Influence of High-Permeability Layers for Enhancing Landfill Gas Capture and Reducing Fugitive Methane Emissions from Landfills

Gas collection systems of various designs have been used to control landfill gas emissions, which can be problematic, particularly before installation of final landfill covers. In this work, an innovative gas collection system that includes a permeable layer near the top surface of landfills was evaluated for enhancing capture of landfill gas and reducing fugitive methane emissions. A computational model that accounts for advective and diffusive fluxes of multiple gas components was used to evaluate the efficiency of this new design for intermediate landfill covers. The utility of the high-permeability gas-conductive layer was illustrated for several conditions of interest including varying refuse permeability, varying degrees of permeability anisotropy, and temporal atmospheric pressure changes. Simulations showed that the permeable layer decreased methane emissions by 43% when the horizontal to vertical permeability ratio for refuse was kh/kv = 3 and the domain average kh = 3×10?12?m2, while reductions in methane emissions decreased to 17% for the same anisotropy but with kh = 10?11?m2. With this design, barometric pressure changes did not significantly affect oxygen intrusion or methane emission rates.     

Biocover Performance of Landfill Methane Oxidation: Experimental Results

An experimental passive methane oxidation biocover (PMOB) was constructed within the existing final cover of the St-Nicéphore landfill. Its substrate consisted of a 0.80-m-thick mixture of sand and compost. The goal of this experiment was to evaluate the performance of the PMOB in reducing CH4 emissions when submitted to an increasing methane load. The CH4 load applied started with 9.3?g?CH4?m?2?d?1. When the site had to be closed for the winter, the CH4 input was 820?g?CH4?m?2?d?1. Throughout the study, practically all the CH4 input was oxidized; absolute removal rates were linearly correlated to methane loading; and the oxidation zone was established between 0.6–0.8 m. These results seem to indicate that the upper limit potential of this PMOB to oxidize CH4 was not necessarily reached during the study period. Surface CH4 concentration scans showed no signs of leaks. The substrate offered excellent conditions for the growth of methanotrophs, whose count averaged 3.91×108?CFU?g?dw?1 soil.     

Characterization of Methane Flux and Oxidation at a Solid Waste Landfill

Methane emissions were measured at several locations at a typical solid waste facility using a static chamber technique. At the entire facility, methane flux varied from ?13.6?to?1,755?g?m?2?day?1. The flux data had an arithmetic mean value of 71.3?g?m?2?day?1 and a geometric mean value of 18.6?g?m?2?day?1. At this site, methane emission was generally lower on the side slopes relative to the flat areas of the landfill. The spatial variability of methane flux was characterized by point kriging and inverse distance weighing (IDW) in an intensive study of a 61×61-m area. The geospatial means in this area obtained by both methods were almost identical (20.9 versus 20.8?g?m?2?day?1). These geospatial means for the area were also similar to the arithmetic mean (24.5?g?m?2?day?1), but 3.4 times the geometric mean (6.5?g?m?2?day?1). Methane oxidation was evaluated at the surface of the landfill and at several depths within the cover soil using stable isotope techniques. The δ?13C of CH4 averaged ?55.4% in the anoxic zone. Methane collected in chambers and in surficial soil probes exhibited more 13C enriched values, ranging from ?55.4 to ?34.5%, due to the preferential uptake of 12CH4 by methanotrophic bacteria. Methane oxidation at the landfill averaged 22% and occurred in the upper 70?cm of the landfill cover soil. Oxidation occurred in all tested locations of the landfill and for all ages of buried waste.     

