Methane is a powerful greenhouse gas that significantly contributes to the Earth’s warming atmosphere. A major source of this gas comes from landfills, where discarded organic materials decompose.
How Landfills Generate Methane
Methane production in landfills begins with the decomposition of organic waste, which includes food scraps, yard trimmings, and paper. This breakdown occurs through a process called anaerobic decomposition, meaning it happens in an environment where oxygen is absent or quickly depleted. Unlike aerobic decomposition, which produces carbon dioxide, the lack of oxygen in landfills creates conditions suitable for specific microorganisms.
These microorganisms, primarily bacteria and archaea, work in stages to break down complex organic matter. Initially, hydrolytic bacteria convert large molecules like carbohydrates, proteins, and fats into smaller, soluble compounds such as sugars and amino acids. Acidogenic bacteria then ferment these smaller molecules, producing organic acids, carbon dioxide, and hydrogen. Finally, methanogenic archaea convert these intermediate products into methane and carbon dioxide. Methane typically constitutes about 50% of landfill gas, with carbon dioxide making up the remaining portion, along with trace amounts of other compounds.
Environmental Impact of Landfill Methane
Methane traps heat in the atmosphere more effectively than carbon dioxide. While carbon dioxide persists in the atmosphere for hundreds to thousands of years, methane has a shorter atmospheric lifetime, typically around a decade. However, its high warming potential makes it a significant contributor to climate change even in that shorter timeframe.
When compared over a 100-year period, methane’s Global Warming Potential (GWP) is estimated to be 27 to 30 times greater than that of carbon dioxide. If measured over a 20-year period, this ratio increases to 81 to 83 times, highlighting its intense, short-term warming effect. Landfills are a substantial source of these emissions, accounting for approximately 14.4% of human-related methane emissions in the United States in 2022. This amount is comparable to the greenhouse gas emissions from over 24 million gasoline-powered passenger vehicles driven for one year.
Strategies to Reduce Landfill Methane
Mitigating methane emissions from landfills involves capturing and utilizing the gas. Landfill gas (LFG) collection systems extract the methane and carbon dioxide mixture that forms underground. These systems often use vertical wells drilled into the waste or horizontal collectors installed during landfill operations to draw the gas to a central point using blowers or vacuums.
Collected landfill gas can be managed in several ways. One method is flaring, where the gas is burned off. While flaring releases carbon dioxide, it converts methane into a less impactful greenhouse gas, significantly reducing its overall warming effect. Enclosed flares, operating at high combustion temperatures, can achieve over 98% destruction efficiency.
A more beneficial approach converts collected landfill gas into energy. This process generates electricity using technologies like internal combustion engines, gas turbines, or fuel cells. Reciprocating internal combustion engines are widely used for electricity generation due to their reliability and lower costs. Landfill gas can also be processed and upgraded to pipeline-grade renewable natural gas, or used directly for heating applications in boilers or combined heat and power systems. Utilizing LFG as an energy source reduces methane emissions and can displace fossil fuels, offering a dual environmental benefit.