Micrometeorological measurements of N2O and CH4 emissions from a municipal solid waste landfill.

Micrometeorological measurements of methane (CH4) and nitrous oxide (N2O) emissions were made at the decommissioned Park Road Landfill in Grimsby, Ontario, Canada between June and August 2002. The influence of precipitation, air temperature, wind speed and barometric pressure on the temporal variability of landfill biogas emissions was assessed. Gas flux measurements were obtained using a micrometeorological mass balance measurement technique [integrated horizontal flux (IHF)] in conjunction with two tunable diode laser trace gas analyser (TDLTGA) systems. This method allows for continuous, non-intrusive measurements of gas flux at high temporal resolution. Mean fluxes of N2O were negligible over the duration of the study (-0.23 to 0.02 microg m(-2) s(-1)). In contrast, mean emissions of CH4 were much greater (80.4 to 450.8 microg m(-2) s(-1)) and varied both spatially and temporally. Spatial variations in CH4 fluxes were observed between grass kill areas (biogas 'hot spots') and the densely grass-covered areas of the landfill. Temporal variations in CH4 fluxes were also observed, due at least in part to barometric pressure, wind speed and precipitation effects.     

Atmospheric emissions and attenuation of non-methane organic compounds in cover soils at a French landfill

In addition to methane (CH(4)) and carbon dioxide (CO(2)), landfill gas may contain more than 200 non-methane organic compounds (NMOCs) including C(2+)-alkanes, aromatics, and halogenated hydrocarbons. Although the trace components make up less than 1% v/v of typical landfill gas, they may exert a disproportionate environmental burden. The objective of this work was to study the dynamics of CH(4) and NMOCs in the landfill cover soils overlying two types of gas collection systems: a conventional gas collection system with vertical wells and an innovative horizontal gas collection layer consisting of permeable gravel with a geomembrane above it. The 47 NMOCs quantified in the landfill gas samples included primarily alkanes (C(2)-C(10)), alkenes (C(2)-C(4)), halogenated hydrocarbons (including (hydro)chlorofluorocarbons ((H)CFCs)), and aromatic hydrocarbons (BTEXs). In general, both CH(4) and NMOC fluxes were all very small with positive and negative fluxes. The highest percentages of positive fluxes in this study (considering all quantified species) were observed at the hotspots, located mainly along cell perimeters of the conventional cell. The capacity of the cover soil for NMOC oxidation was investigated in microcosms incubated with CH(4) and oxygen (O(2)). The cover soil showed a relatively high capacity for CH(4) oxidation and simultaneous co-oxidation of the halogenated aliphatic compounds, especially at the conventional cell. Fully substituted carbons (TeCM, PCE, CFC-11, CFC-12, CFC-113, HFC-134a, and HCFC-141b) were not degraded in the presence of CH(4) and O(2). Benzene and toluene were also degraded with relative high rates. This study demonstrates that landfill soil covers show a significant potential for CH(4) oxidation and co-oxidation of NMOCs.     

The influence of atmospheric pressure on landfill methane emissions

Landfills are the largest source of anthropogenic methane (CH4) emissions to the atmosphere in the United States. However, few measurements of whole landfill CH4 emissions have been reported. Here, we present the results of a multi-season study of whole landfill CH4 emissions using atmospheric tracer methods at the Nashua, New Hampshire Municipal landfill in the northeastern United States. The measurement data include 12 individual emission tests, each test consisting of 5-8 plume measurements. Measured emissions were negatively correlated with surface atmospheric pressure and ranged from 7.3 to 26.5 m3 CH4 min(-1). A simple regression model of our results was used to calculate an annual emission rate of 8.4 x 10(6) m3 CH4 year(-1). These data, along with CH4 oxidation estimates based on emitted landfill gas isotopic characteristics and gas collection data, were used to estimate annual CH4 generation at this landfill. A reported gas collection rate of 7.1 x 10(6) m3 CH4 year(-1) and an estimated annual rate of CH4 oxidation by cover soils of 1.2 x 10(6) m3 CH4 year(-1) resulted in a calculated annual CH4 generation rate of 16.7 x 10(6) m3 CH4 year(-1). These results underscore the necessity of understanding a landfill's dynamic environment before assessing long-term emissions potential.     

