Field Evaluation of a Leachate Collection System Constructed with Scrap Tires Ahmet H. Aydilek1; Edward Tristram Madden2; and M. Melih Demirkan3 Abstract: Landfilling costs and the potential uses of scrap tires have prompted researchers to investigate beneficial reuses. One important application is the use of tire chips as a leachate collection material in municipal solid waste landfills. Laboratory and field studies were conducted to investigate the performance of tire chips as a drainage medium in landfills. The laboratory portion of the program included a series of hydraulic conductivity and compressibility tests. Two field test cells, one with tire chips and another with gravel as the control, were constructed. The tire-chip cell was instrumented with flowmeters, thermistors, and gas collection devices to evaluate the hydraulic performance as well as the potential for spontaneous combustion. Leachate collected from the two cells was analyzed to determine if tire chips would potentially contaminate the groundwater. The results indicated that adequate drainage conditions were present within the tire-chip layer. The presence of insignificant quantities of carbon monoxide, and the lack of oxygen, and recorded low temperatures suggested that a combustion hazard was not present. The field leachate data indicated that tire chips can be safely used as part of a landfill leachate collection layer, even though it may not be suitable to place them near drinking water sources. DOI: 10.1061/共ASCE兲1090-0241共2006兲132:8共990兲 CE Database subject headings: Tires; Leaching; Combustion; Landfills; Waste management.
Introduction Over 290 million scrap tires are generated annually in the United States. At present, only 24% of the scrap tires is beneficially reused in construction, and another 9% is recycled 共Rubber Manufacturers Association 2004兲. Additionally, 2–4 billion scrap tires are stockpiled across the country, which pose severe health and fire hazards 共Reddy and Marella 2001兲. Even though tires comprise only 1.2% of the solid waste in the United States, they present significant problems due to their nonbiodegradable, compaction resistant, and self-combustible nature 共Moo-Young et al. 2003兲. Unless alternative uses of these scrap tires are introduced, an increase in landfilling costs is inevitable under present circumstances. One way of handling waste scrap tires is to use them as drainage material in landfill leachate collection systems. This decreases the cost of landfilling and is an environmentally friendly option. The leachate collection layer is used in modern landfills to remove the water generated by the overlying waste 共i.e., leachate兲 and minimizes the hydraulic head on the composite liner. 1 Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Maryland, 1163 Glenn Martin Hall, College Park, MD 20742 共corresponding author兲. E-mail:
[email protected] 2 Ph.D. Student, Dept. of Civil and Environmental Engineering, Univ. of Maryland, 1163 Glenn Martin Hall, College Park, MD 20742. 3 Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of Maryland, 1163 Glenn Martin Hall, College Park, MD 20742. Note. Discussion open until January 1, 2007. 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 August 4, 2005; approved on February 4, 2006. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 8, August 1, 2006. ©ASCE, ISSN 10900241/2006/8-990–1000/$25.00.
U.S. Environmental Protection Agency 共USEPA兲 Subtitle D regulations specify that this drainage layer should be designed to prevent a leachate head from exceeding 0.3 m above the liner. Additionally, a minimum thickness and hydraulic conductivity of 0.3 m and 1⫻10−5 m/s, respectively, are generally recommended for the design of a leachate collection layer by various state agencies 共Reddy and Saichek 1998a兲. Moreover, the drainage layer must also protect the underlying liner 共e.g., geomembrane兲 under great waste depths 共Warith et al. 2004兲. A uniform gravel or sandy gravel has typically been used in constructing the leachate collection layer. The high cost of gravel associated with its processing and the fact that it is a nonrenewable resource have prompted researchers to look for alternative materials. Scrap tires have been proposed as an alternative due to their high hydraulic conductivities. Several laboratory studies have been conducted to show the effectiveness of scrap tires in a leachate collection layer 共Hall 1991; Edil et al. 1992; Humphrey and Manion 1992; Reddy and Saichek 1998a; Warith et al. 2004; Rowe and McIsaac 2005兲. The drainage performance of scrap tires, as well as their capability in protecting the underlying liner system, was also observed in field studies 共Evans 1997; Reddy and Saichek 1998b; Park et al. 2003兲. According to these studies, scrap tires have hydraulic conductivity one to two orders of magnitude higher than the design performance criterion of 1⫻10−5 m/s, even under significant laboratory confining pressures. Most of the existing work showed that scrap tires did not cause groundwater or surface water contamination, since the measured concentrations of constituents leached from the tires were significantly below the USEPA maximum concentration limits 共Humphrey and Katz 2001; Park et al. 2003兲. Laboratory column studies indicated that scrap tires may be more prone to clogging than uniform gravels in critical zones 共e.g., near leachate sumps兲; however, tires are not likely to pose problems in leachate collection layers provided that uniformly graded large-size tire chips are
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placed at appropriate design thicknesses 共Rowe and Mclsaac 2005兲. Existing research has indicated that scrap tires can be effectively used in landfill leachate collection systems. However, limited information about their field performance, including the hydraulic performance as well as combustibility and leaching behavior in field drainage systems, have prevented their widescale use in landfill applications. The objective of this study was to investigate the field performance of scrap tires in landfill leachate collection systems and provide additional data to demonstrate their effectiveness as a drainage medium. To achieve this objective, laboratory tests were conducted to determine the hydraulic conductivity and compressibility of scrap tires. The drainage performance and environmental suitability of tires, as well as their potential for combustion hazard, were also observed in a field test cell.
