Depletion of Antioxidants from a HDPE Geomembrane in a Composite Liner

The results of two series of accelerated aging tests are reported. Both series of tests were conducted at temperatures of 85, 70, 55, and 26°C over a period of about 3?years. In the simulated liner series, the top of the geomembrane was covered with a geotextile (protection) layer that was exposed to simulated municipal solid waste (MSW) landfill leachate while the bottom of the geomembrane was in contact with a hydrated geosynthetic clay liner. In the immersion series, the geomembrane was immersed in the simulated MSW leachate, and hence, both sides were exposed to leachate. The results from oxidative induction time tests indicate that the antioxidant depletion is about 2.2–4.8 times faster for the leachate immersed geomembrane than for geomembrane in a composite liner. The higher rates are attributed to the higher extraction of antioxidants from two sides of the geomembrane immersed in leachate. The measured antioxidant depletion rates are extrapolated to a range of temperatures (0–60°C) using Arrhenius modeling. At a liner temperature of 35°C, the calculated time for the depletion of antioxidants is about 40?years for a geomembrane in a composite liner compared to 10?years if it is simply immersed in leachate. These tests suggest that to obtain realistic estimates of geomembrane service life one needs data from tests that simulate the expected field conditions and that prediction based on immersion tests may underestimate the service life.     

Effects of Thickness on the Aging of HDPE Geomembranes

The results of an accelerated aging test program to evaluate the effect of thickness on the depletion of antioxidants from high-density polyethylene (HDPE) geomembranes and subsequent degradation of the physical properties are reported. Three commercially available HDPE geomembranes having nominal thicknesses of 1.5, 2.0, and 2.5 mm were examined. The geomembranes were immersed in a synthetic leachate at 85, 70, 55, and 22°C and tested for oxidative induction time, crystallinity, melt index (MI), tensile properties, and stress-crack resistance. The antioxidant depletion rate for the 1.5 mm geomembrane was faster than for the 2.0 and 2.5 mm geomembranes. Antioxidant depletion time was predicted at representative landfill temperatures of 20–60°C using Arrhenius modeling and was found to increase with geomembrane thickness for the three geomembranes examined. Based on the results of crystallinity, MI, and stress-crack resistance, the degradation of the geomembrane was slowest for the thickest geomembrane. These results suggest that a thicker geomembrane may have a longer service life (other things being equal).     

Aging of HDPE Geomembrane in Three Composite Landfill Liner Configurations

Laboratory-accelerated aging experiments conducted to examine the depletion of antioxidant from a geomembrane (GM) underlain by a geosynthetic clay liner (GCL) are described. Three different “protection” layers between the GM and overlying gravel and leachate are examined: (1) A traditional nonwoven geotextile (GT); (2) a GT-GCL; and (3) a GT-sand-GT layer. The GT-GCL protection layer gives an antioxidant depletion rate 0.59 to 0.66 times slower than the GT layer alone. The GT-sand-GT layer gives depletion rates 0.72–0.75 times that of the conventional GT alone. Based on Arrhenius modeling, the time required for depletion of antioxidants at 35°C is estimated to be 65 years for a GM with a GT-GCL protection layer, 50 years for a GT-sand-GT layer, and 40 years for a conventional GT protection layer. These times are all significantly greater than the depletion time for GM immersed in leachate (10 years) for the geomembrane tested.     

Permeation of BTEX through Unaged and Aged HDPE Geomembranes

The effects of aging of high-density polyethylene (HDPE) geomembranes on the diffusion and partitioning of a group of volatile organic compounds (VOCs) are examined. Two different 1.5?mm thick HDPE geomembranes were aged in the laboratory at 85°C by immersing in a synthetic leachate for up to 32?months. The results of partitioning and diffusion tests performed at room temperature on both unaged and aged geomembranes using a dilute aqueous solution containing four VOCs commonly found in landfill leachates [benzene, toluene, ethylbenzene, and xylenes (BTEX)] are reported. The diffusion and partitioning coefficients decreased with increased aging. The calculated permeation coefficients decreased by 36–62% after aging the geomembrane for about 10–32?months. This decrease in diffusion, partitioning, and permeation coefficients is related to the increase in geomembrane crystallinity during aging. A relationship between partitioning, diffusion, and permeation coefficients with the geomembrane crystallinity is established and could potentially be used to evaluate the migration of VOCs through HDPE geomembranes. Aging of HDPE geomembrane did not increase diffusive transport of organic contaminants.     

