A fully coupled model for water-gas-heat reactive transport with methane oxidation in landfill covers

Theoretical assessment of influential factors and application in chlorinated hydrocarbon detection with membrane interface probe

The membrane interface probe (MIP) is an efficient and economical in-situ tool for chlorinated hydrocarbon (CH) contaminated site investigation. Given that the interpretation of MIP test is currently limited to a qualitative level, a theoretical model considering multiphase flow and multifield coupling is firstly proposed to simulate MIP test process. This model can consider phase change, membrane effect, adsorption and dissolution of the CH liquid, gas diffusion, and evaporation. Then, the model is used to study the changes in soil temperature and soil CH concentration during MIP test, as well as the influences of soil CH concentration and soil properties (initial water saturation, soil intrinsic permeability, and thermal properties) on MIP response. Finally, a simplified MIP interpretation model is developed based on parametric analysis results and verified against field and laboratory test data. It is found that the soil CH concentration, rather than soil properties, dominates the MIP response. The simplified interpretation model can deliver practical prediction of the CH concentration through the detected results by MIP, which may improve the applicability of MIP.

Numerical investigation and optimal design of capillary barrier cover with passive gas collection pipes on the performance at limiting landfill gas emissions

Relying solely on soil properties may not fully ensure the performance of capillary barrier covers at limiting landfill gas (LFG) emissions. This study proposed to install passive gas collection pipes in the coarse-grained soil layers of capillary barrier covers to enhance their performance at limiting LFG emissions. First, the LFG generation rate of municipal solid waste and its influencing factors were analyzed based on empirical formulas. This information provided necessary bottom boundary conditions for the analyses of LFG transport through capillary barrier covers with passive gas collection pipes (CBCPPs). Then, numerical simulations were conducted to investigate the LFG transport properties through CBCPPs and reveal relevant influencing factors. Finally, practical suggestions were proposed to optimize the design of CBCPPs. The results indicated that the maximum whole-site LFG generation rate occurred at the end of landfilling operation. The gas collection efficiency (E) of CBCPPs was mainly controlled by the ratio of the intrinsic permeability between the coarse- and fine-grained soil (K2/K1) and the laying spacing between gas collection pipes (D). E increased as K2/K1 increased but decreased as D increased. An empirical expression for estimating E based on K2/K1 and D was proposed. In practice, CBCPPs were supposed to be constructed once the landfilling operation finished. It is best to select the fine- and coarse-grained soils with K2/K1 exceeding 10,000 to construct CBCPPs.

Effects of initial temperature and moisture content on heat generation during degradation of municipal solid waste

Heat generation from degradation of organic matter in municipal solid waste (MSW) often leads to increased landfill temperature. However, it is difficult to measure environmental heat loss in laboratory and field tests; therefore, little research has been conducted to evaluate heat generation during waste degradation under different initial temperatures and moisture contents. In this study, tests were conducted to investigate the effects of initial temperature and moisture content on heat generation during waste degradation. A simple formula for calculating heat generation was proposed. Within 200 h, the waste temperature decreased by about 70%, and lower initial moisture contents were associated with greater temperature decreases. The smallest temperature decrease of 47% and the greatest heat generation occurred when the initial temperature was 40 °C. The initial moisture content increased from 30% to 60% and the heat generation increased from 5% to 36%. The heat generation per unit mass of organic matter during the aerobic and anaerobic stages were 19.44-23.77 and 0.27-0.50 MJ·kg-1, respectively, indicating that the proposed formula for calculation of heat generated from waste degradation was reasonable. The results presented herein provide theoretical support for the prediction of heat generation and the recycling of heat resources in MSW landfill sites.

