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

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.

Model-based interpretation of methane oxidation and respiration processes in landfill biocovers: 3-D simulation of laboratory and pilot experiments

Landfill biocovers are an efficient strategy for the mitigation of greenhouse gas emissions from landfills. A complex interplay between key physical and reactive processes occurs in biocovers and affects the transport of gas components. Therefore, numerical models can greatly help the understanding of these systems, their design and optimal operation. In this study, we developed a 3-D multicomponent modeling approach to quantitatively interpret experimental datasets measured in the laboratory and in pilot-scale landfill biocovers. The proposed model is able to reproduce the observed spatial and temporal dynamics of CH₄, O₂ and CO₂ migration in biocovers under different operating conditions and demonstrates the importance of dimensionality in understanding the propagation of gas flow and migration of gas components in such porous media. The model allowed us to capture the coupled transport behavior of gas components, to evaluate the exchange of gas fluxes at the interface between the biocover surface and free air flow, and to investigate the effects of different gas injection patterns on the distribution of gas components within biocovers. The model also helps elucidating the dynamics and competition between methane oxidation and respiration processes observed in the different experimental setups. The simulation outcomes reveal that increasing availability of methane (i.e., higher injection flow rates or higher fractions of CH₄ in the injected gas composition) results in progressive dominance of methane oxidation in the biocovers and moderates the impact of respiration.

Real-time monitoring of methane oxidation in a simulated landfill cover soil and MiSeq pyrosequencing analysis of the related bacterial community structure

Real-time CH₄ oxidation in a landfill cover soil was studied using automated gas sampling that determined biogas (CH₄ and CO₂) and O₂ concentrations at various depths in a simulated landfill cover soil (SLCS) column reactor. The real-time monitoring system obtained more than 10,000 biogas (CH₄ and CO₂) and O₂ data points covering 32 steady states of CH₄ oxidation with 32 different CH₄ fluxes (0.2-125mol·m^-2·d^-1). The kinetics of CH₄ oxidation at different depths (0-20cm, 20-40cm, and 40-60cm) of SLCS were well fit by a CH₄-O₂ dual-substrate model based on 32 values (averaged, n=5-15) of equilibrated CH₄ concentrations. The quality of the fit (R² ranged from 0.90 to 0.96) was higher than those reported in previous studies, which suggests that real time monitoring is beneficial for CH₄ oxidation simulations. MiSeq pyrosequencing indicated that CH₄ flux events changed the bacterial community structure (e.g., increased the abundance of Bacteroidetes and Methanotrophs) and resulted in a relative increase in the amount of type I methanotrophs (Methylobacter and Methylococcales) and a decrease in the amount of type II methanotrophs (Methylocystis).

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.