Modeling of Clay Liner Desiccation

The potential for the desiccation of clay liner component of composite liners due to temperature field generated by breakdown of organic matter in municipal solid waste landfills is examined using a model proposed by Zhou and Rowe. In these analyses, a set of fully coupled governing equations expressed in terms of displacement, capillary pressure, air pressure, and temperature increase are used, and numerical results are solved by using finite element method with a mass-conservative numerical scheme. The model results are shown to be in encouraging agreement with experimental data for a problem involving heating of a landfill liner. The fully coupled transient fields (temperature, horizontal stress change, suction head, and volumetric water content) are then examined for two types of composite liner system, one involving a geomembrane over a compacted clay liner (CCL) and the other involving a geomembrane over a geosynthetic clay liner (GCL). It is shown that there can be significant water loss and horizontal stress change in both the CCL and GCL liner even with a temperature increase as small as 20°C. The time to reach steady state decreases as boundary temperature increases. Under a 30°C temperature increase, it takes 5 years to reach the steady state water content with a GCL liner but 50 years with a CCL liner. The effects of various parameters, such as hydraulic conductivity and thickness of the liner, on the performance of the liner are discussed.     

Comparison of Solute Transport in Three Composite Liners

Three composite landfill liners were compared in this study based on leakage rate, mass flux, and sorptive capacity. One composite liner consisted of a geomembrane and a geosynthetic clay liner (GCL). The other two had a geomembrane and a thicker soil barrier (61 or 122 cm). The analyses employed one- and three-dimensional numerical models that were developed for analyzing contaminant transport through defects in the geomembrane component of composite liners and diffusion of volatile organic compounds through intact composite liners (i.e., composite liners without holes in the geomembrane). Cadmium was used to represent inorganic leachate constituents and toluene was used to represent organic leachate constituents. The composite liner, having a GCL had the lowest leakage rate of the three composite liners. For cadmium, the mass flow rate and sorptive capacity for the three composite liners varied within an order of magnitude. However, for toluene, the mass flux from the GCL composite liner was two to three orders of magnitude greater than that through composite liners having a thicker soil liner. Additionally, for leachate having similar concentrations of cadmium and toluene, the mass flux of toluene can be as much as seven orders of magnitude greater than that for cadmium. For toluene, the sorptive capacity of thicker liners was an order of magnitude greater than that for the GCL composite liner. Similar behavior is expected for other inorganic and organic solutes.     

Analytical Solution for Diffusion of VOCs through Composite Landfill Liners

Analytical solutions are presented for analyzing volatile organic compound (VOC) diffusion through intact composite landfill liners for two scenarios with boundary conditions at the base of either a VOC concentration of zero or a VOC mass flux of zero. A time-dependent concentration top boundary condition is included in the presented analytical solutions to model typical variations of VOC concentration in the leachate over time. The presented solutions are verified against alternative numerical solutions and applied to analyze dichloromethane diffusion through a composite liner. The analytical solutions are found to provide useful predictions of VOC concentration and mass flux for the design of composite liners. VOC concentrations and fluxes at the base of the composite liner at 30?years predicted by consideration of representative transient variation in leachate concentration, for an example problem, are nearly half of those when a constant leachate concentration assumed.     

Effectiveness of Scrap Tire Chips as Sorptive Drainage Material

Scrap tire disposal is a problem of growing concern. One solution to this problem is innovative methods for the reuse and recycling of scrap tires. Based on batch isotherm tests, scrap tire chips have been identified to be good sorbents of volatile organic compounds (VOCs) and could be used as leachate drainage layer material in solid waste landfills and in other similar applications. To demonstrate the effects of tire chips on the leachate they come in contact with in a drainage layer over a liner, large-scale tank tests simulating the drainage layer and the clay liner and also field tests were performed. Two cells were constructed in a landfill: one with scrap tire chips and the other with gravel leachate collection layer. According to the results of the large-scale tank tests and field tests, shredded tire chips have a significantly positive impact on the quality of the leachate with which they come in contact. The use of scrap tires in landfills would reduce the magnitude of the current tire disposal problem (a 1 ha landfill requires approximately 300,000 tires to fill 0.3 m of a leachate collection layer) and convert one waste into a beneficial construction material and simultaneously mitigate the problem of VOC transport from through landfill liners.     