Study on the effect of landfill gas on aerobic municipal solid waste degradation: Lab-scale model and tests

Aeration is of great importance in landfill remediation. However, most existing studies on aerobic waste degradation ignore the presence of landfill gases. In this study, gas characteristics during aerobic waste degradation in the presence of landfill gas in lab-scale lysimeters were investigated. Oxygen (O₂) was intermittently injected into municipal solid waste. Changes in the gas concentration and reaction rate of methane (CH₄), carbon dioxide (CO₂), and O₂ during the reaction process were monitored and calculated. The results showed that all reactions, including aerobic degradation, CH₄ oxidation, and anaerobic waste degradation, occurred simultaneously during landfill aeration. The maximum O₂ consumption rate was 0.013 mol day^-1 kg^-1 dry waste. CH₄ production was stimulated after the O₂ content was insufficient to sustain the aerobic environment. Higher CH₄ production was likely attributed to the remaining substrate and biomass from dead aerobic microorganisms decomposed by growing anaerobic microorganisms. Based on the biochemical reaction and principle of mass conservation, a gas balance model during waste aeration was established to analyze the proportions of aerobic waste degradation, CH₄ oxidation, and anaerobic waste degradation. The CH₄ oxidation reaction was more advantageous than the aerobic waste degradation reaction during aeration. With an increase in gas injection times, the anaerobic reaction gradually weakened. The maximum proportion of CH₄ oxidation reaction could achieve at 21.4 % during aeration, which is of great significance for the waste degradation reaction. The maximum proportion of aerobic waste degradation and the minimum proportion of anaerobic waste degradation were approximately 16.0 % and 74.2 %, respectively. The results show that landfill gas should be considered in the progress of landfill aeration. This study provides a novel approach for calculating the proportion of reactions during landfill aeration, which deepens the understanding of the reaction process and contributes to the design of aerobic landfill projects.

Gas transport in landfill cover system: A critical appraisal

Landfill gas (LFG) emission is gaining more attention from the scientific fraternity and policymakers recently due to its threat to the atmosphere and human health of the populace living in surrounding premises. Though landfill cover (LFC) (viz., daily, intermittent and final cover) is widely used by landfill operators to mitigate or reduce these emissions, their overall performance is still under question. A critical analysis of available literature, primarily pertaining to (i) the composition of the landfill gases and their migration in the LFC system, (ii) experimental and mathematical investigations of the transport mechanism of gas and (iii) the impact of additives to cover soils on transport and fate of gas, has been conducted and presented in this manuscript. Investigation of the efficiency of modified soil was mainly focused on laboratory test. More field tests and application of amended cover soils should be conducted and promoted further. Studies on nitrous oxide and emerging pollutants, including poly-fluoroalkyl substances transport in landfill cover system are limited and need further research. The transport mechanisms of these unconventional contaminants should be considered regarding the selection of LFC materials including geomembrane and geosynthetic clay liners. The existing analytical and numerical models can provide a basic understanding of LFG transport mechanisms and are able to predict the migration behaviour of LFG; however, there are still knowledge gaps concerning the interaction between different species of the gas molecule when modeling multi-component gas transport. Gas transport through fractured cover should also be considered when evaluating LFG emission in the future. Simplified design method for landfill cover system regarding LFG emission based on analytical models should be proposed. Overall, mathematical models combined with experiments can facilitate more visualized and intensive insights, which would be instrumental in devising climate adaptive landfill covers.

Numerical model for static chamber measurement of multi-component landfill gas emissions and its application

The quantitative assessment of landfill gas emissions is essential to assess the performance of the landfill cover and gas collection system. The relative error of the measured surface emission of landfill gas may be induced by the static flux chamber technique. This study aims to quantify effects of the size of the chamber, the insertion depth, pressure differential on the relative errors by using an integrated approach of in situ tests, and numerical modeling. A field experiment study of landfill gas emission is conducted by using a static chamber at one landfill site in Xi'an, Northwest China. Additionally, a two-dimensional axisymmetric numerical model for multi-component gas transport in the soil and the static chamber is developed based on the dusty-gas model (DGM). The proposed model is validated by the field data obtained in this study and a set of experimental data in the literature. The results show that DGM model has a better capacity to predict gas transport under a wider range of permeability compared to Blanc's method. This is due to the fact that DGM model can explain the interaction among gases (e.g., CH4, CO2, O2, and N2) and the Knudsen diffusion process while these mechanisms are not included in Blanc's model. Increasing the size and the insertion depth of static chambers can reduce the relative error for the flux of CH4 and CO2. For example, increasing the height of chambers from 0.55 to 1.1 m can decrease relative errors of CH4 and CO2 flux by 17% and 18%, respectively. Moreover, we find that gas emission fluxes for the case with positive pressure differential (∆Pin-out) are greater than that of the case without considering pressure fluctuations. The Monte Carlo method was adopted to carry out the statistical analysis for quantifying the range of relative errors. The agreement of the measured field data and predicted results demonstrated that the proposed model has the capacity to quantify the emission of landfill gas from the landfill cover systems.

