 0.84, respectively), which only operated for 100 days after the influent strength was increased at 1720 days. Column U1 was terminated at this time to provide an indication of clogging that had occurred to date and to guide decisions about the time to terminate the other columns. The high effluent concentrations during this period are likely the result of the inability of the mixed

13 McIsaac and Rowe 1057 Fig. 11. Drainable porosity profiles in the columns with time. population of bacteria to adapt to the increased concentration within a short period of time. It took longer for the bacteria in the unsaturated columns examined in this study to adjust to an increase in COD and calcium concentrations (after 1720 days) than it did for the bacteria in the saturated column examined by McIsaac and Rowe (2005). The slow adjustment is likely due to the faster rate of moving leachate through the gravel for the unsaturated columns than for the saturated columns, combined with the sporadic distribution of biofilm in the unsaturated columns. Effluent COD and calcium concentrations were similar throughout the operation of the columns except for a short period of time after the influent concentration was increased at 1720 days. Thus, it can be inferred that the full potential of either the 400 or 600 mm filled columns to treat the leachate was not realized even after the influent COD and calcium concentrations were elevated at 1720 days. The results show that the treatment of the leachate (i.e., reductions in the organic (COD) and inorganic (calcium) concentrations) does occur within the unsaturated portion of a LCS and the reduction can be large (Fig. 4). For example, after 1720 days, the amount of COD and calcium removed was as high as and 3000 mg/l, respectively, as the leachate had flowed through the columns. The actual mass of COD and calcium removed depended on the dosed volume and the thickness of gravel through which the leachate percolated, as discussed later. Even before the leachate reached the saturated layer of a LCS, the leachate had undergone considerable treatment. The treatment of leachate in the unsaturated layer provides an explanation for why the end of the pipe leachate strength at a municipal solid waste landfill is only a fraction of the strength that is leaving the waste layer and entering the drainage layer of a LCS. Because of the potential of the unsaturated gravel zone to treat leachate with very little clog formation, the LCSs should be designed and operated such that the drainage material remains unsaturated for as long as possible. If followed, this recommendation also has the advantage of minimizing the head on the liner and hence minimizing leakage through the liner system. To illustrate the fact that the magnitude of the dose volumes has some impact on the normalized COD and calcium concentrations for each of the different drainage layer thicknesses (i.e., each column), results are given in Figs. 5, 6, and 7 for dose volumes less than and greater than 25 ml. The mass of COD and calcium removed from each dose after flowing through the different thicknesses of drainage gravel for doses less than and greater than 25 ml are shown in Figs. 8, 9, and 10. At dose volumes less than 25 ml, there was a relatively high proportion of COD and calcium removed from the leachate as it percolated through the gravel (Figs. 5a, 6a, and 7a); however, due to the small volume (i.e., <25 ml) of leachate being treated, the actual mass of both COD and calcium removed was small (Figs. 8a, 9a,