Predicting Leakage through Composite Landfill Liners

Leakage through composite landfill liners having various characteristics was analyzed using existing analytical and numerical models developed for the study. Three-dimensional numerical models were used to analyze leakage through circular defects and two-dimensional numerical models were used to analyze leakage from defective seams. Leakage rates predicted with the numerical models were compared to leakage rates predicted using existing equations and analytical models currently being used. These comparisons show that existing equations and analytical models all have limitations and no universal equation or method is available for predicting leakage rates. To overcome some of the deficiencies in the existing equations and models, new equations were developed based on results from the numerical models. Recommendations are made for using the new equations, existing equations, and analytical models to predict leakage rates in thick composite liners having a geomembrane overlaying a compacted soil liner and thin composite liners having a geomembrane overlaying a geosynthetic clay liner.     

Performance of North American Bioreactor Landfills. I: Leachate Hydrology and Waste Settlement

An assessment of state-of-the-practice at five full-scale North American landfills operating as bioreactors is presented in this two-paper set. This paper focuses on effectiveness of liners and leachate collection systems, leachate generation rates, leachate recirculation practices and rates, effectiveness in moistening the waste, and settlement of the waste over time. Except in one case, the liner and leachate collection systems at the bioreactor landfills were similar to those used for landfills operated conventionally. Leachate generation rates increased approximately linearly with recirculation rate, but in all cases, the leachate generation rate was <300?L/m2?year. Leachate depths generally were maintained within regulatory requirements, even with the highest recirculation rates. Leakage rates from liners at bioreactor landfills, including alternative liner designs employing geosynthetic clay liners, are comparable to leakage rates from conventional landfills. Thus, based on the information gathered in this study, additional requirements or features for liners or leachate collection systems are not warranted for bioreactor landfills. Diminishing capacity of horizontal recirculation trenches is common. Experience at one landfill suggests that small doses at high frequency under substantial injection pressure can deter loss of trench capacity. Only those landfills that were aggressive in recirculation had achieved water contents near the field capacity. Increasing the amount of liquid that is added may be required to achieve field capacity at some landfills, particularly if a final cover is placed soon after waste grades are reached. The rate of time-dependent waste settlement attributed to biodegradation is about 1.6 times larger in bioreactor landfills than in conventional landfills, and increases as the recirculation dosage increases.     

Estimation of Mass Transport Parameters of Organic Compounds through High Density Polyethylene Geomembranes Using a Modifield Double-Compartment Apparatus

A modified double-compartment apparatus (MDCA) is used to estimate mass transport parameters of organic compounds through high density polyethylene (HDPE) geomembranes and to investigate the effects of aging and external tension of HDPE geomembranes on the mass transport of organic compounds. A developed one-dimensional partition–diffusion mass transport model successfully explains the mass transport of the organic compounds through the HDPE geomembranes in a dilute aqueous solution–geomembrane system. Similar to batch immersion tests, the HDPE–water partition coefficient (KHDPE–W) values of organic compounds are found to have close relationships with the octanol–water partition coefficient and the aqueous solubility; furthermore, the diffusion coefficient (D) values decrease with the increase of their molecular diameter. For HDPE geomembranes served in the landfill liner for 5 years and stretched by 8% of their initial length, KHDPE–W values for organic compounds increase by 5–58%, D values for organic compounds increase by 10–86%, and breakthrough times are faster, indicating more amounts of organic compounds may break through the HDPE geomembrane in fields than expected. The mass transport parameters from MDCA tests could be used with those from batch immersion tests interchangeably after mass loss and immobilization of organic compounds in MDCA tests are considered.     

Leachate Recirculation in Bioreactor Landfills Using Geocomposite Drainage Material