Approximate analytical model for transient transport and oxygen-limited biodegradation of vapor-phase petroleum hydrocarbon compound in soil

Volatile organic compounds (VOCs) contamination may occur in subsurface soil due to various reasons and pose great threat to people. Petroleum hydrocarbon compound (PHC) is a typical kind of VOC, which can readily biodegrade in an aerobic environment. The biodegradation of vapor-phase PHC in the vadose zone consumes oxygen in the soil, which leads to the change in aerobic and anaerobic zones but has not been studied by the existing analytical models. In this study, a one-dimensional analytical model is developed to simulate the transient diffusion and oxygen-limited biodegradation of PHC vapor in homogeneous soil. Laplace transformation and Laplace inversion of the Talbot method are adopted to derive the solution. At any given time, the thickness of aerobic zone is determined by the dichotomy method. The analytical model is verified against numerical simulation and experimental results first and parametric study is then conducted. The transient migration of PHC vapor can be divided into three stages including the pure aerobic zone stage (Stage I), aerobic-anaerobic zones co-existence stage (Stage II), and steady-state stage (Stage III). The proposed analytical model should be adopted to accommodate scenarios where the transient effect is significant (Stage II), including high source concentration, deep contaminant source, high biodegradation capacity, and high water saturation. The applicability of this model to determine the breakthrough time for better vapor intrusion assessment is also evaluated. Lower first-order biodegradation rate, higher source concentration, and shallower source depth all lead to smaller breakthrough time.

Experimental analysis and modeling of the methane degradation in a three stage biofilter using composted sawdust as packing media

Methane gas is a very effective greenhouse gas and the second-largest contributor to global warming. Biofiltration is an effective technology that uses microorganisms to degrade the pollutant by oxidizing it. In this work, the performance of a biofilter with supporting filter media, consisting of composted sawdust, is evaluated at three different sampling ports. Furthermore, a transient model is developed to predict methane concentration at various heights and times. The developed model is validated with the experimental data and shows good agreement with the experimental data. The highest removal efficiency and elimination capacity was found to be 72% and 0.108 g m^-3 h^-1 respectively. The effect of parameters such as specific surface area, the reaction rate constant, biofilm thickness and airflow rate were studied on the outlet methane concentration. Under similar conditions, the simulations showed that the removal efficiency of 95% might be achieved for the height of 2 m.

Comprehensive overview of numerical modeling of coupled landfill processes

Landfilling is the primary method used for municipal solid waste (MSW) disposal. To design, optimize, and manage landfills with a life span of several decades, a deeper understanding of long-term MSW behaviors is necessary and worthwhile. These behaviors should be modeled using approaches that account for coupled processes so as to capture the evolutionary mechanisms that are mainly dominated by biochemical, mechanical, hydraulic, and thermal processes, as well as the complex interactions among them. Many mathematical models have been developed over the past three decades to address this issue. However, most of them only emphasize some of these processes, with only few models accounting for all the processes. In this review, we present a comprehensive overview of the mathematical and numerical formulations of this coupled problem. Each process occurring in landfills is interpreted in detail using different sub-models and the corresponding parameter values. Then, the existing coupled models for MSW are reviewed, and the challenges and perspectives related to the modeling of the long-term behaviors of MSW are highlighted. We conclude that more reliable constitutive formulations based not only on well-designed laboratory tests but also on field tests are necessary to improve the modeling of MSW behaviors in future.