Factors related to the diversity and distribution of soil fauna on Gin Drinkers' Bay landfill,Hong Kong

The major potential problems from a landfill are biogas and leachate generated from the wastes deposited. These factors can also exert harmful effects on the growth of vegetation and influence the diversity and distribution of soil animals, either directly or indirectly. This paper describes investigations undertaken at the completed Gin Drinkers' Bay landfill, Hong Kong. The absence of plant cover at the site limited the occurrence of some common soil animals such has Diplopoda, Hemiptera, Isopoda and Isoptera. This was possibly due to the shortage of plants as shelter and the lack of organic matter as food. One area (site H), with a high level of landfill gas in the soil zone, had a lower diversity of soil animals than another area (site L) with a lower level of landfill gas. Some edaphic factors such as moisture content and concentrations of K, Na and Mg in the “contaminated” site H may also have resulted in lower numbers of species and individuals.     

Estimation of mass transport parameters of gases for quantifying CH4 oxidation in landfill soil covers

Methane (CH(4)), which is one of the most abundant anthropogenic greenhouse gases, is produced from landfills. CH(4) is biologically oxidized to carbon dioxide, which has a lower global warming potential than methane, when it passes through a cover soil. In order to quantify the amount of CH(4) oxidized in a landfill cover soil, a soil column test, a diffusion cell test, and a mathematical model analysis were carried out. In the column test, maximum oxidation rates of CH(4) (V(max)) showed higher values in the upper part of the column than those in the lower part caused by the penetration of O(2) from the top. The organic matter content in the upper area was also higher due to the active microbial growth. The dispersion analysis results for O(2) and CH(4) in the column are counter-intuitive. As the upward flow rate of the landfill gas increased, the dispersion coefficient of CH(4) slightly increased, possibly due to the effect of mechanical dispersion. On the other hand, as the upward flow rate of the landfill gas increased, the dispersion coefficient of O(2) decreased. It is possible that the diffusion of gases in porous media is influenced by the counter-directional flow rate. Further analysis of other gases in the column, N(2) and CO(2), may be required to support this hypothesis, but in this paper we propose the possibility that the simulations using the diffusion coefficient of O(2) under the natural condition may overestimate the penetration of O(2) into the soil cover layer and consequently overestimate the oxidation of CH(4).     

Methane oxidation in a landfill cover with capillary barrier

The methane oxidation potential of a landfill cover with capillary barrier was investigated in an experimental plant (4.8 m x 0.8 m x 2.1m). The cover soil consisted of two layers, a mixture of compost plus sand (0.3 m) over a layer of loamy sand (0.9 m). Four different climatic conditions (summer, winter, spring and fall) were simulated. In and outgoing fluxes were measured. Gas composition, temperature, humidity, matrix potential and gas pressure were monitored in two profiles. CH4 oxidation rate within the investigated top cover ranged from 98% to 57%. The minimum was observed for a short time after irrigation. Temperature distribution, gas concentration profiles and lab-scaled batch experiments indicate that before irrigation the highest oxidising activity took place in a depth of about 30 cm. After irrigation the oxidising horizon seemed to migrate upwards since methanotrophic bacteria develop better there due to an adequate supply with oxygen. It can be assumed that the absence of oxygen is one of the most important limiting factors for the CH4 oxidation process. Abrupt cross-overs between horizons of different soil material may lead to zones of increased water saturation and decreased soil respiration.     

Mitigation of global greenhouse gas emissions from waste: conclusions and strategies from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. Working Group III (Mitigation).