Laboratory Tests Material Characterization The tire chips used in this study were obtained from a material supplier in Baltimore, Md. Conventional grain size distribution analysis 共ASTM D 422兲 was not applicable to tire chips used in the current study due to their irregular sizes. The size of tire chips ranged from 25 to 100 mm in length, with an average chip size of 50 mm. A small percentage 共⬃5%兲 of the chips had sizes of 100–200 mm in length, and these tire pieces were eliminated before the laboratory tests. Additionally, small size crumb rubber present in the chips 共about 3–5%兲 was eliminated by sieving through a 2-mm sieve 共U.S. Sieve size No. 10兲. The uncompacted 共loose兲 dry unit weight of the tire chips ranged from 4.1 to 5.2 kN/m3. The water content of the material was 2.7%. Methodology A large-scale permeameter 共280 mm in diameter and 508 mm in height兲 was constructed from acrylic to determine the hydraulic conductivity of the tire chips 共Fig. 1兲. A loading piston with a capacity of 960 kPa applied varying stresses on the specimens during the hydraulic conductivity test. Three layers of 0.15-mmthick plastic sheet were placed between the tire chips and the acrylic to minimize side-wall friction based on the suggestions of McIsaac and Rowe 共2005兲. As a result of this treatment, approximately 85% of the applied load was transmitted to the base of the column. The test specimens were mostly comprised of wire-free tire chips, but also included small percentages 共⬃15%兲 of wirecontaining chips 共1–12% of the chips included wires greater than 57 mm in length兲. Before starting the tests, the load was varied on the tire chips that were loosely placed in the permeameter, and the change in height and density of the specimen at the end of each loading cycle was recorded to analyze its compressibility behavior. A maximum compressive stress of 219–244 kPa was selected. This pressure corresponded to a waste height of 30–33 m in a landfill, assuming an average unit weight of 7.35 kN/m3 for moderately to well-compacted solid waste 共Warith et al. 2004兲. A constant head was maintained on the specimens by sending the flow from the bottom and collecting from the top. Tap water was used as the influent. Each test was terminated after ensuring the stabilization of flow. All tests were conducted under controlled temperature
Fig. 1. Laboratory hydraulic conductivity test device
conditions 共24±0.5°C兲. Four sets of measurements were conducted on tire chip specimens collected from the sample batch.
Field Study Construction of Cells To investigate the performance of tire chips in a landfill leachate collection system, two 2.4-ha cells were constructed in Garret County Landfill located in Oakland, Md. Cell 1 served as a control cell and included a 0.6-m-thick layer of gravel as its leachate collection layer. The gravel was crushed from local limestone 共Greenbriar formation兲. Particle size distribution analysis indicated that the D85 and D10 sizes of the gravel ranged from 19 to 23 and from 5 to 8.5 mm, respectively. The material had a coefficient of uniformity of 2.5. Cell 2 was comprised of tire chips. The chips were placed on a geocomposite drainage layer, which was underlain by a 1.5-mm-thick 共60-mil兲 textured high density polyethylene 共HDPE兲 geomembrane, and a geosynthetic clay liner 共GCL兲. Therefore, the tire chips acted as a filtration/ protection layer in addition to their assistance to drainage by the geocomposite. The geocomposite was formed of a geonet with a needle-punched nonwoven geotextile heat-bonded to its two sides. The GCL was manufactured by needle punching the bentonite through upper and lower nonwoven needle-punched geotextiles. The hydraulic conductivity and hydrated internal shear strength of the GCL were 5⫻10−11 m/s and 24 kPa based on the laboratory tests performed per ASTM D 5084 and D 5321, respectively. The tests conducted following the guidelines outlined in ASTM D 4833 indicated that the geomembrane has a puncture resistance of 0.36 kN. The details of the liner system are shown in Fig. 2共a兲. In order to protect the geomembrane liner, the tire chips were placed in two separate lifts: a 0.25-m-thick wire-free chip layer directly placed on the geocomposite layer that was overlain by a 0.75-m-thick wire-containing tire-chip layer. The thickness of the tire-chip layer was higher than the ones used in previous field
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Fig. 2. Details of: 共a兲 liner system; 共b兲 gas collection system 共not to scale兲
investigations 共Park et al. 2003; Edil et al. 2004兲; however, it was still lower than the recommended maximum limit of 3 m by ASTM D 6270. The construction of the cells was completed in November 1999, and in 2 years municipal refuse was placed over the cells in two lifts to a total height of about 6 m. Monitoring Program A 4-year monitoring program was established to evaluate the performance of the tire-chip layer. The evaluation was based on the criteria that the tire chips would not spontaneously combust and the drainage performance of the tire layer would remain satisfactory to prevent ponding of leachate on top of the tire-chip layer. Moreover, the tire chips should not affect the chemical composition of the leachate in a manner that would prohibit using wellestablished, economical leachate treatment techniques. In order to satisfy these criteria, the leachate generation rates of the two cells were compared, and the leachate collected periodically from both cells was analyzed for chemical constituents. A gas vault was built in Cell 2 to collect and measure gases, which may potentially cause combustion in the tire-chip layer. Furthermore, thermistors were installed in Cell 2 to record the changes in temperatures. Fig. 3 provides a plan view of Cell 2 and schematic of the instrumentation setup. The hydraulic performance of the tire-chip layer was monitored through measuring the flow rate of leachate generated by each cell using a flowmeter. Additionally, the Garrett County Landfill authority collected leachate samples monthly to confirm the rates recorded by the flowmeters. The samples were collected via a wet cell for Cell 1 and a manhole for Cell 2. Two concrete lagoons were built to store the water that cannot be held by the wet well in case of high precipitation in the area. Preliminary