Development of EPA OTM 10 for Landfill Applications

In 2006, the U.S. Environmental Protection Agency posted a new test method on its website called Other Test Method 10 (OTM 10) which describes direct measurement of pollutant mass emission flux from area sources using ground-based optical remote sensing. The method has validated application to relatively small bounded area sources but additional guidance is needed for large area sources, such as landfills, where the emission zone can exceed the size of optical configuration leading to difficulties in relating measured fluxes to emissions per unit area. This paper presents the findings of a series of tracer-release experiments designed to improve the understanding of OTM 10 in landfill applications. OTM 10 plume capture efficiency data acquired at a variety of landfill sites under a range of meteorological conditions and measurement configurations are presented. Experiments indicate an overall capture efficiency factor of 0.81 with a standard deviation of 0.33. Lower capture efficiencies from side slope releases are noted (0.69). The combined data set is analyzed for factors influencing capture efficiency. A multiple linear regression is used to model the capture efficiency as a function of primary parameters including distance of the tracer release from the observing plane and wind speed. A simplified model based on the regression analysis is described and its use for approximating the area contributing to flux is presented.     

Modeling of Biodegradation in an Old Unregulated European Landfill

In this note an example of modeling of biodegradation processes of an old abandoned municipal solid waste landfill for its simulation is illustrated using the landfill dynamic simulation tool MODUELO. In this program the waste biodegradation model is based on the quantification of organic matter, its chemical composition, biodegradability, accessibility to microorganisms, and the ratio nonbiodegradable leachable organic matter to gasifiable matter. Data from a characterization campaign, presented elsewhere, were used to determine these parameters. The experimental information was completed with safety factors (to compensate for sampling uncertainty) and literature values. The degradation rates were determined, after having calibrated the hydrological model, by fitting the temporal series of pollutants’ concentration measured in the leachate and the biogas composition. The achieved fit of the simulated series compared to the measured data is reported as the result of this work. Given the limited information available, the obtained simulation model is considered an acceptable tool to study the future evolution of the landfill in different circumstances.     

Field Performance of a Compacted Clay Landfill Final Cover at a Humid Site

A study was conducted in southern Georgia, USA, to evaluate how the hydraulic properties of the compacted clay barrier layer in a final landfill cover changed over a 4-year service life. The cover was part of a test section constructed in a large drainage lysimeter that allowed continuous monitoring of the water balance. Patterns in the drainage (i.e., flow from the bottom of the cover) record suggest that preferential flow paths developed in the clay barrier soon after construction, apparently in response to desiccation cracking. After four years, the clay barrier was excavated and examined for changes in soil structure and hydraulic conductivity. Tests were conducted in situ with a sealed double-ring infiltrometer and two-stage borehole permeameters and in the laboratory on hand-carved blocks taken during construction and after four years of service. The in situ and laboratory tests indicated that the hydraulic conductivity increased approximately three orders of magnitude (from ≈ 10?7?to? ≈ 10?4?cm?s?1) during the service life. A dye tracer test and soil structure analysis showed that extensive cracking and root development occurred throughout the entire depth of the barrier layer. Laboratory tests on undisturbed specimens of the clay barrier indicated that the hydraulic conductivity of damaged clay barriers can be underestimated significantly if small specimens (e.g., tube samples) are used for hydraulic conductivity assessment. The findings also indicate that clay barriers must be protected from desiccation and root intrusion if they are expected to function as intended, even at sites in warm, humid locations.     

Application of Fuzzy Logic to Estimate Flow of Methane for Energy Generation at a Sanitary Landfill

This paper proposes the use of a multicriteria assessment technique to evaluate the methane flow during gas extraction from a sanitary landfill. A number of parameters determine the gas generation and the feasibility for its extraction from a landfill. These parameters form a complex set of information with unknown mathematical interrelationships making potential gas flow evaluations difficult and elusive. In addition, the data available for a particular landfill are very often imprecise, uncertain, or subjective, making it even more difficult to evaluate the potential for gas extraction without conducting pilot tests. The method proposed in this paper uses fuzzy composite programming that allows for the use of imprecise information. A landfill gas potential index has been defined, which can be determined by easily obtainable climatological, geological, and landfill parameters. The landfill gas (LFG) potential index is related to the landfill gas flow using an empirical equation. The LFG potential model was calibrated and verified using data obtained from 61 landfills where gas extraction is being conducted. A sensitivity analysis was done to study the impact of variations in the input data on model output.     