14 1058 Can. Geotech. J. Vol. 45, 2008 and 10a) compared with that for larger doses (i.e., >25 ml, Figs. 8b, 9b, and 10b). At dose volumes greater than 25 ml, there was less relative treatment of the leachate (i.e., the biological reactors were less efficient at high dose volumes) than for dose volumes less than 25 ml; however, the total mass of COD and calcium removed per dose was greater for the higher doses (i.e., >25 ml, Figs. 8b, 9b, and 10b) than for the smaller doses (<25 ml, Figs. 8a, 9a, and 10a). Thus, the results indicate that the greatest mass of COD and calcium were removed from the leachate and deposited in the gravel for the large doses. The same trend was observed after the influent concentrations were increased at 1720 days (Figs. 5 10). Therefore, minimizing the amount of leachate percolating through the gravel (e.g., by controlling infiltration through the landfill cover) or reducing the mass of nutrients and inorganic material for biological activity and precipitation (e.g., by treating the waste, minimizing the height of waste above the leachate collection system, or by placing new waste over old waste; Armstrong and Rowe 1999) will limit the amount of mass removed and thus the amount of clogging within the leachate collection system. For dosed volumes less than 25 ml, the thickness of gravel through which the leachate percolated made very little difference to the amount of treatment the leachate received. The normalized COD and calcium values (Figs. 5a, 6a, 7a) and the amount of COD and calcium removed (Figs. 8a, 9a, 10a) were similar for all three columns. For the 200 mm of unsaturated gravel (column U1), the removal of COD and calcium was almost the same as for 400 mm (column U2) and 600 mm (column U3) of gravel. At dose volumes greater than 25 ml, an increase in the thickness of unsaturated gravel through which the leachate percolated did reduce the strength of the effluent leachate. Column U1 was generally less efficient at removing organic and inorganic mass from the leachate, which resulted in less COD and calcium removed per dose. Columns U2 and U3 with the thicker gravel layer resulted in the greatest treatment of the leachate (i.e., more consistently lower normalized values and larger mass of COD and Ca removed). These columns gave the greatest treatment because the greater thickness of gravel resulted in greater retention times for the leachate and greater surface area for biofilm growth and hence greater exposure of the leachate to active biomass and therefore more treatment of the leachate. The fact that U2 and U3 behaved similarly after 1720 days indicates that a gravel thickness of 400 mm was as efficient as a thickness of 600 mm in reducing the strength for the leachate flows and concentrations examined. Acetic, propionic, and butyric acid concentrations in the influent and effluent leachate were measured with time. Similar reductions in acid concentrations were observed in all of the columns. All of the columns were more efficient at removing acetic acid than propionic acid throughout the testing period. The variations in the quality of the leachate as it passed through the unsaturated gravel-filled columns are consistent with the leachate chemistry study by Rittmann et al. (1996). Rittmann et al. (1996) showed that the reduction in COD (primarily due to fermentation of acetic acid to carbonic acid) and production of carbonate resulted in a shift in ph to higher values and in conjunction with the increased carbonate concentration promoted the development of inorganic clog material (predominantly calcium carbonate). Clogging The drainable porosity measurements were performed infrequently to minimize disturbance to the biofilm that developed. The initial average drainable porosities (Fig. 11) measured over the entire length of drainage gravel in U1, U2, and U3 were 0.43, 0.41, and 0.42, respectively. From the drainable porosity measurements, limited clogging occurred within the gravel. The maximum measured reduction in drainable porosity from its original value was 14% within the mm interval of U2 (Fig. 11). The average reduction for columns U2 and U3 after 8 years of operation was 8%. The reduction per unit time in each drained interval in almost all cases was greater during the time in which the columns were fed high strength leachate (after 1720 days). The change in drainable porosity per year (averaged over the top 400 mm of both U2 and U3) before and after 1720 days was and 0.009, respectively. There was some dissolution of the limestone directly under the influent ports owing to the low ph of the leachate introduced to columns U2 and U3 after 1720 days, and this dissolution resulted in an increase in drainable porosity in the uppermost layer. The increase in drainable porosity is not manifest in the VVO results discussed later, since they are based on the amount of clog removed from the gravel at the time of termination and the initial volume of voids measured before the start of the tests. Figure 12 shows the wet, dry, and volatile masses per unit volume of gravel removed at the end of the tests. Also shown is the percentage of volatile clog, the VVO (i.e., the proportion of the original void space that was filled with clog material), and the calculated total porosity n at the time of column disassembly. Very little clogging occurred within the unsaturated gravel. The VVO in U1 was less than 5% after 5.2 years and falls within the 0% 20% range observed in the field after 5 years by Fleming et al. (1999). Even after 8 years of operation and exposure to high strength leachate, very little clog had developed in the unsaturated gravel and the VVOs in U2 and U3 were less than 13% within the top 150 mm of the columns and less than 8% in the remainder. Because U2 and U3 operated 1.7 times longer than U1 and were exposed to the higher strength leachate during this time, there was a greater accumulation of clog material in these columns than in column U1. The distribution of hard cementatious clog and the degree of cementation was greatest at the top and in the centre of the gravel owing to the flow distribution effect. Columns U2 and U3 had larger amounts of retained mass at the influent end (top) of the columns (within the top 150 mm) than in the base (Fig. 12). This larger retained mass is primarily due to the fact that initially there is a good distribution of influent leachate over the top surface of the gravel through the eight influent ports, but at depths greater than 150 mm the majority of the flow through the gravel occurs in a few free-draining flow pathways. In U2 and U3, for almost the entire cross-sectional area (approximately 85%) within the top 150 mm, the gravel particles were severely cemented together at the points of contact, although the pores remained

