Landfill methane (CH4) is microbially oxidized to carbon dioxide. Thereby, the risk of explosion and the emissions of the greenhouse gas CH4 are reduced.
Degradation of waste generates CH4 and CO2. Over time, landfill gas production decreases to a level where energy conversion and flaring are no longer viable. However, gas production continues for decades to centuries, and risks to safety and greenhouse gas emissions persist. In this phase as additional measure, landfill gas can be treated biologically in methane oxidation systems (MOS; filters, windows, entire covers). These consist of a methane oxidation layer (MOL), the top part of which is vegetated to prevent erosion, above a gas distribution layer (GDL).
MOS are connected to a gas well or receive gas directly from the waste body, depending on the presence of gas extraction and/or liner systems. Landfill gas is actively (pumping) or passively (pressure difference between waste body and atmosphere) supplied via gas inlets implemented in the GDL, oxygen reaches the MOL by diffusion from the atmosphere. Design goals focus on
– Spatial homogeneity of CH4 load, design parameters: MOL permeability (depending on texture & compaction) and its difference to GDL permeability; number of gas inlet points;
– Dimensioning adapted to the soil’s CH4 oxidation capacity;
– Soil chemical properties, required to sustain demands of methanotrophic bacteria and vegetation.
Use of mineral soils is preferred, organic materials (e.g. composts) are microbially degradable, causing settlement and loss of permeability.
Main stakeholders & beneficiaries: landfill operators, regulator
MOS substitute a part of the landfill cover system. Capital costs increase marginally, mainly dependingnd on the availability of suitable soils and the construction practice needed to reach target soil physical properties. After construction, costs are mainly determined by the monitoring acivities.
Evidence of success
This practice reduces the emissions of greenhouse gases from landfills where CH4 formation is too low for economically viable technical gas treatment. Without this practice, landfill gas would migrate into the environment in an uncontrolled manner, endangering safety and releasing the greenhouse gas CH4 for a long period of time. Factual evidence of success or failure can be provided by measurement of surface CH4 and CO2 concentrations and fluxes, soil gas composition and visual inspections.
CH4 flux has to be determined in advance for dimensioning (empirically, modelling). Spatial homogeneity of gas flux is the main design challenge. Choice of correct soil / construction practice to reach target properties is crucial. Organic materials used (not recommended) need regular replacement.
Potential for learning or transfer
Methane oxidation systems are an adequate tool to convert CH4 into CO2 when landfill gas production is too low to enable active extraction for energy conversion or flaring. They can also be implemented as additional measure to reduce gas emissions. The application potential is large as landfills are still a central component of waste management in many EU member states. This practice can hence be transferred to other EU member states, especially to those with moderate to high precipitation rates and moderate temperatures. The practice is less sustainable in regions low in precipitation, with long dry seasons, or strong winters as drought or freezing promote cracks, where gas can easily escape, and lower microbial activity.
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