Fig. 3. 共a兲 Construction plan details of Cell 2 关land contours are in feet 共1 ft⫽0.304 m兲兴; 共b兲 instrumentation details for each location 共A1–C4兲 共not to scale兲
analyses indicated that the leachate flow rates measured by the two methods were comparable, and therefore the data recorded by the flowmeters were used to evaluate the hydraulic performance. To investigate the possibility of spontaneous combustion in Cell 2, the conditions that are likely to promote the combustion hazard were monitored. These conditions included the temporal variations in temperature and presence of combustible gases, including oxygen and carbon monoxide. Gas composition was monitored every 3–4 months by withdrawing gas from a gas vault placed within the tire-chip layer between Locations C3 and C4. The vault was formed of a 305-mm-diameter, perforated HDPE pipe with end caps. Attached to this was a 25-mm-diameter HDPE pipe that protected the sampling tube, which was extended from the sampling vault to the landfill perimeter berm where the sampling point was located 关Fig. 2共b兲兴. The sampling tube was a polyvinyl chloride 共PVC兲 vacuum tubing, with a 6.4 mm inside diameter and a 15.6 mm outside diameter. PVC is generally suspected to retain gas molecules on its surface and is not recommended for analyses below the parts per million 共ppm兲 range; however, this was not a concern for the carbon dioxide, methane, and oxygen analyses, because these gases were measured in the percent by volume range. The potential for the PVC tubing to affect the analysis of carbon monoxide in the ppm range was minimized by “conditioning” the tubing to fill receptor sites on the tubing surface with gas molecules prior to actual gas sampling and analysis. Conditioning was accomplished simply by pumping through the tubing a volume of the sample gas equal to several
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times the volume of the tubing, and measuring steady-state effluent concentrations over a period of time. The samples were analyzed for methane, carbon dioxide, and oxygen using a VRAE PGM-7800 gas analyzer 共multigas monitor兲 and the Bacharach GA 94 landfill monitor. A sample of gas from the exhaust port of either device was collected in a Tedlar bag and its carbon monoxide concentrations were measured using colorimetric tubes. The monitoring of air temperatures and the temperatures in Cell 2 was performed using 25 permanently installed Roctest Model TH-1 thermistors. Thermistors were placed at the top and bottom of the tire-chip layer at all 12 locations 共i.e., A1–C4兲. Two thermistors were installed at top of the layer at Location A2 for quality control. The data logger recorded the temperatures daily. The purposes of the thermistors were: 共1兲 to evaluate the likelihood that temperatures above the action thresholds would develop before the entire cell was covered with waste; and 共2兲 to measure the normal variation of temperature within a small area to aid in interpreting the variation of temperatures within the entire tirechip layer. The effect of tire chips on leachate composition was evaluated by comparing analytical results for leachate collected from Cells 1 and 2. Leachate samples were collected from sumps of the two cells every 3–4 months via the wet cell for Cell 1 and the manhole for Cell 2. The only exception was for the first 6 months of the monitoring period, when leachate from Cell 2 was collected via an outlet to a stormwater management area. Grab samples of leachate were collected following the procedures described in ASTM D 6759. The collected leachate was relatively fresh since the wet cell or concrete lagoons were drained periodically as they were sampled. The leachate samples were analyzed for the total as well as dissolved concentrations of the following 13 metals: silver 共Ag兲, arsenic 共As兲, barium 共Ba兲, cadmium 共Cd兲, chromium 共Cr兲, copper 共Cu兲, iron 共Fe兲, mercury 共Hg兲, manganese 共Mn兲, nickel 共Ni兲, lead 共Pb兲, selenium 共Se兲, and zinc 共Zn兲. These constituents were selected because they fall in one or more of the following groups: 共1兲 metals for which a regulatory toxicity characteristic concentration has been established, as identified in USEPA 40 CFR 261.24 共i.e., Ag, As, Ba, Cd, Cr, Hg, Pb, Se兲; 共2兲 metals contained in the National Primary Drinking Water Standards as identified in USEPA 40 CFR 141.11 共i.e., As, Ba, Cd, Cr, Cu, Hg, Ni, Pb, Se兲; and 共3兲 typical metals for which acceptance criteria have been established by leachate treatment facilities 共i.e., Cd, Cu, Hg, Ni, Pb, Zn兲. Iron and manganese were added to the list of analyses because an increase in the concentrations of these two metals would likely indicate that oxidation of the steel wires in the tire chips was occurring. Additionally, the leachate samples were analyzed for nitrite/nitrate, sulfate, and 37 different volatile organic compounds 共VOCs兲, which are likely to leach either from the tire chips or the refuse. A Hewlett-Packard 4500 inductively coupled argon plasma mass spectrometer was used for analyzing the cations following the procedure outlined in the USEPA Method 6020. The USEPA Method 300 was followed to analyze anions 共nitrate, nitrite, and sulfates兲 using an ion chromatograph. Mercury was analyzed according to the USEPA Method 7471 using a cold vapor atomic absorption spectrophotometer. VOCs were analyzed using a gas chromatography/mass spectrometry system. The guidelines in the USEPA Method 8260B or 8270C were followed for VOC analysis, depending on the volatility level of the contaminant. A Horiba U-10 meter was used to measure the pH and electrical conductivity of the samples.
