Abstract: Climate change is one of the greatest challenges of our time and is primarily driven by anthropogenic greenhouse gas emissions. While carbon dioxide has received the most attention, other gases also play a substantial role. Methane is the second most significant anthropogenic greenhouse gas after carbon dioxide, and its atmospheric concentration continues to rise steadily. The main sources of methane emissions associated with human activity include agriculture (livestock farming and rice cultivation), the energy sector (extraction, processing, distribution natural gas, oil, and coal), as well as waste management practices (landfill gas emissions). Due to its relatively short atmospheric lifetime and its high heat-trapping capacity, reducing methane emissions offers a rapid and effective means of slowing the rate of global warming.
Strategies for mitigating anthropogenic methane emissions are already well established and implemented across various sectors. In waste management, technologies that enable the capture and utilization of generated landfill gas (primary composed of methane) or the reduction its surface emissions are becoming increasingly popular. Although capturing landfill methane for flaring or conversion into renewable energy are both effective approaches, they require stable gas production, condition typically found only in landfills that are no more than 8–15 years old. These approaches are generally less economical and technologically feasible for smaller or older landfills, where methane emissions are lower and more diffuse.
An alternative approach is the use of Microbial Methane Oxidation Systems (MMOS)—technologies that employ methanotrophic bacteria for the biological elimination of methane from landfill gas.
Methanotrophs are Gram-negative bacteria capable of oxidizing methane. They constitute a highly heterogeneous group, characterized by great enzymatic and metabolic diversity, which enables them effectively utilize methane across a wide range of ecological niches.
During the biochemical transformations within bacterial cells, carbon from methane can be utilized as an energy source, leading to CO₂ production, or assimilated into biomass.
One of the configuration in which MMOS are used are biocovers created from a layer of permeable material such as compost, a mixture of soil and organic matter, or specially selected porous materials.
A typical biocover applied to a landfill site consists of several layers:
- foundation layer formed from the top layer of the landfill
- gas distribution layer – ensures the uniform distribution of landfill gas;
- filtration layer (optional) – supports uniform gas diffusion and protects the biofilm;
- methane oxidation layer – the zone in which the actual biological oxidation of methane occurs.
The efficiency of methane oxidation in biocover depends on a number of environmental factors that may influence the process indirectly, by affecting methane and oxygen diffusion, or directly, by modulating the structure and activity of the microbial community. The key factors include:
- temperature and humidity,
- pH of the environment,
- availability of oxygen and methane,
- presence of nitrogen and other essential nutrients.
To ensure optimal process performance, the methane oxidation layer should:
- enable efficient upward transport of gases (CH₄, CO₂) and downward transport of O₂,
- ensure adequate water retention to support microbial activity,
- provide a physical environment promoting biofilm development by methanotrophic bacteria,
- ensure appropriate insulation to maintain the optimal temperature for bacterial activity
- provide the necessary nutrients to sustain the activity of microorganisms
Field studies at the Kudjape (Estonia) [8] and Aikkala (Finland) [9] landfills confirmed that biocovers effectively reduce methane emissions, with observed seasonal variations linked to environmental conditions. Long-term monitoring of the biocover system at the Klintholm landfill (Denmark) [12] further demonstrated that high CH₄ oxidation efficiency can be maintained for years, even as the physical properties of the cover material change over time.