Greenhouse gas (GHG) emissions from post-consumer waste and wastewater are a small contributor (about 3%) to total global anthropogenic GHG emissions. Emissions for 2004-2005 totalled 1.4 Gt CO2-eq year(-1) relative to total emissions from all sectors of 49 Gt CO2-eq year(-1) [including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and F-gases normalized according to their 100-year global warming potentials (GWP)]. The CH4 from landfills and wastewater collectively accounted for about 90% of waste sector emissions, or about 18% of global anthropogenic methane emissions (which were about 14% of the global total in 2004). Wastewater N2O and CO2 from the incineration of waste containing fossil carbon (plastics; synthetic textiles) are minor sources. Due to the wide range of mature technologies that can mitigate GHG emissions from waste and provide public health, environmental protection, and sustainable development co-benefits, existing waste management practices can provide effective mitigation of GHG emissions from this sector. Current mitigation technologies include landfill gas recovery, improved landfill practices, and engineered wastewater management. In addition, significant GHG generation is avoided through controlled composting, state-of-the-art incineration, and expanded sanitation coverage. Reduced waste generation and the exploitation of energy from waste (landfill gas, incineration, anaerobic digester biogas) produce an indirect reduction of GHG emissions through the conservation of raw materials, improved energy and resource efficiency, and fossil fuel avoidance. Flexible strategies and financial incentives can expand waste management options to achieve GHG mitigation goals; local technology decisions are influenced by a variety of factors such as waste quantity and characteristics, cost and financing issues, infrastructure requirements including available land area, collection and transport considerations, and regulatory constraints. Existing studies on mitigation potentials and costs for the waste sector tend to focus on landfill CH4 as the baseline. The commercial recovery of landfill CH4 as a source of renewable energy has been practised at full scale since 1975 and currently exceeds 105 Mt CO2-eq year(-1). Although landfill CH4 emissions from developed countries have been largely stabilized, emissions from developing countries are increasing as more controlled (anaerobic) landfilling practices are implemented; these emissions could be reduced by accelerating the introduction of engineered gas recovery, increasing rates of waste minimization and recycling, and implementing alternative waste management strategies provided they are affordable, effective, and sustainable. Aided by Kyoto mechanisms such as the Clean Development Mechanism (CDM) and Joint Implementation (JI), the total global economic mitigation potential for reducing waste sector emissions in 2030 is estimated to be > 1000 Mt CO2-eq (or 70% of estimated emissions) at costs below 100 US$ t(-1) CO2-eq year(-1). An estimated 20-30% of projected emissions for 2030 can be reduced at negative cost and 30-50% at costs < 20 US$ t(-) CO2-eq year(-1). As landfills produce CH4 for several decades, incineration and composting are complementary mitigation measures to landfill gas recovery in the short- to medium-term--at the present time, there are > 130 Mt waste year(-1) incinerated at more than 600 plants. Current uncertainties with respect to emissions and mitigation potentials could be reduced by more consistent national definitions, coordinated international data collection, standardized data analysis, field validation of models, and consistent application of life-cycle assessment tools inclusive of fossil fuel offsets.     

Life Cycle Assessment of landfill biogas management: Sensitivity to diffuse and combustion air emissions

Goal and scopeThe life cycle inventory of landfill emissions is a key point in Life Cycle Assessment (LCA) of waste management options and is highly subject to discussion. Result sensitivity to data inventory is accounted for through the implementation of scenarios that help examine how waste landfilling should be modeled in LCA.MethodFour landfill biogas management options are environmentally evaluated in a Life Cycle Assessment perspective: (1) no biogas management (open dump), conventional landfill with (2) flaring, (3) combined heat and power (CHP) production in an internal combustion engine and (4) biogas upgrading for use as a fuel in buses. Average, maximum and minimum literature values are considered both for combustion emission factors in flares and engines and for trace pollutant concentrations in biogas.ResultsBiogas upgrading for use as a fuel in buses appears as the most relevant option with respect to most non-toxic impact categories and ecotoxicity, when considering average values for trace gas concentrations and combustion emission factors. Biogas combustion in an engine for CHP production shows the best performances in terms of climate change, but generates significantly higher photochemical oxidant formation and marine eutrophication impact potentials than flaring or biogas upgrading for use as a fuel in buses.Interpretation and discussionHowever the calculated environmental impact potentials of landfill biogas management options depend largely on the trace gas concentrations implemented in the model. The use of average or extreme values reported in the literature significantly modifies the impact potential of a given scenario (up to two orders of magnitude for open dumps with respect to human toxicity). This should be taken into account when comparing landfilling with other waste management options. Also, the actual performances of a landfill top cover (in terms of oxidation rates) and combustion technology (in terms of emission factors) appear as key parameters affecting the ranking of biogas management options.     