Performance of a compacted loess/gravel cover as a capillary barrier and landfill gas emissions controller in Northwest China

Loess is widely distributed in Northwest China where the rainy season coincides with the warm and vegetation growth period. The use of loess as a capillary barrier cover (CBC) material is promising. However, how the loess/gravel CBC perform as a capillary barrier and landfill gas emissions controller remains elusive. In this study, the performance of a designed CBC comprised 1.3 m-thick compacted loess underlain by 0.3 m-thick gravel in extremely wet and dry years of Xi'an city from 1950 to 2000 was analyzed using numerical modeling. An instrumented CBC test section comprised 0.9 m-thick compacted loess underlain by 0.3 m-thick gravel was constructed to show the hydraulic responses in real conditions from January 2015 to January 2017. The numerical results indicated that the designed CBC performed well as a capillary barrier as no percolation occurred during the extremely wet periods. Despite adopting a CBC of 0.4 m thinner than the designed one, the test section produced only 16.16 mm percolation during the two-year monitoring period, and that can meet the recommended limit of 30 mm/yr. The effect of the capillary break on increasing the water storage within the CBC was observed at the test section in fall. The increased water storage can significantly decrease the gas permeability, and thus improve the performance of the CBC as a LFG emissions controller. Furthermore, the LFG emissions can be controlled to meet the limit set by the Australian guideline by decreasing the bottom gas pressure and artificial watering. Finally, a procedure was proposed to enhance the performance of CBCs.

Full-scale experimental study of methane emission in a loess-gravel capillary barrier cover under different seasons

The methane emission in a loess-gravel capillary barrier cover (CBC) in winter and summer was investigated by constructing a full-scale testing facility (20 m × 30 m) with a slope angle of 14.5° at a landfill in Xi'an, China. Weather conditions, methane emission, gas concentration, temperature, and volumetric water content (VWC) in the CBC were measured. The temperature and moisture in the CBC showed a typical seasonal pattern of warm and dry in summer and cold and wet in winter. Accordingly, the maximum methane oxidation rate and methane emission were higher in summer. The mean methane influx and methane emission decreased significantly as the VWC increased beyond 40% (i.e., a degree of saturation 0.85) at a depth of 0.85 m, which was near the loess/gravel interface. At this depth, more water was presented in the loess layer in the downslope direction due to capillary barrier effects, which increased the upslope methane emission. More dominant methane emission in the middle- and upper-section of the CBC occurred in summer than in winter as there was less soil moisture to facilitate methane transfer. The LFG balance showed that a significant fraction of the loaded LFG was not accounted in the flux chamber measurements due to the preferential flow along the edges of the CBC. The maximum methane oxidation rate was 93.3 g CH₄ m^-2 d^-1, indicating the loess-gravel CBC could mitigate methane emissions after landfill closure.

Effects of thermal boundary condition on methane oxidation in landfill cover soil at different ambient temperatures

Microbial aerobic methane oxidation (MAMO) has been considered as an environmental-friendly method for mitigating methane emission from municipal landfill sites. Soil column has in a landfill cover under one-dimensional (1-D) condition. However, most of the published soil column tests failed to simulate 1-D heat transfer due to the use of thermal conductive boundary at the sidewall. In the present study, a heavily instrumented soil column was developed to quantify the effects of thermal boundary condition on the methane oxidation efficiency under different ambient temperatures in landfill cover soil. The sidewall of the soil column was thermally insulated to ensure 1-D heat transport as would have been typically expected in the field condition. Two soil column tests with and without thermal insulation were conducted at a range of controlled ambient temperatures from 15 to 30°C, for studying how soil moisture, matric suction, gas pressure, soil temperature and gas concentration evolve with MAMO. The test results reveal that ignoring thermal insulation in a soil column test would result in a greater loss of soil heat generation by MAMO and hence oxidation efficiency by up to 100% for the range of temperature considered. When the ambient temperature increased to 30°C (but less than the optimum temperature for MAMO), the MAMO efficiency increased abruptly at first but then decreased substantially with time, and this is likely due to the accumulation of biomass generated by MAMO.