Fig. 4. 共a兲 Compressibility versus compressive stress; 共b兲 hydraulic conductivity versus compressive stress relationships for tire chips tested
Results Laboratory Test Results Fig. 4共a兲 shows the compressibility 共i.e., strain兲 behavior of tire chips tested in the laboratory. The tire chips are highly compressible as observed by the increase in compressibility and dry unit weight with increasing compressive stress. Hall 共1991兲 reported 36% compression for tire chips 共19–38 mm in size兲 when compressed under 70 kPa. Warith et al. 共2004兲 reported that the compressibilities of 75-mm-long tire chips increased from 30 to 50% with increasing stresses from 65 to 340 kPa regardless of the quality of the chips 共i.e., edges cut in regular shapes versus torn or shredded兲. Rowe and McIsaac 共2005兲 observed that tire chips exhibited a compressibility of 44–48% under an applied load of 150 kPa, which is comparable to a compressibility of 40–46% recorded in the current study. Similar trends, i.e., increase in compressibility with increasing stress, were also observed by Edil et al. 共1992兲, Humphrey and Manion 共1992兲, Donovan et al. 共1996兲, and Reddy and Saichek 共1998a兲. The trends in the compressibility versus stress curve as well as the values measured in the current study, are comparable with those reported by previous researchers. As seen in Fig. 4共b兲, the hydraulic conductivity of tire chips decreased more than an order of magnitude when the stress was
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Fig. 5. 共a兲 Volume of leachate generated by two cells; 共b兲 precipitation versus time in vicinity of landfill
increased from 0 to 244 kPa. The initial and final hydraulic conductivities of the chips ranged from 0.98 to 1.3, and from 0.02 to 0.05 m/s, respectively, indicating a 20–65 times decrease in hydraulic conductivity. Similar to the observations made by Edil et al. 共1992兲, Reddy and Saichek 共1998a兲, and Warith et al. 共2004兲, the results of the current laboratory study indicates that, even at high compressive stresses, tire chips possess a hydraulic conductivity of greater than 1⫻10−5 m/s, a value generally recommended for the design of landfill leachate collection systems. The hydraulic conductivity value was much higher at 45 kPa, a pressure that is likely to be exerted on tire chips in Cell 2 due to a placement of 6 m of waste. Field Hydraulic Performance The hydraulic performance of the tire-chip layer was evaluated based on the flow rate of leachate drained periodically from the two cells. Fig. 5共a兲 presents the leachate flow rates. Due to a large number of data points, the flow rates at selected intervals are presented. The data indicate that the leachate flow rates generated by Cell 2 are comparable to those generated by Cell 1. Moreover, the total volumes of leachate collected from Cells 1 and 2 in 4 years were 4.2⫻104 and 4.1⫻104 L, respectively. These data indicate that no significant change in drainage conditions occurred due to use of tire chips instead of gravel as the leachate collection
Fig. 6. Temporal changes in concentrations of gases collected from Cell 2
layer material. The precipitation had an effect on the generated leachate flow rates, as shown in Fig. 5共b兲. For instance, the flow rates dropped in February 2002 due to low rainfall and increased in February 2003 and November 2003 due to high rainfall. Since both cells were filled with similar amounts of waste and the filling rates were nearly the same, the fluctuations in leachate flow rates were attributed to precipitation. The comparable leachate flow rates generated by the two test cells during the 4-year monitoring period indicated that adequate drainage conditions were present within the tire-chip layer. Field Spontaneous Combustion Gas concentrations in Cell 2 were measured during each of the monitoring events. The methane concentrations were not detectable before waste filling 共⬍0.05%兲, with an increase to about 20% by July 2001, as seen in Fig. 6. After this date, the methane concentrations exhibited a decreasing trend and the recorded concentration on July 2003 was 18%. A similar trend, initial increase followed by a decrease, was also observed for carbon dioxide concentrations. On the other hand, carbon monoxide and oxygen concentrations decreased to below detection limits by July 2003. The temporal fluctuations in concentrations of the four chemicals are attributed to active filling of refuse in the site. The measured methane, oxygen, and carbon dioxide concentrations are highly comparable with a range of values measured in municipal solid
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Fig. 7. Temperatures registered by thermisters located at different depths and locations inside Cell 2: A⫽first lift of waste; B⫽second lift on A4; C⫽first lift on B4 and C4; D⫽first lift on A3, B2, and C2; E⫽complete cell covered with waste; and F⫽power outage