Comparison of Metals Leaching from CCA- and ACQ-Treated Wood in Simulated Construction and Demolition Debris Landfills

Pressure-treated wood is often disposed of in landfills in the United States, very frequently in construction and demolition (C&D) debris landfills. C&D debris landfills in many states are not equipped with liner systems to protect groundwater. With the voluntary withdrawal of chromated copper arsenate (CCA) treated wood for most residential applications in January 2004, copper-based wood preservatives, including alkaline copper quaternary (ACQ), are more widely used. To evaluate the impact of metal losses from ACQ-treated wood disposed in C&D debris landfills and compare to those of CCA-treated wood under similar conditions, leachates from three simulated C&D debris landfills (lysimeters) were collected and analyzed for over a period of one year. The wood component in one lysimeter (the control lysimeter) contained pallet wood; the second lysimeter contained CCA-treated wood, and the third contained ACQ-treated wood. Each lysimeter was buried in an active landfill for temperature control. Several batch leaching tests [including the standardized toxicity characteristic leaching procedure (TCLP) and the synthetic precipitation leaching procedure (SPLP)] were also conducted for comparison purposes. Although the two lysimeters containing treated wood had elevated copper concentrations within the waste matrix, the concentration in the leachate samples from these lysimeters was below detection for Cu (<4?μg/L) throughout the duration of the experiment, likely a result of precipitation as copper sulfide mineral in the reducing conditions of the simulated C&D landfills. As expected, the lysimeter containing CCA-treated wood showed elevated concentrations of arsenic and chromium, with maximum concentrations of 1.16 mg/L and 0.2 mg/L respectively. Greater amounts of boron (B) leached from ACQ-treated wood than CCA-treated wood or pallet wood debris. The results suggest that copper leaching will not be a major concern upon the disposal of ACQ-treated wood in C&D debris landfills. Arsenic leaching from CCA-treated wood remains a concern for unlined C&D debris landfills.     

Parameter Evaluation and Performance Comparison of MSW Settlement Prediction Models in Various Landfill Types

Estimation of settlement for municipal solid waste (MSW) landfills is critical to the successful site operation and maintenance as well as to the future development of the sites. In this paper, the six existing settlement prediction models are analyzed according to the three landfill types. 15 MSW landfill sites are classified into three categories, Types I, II, and III depending on their settlement activity levels and fill age. Measured settlement data of each landfill site are plotted as a strain curve on a logarithmic time scale, and the slope of the curve is used to determine a settlement activity level. First, the parameters of each prediction model are evaluated to determine their typical values for each type of the landfills. The results will serve as a basis for engineers to determine if their calculated model parameters are within the typical ranges. Second, each model’s performance of predicting settlement for the landfill sites is analyzed against the measured settlement data to compare the model’s applicability according to the landfill types. The results recommend specific prediction models to be used in a certain landfill type(s).     

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.     

Composition and Characteristics of Excavated Materials from a New Jersey Landfill

The composition of material excavated from the Burlington County landfill in New Jersey was determined, and the major reclaimed fractions characterized. Based on a waste age map, 98 samples (80?kg each) collected from 13 gas extraction well borings were handsorted into 14 fractions and fines (<2.54?cm) that fell through the screen were collected. At least 50%, by weight, of the material was fines. The most abundant oversize materials (overs) fractions, by weight, were miscellaneous items, wood, other plastics [not polyethylene terephthalate or high density polyethylene containers], and paper. Less paper was found in the oldest (7.5–11.5?years) section of the landfill (P<0.10), most likely due to microbial degradation. Several of the characteristics of the materials excavated from the landfill, such as temperature, particle size, bulk density, volatile solids, and contamination were correlated with the age of the deposits made. High levels of adherent soil will likely prove to be an insurmountable obstacle to recycling most excavated waste fractions other than fines unless further processing is pursued.     