Numerical model of aerobic bioreactor landfill considering aerobic-anaerobic condition and bio-stable zone development

Aeration by airflow technology is a reliable method to accelerate waste biodegradation and stabilization and hence shorten the aftercare period of a landfill. To simulate hydro-biochemical behaviors in this type of landfills, this study develops a model coupling multi-phase flow, multi-component transport and aerobic-anaerobic biodegradation using a computational fluid dynamics (CFD) method. The uniqueness of the model is that it can well describe the evolution of aerobic zone, anaerobic zone, and temperature during aeration and evaluate aeration efficiency considering aerobic and anaerobic biodegradation processes. After being verified using existing in situ and laboratory test results, the model is then employed to reveal the bio-stable zone development, aerobic biochemical reactions around vertical well (VW), and anaerobic reactions away from VW. With an increase in the initial organic matter content (0.1 to 0.4), the bio-stable zone expands at a decreasing speed but with all the horizontal ranges larger than 17 m after an intermittent aeration for 1000 days. When waste intrinsic permeability is equal or greater than 10^-11 m², aeration using a low pressure between 4 and 8 kPa is appropriate. The aeration efficiency would be underestimated if anaerobic biodegradation is neglected because products of anaerobic biodegradation would be oxidized more easily. A horizontal spacing of 17 m is suggested for aeration VWs with a vertical spacing of 10 m for screens. Since a lower aeration frequency can give greater aeration efficiency, a 20-day aeration/20-day leachate recirculation scenario is recommended considering the maximum temperature over a reasonable range. For wet landfills with low temperature, the proportion of aeration can be increased to 0.67 (20-day aeration/10-day leachate recirculation) or an even higher value.

A steady-state analytical profile method for determining methane oxidation in landfill cover

Gas concentration profiles of carbon dioxide (CO₂), oxygen (O₂), methane (CH₄) and nitrogen (N₂) are usually measured during tests investigating microbial aerobic methane oxidation in landfill cover. However, only qualitative/limited information can be obtained from gas concentration profiles by existing methods. A new method is proposed to determine methane oxidation in soil quantitatively and comprehensively, including methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions. Governing equations are established based on mass balance for O₂, CO₂, CH₄ and N₂ at one-dimensional and steady-state condition. Gas transfer mechanisms considered include gas diffusion, advection and gas reaction. The method utilizes gas concentration profiles to determine gas diffusion for each gas component according to Fick's law. Then gas advections and reactions can be determined by mass balance. The method is validated by (i) published soil column tests investigating methane oxidation and (ii) a calibrated numerical model based on a selected soil column test. The new method is capable of determining methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions for CH₄, CO₂ and O₂.

Environmental factors influencing landfill gas biofiltration: Lab scale study on methanotrophic bacteria growth

The post-management of landfills represents an important challenge for landfill gas treatment. Traditional systems (energy recovery, flares, etc.) present technical problems in treating flow with low methane (CH₄) concentrations. The objective of this study was to isolate methanotrophic bacteria from a field-scale biofilter in order to study the bacteria in laboratories and evaluate the environmental factors that mostly influence Microbial Aerobic Methane Oxidation (MAMO). The soil considered was sampled from the biofilter located in the landfill of Venosa (Basilicata Region, Italy) and it was mainly composed of wood chips and compost. The results showed that methanotrophic microorganisms are mainly characterized by a slow growth and a significant sensitivity to CH₄ levels. Temperature and nitrogen (N) also have a very important role on their development. On the basis of the results, biofilters for biological CH₄ oxidation can be considered a viable alternative to mitigate CH₄ emissions from landfills.

A Simulation model for estimating methane oxidation and emission from landfill cover soils

Quantification of methane (CH₄) oxidation and emission from landfill cover soils is important for evaluating measures to mitigate anthropogenic greenhouse gas emissions. In this study, a model that combines the multicomponent diffusive equation and Darcy's law, coupled with the dual Monod kinetic equation, was established to simulate CH₄ transport, oxidation and emission in landfill cover soils. Sensitivity analysis was performed to illustrate the influence of model parameters on CH₄ transport, oxidation and emission. The model was then applied to predict CH₄ emissions from several column experiments. The results of the sensitivity analysis showed that a high CH₄ oxidation rate can be obtained with a high V_max of cover soil, even for a low cover soil thickness, and that oxidation efficiency is constant when the thickness of the cover soil becomes greater than a threshold value. The simulated results fitted well with the measured values, confirming that the new model provides a reliable method for estimating CH₄ emissions from landfills.