waste landfills in the United States 共Qian et al. 2002兲. These observations indicated that normal anaerobic decomposition of the municipal solid waste was occurring within Cell 2. The presence of insignificant quantities of carbon monoxide and lack of oxygen suggested that a combustion hazard was not likely to exist. In order to further investigate the possibility of spontaneous combustion, temperatures at different depths inside Cell 2 were monitored during waste filling. Thermistors that were placed at the bottom of the tire-chip layer at Locations A1 and B4 recorded erroneous data from the beginning of the construction, and were eliminated from the monitoring program. Fig. 7 presents the temporal variations in temperature along with the corresponding ambient air temperatures. For clarity of presentation, the data were separated into four groups. The maximum temperatures inside the tire chips are between 28 and 61°C, which is comparable to the temperatures observed in solid waste landfills 共Ham and Barlaz 1987兲. Furthermore, the temperatures are well below 204°C, the approximate threshold temperature for combustion of tire chips 共PSI 1994兲. This is mainly due to the fact that the thickness of the tire-chip layer was less than 3 m and large shreds with a minimum of rubber fines were used in construction, as recommended in ASTM D6270. The two major fire events with use of tire chips occurred in 1996. Roadbeds constructed with 8 and 15-m-thick sections of tire chips experienced catastrophic internal heating reactions due to retention of heat caused by the high insulating value of tire shreds in large fills 共Humphrey 1996; Moo-Young et al. 2003兲. Due to these events, Humphrey et al. 共1997兲 recommended a decrease in the thickness of tire-chip layers in geotechnical
construction. Other probable causes of fires include microbes that consume exposed steel wires or liquid petroleum products from the surface of tire chips, microbes creating an acidic environment that increases the rate of oxidation of steel wires, and oxidation of exposed steel wires or rubber exposed at the cut edges of tire chips due to free access to air and water 共Rubber Manufacturers Association 1997兲. Although these processes may potentially occur to some degree in Cell 2, the small layer thickness and lack of oxygen in the tire-chip layer in the current study suggest that the combustion hazard is not likely to exist. Donovan et al. 共1996兲 presented several case studies in regard to tire chip reuse in landfill applications and concluded that tire chips placed in thin layers are much less prone to spontaneous combustion than those placed in very thick lifts. The data given in Fig. 7 indicate that once the waste was placed over the entire area of the cell, the temperature decreased a little and stayed at an equilibrium temperature for the rest of the monitoring period. For instance, filling of A4 started in February 2000, and the temperatures at the top of the tire-chip layer increased from 10 to about 31°C 关Point A in Fig. 7共a兲兴. Small increases in temperature can be seen when the second lift of waste was placed on A4 in December 2000 共Point B兲 and the first lift was placed on B4 and C4 in April 2001 共Point C兲. The temperatures increased to about 35°C when a lift was placed on A3, B2, and C2 in July 2001 共Point D兲. When the entire cell was covered with waste in December 2001, the temperatures started to decline 共Point E兲, and dropped to about 17°C after 2 years since the solid waste acted as an insulator. Similar trends can be observed for the temperatures recorded at the bottom of the tire-chip layer, with
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Fig. 8. Temporal changes in: 共a兲 pH; 共b兲 electrical conductivity of leachate collected from two cells
the final temperatures ranging from 16 to about 24°C 2 years after the entire cell was covered with waste. The effect of waste filling history was less pronounced on the thermistors located at the bottom, mainly due to insulating effects of the overlying waste 关Fig. 7共d兲兴. During the summer months, the thermister readings at the bottom of the tire-chip layer were near ambient temperatures, but were generally 10–20°C warmer than the air temperatures during the winter months. The thermistors at the top of the layer tended to more closely follow air temperatures until they were buried with solid waste in December 2001. Field Leachate Quality Leachate samples were collected from the two cells periodically and transported to the laboratory for chemical analyses. The pH and electrical conductivity 共EC兲 measurements conducted on the samples are given in Fig. 8. The relatively high pH values 共pH⬎7兲 in March 2000 indicate that the waste was still in an aerobic state of decomposition in Cell 2, as only a fraction of the final amount of waste has been placed above the drainage material for a short period of time 共⬃8 months兲. This observation can further be supported by the relatively low leachate flow rates, low methane and carbon dioxide concentrations, and high oxygen concentrations as shown in Figs. 5 and 6, respectively. By July 2003, on the other hand, the soil has probably reached an