Effect of Apparent Cohesion on Translational Failure Analyses of Landfills

The scope of this paper is to develop a new analysis method introducing the effect of apparent cohesion of the waste and/or liner materials on the translational failure of landfills. The apparent cohesion of the liner materials seriously affects the factor of safety (FS), especially for a liner interface with a low friction angle and high apparent cohesion. Obtaining accurate and reliable apparent cohesion for various liner materials becomes an important approach for translational failure analysis. The relationship between B and FSave for c>0 is different with that for c = 0. Using an equivalent friction angle to represent the apparent cohesion of the liner materials may lead to an unsafe result. Also, a waste filling sequence, to keep the FS above a stipulated value during the landfilling process, can be generated. The values of FSV/FSave varies between 1.9 and 2.1 and are concentrated around 2.0 when FSave ≥ 0.95. The approximate solutions of FS with adequate accuracy can be obtained by assuming FSV = 2.0?FS when FS?0.95.     

Unit Weight Determination of Landfill Waste Using Sonic Drilling Methods

The total unit weight of waste in a 40.5?ha landfill, up to 34?m thick, was estimated based on tube samples taken from borings made using sonic drilling technology. The waste’s unit weight was needed for input into slope stability calculations, which were part of the landfill’s closure design. The sonic drill was capable of penetrating and obtaining samples of all types of materials in the landfill. No significant correlation of waste unit weight, with depth, was found. Instead, the waste samples’ constituents were observed to have the greatest effect on their unit weights. The site also offered the unique opportunity to back-calculate the waste’s overall unit weight using a numerical model, and knowledge of the in situ stresses in the underlying clay as determined by oedometer testing. These calculations indicated that samples taken, using sonic drilling, gave a good a approximation of the waste’s overall unit weight at each boring location.     

Evaluation of Decomposition Effect on Long-Term Settlement Prediction for Fresh Municipal Solid Waste Landfills

A considerable amount of settlement occurs due to the decomposition of municipal solid waste (MSW) in landfills over a period of years. Therefore, the effect of biological decomposition governs the long-term settlement characteristics of municipal solid waste landfills. In this study, we investigated the long-term settlement characteristics by applying a number of prediction methods to fresh MSW sites and predicting the settlement curves. Most proposed methods, excluding the power creep law, successfully predicted long-term settlement only if accelerated logarithmic compression due to decomposition of biodegradable MSW was included in the settlement prediction.     

Air Permeability of Waste in a Municipal Solid Waste Landfill

The permeability of compacted municipal solid waste in a landfill with respect to air (or gas) flow was estimated using a short-term air injection test. Air was added to 134 vertical wells installed at three different depths at flow rates in the range of 0.14?–1.4?m3?min?1 and the corresponding steady state pressures were recorded. The permeability of the waste with respect to airflow (described here as the air permeability) was estimated for different anisotropy ratios (kr/kz = 1, 10, and 100) using a steady state, two-dimensional, axisymmetric analytical fluid flow model in conjunction with the measured flow and pressure data. The air permeability of landfilled municipal solid waste modeled as an isotropic medium was found to range from 1.6×10?13 to 3.2×10?11?m2. The estimated air permeability results were on the low end of values previously applied to model landfill gas flow. Estimated air permeability decreased significantly with increasing waste depth. The lower permeability encountered in the deeper layers was primarily attributed to the lower porosity of the waste caused by higher overburden pressures and higher moisture content of waste in deeper layers of the landfill than in shallow layers. The results suggest that multiple wells screened at different depths provide greater control of air distribution within the landfill. Leachate recirculation was documented to impact the ability to add air. In addition to limitations posed by standing water in many of the deeper wells, waste exposed to leachate recirculation was found to be significantly less permeable to air when compared to original conditions.     

Antioxidant Depletion from a High Density Polyethylene Geomembrane under Simulated Landfill Conditions

Accelerated aging tests to evaluate the depletion of antioxidants from a high density polyethylene geomembrane are described. The effects of temperature, high pressure, and continuous leachate circulation on the aging of geomembranes in composite liner systems are examined. The antioxidant depletion rates (0.05, 0.19, and 0.41?month?1 at 55, 70, and 85°C, respectively) obtained for the simulated landfill liner at 250 kPa vertical pressure are consistently lower than that obtained from traditional leachate immersion tests on the same geomembrane (0.12, 0.39, and 1.1?month?1 at 55, 70, and 85°C). This difference leads to a substantial increase in antioxidant depletion times at a typical landfill liner temperature (35°C) with 40 years predicted based on the data from the landfill liner simulators tests, compared to 15 years predicted for the same geomembrane based on leachate immersion tests. In these tests, the crystallinity and tensile yield strain of the geomembrane increased in the early stages of aging and then remained relatively constant over the testing period. There was no significant change in other geomembrane properties within the testing period.     