The key purpose of this study was to test the use of a permeable blanket made up of a geocomposite drainage layer (GDL) for leachate recirculation in municipal solid waste (MSW) landfills and to predict the observed leachate travel in the blanket using a numerical model. A 34?m long by 12?m wide permeable blanket made up of GDL was constructed at an active MSW landfill located in Michigan. Leachate was injected in the GDL using a perforated pipe placed centrally above the GDL along its length. Moisture content sensors, pressure transducers, thermistors, thermocouple sensors, and a vertical load sensor were embedded immediately below the GDL blanket to monitor the flow of injected leachate. After the blanket was covered with waste, leachate was injected into the blanket at rates ranging from 0.9 to 2.6?m3/h per meter length of the blanket. Data collected from the embedded sensors indicated that the injected leachate traveled at rates ranging from 5 to 18?m/h through the blanket depending upon the leachate injection rate. Only pressure transducers and thermistors were consistently able to detect migration of injected leachate once the blanket got saturated. Moisture content sensors could not register any change in readings once the blanket became saturated. Leachate injection pressure monitored over a period of about 12 months indicated no signs of clogging of the blanket. The leachate pressures measured immediately below the blanket were less than the net leachate injection pressure indicting that there was a head loss in the GDL blanket. Numerical modeling of liquid flow in the blanket indicated that predicted leachate travel in the blanket was consistent with the field data for assumed values of the waste hydraulic conductivity. In the absence of measured representative hydraulic properties of the waste, absolute verification of the field data was not possible.     

Operation and Performance of Horizontal Wells for Leachate Control in a Waste Landfill

This paper presents data on gas and leachate flow rates and leachate levels obtained during a 600?day pumping trial on three retrofitted horizontal wells in a domestic waste landfill at Rainham, United Kingdom. The changes in gas and leachate flow rate with time and atmospheric pressure, and the interaction between the two flows, are discussed. The spatial variability of the response of the leachate levels within the landfill is explored with reference to the anisotropy and heterogeneity of the permeability of the waste. It is shown that horizontal wells can be an effective means of controlling leachate heads near the base of a landfill, and that leachate levels must be measured using piezometers with a discrete response zone rather than fully screened observation wells if meaningful results are to be obtained. It is argued that the large amounts of gas pumped from the nominally saturated zone of the landfill must have come from the ongoing degradation of the waste within the zone of influence of the well.     

Effect of Gas on Pore Pressures in Wet Landfills

The waste in a landfill may become saturated due to many reasons, including leachate recirculation or extreme precipitation. As high saturation levels in waste are achieved, the permeability of the waste to landfill gas decreases. This may result in pore pressures that are greater than what would be predicted by fluid statics. A theoretical model for estimating the excess pore pressure at the bottom of saturated waste is derived. A finite difference procedure is then presented as an approximate solution to the model. It was found that below the level of saturation, the steady-state excess pore pressure distribution increases linearly similar to a hydrostatic distribution. Combining its effect with the static water pressure, the excess pore pressure may be accounted for by using an equivalent unit weight of fluid that is artificially higher than water. A parametric study of the input parameters showed that the equivalent unit weight of the pore fluid was highly dependent on the hydraulic conductivity of the waste, particularly if the hydraulic conductivity of the waste is lower than about 2×10?6?m/s.     

Numerical Examination of a Method for Reducing the Temperature of Municipal Solid Waste Landfill Liners

A method to control the increase in landfill liner temperature due to the heat generated by the waste is examined. The design involves installation of an array of cooling pipes beneath the waste. The feasibility of this system for cooling the liner was examined by performing a series of analyses for conditions based on the Tokyo Port Landfill. The results suggest that the introduction of a cooling system can substantially reduce liner temperature and consequently significantly increase the service life of a high-density polyethylene (HDPE) geomembrane liner in an engineered barrier system. The effects of pipe layout, pipe spacing, and coolant flow rate are examined. It is shown that a periodic pipe layout is the most efficient. Liner temperature decreases with increased coolant transfer flow rate     

Pilot-Scale Simulation of Landfill Bioreactor and Controlled Dumping of Fresh and Partially Stabilized Municipal Solid Waste in a Tropical Developing Country

Four pilot-scale lysimeters were used to study the benefits of landfill operation with and without leachate recirculation in tropical weather conditions. Young and old landfills were simulated by filling lysimeters with a segregated fraction of fresh municipal solid waste (MSW) and MSW mined from an open dump site, respectively, and periodically monitoring leachate quantity and quality and biogas quality. For each substrate, one lysimeter was operated as a bioreactor with leachate recirculation and another lysimeter was operated as a controlled dump, for a period of 10 months. Densities between 652 and 825??kg/m3 could be achieved with fresh and mined MSW. Despite such compaction during waste placement, bioreactor technology helps in leachate management, especially in the case of the young landfill lysimeter operated in tropical weather. The benefits of leachate recirculation in the young landfill lysimeter were evident from the significant decrease in chemical oxygen demand (COD) (86%), biochemical oxygen demand (BOD) (82%), dissolved organic carbon (DOC) (85%), and volatile solids (75%) in leachates. However, ammonia nitrogen (amm-N) and chlorides in the leachates accumulated in bioreactor landfills. Operating an old landfill lysimeter as a bioreactor seemed to have no exceptional advantage in the context of leachate management, although leachate recirculation enhanced the methane potential of both fresh and mined MSW.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.     

