Biochar-Based Biocovers for Landfill Methane Mitigation: Quantifying Adsorption, Transport & Oxidation

One of the major environmental challenges associated with landfills is the generation of landfill gas which comprise 50% CH4 and 50% CO2. To mitigate the continually rising levels of CH4 emissions from landfills, engineered, bio-based cover systems in combination with efficient gas collection system during the landfill’s active phase, or just the bio-based cover systems in case of an old/abandoned landfill can be employed. Microbially mediated biochemical CH4 oxidation is the key CH4 mitigation mechanism in landfill cover systems and the use of organic materials as cover amendments was found to enhance this process. Compost has been extensively researched and advocated for use as a biocover material. Albeit, application-oriented challenges still exist due to the material’s insufficient porosity and the possibility of undergoing self-degradation (if not sufficiently mature) thereby, lowering its performance. In order to design an efficient biocover system, a stable material with sufficiently good porosity and methane adsorption capacity which can promote microbial methane oxidation process needs to be selected. One such material that could potentially enhance the methane mitigation in landfill cover system is biochar. Biochar is a carbon-rich, highly porous material generated as a byproduct in the process of producing bio-energy from raw biomass (discarded wood, manure or agricultural crop residues) through pyrolysis or gasification. The physical-chemical and surface characteristics of biochars are strongly influenced by the type and composition of feedstock (raw biomass), production conditions (reactor residence time, temperature) and treatment processes (activation, particle screening, etc.). Preliminary studies show that amending biochar to soils containing a high fraction of clay content, can increase the aeration and thus limit the fraction of anoxic/anaerobic pore volume and subsequently promote microbial methane oxidation by methanotrophs. The purpose to this study is to model the effects of amending seven different types of hardwood biochars to landfill cover soil (silty clay) on the stability of cover slopes, methane adsorption and oxidation capacities under varying levels of amendment ratios and environmental conditions such as moisture and temperature. Biochars were characterized for their physical-chemical and hydraulic properties such as particle size distribution, specific gravity, density, porosity, surface area, hydraulic conductivity, water holding capacity, organic matter and organic carbon contents, pH, oxidation-reduction potential and electrical conductivity, zeta potential, carbon, nitrogen and hydrogen (CHN) elemental composition. The biochar properties were correlated to methane adsorption capacities to develop a selection criterion for woody biochars for application as landfill cover amendments. Geotechnical properties such as compressibility and shear strength of biochars and biochar-amended cover soils were characterized to evaluate the stability aspects of the biochar-based biocover system. A series of batch and small-scale column adsorption tests were conducted to model the reactive transport of methane (1-D advection-dispersion equation) through biochars and biochar-amended soils under varying methane inflow rates and moisture levels. Batch incubation testing was performed on selected biochar-amended soils to quantify the combined methane adsorption and microbial methane oxidation capacities and model the effects of varying biochar types, biochar to soil amendment ratios, moisture and temperature on the methane oxidation rates. Additionally, a life cycle analysis was performed to evaluate the sustainability metrics (i.e. environmental, economic and social aspects) of biochar-based landfill cover systems and conventional soil cover systems that were designed to achieve zero methane emissions for a hypothetical landfill site located in Northeastern Illinois region. Following this, Monte-Carlo simulations were used to assess the uncertainty levels in the life cycle impact assessment results by varying the methane oxidation rates of the cover materials. Finally, design recommendations were made for the most sustainable, field-scale implementation of biochar-based cover systems which can achieve complete oxidation of methane in landfills located in Northeastern Illinois. Overall, this study presents valuable information that can be critical to the development of design guidelines for field-scale implementation of biochar-based landfill cover systems with the aim of achieving optimum CH4 mitigation.