An analytical model for estimating the reduction of methane emission through landfill cover soils by methane oxidation

Methane removal efficiencies of biochar-mediated landfill soil cover with reduced depth

Biochar amendment for landfill soil cover has the potential to enhance methane removal efficiency while minimizing the soil depth. However, there is a lack of information on the response of biochar-mediated soil cover to the changes in configuration and operational parameters during the methane transport and transformation processes. This study constructed three biochar-amended landfill soil covers, with reduced soil depths from 75 cm (C2) to 55 cm (C3) and 45 cm (C4), and the control group (C1) with 75 cm and no biochar. Two operation phases were conducted under two soil moisture contents and three inlet methane fluxes in each phase. The methane removal efficiency increased for all columns along with the increase in methane flux. However, increasing moisture content from 10% to 20% negatively influenced the methane removal efficiency due to mass transfer limitation when at a low inlet methane flux, especially for C1; while this adverse effect could be alleviated by a high flux. Except for the condition with low moisture content and flux combination, C3 showed comparable methane removal efficiency to C2, both dominating over C1. As for C4 with only 45 cm, a high moisture content combined with a high methane flux enabled its methane removal efficiency to be competitive with other soil depths. In addition to the geotechnical reasons for gas transport processes, the evolution in methanotroph community structure (mainly type I methanotrophs) induced by biochar amendment and variations in soil properties supplemented the biological reasons for the varying methane removal efficiencies.

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.

Quantification and control of gaseous emissions from solid waste landfill surfaces

Landfilling is the most common option for solid waste disposal worldwide. Landfill sites can emit significant quantities of greenhouse gases (GHGs; e.g., methane, carbon dioxide, and nitrous oxide) and release toxic and odorous compounds (e.g., sulfides). Due to the complex composition and characteristics of landfill surface gas emissions, the quantification and control of landfill emissions are challenging. This review attempts to comprehensively understand landfill emission quantification and control options by primarily focusing on GHGs and odor compounds. Landfill emission quantification was highlighted by combining different emissions monitoring approaches to improve the quality of landfill emission data. Also, landfill emission control requires a specific approach that targets emission compounds or a systematic approach that reduces overall emissions by combining different control methods since the diverse factors dominate the emissions of various compounds and their transformation. This integrated knowledge of emission quantification and control options for GHGs and odor compounds is beneficial for establishing field monitoring campaigns and incorporating mitigation strategies to quantify and control multiple landfill emissions.

Design method of a modified layered aerobic waste landfill divided by coarse material

To overcome the weaknesses of traditional landfills, a modified aerobic landfill concept with intermediate covers of coarse material between waste layers functioning as facilities of drainage and aeration has been proposed recently. In this study, a one-dimensional coupled model, including aerobic biodegradation, oxygen diffusion, and advection, is proposed to describe oxygen distribution in this new type of landfill. Homotopy analysis method and perturbation method are applied to solve this model at passive aeration and active aeration, respectively. The model has six input variables, that is, oxygen diffusion coefficient, gas permeability, maximum oxygen consumption rate, layer thickness of waste, and injection pressure and extraction pressure. A combination of their typical values gives rise to over 700,000 scenarios which can be calculated by the proposed solution. The coupled effect of the above variables on oxygen migration is quantitatively investigated, followed by an estimation formula of the minimum oxygen concentration in waste layer. The maximum waste layer thickness is defined as a function of other variables for a given aeration target of oxygen volume concentration larger than 5%. A generalized design method of waste layer thickness, injection pressure, and extraction pressure is then developed for the newly proposed modified layered aerobic landfill, which can promote its popularization and application.