Organic Removal of Anaerobically Treated Leachate by Fenton Coagulation

The leachate from a Hong Kong landfill, containing 15,700 mg∕L of chemical oxygen demand (COD) and 2,260 mg∕L of ammonia nitrogen (NH3–N), was first treated in a UASB (upflow anaerobic sludge blanket) reactor at 37°C. The process on average removed 90.4% of COD with 6.6 days of hydraulic retention at an organic loading rate of 2.37 g of COD∕L?day. The UASB effluent was further treated by the Fenton coagulation process using H2O2 and Fe2+. Under the optimal condition of 200 mg of H2O2∕L and 300 mg of Fe2+∕L and an initial pH of 6.0, 70% of residual COD in the UASB effluent was removed, of which 56% was removed by coagulation∕precipitation and only 14% by free radical oxidation. It is obvious that H2O2 and Fe2+ had a strong synergistic effect on coagulation. The average COD in the final effluent was 447 mg∕L. Removing each gram of COD required 0.28 g of Fe2+ and 0.18 g of H2O2.     

Leachate Recirculation Using Permeable Blankets in Engineered Landfills

Subsurface leachate recirculation or liquid injection methods for municipal solid waste (MSW) landfills are horizontal trenches, vertical wells, and permeable blankets. In this study, results of field-scale testing and numerical modeling of a recently developed subsurface leachate recirculation system called permeable blankets have been presented. In the field, at a MSW landfill located in Michigan, the travel of injected leachate in a 60-m-wide by 9-m-long by 0.15-m-deep blanket made up of crushed recycled glass was measured using an automated sensing system consisting of sensors embedded in the blanket. Leachate injection rates used in the field and simulated in this study ranged from 1.1 to 3.6?m3/h per meter length of the injection pipe embedded in the permeable blanket. HYDRUS-2D was used to simulate the travel and pressure head of injected leachate in permeable blankets. The influence of the following parameters on the hydraulic performance of permeable blankets was evaluated: (1) hydraulic properties of permeable blanket and waste; (2) geometry of permeable blanket; (3) settlement of permeable blanket; (4) leachate dosing frequency; and (5) initial degrees of saturation of permeable blanket and waste. The key findings of the study are: (1) the rate and maximum distance of travel of injected leachate are a strong function of the relative hydraulic properties of the permeable blanket and underlying waste and the rate and frequency of leachate injection; and (2) the maximum pressure head in the blanket due to liquid injection does not exceed the injection pressure. The field data and the numerical modeling results indicated that permeable blankets can be designed to inject liquids or recirculate leachate in MSW landfills. Long-term performance of such blankets needs to be evaluated.     

Physical Response of Geomembrane Wrinkles Overlying Compacted Clay

The short-term physical response of a 1.5-mm-thick, high-density polyethylene geomembrane with an artificially formed wrinkle and overlying three different subgrade materials (sand and compacted clay at two initial water contents) are reported. The influence of the subgrade, protection layer, backfill, and applied pressure on the fate of the gap beneath the wrinkle, wrinkle deformations, and local geomembrane indentations is investigated. The gap beneath the geomembrane wrinkle was observed to remain with sand above and below the geomembrane, even at applied pressures of 1,100?kPa. The gap was eliminated with compacted clay as the subgrade, depending on the applied pressure and the clay water content. When the clay was compacted at a water content equal to the standard Proctor optimum (ωopt)+4%, the gap was eliminated at pressures greater than 100?kPa, whereas the gap remained at 250?kPa and was eliminated at 500?kPa and larger when compacted at ωopt+1%. It was found that the presence of a wrinkle increases the maximum geomembrane strain due to local gravel indentations by 10% as compared to a flat geomembrane. The protection layers tested did not significantly influence the change in height and width of the wrinkle, but did influence the local geomembrane strain. The maximum strain in the geomembrane (at 250?kPa with 50?mm gravel backfill and the softer clay subgrade) was 42% without protection; 15 and 11% with nonwoven needle-punched geotextiles with mass per unit area of 390 and 1,200?g/m2, respectively; and 2% with a 150-mm-thick sand protection layer.