Rehabilitation of El Yahoudia dumping site, Tunisia

As in all developing countries, cities in Tunisia face serious problems of environmental pollution caused mainly by the inadequate and inefficient final disposal of their generated solid wastes. The Tunisian government launched a development program including the construction of landfills in the main cities and the closure of the contaminated sites issued from solid wastes landrising practice. The project of the Henchir El Yahoudia landfill restoration is the first experience in this programme. It has been suggested to convert the site to a green park and to implement an ornamental plant nursery. The whole surface of the landfill is approximately 100 ha from which 30 ha have been already transformed to an urban recreational area and the remaining 70 ha have to be characterized for the project extension. A field investigation by boring was conducted in order to define the geological and the hydrogeological conditions, the vertical and horizontal wastes layer extension, content and degree of decomposition and the composition and quantities of leachate and landfill gas. Representative samples of waste, soil, groundwater and leachate were collected for laboratory analyses. Several of these borings were converted to piezometers to define the flow regime in the site. The results showed that the biogas (CH4, H2S, and CO2), leachate and waste, distribution in the site is mainly affected by the temporal variation of the site operating method. The underlying fissured clay layer facilitated leachate infiltration into the groundwater where high BOD, COD and nitrogen concentrations were registered.     

Use of a biologically active cover to reduce landfill methane emissions and enhance methane oxidation

Biologically-active landfill cover soils (biocovers) that serve to minimize CH4 emissions by optimizing CH4 oxidation were investigated at a landfill in Florida, USA. The biocover consisted of 50 cm pre-composted yard or garden waste placed over a 10-15 cm gas distribution layer (crushed glass) over a 40-100 cm interim cover. The biocover cells reduced CH4 emissions by a factor of 10 and doubled the percentage of CH4 oxidation relative to control cells. The thickness and moisture-holding capacity of the biocover resulted in increased retention times for transported CH4. This increased retention of CH4 in the biocover resulted in a higher fraction oxidized. Overall rates between the two covers were similar, about 2g CH4 m(-2)d(-1), but because CH4 entered the biocover from below at a slower rate relative to the soil cover, a higher percentage was oxidized. In part, methane oxidation controlled the net flux of CH4 to the atmosphere. The biocover cells became more effective than the control sites in oxidizing CH4 3 months after their initial placement: the mean percent oxidation for the biocover cells was 41% compared to 14% for the control cells (p<0.001). Following the initial 3 months, we also observed 29 (27%) negative CH4 fluxes and 27 (25%) zero fluxes in the biocover cells but only 6 (6%) negative fluxes and 22 (21%) zero fluxes for the control cells. Negative fluxes indicate uptake of atmospheric CH4. If the zero and negative fluxes are assumed to represent 100% oxidation, then the mean percent oxidation for the biocover and control cells, respectively, for the same period would increase to 64% and 30%.     