15 McIsaac and Rowe 1059 Fig. 12. Clog properties at disassembly of the columns.

16 1060 Can. Geotech. J. Vol. 45, 2008 Fig. 13. Sporadic distribution of clog on cemented gravel after being air dried. open for the passage of leachate. Significant force and a pry bar were required to break apart the cemented gravel particles. Only a few stones in contact with the perimeter of the column walls could be easily removed by hand. From 150 mm to the base of the gravel, approximately 25% 35% of the gravel was severely cemented. The gravel outside of this cemented area could be removed by hand with very little effort. There was no cementation of the gravel at the perimeter of the columns because the majority of leachate flow occurred within the free-draining flow pathways located nearer to the centre of the columns. Although not evident from the wet solids plot in Fig. 12, the same trend was observed in U1 (terminated after 5.2 years) at the time of disassembly with more inorganic clog and more cementation of gravel at the top than at the bottom. Inspection of Figs. 12a and 12b indicates that in columns U2 and U3 the greatest wet and dry mass of clog material was at the top of the column where the leachate entered the gravel. The mass reduced essentially linearly with depth to about 200 mm and then remained relatively constant with further depth (especially below 300 mm). The results for both columns were similar, with slightly more mass per interval being removed in U2 than in U3. The VVO values are derived from, and hence follow, the masses of clog material. These results suggest that the majority of the clogging would occur in the mm typical unsaturated gravel thickness in a LCS. Increasing the thickness of the gravel results in more mass removal, but the reduction is modest. Clog developed on the gravel where leachate could be retained. As a result, only a fraction of the total surface area of the unsaturated gravel was available for leachate retention and biofilm growth (Fig. 13). Leachate was retained on the top, relatively horizontal surfaces of the gravel and within a capillary miniscus at gravel-to-gravel contacts where the interface gap between two particles was small (approximately 1 3 mm). Once the void distance between two particles was greater than 2 3 mm, the capillary action and menisci ceased. The sporadic distribution of active biofilm on the unsaturated gravel in addition to the flow distribution effect limited the contact between the biofilm and the leachate as the leachate flowed through the unsaturated gravel and thus limited the biologically induced clogging under unsaturated conditions. Inorganic material was the dominant fraction within the clog. The percentages of active degradable organic material to inactive nondegradable inorganic material (percent volatile in Fig. 12d) generally were less than 12% for all the columns, and coincides with the relatively low amount of biofilm observed within the columns. The percent volatile was uniform along the gravel columns and similar between different columns, indicating the clog that developed was similar in all columns. Dry and ash film densities were also relatively uniform throughout the columns (Fig. 14). The clog composition was relatively uniform along the lengths of the columns and similar in composition from column to column (Table 1). Based on dry mass, the majority of the clog material was calcium carbonate (CaCO 3 ) for all columns. The clog had 27% 32% calcium, 48% 52% carbonate, 1% 2% iron, and 1% 5% magnesium. The average fractional value of calcium and carbonate found in the clog material was very similar to values obtained by McIsaac and Rowe (2006) in an experimental clogging study designed to mimic the two dimensional leachate flow conditions adjacent to a leachate collection pipe in a primary LCS, indicating that similar biologically induced clogging is occurring in the unsaturated gravel as in the saturated gravel of a LCS.

17 McIsaac and Rowe 1061 Fig. 14. Clog densities within columns at disassembly. Conclusions Changes in leachate characteristics and drainable porosity caused by clogging were monitored over a period of up to 8 years in three unsaturated columns filled with 50 mm gravel and permeated with municipal solid waste leachate. The gravel used was the same as that used in stages 3 and 4 of the KVL. The leachate used was obtained from the KVL (average influent ph = 7), and all the results prior to 1720 days may be considered applicable to landfills such as the KVL. After 1720 days, the leachate strength was increased (and the influent ph decreased to an average of 4.6) to examine the effect of more acidic leachate and these results would be applicable to landfills with acidic leachate. Three different gravel thicknesses (200, 400, and 600 mm) were examined. The following conclusions were drawn from the results of these tests. (1) Partial treatment of leachate does occur within unsaturated gravel used as part of a LCS at the base of a landfill. Large reductions in the organic and inorganic concentrations in the leachate occurred after traveling through the unsaturated gravel. Even before leachate reaches the saturated layer of a LCS, the leachate has undergone significant treatment. This finding provides further evidence that effluent leachate strength at a municipal solid waste landfill is only a fraction of the strength (in terms of concentration of organic contaminants, as represented by COD, and some inorganic contaminants, as represented by calcium) of the leachate that is leaving the waste layer and entering the drainage layer of a LCS. (2) Although the unsaturated gravel was more efficient at lower dose volumes (i.e., the reduction in COD and calcium concentrations were greater than those for larger doses), a greater mass of organic and inorganic constituents was removed per dose for larger dose volumes. Thus, a reduction in the volume of leachate generated may both reduce the amount of clogging and the concentration or organics (and hence COD) and some inorganincs (e.g., calcium) in the leachate. (3) Very little clogging occurred within the unsaturated gravel. The average reduction in drainable porosity from its original value was 8% after 8 years of operation. The development of biofilm was limited to areas on the