Theoretical analysis of coupled effects of microbe and root architecture on methane oxidation in vegetated landfill covers

Reduction of soil moisture by plant root-water uptake could improve soil aeration for microbial aerobic methane oxidation (MAMO) in a landfill cover, but excessive soil moisture removal could suppress microbial activity due to water shortage. Existing models ignore the coupled microbe-vegetation interaction. It is thus not known whether the presence of plants is beneficial or adverse to MAMO. This study proposes a newly-improved theoretical model that couples the effects of root-water uptake and microbial activity for capturing water-gas flow and MAMO in unsaturated soils. Parametric studies are conducted to investigate the effects of root characteristics and transpiration rate on MAMO efficiency. Uniform, parabolic, exponential and triangular root architectures are considered. Ignoring the effects of water shortage on microbe over-predicts the MAMO efficiency significantly, especially for plants with traits that give high root-water uptake ability (i.e., uniformly-rooted and long root length). The effects of plants on MAMO efficiency depends on the initial soil moisture strongly. If the soil is too dry (i.e., close to the permanent wilting point), plant-water uptake, with any root architecture considered, would reduce MAMO efficiency as further soil water removal by plants suppresses microbial activity. Plants with exponential or triangular root architectures could preserve 10% higher MAMO than the other two cases. These two architectures are more capable of minimizing the adverse effects of root-water uptake due to microbial water shortage. This implies that high-water-demand plants such as those with long root length and with uniform or parabolic root architectures require more frequent irrigation to prevent from excessive reduction of MAMO efficiency.

Numerical modelling of methane oxidation efficiency and coupled water-gas-heat reactive transfer in a sloping landfill cover

Microbial aerobic methane oxidation in unsaturated landfill cover involves coupled water, gas and heat reactive transfer. The coupled process is complex and its influence on methane oxidation efficiency is not clear, especially in steep covers where spatial variations of water, gas and heat are significant. In this study, two-dimensional finite element numerical simulations were carried out to evaluate the performance of unsaturated sloping cover. The numerical model was calibrated using a set of flume model test data, and was then subsequently used for parametric study. A new method that considers transient changes of methane concentration during the estimation of the methane oxidation efficiency was proposed and compared against existing methods. It was found that a steeper cover had a lower oxidation efficiency due to enhanced downslope water flow, during which desaturation of soil promoted gas transport and hence landfill gas emission. This effect was magnified as the cover angle and landfill gas generation rate at the bottom of the cover increased. Assuming the steady-state methane concentration in a cover would result in a non-conservative overestimation of oxidation efficiency, especially when a steep cover was subjected to rainfall infiltration. By considering the transient methane concentration, the newly-modified method can give a more accurate oxidation efficiency.

Methane oxidation in a biofilter (Part 1): Development of a mathematical model for designing and optimization

The aim of this work is the evaluation of the efficiency of such a biofilter, through the application of a mathematical model which describes the biological oxidation process. This mathematical model is able to predict the efficiency of the system under varying operating conditions. Literature data have been used in order to build the model. The factors that mostly affect the process and which actually regulate the entire process have been highlighted in this work. Specifically, it was found that temperature, flow and methane concentration are the most important parameters that influence the system. The results obtained from the mathematical model showed also that the biofilter system is simple to implement and manage and allows the achievement of high efficiency of methane oxidation. In the optimal conditions for temperature (between 20-30°C), residence time (between 0.7-0.8 h) and methane molar fraction (between 20-25%) the efficiency of methane oxidation could be around 50%.

Methane oxidation in a biofilter (Part 2): A lab-scale experiment for model calibration

In this study an experimental study on a biological methane oxidation column presented with the aim to calibrate a mathematical model developed in an earlier study. The column was designed to reproduce at lab-scale a real biofilter trying to consider the more probable landfill boundary conditions. Although the methane oxidation efficiency in the column was lower than the expected (around 35%), an appropriate model implementation showed an acceptable agreement between the outcomes data of the model simulation and the experimental data (with Theil's Inequality Coefficient value of 0.08). A calibrated model allows a better management of the biofilter performance in terms of methane oxidation.