anaerobic decomposition stage as evidenced by a decrease in pH 共pH⬍7兲 and oxygen concentrations, as well as an increase in flow rates and carbon dioxide and methane concentrations. The pH of Cell 2 leachate was typically very close to that of Cell 1 leachate, indicating that tire chips did not leach out any acidic or alkaline compounds. In general, the pH in leachate of either of the cells was between 6 and 8, comparable with the range of pH recommended in the USEPA National Secondary Drinking Water Standards 共USEPA 2002兲. The electrical conductivity of the leachate produced from the tire-chip layer was generally lower than that of the one from the gravel layer. The EC of the leachate from Cell 1 was 10.8 mS/cm in May 2001 and increased to about 13 mS/cm in October 2001, most probably due to variations in refuse type placed in that cell. However, the EC values dropped back to around 6.2 and 5.8 mS/cm for Cells 1 and 2, respectively, by July 2003. A summary of the analytical data for the 4-year monitoring period is presented in Tables 1 and 2. For comparison purposes, only the data from the three sampling events are presented in Tables 1 and 2, even though samples from the test cells were collected and concentration measurements were performed at every 3–4 months. Silver 共Ag兲, mercury 共Hg兲, cadmium 共Cd兲, and 20 of the VOCs tested were not detected in leachate collected from either cells during any of the sampling events. In general, the leachate collected from the tire-chip layer has lower total and dissolved metal concentrations than those collected from the gravel layer. A comparison of total concentrations on July 2000 and July 2003 from Cell 2 indicates that concentrations of iron and zinc have increased after passing through the tire chips. Similar trends can be observed for copper, chromium, and barium to a lesser degree. Although this trend was not observed when the July 2000 data were compared to the January 2002 data, the samples collected on January 2002 were preceded by significantly less rain than either July 2000 or July 2003 dates which may have affected the leachability of these metals from either the decomposing waste or as they passed through the tire chips. However, these metals have precipitated out of solution 共likely onto suspended solids in the leachate兲 as evidenced by a decrease in dissolved concentrations of iron and an insignificant change in dissolved concentrations of the other three metals when July 2000 and July 2003 data of Cell 2 are compared. The total metal concentrations include the precipitates of ions and can be considered less mobile with respect to groundwater contamination, therefore dissolved metal concentrations would be more appropriate to assess the potential contamination of groundwater by tire-chip leachate 共Park et al. 2003兲. The dissolved concentrations of most of the inorganics and VOCs leached from the tire-chip layer were below the USEPA maximum concentration limits 共MCLs兲 and water quality limits 共WQLs兲. The only exceptions from the inorganics were iron and manganese, which were also reported to leach from tire chips under laboratory or field conditions 共Grefe 1989; Edil and Bosscher 1992兲. However, it is difficult to say that the tire chips used in the current study leached out iron and manganese, since the dissolved concentrations of these constituents are also high in the leachate collected from Cell 1 when end-of-monitoring conditions are considered 共i.e., July 2003兲. Similar observations can be made for some of the VOCs 共e.g., acetone, 2-butanone, 4-methyl-2-pentanone, vinyl chloride兲. It is likely that most of the metals and VOCs originated from the municipal refuse. Precipitation would carry the contaminants through the gravel or tire-chip layer and would exit through the well or the manhole.
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Table 1. Concentrations of Inorganics Collected from Two Test Cells 共all Concentrations are in mg/L兲 July 2000 Metal Arsenic Barium Chromium Copper Iron Lead Manganese Nickel Selenium Zinc
January 2002
July 2003
USEPA MCL
USEPA WQL
Cell 1 共gravel兲
Cell 2 共tire chips兲
Cell 1 共gravel兲
Cell 2 共tire chips兲
Cell 1 共gravel兲
Cell 2 共tire chips兲
0.01 2.0 0.1 1.3 0.3 0.015 0.05 NR 0.05 5
0.34 NR 0.57 0.013 1.0 0.065 NR 0.47 0.05 NR
0.008 0.24 0.04 0.007 40 ND 2.6 0.16 ND 0.11
共a兲 Total metals ND 0.12 ND 0.004 28 ND 1.2 ND ND 0.017
0.027 0.16 0.03 0.006 20 ND 1.1 0.16 ND 0.28
0.02 0.15 0.04 0.02 4.1 ND 2.1 ND ND 0.12
0.016 0.2 0.13 ND 230 0.007 22.4 0.2 0.01 17
0.009 0.2 0.09 0.01 156 0.007 6.8 0.1 0.007 1.5
共b兲 Dissolved metals ND 0.026 0.02 0.008 0.008 ND 0.16 0.14 0.008 0.008 ND 0.03 0.036 0.08 0.04 ND ND ND ND 0.005 26 3.2 1.6 128 2.4 ND ND ND ND ND 1.2 1.1 2.1 22.4 6.5 ND 0.16 ND 0.18 0.09 ND ND ND 0.009 0.006 ND 0.006 0.016 12 0.04 共c兲 Other Nitrate/nitrite 10/1 NR ND ND ND ND ND ND Sulfate 25.0 NR ND 17 34 69 275 8.2 Note: NR⫽not reported; and ND⫽not detected. Limits for iron, manganese, zinc, and sulfate are based on the USEPA National Secondary Drinking Water Standards, which are nonenforceable guidelines regulating contaminants that may cause aesthetic effects 共such as taste, odor, or color兲 in drinking water. The limits for the remaining constituents are based on USEPA maximum contaminant level 共MCL兲, which is defined as the highest level of a contaminant that is allowed in drinking water. MCLs are enforceable standards. USEPA water quality limits 共WQLs兲 are the allowable limits for nondrinking freshwaters. Arsenic Barium Chromium Copper Iron Lead Manganese Nickel Selenium Zinc