CH4 and N2O from mechanically turned windrow and vermicomposting systems following in-vessel pre-treatment

Methane (CH4) and nitrous oxide (N2O) are included in the six greenhouse gases listed in the Kyoto protocol that require emission reduction. To meet reduced emission targets, governments need to first quantify their contribution to global warming. Composting has been identified as an important source of CH4 and N2O. With increasing divergence of biodegradable waste from landfill into the composting sector, it is important to quantify emissions of CH4 and N2O from all forms of composting and from all stages. This study focuses on the final phase of a two stage composting process and compares the generation and emission of CH4 and N2O associated with two differing composting methods: mechanically turned windrow and vermicomposting. The first stage was in-vessel pre-treatment. Source-segregated household waste was first pre-composted for seven days using an in-vessel system. The second stage of composting involved forming half of the pre-composted material into a windrow and applying half to vermicomposting beds. The duration of this stage was 85 days and CH4 and N2O emissions were monitored throughout for both systems. Waste samples were regularly subjected to respirometry analysis and both processes were found to be equally effective at stabilising the organic matter content. The mechanically turned windrow system was characterised by emissions of CH4 and to a much lesser extent N2O. However, the vermicomposting system emitted significant fluxes of N2O and only trace amounts of CH4. In-vessel pre-treatment removed considerable amounts of available C and N prior to the second stage of composting. This had the effect of reducing emissions of CH4 and N2O from the second stage compared to emissions from fresh waste found in other studies. The characteristics of each of the two composting processes are discussed in detail. Very different mechanisms for emission of CH4 and N2O are proposed for each system. For the windrow system, development of anaerobic zones were thought to be responsible for CH4 release. High N2O emission rates from vermicomposting were ascribed to strongly nitrifying conditions in the processing beds combined with the presence of de-nitrifying bacteria within the worm gut.     

Estimation of biogas produced by the landfill of Palermo, applying a Gaussian model

In this work, a procedure is suggested to assess the rate of biogas emitted by the Bellolampo landfill (Palermo, Italy), starting from the data acquired by two of the stations for monitoring meteorological parameters and polluting gases. The data used refer to the period November 2005-July 2006. The methane concentration, measured in the CEP suburb of Palermo, has been analysed together with the meteorological data collected by the station situated inside the landfill area. In the present study, the methane has been chosen as a tracer of the atmospheric pollutants produced by the dump. The data used for assessing the biogas emission refer to night time periods characterized by weak wind blowing from the hill toward the city. The methane rate emitted by the Bellolampo dump has been evaluated using a Gaussian model and considering the landfill both as a single point source and as a multiple point one. The comparison of the results shows that for a first approximation it is sufficient to consider the landfill of Palermo as a single point source. Starting from the monthly percentage composition of the biogas, estimated for the study period, the rate of biogas produced by the dump was evaluated. The total biogas produced by the landfill, obtained as the sum of the emitted component and the recovered one, ranged from 7519.97 to 10,153.7m3/h. For the study period the average monthly estimations of biogas emissions into the atmosphere amount to about 60% of the total biogas produced by the landfill, a little higher than the one estimated by the company responsible for the biogas recovery plant at the landfill.     

Evaluating effects of wind-induced pressure fluctuations on soil-atmosphere gas exchange at a landfill using stochastic modelling.

The impact of wind turbulence-induced pressure fluctuations at the soil surface on landfill gas transport and emissions to the atmosphere at an old Danish landfill site was investigated using stochastic modelling combined with soil property and gas transport data measured at the site. The impacts of soil physical properties (including air permeability and volumetric water content) and wind-induced pressure fluctuation properties (amplitude and temporal correlation) on landfill gas emissions to the atmosphere were evaluated. Soil-air permeability and pressure fluctuation amplitude were found to be the most important parameters. Wind-induced gas emissions were further compared with gas emissions caused by diffusion and by long-term pressure variations (due to passing weather systems). Here diffusion and wind-induced gas transport were found to be equally important with wind-induced gas transport becoming the most important at lower soil-air contents.     