Low O2 level enhances CH4-derived carbon flow into microbial communities in landfill cover soils

CH₄ oxidation in landfill cover soils plays a significant role in mitigating CH₄ release to the atmosphere. Oxygen availability and the presence of co-contaminants are potentially important factors affecting CH₄ oxidation rate and the fate of CH₄-derived carbon. In this study, microbial populations that oxidize CH₄ and the subsequent conversion of CH₄-derived carbon into CO₂, soil organic C and biomass C were investigated in landfill cover soils at two O₂ tensions, i.e., O₂ concentrations of 21% ("sufficient") and 2.5% ("limited") with and without toluene. CH₄-derived carbon was primarily converted into CO₂ and soil organic C in the landfill cover soils, accounting for more than 80% of CH₄ oxidized. Under the O₂-sufficient condition, 52.9%-59.6% of CH₄-derived carbon was converted into CO₂ (CE_CO2-C), and 29.1%-39.3% was converted into soil organic C (CE_organic-C). A higher CE_organic-C and lower CE_CO2-C occurred in the O₂-limited environment, relative to the O₂-sufficient condition. With the addition of toluene, the carbon conversion efficiency of CH₄ into biomass C and organic C increased slightly, especially in the O₂-limited environment. A more complex microbial network was involved in CH₄ assimilation in the O₂-limited environment than under the O₂-sufficient condition. DNA-based stable isotope probing of the community with ¹³CH₄ revealed that Methylocaldum and Methylosarcina had a higher relative growth rate than other type I methanotrophs in the landfill cover soils, especially at the low O₂ concentration, while Methylosinus was more abundant in the treatment with both the high O₂ concentration and toluene. These results indicated that O₂-limited environments could prompt more CH₄-derived carbon to be deposited into soils in the form of biomass C and organic C, thereby enhancing the contribution of CH₄-derived carbon to soil community biomass and functionality of landfill cover soils (i.e. reduction of CO₂ emission).

Effects of plant radial oxygen loss on methane oxidation in landfill cover soil: A simulative study

Radial oxygen loss (ROL) by the spreading root systems of vegetation can improve soil aeration for subsequent oxidation of methane (CH₄) by microbes in landfill cover soils. This study proposes a theoretical model that elucidated the effects of ROL on microbial oxidation of CH₄ to understand landfill gas transportation and oxidation in landfill cover soils. Parametric analyses were conducted to investigate the effects of root depth, root architecture, and ROL rate on the CH₄ oxidation efficiency of landfill cover soils. The simulation results suggested that disregarding O₂ emissions by plants root systems could underestimate the CH₄ oxidation efficiency, especially when the water content ranged from 20% to 35%. Additionally, plants with a parabolic root architecture indicated 7-13% higher CH₄ oxidation efficiency than other root architectures, i.e., uniform, triangular, and exponential. The CH₄ oxidation efficiency increased rapidly at root depths less than 0.25 m. Therefore, plants characterized by a parabolic root architecture, longer root length, and higher ROL capacity should be selected as the preferred species for mitigating CH₄ emissions from landfills in humid areas.

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.

A simulation model for methane emissions from landfills with interaction of vegetation and cover soil

Global climate change and ecological problems brought about by greenhouse gas effect have become a severe threat to humanity in the 21st century. Vegetation plays an important role in methane (CH₄) transport, oxidation and emissions from municipal solid waste (MSW) landfills as it modifies the physical and chemical properties of the cover soil, and transports CH₄ to the atmosphere directly via their conduits, which are mainly aerenchymatous structures. In this study, a novel 2-D simulation CH₄ emission model was established, based on an interactive mechanism of cover soil and vegetation, to model CH₄ transport, oxidation and emissions in landfill cover soil. Results of the simulation model showed that the distribution of CH₄ concentration and emission fluxes displayed a significant difference between vegetated and non-vegetated areas. CH₄ emission flux was 1-2 orders of magnitude higher than bare areas in simulation conditions. Vegetation play a negative role in CH₄ emissions from landfill cover soil due to the strong CH₄ transport capacity even though vegetation also promotes CH₄ oxidation via changing properties of cover soil and emitting O₂ via root system. The model will be proposed to allow decision makers to reconsider the actual CH₄ emission from vegetated and non-vegetated covered landfills.