Hydraulic Conductivity of Geosynthetic Clay Liners Exhumed from Landfill Final Covers

Samples of geosynthetic clay liners (GCLs) from four landfill covers were tested for water content, swell index, hydraulic conductivity, and exchangeable cations. Exchange of Ca and Mg for Na occurred in all of the exhumed GCLs, and the bentonite had a swell index similar to that for Ca or Mg bentonite. Hydraulic conductivities of the GCLs varied over 5 orders of magnitude regardless of cover soil thickness or presence of a geomembrane. Hydraulic conductivity was strongly related to the water content at the time of sampling. Controlled desiccation and rehydration of exhumed GCLs that had low hydraulic conductivity (10?9?to?10?7?cm/s) resulted in increases in hydraulic conductivity of 1.5–4 orders of magnitude, even with overburden pressure simulating a 1-m-thick cover. Comparison of these data with other data from the United States and Europe indicates that exchange of Ca and/or Mg for Na is likely to occur in the field unless the overlying cover soil is sodic (sodium rich). The comparison also shows that hydraulic conductivities on the order of 10?6?to?10?4?cm/s should be expected if exchange occurs coincidently with dehydration, and the effects of dehydration are permanent once the water content of the GCL drops below approximately 100%. Evaluation of the field data also shows that covering a GCL with a soil layer 750–1,000?mm thick or with a geomembrane overlain by soil does not ensure protection against ion exchange or large increases in hydraulic conductivity.     

Hydraulic Performance of Geosynthetic Clay Liners in a Landfill Final Cover

Percolation from a landfill final cover containing a geosynthetic clay liner (GCL) as the hydraulic barrier is described. The GCL was covered with 760?mm of vegetated silty sand and underlain with two gravel-filled lysimeters to monitor percolation from the base of the cover. Higher than anticipated percolation rates were recorded in both lysimeters within 4–15?months after installation of the GCL. The GCL was subsequently replaced with a GCL laminated with a polyethylene geofilm on one surface (a “composite” GCL). The composite GCL was installed in two ways, with the geofilm oriented upwards or downwards. Low percolation rates (2.6–4.1?mm/year) have been transmitted from the composite GCL for more than 5?years regardless of the orientation of the geofilm. Samples of the conventional GCL that were exhumed from the cover ultimately had hydraulic conductivities on the order of 5×10?5?cm/s. These high hydraulic conductivities apparently were caused by exchange of Ca and Mg for Na on the bentonite combined with dehydration. The overlying and underlying soils likely were the source of the Ca and Mg involved in the exchange. Column experiments and numerical modeling indicated that plant roots and hydraulic anomalies caused by the lysimeters were not responsible for the high hydraulic conductivity of the GCL. Despite reports by others, the findings of this study indicate that a surface layer 760?mm thick is unlikely to protect conventional GCLs from damage caused by cation exchange and dehydration. Accordingly, GCLs should be used in final covers with caution unless if cation exchange and dehydration can be prevented or another barrier layer is present (geomembrane or geofilm).     

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.     

Determination of Source Strength of Landfill Gas: A Numerical Modeling Approach

An accurate estimation of source strength of gas in a landfill is necessary to design gas extraction systems. A one-dimensional mathematical model to estimate source strength of landfill gas is presented. The model is based on fundamental principles of gas transport through porous media, and incorporates methane oxidation in landfill cover soils. The nonlinear mathematical equation for gas migration through layered soil was linearized and solved using the partially implicit Crank Nicolson technique. Apart from source strength, concentration and pressure as functions of depth and gas emissions into the atmosphere were computed. Laboratory experiments were conducted to generate data for model calibration and verification for mono- and two-layered systems. Other model parameters were estimated using information from published literature. The model data were in agreement with laboratory test results obtained for both systems.