Landfill gas is a mixture of gases produced when organic waste breaks down inside a landfill. It’s roughly 45% to 60% methane and 40% to 60% carbon dioxide by volume, with a small fraction of other compounds like sulfides. Because methane is both a potent greenhouse gas and a usable fuel, landfill gas is simultaneously an environmental hazard and an energy resource.
What’s in Landfill Gas
The two dominant components are methane and carbon dioxide, which together make up more than 90% of the gas. The remaining 2% to 9% consists of other gases, including hydrogen sulfide (which gives landfills their rotten-egg smell), nitrogen, oxygen, and a range of volatile organic compounds. These trace compounds come from items like cleaning products, paints, and solvents that were tossed in with household trash.
The methane content is what makes landfill gas valuable as a fuel. Natural gas from a pipeline is mostly methane, so landfill gas is essentially a lower-grade version of the same thing. It’s also what makes landfill gas dangerous: methane is flammable, odorless, and a powerful heat-trapping gas when released into the atmosphere.
How Landfill Gas Forms
Landfill gas doesn’t appear all at once. It develops in four overlapping phases as different types of bacteria take turns breaking down the waste.
In the first phase, bacteria that need oxygen consume it while breaking down food scraps, paper, yard waste, and other organic material. The main byproduct at this stage is carbon dioxide, and nitrogen levels are high because air is still trapped in the waste. This phase ends when the available oxygen runs out.
In the second phase, bacteria that thrive without oxygen take over. They convert the partially broken-down material into organic acids and alcohols, releasing carbon dioxide and hydrogen gas in the process. The environment inside the landfill becomes increasingly acidic.
The third phase is a transitional period. A new group of bacteria begins consuming those organic acids, which neutralizes the environment and makes conditions hospitable for methane-producing microbes. These methane producers start feeding on the carbon dioxide and acetate left behind, and methane concentrations begin to climb.
By the fourth phase, gas composition and production rates stabilize. This is when the landfill reaches its steady-state output of roughly half methane, half carbon dioxide. A large landfill can remain in this phase for decades, continuously producing gas as long as decomposable material remains.
Why Landfill Gas Is a Climate Concern
Methane is a far more effective heat-trapping gas than carbon dioxide. Over a 100-year window, methane has a global warming potential of 27 to 30, meaning each ton warms the atmosphere about 28 times as much as a ton of carbon dioxide. Over a 20-year window, that number jumps to 81 to 83, because methane is especially potent in the short term before it breaks down in the atmosphere.
Landfills are one of the largest human-caused sources of methane emissions. When gas simply seeps out of a landfill without being captured or burned, all of that methane enters the atmosphere directly. Even burning it off in a flare is better from a climate perspective, because combustion converts methane into carbon dioxide, which is a much weaker greenhouse gas.
Safety Hazards
Methane becomes explosive when it makes up between about 5% and 15% of the air in an enclosed space. Below 5%, there isn’t enough fuel to ignite. Above 15%, there isn’t enough oxygen. That narrow window is called the explosive range, and it’s a real concern in buildings, utility corridors, or basements near landfills where gas can migrate underground and accumulate.
Landfill gas can also displace oxygen in confined spaces, creating a suffocation risk. The trace compounds, while present in small amounts, include some that can cause headaches, nausea, or irritation at elevated concentrations. This is one reason modern landfills are required to monitor and control gas migration beyond their boundaries.
How Landfills Collect the Gas
Modern landfills use active collection systems to capture gas before it escapes. The basic setup involves a network of vertical wells or horizontal trenches drilled into the waste mass, lined with perforated PVC pipe and surrounded by gravel to allow gas to flow in. These wells connect to a header pipe that runs along the surface, and a blower creates suction to pull the gas out continuously.
Because the gas is warm and moist, condensation forms inside the pipes. Condensate traps are placed throughout the system to remove this water and keep the gas flowing. At the end of the collection network, the gas reaches either a flare, where it’s burned off, or a processing facility, where it’s put to use.
Turning Landfill Gas Into Energy
The simplest use is burning the gas directly in boilers or industrial furnaces as a replacement for natural gas or other fossil fuels. Many landfills also feed their gas into generators that produce electricity, which can power the landfill’s own operations or be sold to the grid.
A more advanced option is upgrading landfill gas into what’s called renewable natural gas. This involves removing the carbon dioxide, nitrogen, oxygen, and volatile organic compounds until the gas is nearly pure methane, matching pipeline-quality natural gas. Once compressed, it can be injected into existing natural gas pipelines or used as vehicle fuel. This process is more expensive but produces a higher-value product and displaces fossil natural gas on a one-to-one basis.
The economics depend largely on the size of the landfill and how much gas it produces. Larger, actively receiving landfills generate enough gas to justify the infrastructure investment, while smaller or older sites may only produce enough to warrant a simple flare.