Methane oxidation in a landfill cover soil reactor: Changing of kinetic parameters and microorganism community structure

Changing of CH₄ oxidation potential and biological characteristics with CH₄ concentration was studied in a landfill cover soil reactor (LCSR). The maximum rate of CH₄ oxidation reached 32.40 mol d^-1 m^-2 by providing sufficient O₂ in the LCSR. The kinetic parameters of methane oxidation in landfill cover soil were obtained by fitting substrate diffusion and consumption model based on the concentration profile of CH₄ and O₂. The values of [Formula: see text] (0.93-2.29%) and [Formula: see text] (140-524 nmol kg_soil-DW^-1·s^-1) increased with CH₄ concentration (9.25-20.30%), while the values of [Formula: see text] (312.9-2.6%) and [Formula: see text] (1.3 × 10^-5 to 9.0 × 10^-3 nmol mL^-1 h^-1) were just the opposite. MiSeq pyrosequencing data revealed that Methylobacter (the relative abundance was decreased with height of LCSR) and Methylococcales_unclassified (the relative abundance was increased expect in H 80) became the key players after incubation with increasing CH₄ concentration. These findings provide information for assessing CH₄ oxidation potential and changing of biological characteristics in landfill cover soil.

Microbial community structures and metabolic profiles response differently to physiochemical properties between three landfill cover soils

Landfills are always the most important part of solid waste management and bear diverse metabolic activities involved in element biogeochemical cycling. There is an increasing interest in understanding the microbial community and activities in landfill cover soils. To improve our knowledge of landfill ecosystems, we determined the microbial physiological profiles and communities in three landfill cover soils (Ninghai: NH, Xiangshan: XS, and Fenghua: FH) of different ages using the MicroResp(TM), phospholipid fatty acid (PLFA), and high-throughput sequencing techniques. Both total PLFAs and glucose-induced respiration suggested more active microorganisms occurred in intermediate cover soils. Microorganisms in all landfill cover soils favored L-malic acid, ketoglutarate, and citric acid. Gram-negative bacterial PLFAs predominated in all samples with the representation of 16:1ω7, 18:1ω7, and cy19:0 in XS and NH sites. Proteobacteria dominated soil microbial phyla across different sites, soil layers, and season samples. Canonical correspondence analysis showed soil pH, dissolved organic C (DOC), As, and total nitrogen (TN) contents significantly influenced the microbial community but TN affected the microbial physiological activities in both summer and winter landfill cover soils.

Potential application of biocover soils to landfills for mitigating toluene emission

Biocover soils have been demonstrated to be a good alternative cover material to mitigate CH4 emission from landfills. To evaluate the potential of biocover soil in mitigating emissions of non-methane volatile organic compounds (NMVOCs) from landfills, simulated cover soil columns with the influx of toluene (chosen as typical of NMVOCs) concentrations of 102-1336 mg m(-3) in the presence or absence of the major landfill gas components (i.e., CH4 and CO2) were conducted in this study. In the two experimental materials (waste biocover soils (WBS) and landfill cover soils (LCS)), higher toluene reduction was observed in WBS with respect to LCS. After the introduction of landfill gas, an increase of microbial diversity and relative abundance of toluene-degrading bacteria and methanotrophs occurred in WBS. To illustrate the role of toluene-degrading activity in mitigating toluene emissions through landfill covers, an analytical model was developed by incorporating the steady-state vapor transport with the first-order kinetics of aerobic biodegradation limited by O2 availability. This study demonstrated that biocover soils have great potential in applying to landfills for mitigating toluene emission to the atmosphere.

Toxicity and bioaccumulation of bromadiolone to earthworm Eisenia fetida

Bromadiolone, a potent second-generation anticoagulant rodenticide, has been extensively used worldwide for the field control of rodents. Invertebrates may be at risk from primary poisoning as a result of bromadiolone bait applications. However, there are few data regarding the toxicity and bioaccumulation of bromadiolone to earthworms. In this study, we reported that bromadiolone was toxic to earthworms at 1mg/kg soil, which is a likely concentration in the field following application of bromadiolone baits. Exposure to bromadiolone resulted in a significant inhibition of earthworm growth. The antioxidant activities of superoxide dismutase and catalase were slightly increased in earthworms, while malondialdehyde content (as a molecular marker indicative of the damage to lipid peroxidation) was dominantly elevated over the duration of exposure. Bromadiolone in soil is bioaccumulative to earthworms. The biota to soil accumulation factors (BSAFs) of bromadiolone were concentration dependent and BSAFs decreased as the level of bromadiolone in soil increased. These results suggest earthworms are not only the potential subject to primary poisoning but also the source of secondary exposure for insectivores and scavengers following application of bromadiolone.