Above- and below-ground methane fluxes and methanotrophic activity in a landfill-cover soil

Landfills are a major anthropogenic source of the greenhouse gas methane (CH(4)). However, much of the CH(4) produced during the anaerobic degradation of organic waste is consumed by methanotrophic microorganisms during passage through the landfill-cover soil. On a section of a closed landfill near Liestal, Switzerland, we performed experiments to compare CH(4) fluxes obtained by different methods at or above the cover-soil surface with below-ground fluxes, and to link methanotrophic activity to estimates of CH(4) ingress (loading) from the waste body at selected locations. Fluxes of CH(4) into or out of the cover soil were quantified by eddy-covariance and static flux-chamber measurements. In addition, CH(4) concentrations at the soil surface were monitored using a field-portable FID detector. Near-surface CH(4) fluxes and CH(4) loading were estimated from soil-gas concentration profiles in conjunction with radon measurements, and gas push-pull tests (GPPTs) were performed to quantify rates of microbial CH(4) oxidation. Eddy-covariance measurements yielded by far the largest and probably most representative estimates of overall CH(4) emissions from the test section (daily mean up to ～91,500μmolm(-2)d(-1)), whereas flux-chamber measurements and CH(4) concentration profiles indicated that at the majority of locations the cover soil was a net sink for atmospheric CH(4) (uptake up to -380μmolm(-2)d(-1)) during the experimental period. Methane concentration profiles also indicated strong variability in CH(4) loading over short distances in the cover soil, while potential methanotrophic activity derived from GPPTs was high (v(max)～13mmolL(-1)(soil air)h(-1)) at a location with substantial CH(4) loading. Our results provide a basis to assess spatial and temporal variability of CH(4) dynamics in the complex terrain of a landfill-cover soil.     

Differences in volatile methyl siloxane (VMS) profiles in biogas from landfills and anaerobic digesters and energetics of VMS transformations

The objectives of this study were to compare the types and levels of volatile methyl siloxanes (VMS) present in biogas generated in the anaerobic digesters and landfills, evaluate the energetics of siloxane transformations under anaerobic conditions, compare the conditions in anaerobic digesters and municipal solid waste (MSW) landfills which result in differences in siloxane compositions. Biogas samples were collected at the South District Wastewater Treatment Plant and South Dade Landfill in Miami, Florida. In the digester gas, D4 and D5 comprised the bulk of total siloxanes (62% and 27%, respectively) whereas in the landfill gas, the bulk of siloxanes were trimethylsilanol (TMSOH) (58%) followed by D4 (17%). Presence of high levels of TMSOH in the landfill gas indicates that methane utilization may be a possible reaction mechanism for TMSOH formation. The free energy change for transformation of D5 and D4 to TMSOH either by hydrogen or methane utilization are thermodynamically favorable. Either hydrogen or methane should be present at relatively high concentrations for TMSOH formation which explains the high levels present in the landfill gas. The high bond energy and bond distance of the Si–O bond, in view of the atomic sizes of Si and O atoms, indicate that Si atoms can provide a barrier, making it difficult to break the Si–O bonds especially for molecules with specific geometric configurations such as D4 and D5 where oxygen atoms are positioned inside the frame formed by the large Si atoms which are surrounded by the methyl groups.     

Induction of enhanced methane oxidation in compost: temperature and moisture response

Landfilling is one of the most common ways of municipal solid waste disposal. Degradation of organic waste produces CH(4) and other landfill gases that significantly contribute to global warming. However, before entering the atmosphere, part of the produced CH(4) can be oxidised while passing through the landfill cover. In the present study, the oxidation rate of CH(4) was studied with various types of compost as possible landfill cover. The influence of incubation time, moisture content and temperature on the CH(4) oxidation capacity of different types of compost was examined. It was observed that the influence of moisture content and temperature on methane oxidation is time-dependent. Maximum oxidation rates were observed at moisture contents ranging from 45% to 110% (dry weight basis), while the optimum temperature ranged from 15 to 30 degrees C.     