0.01 2.0 0.1 1.3 0.3 0.015 0.05 NR 0.05 5
0.34 NR 0.57 0.013 1.0 0.065 NR 0.47 0.05 NR
0.006 0.16 0.03 0.003 1.1 ND 2.6 0.15 ND ND
It is very typical for a municipal landfill to contribute to the concentration of metals and other VOCs, most likely due to the presence of batteries, electrical equipment, household appliances, rubber, and paint 共Park et al. 2003兲. The temporal variations in the concentration of inorganic compounds and VOCs did not yield noticeable trends and, therefore, are not presented herein. In order to provide a comparison between the leaching performances of the two cells, the average concentration of each chemical compound in a given cell during 14 sampling events within the 4-year monitoring period was calculated. Fig. 9 provides a ratio of average concentrations of inorganic compounds or VOCs in Cell 2 relative to Cell 1. Some of the VOCs were not present in Cell 2 共e.g., 1,1-dicholoroethene兲, and therefore were not included in Fig. 9. In general, leachate samples from the tire-chipcontaining cell have lower total metal, nitrite/nitrate, and sulfate concentrations than those from the gravel-containing cell. The exceptions are chromium and copper. Arsenic, nickel, lead, and zinc appear to be sorbed onto tire chips. Dissolved metal concentrations indicated that chromium is likely to dissolve into groundwater or surface water under field conditions. Possible chromium leaching from tire chips was also reported in several studies 共Rubber Manufacturers Association l990; Humphrey and Katz 1995; Park et al. 2003兲. However, the dissolved chromium concentrations in the current study ranged from 0.01 to 0.08 mg/L, which are lower than the USEPA MCL or WQLs.
Fig. 9 shows that, in general, the leachate collected from the tire-chip layer has average VOC concentrations lower than those collected from the gravel layer. The only exceptions are acetone, 4-methyl-2-pentanone 共MIBK兲, and cis 1,2-dichloroethene. However, the average as well as the final 共as of July 2003兲 concentrations of cis 1,2-dichloroethene determined in the leachate of Cell 2 are extremely low 共5 and 1 g/L, respectively兲, which indicates that the groundwater contamination is not likely to occur with this VOC due to tire chip placement. There may be two reasons for the phenomenon observed for acetone and MIBK. Either the municipal refuse placed on tire chips may have contained these chemicals or they may have directly leached from the tire chips. The former is the more likely reason because since very low concentrations of styrene, naphthalene, 1,2-dichlorobenzene and 1,4-dichlorobenzene 共average concentrations of 3.5, 3.1, 6.9, and 1.21 g/L, respectively兲 were only detected in the leachate collected from the cell that contained gravel 共Cell 1兲, indicating that the municipal refuse may include different VOCs at different locations. Moreover, the concentrations of these two chemicals fall in a range of data reported for municipal solid waste landfills in the United States 共Qian et al. 2002兲. Humphrey and Katz 共2001兲 reported elevated concentrations of acetone and MIBK from the tire chips placed below the water table; however, the concentrations decreased by time and were not considered as a major health concern. Furthermore, a detailed review of existing leaching test
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Table 2. Concentrations of Organics Collected from Two Test Cells 共all Concentrations are in g / L兲 July 2000 Volatile or semi-volatile organic compounds
MCL
USEPA WQL
Cell 1 共gravel兲
Cell 2 共tire chips兲
January 2002 Cell 1 共gravel兲
Cell 2 共tire chips兲
July 2003 Cell 1 共gravel兲
Cell 2 共tire chips兲
NR 2,400 230 560 2,900 3,790 5,040 Acetone 61a 51 1.7 ND ND 1.6 4.6 ND Benzene 5a,b NR 5,400 420 3,580 547 5,910 7,060 2-Butanone 190a NR 10 0.9 ND ND 5.9 1.5 Chloroethane 3.6a NR ND ND ND ND 5.4 ND 1,2-Dicholorobenzene 600a NR ND ND ND ND 1.4 ND 1,4-Dicholorobenzene 75a NR 1.9 6.8 ND ND 6.5 ND 1,1-Dicholoroethene 80a NR 2.2 ND ND ND 1.4 1.0 cis,1,2-Dicholoroethene 70a NR 4.7 ND ND ND 67 2.5 Dichloromethane 5a,b Ethylbenzene 700 2,100 3.0 ND ND 1.1 18 1.1 NR ND ND ND ND 61 137 4-mthyl-2-pntanone 50a NR ND ND ND 6.2 21 11 Methyl-tert-butyl-ether 40a NR ND ND ND ND 3.7 ND Naphthalene 10a NR ND ND ND ND 4.6 ND Styrene 100a,b 15,000 140 8.8 14 16 186 35 Toluene 1,000a,b 2.4 1.3 ND ND ND 2.0 1.9 Vinylchloride 2a,b NR 12 ND 3.3 ND 61 4.3 Xylenes 10,000a,b Note: NR⫽not reported; and ND⫽not detected. USEPA or Maryland Department of Environment 共MDE兲 maximum contaminant level 共MCL兲 is defined as the highest level of a contaminant that is allowed in drinking water. MCLs are enforceable standards. The MDE limits were adopted when no MCL was reported by the USEPA. The reported USEPA water quality limits 共WQLs兲 are the allowable limits for human health for consumption of organisms only. a MDE. b USEPA.