18 1062 Can. Geotech. J. Vol. 45, 2008 Table 1. Composition of clog removed from the unsaturated columns. U1 (200 mm) U2 (400 mm) U3 (600 mm) Distance from top of column (mm) Distance from top of column (mm) Distance from top of column (mm) Water content (%/wet) Organic matter (TVS; %/dry) Carbonate as CO 3 (%/dry) Calcium, Ca (%/dry) Magnesium, Mg (%/dry) Silicon, Si (%/dry) Iron, Fe (%/dry) Sodium, Na (%/dry) Aluminum, Al (%/dry) Potassium, K (%/dry) Phosphorus, P (%/dry) Titanium, Ti (%/dry) Manganese, Mn (%/dry) Strontium, Sr (mg/kg) Zinc, Zn (mg/kg) Barium, Ba (mg/kg) Ca/CO Note: TVS, total volatile solids. gravel where leachate could be retained (e.g., flat surfaces and contact points between gravel particles). As a result, only a fraction of the total surface area of the unsaturated gravel was covered with biofilm. The sporadic distribution of biofilm 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 within the unsaturated gravel. (4) Although the upper mm of unsaturated gravel experienced the greatest deposition of inorganic mass and consequent clogging, increasing the gravel thickness resulted in the removal of larger amounts of organic and inorganic mass from the leachate. The greater thickness of gravel resulted in higher retention times of the leachate within the column, a higher surface area for biofilm growth and thus more exposure of the leachate to active biomass and therefore more treatment of the leachate. With the potential to treat leachate with very little clog formation, LCSs should be designed and operated such that the drainage material of the LCS remains unsaturated for as long as possible. If followed, this recommendation also has the advantage of minimizing the head on the liner and hence minimizing leakage through the liner system. (5) The flow within the unsaturated gravel was heterogeneous, rapid, and predominantly occurred in a few freedraining flow pathways. The short leachate retention times resulted in limited contact time between the gravel (potential surface for bacterial growth) and the leachate, which limited biological induced clogging within the unsaturated gravel; however, it was sufficient to result in significant drops in concentrations of COD and calcium in the leachate. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada. The authors are grateful to Eugene Benda and Bernie Chau from the City of Toronto for their valuable support and assistance. The value of discussions and assistance of Gary Lusk is gratefully acknowledged. References Armstrong, M.D., and Rowe, R.K Effect of landfill operations on the quality of municipal solid waste leachate. In Sardina 1999, Proceedings of the 7th International Waste Management and Landfill Symposium, Santa Margherita di Pula, Cagliari, Italy, 4 8 October Edited by T.H. Christensen, R. Cossu, and R. Stegmann. CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy. Vol. 2, pp ASTM Standard test methods for specific gravity of soil solids by water pycnometer (D854-98). In Annual book of ASTM standards, Vol American Society for Testing and Materials (ASTM), West Conshohocken, Penn. Bouchez, T., Munoz, M.-L., Vessigaud, S., Bordier, C., Aran, C., and Duquennoi, C Clogging of MSW landfill leachate collection systems: prediction methods and in situ diagnosis. In Sardina 2003, Proceedings of the 9th International Waste Management and Landfill Symposium, Santa Margherita di Pula, Cagliari, Italy, 6 10 October Edited by T.H. Christensen, R. Cossu, and R. Stegmann. CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy. CD-ROM. Brune, M., Ramke, H.G., Collins, H.J., and Hanert, H.H Incrustation problems in landfill drainage systems, landfilling of waste: barriers. E&FN Spon, London, UK. pp Fleming, I.R., Rowe, R.K., and Cullimore, D.R Field observations of clogging in a landfill leachate collection system. Ca-

19 McIsaac and Rowe 1063 nadian Geotechnical Journal, 36(4): doi: /cgj Maliva, R.G., Missimer, T.M., Leo, K.C., Statom, R.A., Dupraz, C., Lynn, M., and Dickson, J.A.D Unusual calcite stromatolites and pisoids from a landfill leachate collection system. Geology, 28(10): doi: / (2000) 28<931:UCSAPF>2.0.CO;2. McIsaac, R An experimental investigation of clogging in landfill leachate collection systems, Ph.D. thesis, University of Western Ontario, London, Ont. McIsaac, R., and Rowe, R.K Change in leachate chemistry and porosity as leachate permeates through tire shreds and gravel. Canadian Geotechnical Journal, 42(4): doi: /t McIsaac, R., and Rowe, R.K Effect of filter-separators on the clogging of leachate collection systems. Canadian Geotechnical Journal, 43(7): doi: /t Ontario Regulation Ontario Regulation 232/98, Ontario Ministry of the Environment, Environmental Protection Act Ontario, PIBS 3651E. Rittmann, B.E., Fleming, I.R., and Rowe, R.K Leachate chemistry: its implications for clogging. In Proceedings of the North American Water and Environment Congress 96, June Anaheim, Calif. Edited by C.T. Bathala. American Society of Civil Engineers, New York. CD-ROM. Rowe, R.K., Armstrong, M.D., and Cullimore, D.R Mass loading and the rate of clogging due to municipal solid waste leachate. Canadian Geotechnical Journal, 37(2): doi: /cgj