Impact of different plants on the gas profile of a landfill cover

Methane is an important greenhouse gas emitted from landfill sites and old waste dumps. Biological methane oxidation in landfill covers can help to reduce methane emissions. To determine the influence of different plant covers on this oxidation in a compost layer, we conducted a lysimeter study. We compared the effect of four different plant covers (grass, alfalfa + grass, miscanthus and black poplar) and of bare soil on the concentration of methane, carbon dioxide and oxygen in lysimeters filled with compost. Plants were essential for a sustainable reduction in methane concentrations, whereas in bare soil, methane oxidation declined already after 6 weeks. Enhanced microbial activity - expected in lysimeters with plants that were exposed to landfill gas - was supported by the increased temperature of the gas in the substrate and the higher methane oxidation potential. At the end of the first experimental year and from mid-April of the second experimental year, the methane concentration was most strongly reduced in the lysimeters containing alfalfa + grass, followed by poplar, miscanthus and grass. The observed differences probably reflect the different root morphology of the investigated plants, which influences oxygen transport to deeper compost layers and regulates the water content.     

Assessing methane oxidation under landfill covers and its contribution to the above atmospheric CO₂ levels: the added value of the isotope (δ¹³C and δ¹⁸O CO₂; δ¹³C and δD CH₄) approach

We are presenting here a multi-isotope approach (δ¹³C and δ¹⁸O of CO₂; δ¹³C and δD of CH₄) to assess (i) the level(s) of methane oxidation during waste biodegradation and its migration through a landfill cover in Sonzay (France), and (ii) its contribution to the atmospheric CO₂ levels above the surface. The isotope approach is compared to the more conventional mass balance approach. Results from the two techniques are comparable and show that the CH₄ oxidation under the landfill cover is heterogenous, with low oxidation percentages in samples showing high biogas fluxes, which was expected in clay covers presenting fissures, through which CH₄ is rapidly transported. At shallow depth, more immobile biogas pockets show a higher level of CH₄ oxidation by the methanotrophic bacteria. δ¹³C of CO₂ samples taken at different heights (from below the cover up to 8 m above the ground level) were also used to identify and assess the relative contributions of its main sources both under the landfill cover and in the surrounding atmosphere.     

Effectiveness of compacted soil liner as a gas barrier layer in the landfill final cover system

A compacted soil liner (CSL) has been widely used as a single barrier layer or a part of composite barrier layer in the landfill final cover system to prevent water infiltration into solid wastes for its acceptable hydraulic permeability. This study was conducted to test whether the CSL was also effective in prohibiting landfill gas emissions. For this purpose, three different compaction methods (i.e., reduced, standard, and modified Proctor methods) were used to prepare the soil specimens, with nitrogen as gas, and with water and heptane as liquid permeants. Measured gas permeability ranged from 2.03x10(-10) to 4.96x10(-9)cm(2), which was a magnitude of two or three orders greater than hydraulic permeability (9.60x10(-13) to 1.05x10(-11)cm(2)). The difference between gas and hydraulic permeabilities can be explained by gas slippage, which makes gas more permeable, and by soil-water interaction, which impedes water flow and then makes water less permeable. This explanation was also supported by the result that a liquid permeability measured with heptane as a non-polar liquid was similar to the intrinsic gas permeability. The data demonstrate that hydraulic requirement for the CSL is not enough to control the gas emissions from a landfill.     

Modelling of stable isotope fractionation by methane oxidation and diffusion in landfill cover soils

A technique to measure biological methane oxidation in landfill cover soils that is gaining increased interest is the measurement of stable isotope fractionation in the methane. Usually to quantify methane oxidation, only fractionation by oxidation is taken into account. Recently it was shown that neglecting the isotope fractionation by diffusion results in underestimation of the methane oxidation. In this study a simulation model was developed that describes gas transport and methane oxidation in landfill cover soils. The model distinguishes between (12)CH(4), (13)CH(4), and (12)CH(3)D explicitly, and includes isotope fractionation by diffusion and oxidation. To evaluate the model, the simulations were compared with column experiments from previous studies. The predicted concentration profiles and isotopic profiles match the measured ones very well, with a root mean square deviation (RMSD) of 1.7vol% in the concentration and a RMSD of 0.8 per thousand in the delta(13)C value, with delta(13)C the relative (13)C abundance as compared to an international standard. Overall, the comparison shows that a model-based isotope approach for the determination of methane oxidation efficiencies is feasible and superior to existing isotope methods.