results by Tatlisoz et al. 共1996兲 did not report any leaching of acetone or MIBK from tire chips. These results suggest that most of the VOCs originated from the municipal solid waste and the tire chips have negligible effect on water quality. The field leachate quality analysis of the current study supports the findings of the existing research. The toluene concentrations ranged from 2 to 194 g/L and from 1 to 97 g/L for Cells 1 and 2, respectively. These ranges corresponded to an average concentration ratio of 0.35 共Fig. 9兲, which indicates that the tire chips may have sorbed toluene. Sorption of toluene onto tire rubber was also observed in large-scale laboratory tests conducted by Park et al. 共1996兲. An extensive literature review in regard to evaluation of the environmental suitability of tire chips was made by Park et al. 共2003兲. Their study indicated that chromium, cadmium, iron, and manganese were the only inorganic compounds that were sporadically detected in laboratory leaching tests or during field evaluations. Rowe and McIsaac 共2005兲 reported possible leaching of very low concentrations of aluminum, zinc, iron, and copper; however, they concluded that these constituents were more likely to precipitate into the clog material and get trapped in the tire-chip layer in a landfill system. All laboratory tests indicated that the concentrations of analyzed metals were below the drinking water standard limits, except the EP-toxicity test study conducted by Grefe 共1989兲 which suggested that the iron and manganese concentrations may be above or at the standard limits. Even though several inorganic and organic compounds were detected in tire-chip leachates, concentrations were generally below laboratory toxicity characteristic leaching procedure 共TCLP兲 test limits and were not significantly higher than the USEPA MCLs. Furthermore, neither of the two field studies conducted by the Minnesota Pollution Control Agency 共1990兲 and Humphrey and Katz 共1995兲 indicated the presence of groundwater or surface water contamination due to tire-chip placement. Additional field
data provided by Park et al. 共2003兲 and Edil et al. 共2004兲 along with the results obtained in the current study suggest that tire chips can be safely used as part of a landfill leachate collection layer, even though it may not be suitable to place tire chips near drinking water sources.
Conclusions A study was conducted to investigate the performance of tire chips as leachate collection material in municipal solid waste landfills. Laboratory tests were performed to study the compressibility and hydraulic conductivity of tire chips under different stress levels. A subsequent field study was conducted to monitor the likelihood for spontaneous combustion as well as hydraulic performance and environmental suitability of leachate collection systems constructed from scrap tires. The following conclusions are presented: 1. The laboratory tests indicated that tire chips are highly compressible. A maximum compressibility of 46% was recorded under a compressive stress of 244 kPa by the end of the tests. The minimum hydraulic conductivity recorded at this stress level was 0.02 m/s; however, this value was still orders of magnitude higher than a recommended limit of 1⫻10−5 m/s. The values obtained in the current laboratory testing program are applicable to waste heights of up to 33 m in a landfill, and further testing should be conducted to define the trends for landfills with greater waste heights. 2. The leachate flow rates and total leachate volumes generated by the two field test cells are comparable indicating no retardation of leachate drainage due to use of tire chips instead of gravel as the leachate collection layer material.
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5.
rather than to tire chips as concentrations of these constituents are also high in the leachate collected from the gravel layer when end-of-monitoring conditions are considered. The data collected from the field test cell suggested that adequate drainage conditions were present within the tire-chip layer, a combustion hazard potential was not present, and that tire chips can be safely used as part of a landfill leachate collection layer, even though it may not be suitable to place them near drinking water sources. However, it should be noted that measured leachate flow rates, temperatures, and chemical concentrations are valid for the test cell constructed herein, and further analysis should be considered when extending the results to landfills with greater waste heights. The two test cells are currently being monitored for collection of information about the long-term performance of tire chips in leachate collection systems.
Acknowledgments The writers would to like extend their appreciation to Mr. Gary Love of the Garrett County Landfill for his help and supervision during the field work and suggestions for improvement of the manuscript. Dr. Richard McCuen is thanked for reviewing an initial draft of this paper.
References
Fig. 9. Comparison of concentrations of inorganics and VOCs in leachate collected from two cells
3.
4.
The field temperatures inside the tire chips were between 28 and 61°C, which is comparable to the temperatures observed in solid waste landfills. Moreover, the temperatures were well below the approximate threshold temperature for potential combustion of tire chips. The presence of insignificant quantities of carbon monoxide and the lack of oxygen suggested that a combustion hazard was not likely to exist, mainly due to the fact that the thickness of the tire-chip layer was less than 3 m and large shreds with a minimum of rubber fines were used in construction. The leachate collected from the tire-chip layer had lower inorganic compound, dissolved metal and VOC concentrations than those collected from the gravel layer. Furthermore, the concentrations of the inorganics and VOCs of samples collected from the tire-chip cell were below the USEPA maximum concentration and water quality limits. Relatively higher concentrations of some of the compounds 共e.g., iron, manganese, acetone, 4-methyl-2-pentanone兲 in the leachate collected from the tire-chip layer are attributed to differences in the chemical composition of refuse placed on the two cells
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