Landfill gas (LFG) is a natural byproduct of the decomposition of organic material within a landfill environment. This gas is principally a mix of methane (\(\text{CH}_4\)) and carbon dioxide (\(\text{CO}_2\)), with methane typically comprising 40% to 60% of the total volume. Methane is generated when bacteria break down organic waste, such as food scraps and paper, through anaerobic decomposition. Methane is a potent greenhouse gas, possessing a warming potential 28 to 36 times greater than carbon dioxide over a 100-year period. Landfills are a significant source of human-related methane emissions, requiring a multi-pronged approach that targets both the generation and the capture of the gas.
Diverting Organic Materials from Landfills
The most direct way to reduce methane emissions is to prevent the formation of landfill gas in the first place. Methane-producing bacteria require organic material to fuel their anaerobic decomposition. Therefore, diverting items such as food waste, yard trimmings, and soiled paper from the landfill removes the source material for methane generation.
Source separation programs are implemented to collect these organic materials outside of the mixed waste stream. These collected organics can then be managed through alternative processing methods that utilize aerobic decomposition, which produces carbon dioxide instead of methane. Composting is one such method, where organic matter breaks down in the presence of oxygen, resulting in a stable, nutrient-rich soil amendment.
Another processing pathway is anaerobic digestion (AD), which uses a controlled, oxygen-free environment to break down organics. While this process does produce biogas that contains methane, the gas is captured in a sealed system and can be utilized for energy production rather than escaping into the atmosphere. Diverting this waste stream eliminates the uncontrolled methane release that would occur if the material were buried in a traditional landfill. The logistical shift to these upstream management practices is an effective strategy for preventing future emissions.
Designing Landfill Gas Collection Infrastructure
For existing landfills that contain decades of buried organic waste, managing methane requires an engineered collection system. An active Landfill Gas Collection System (LFGCS) is the primary technical solution, designed to draw gas out of the waste mass before it can vent to the atmosphere. This system consists of a network of vertical extraction wells drilled deep into the waste and connected by horizontal collector pipes.
The network is connected to a central header pipe and a high-capacity vacuum blower system. This blower creates a slight negative pressure, which actively pulls the landfill gas from the pores within the waste mass. The effectiveness of the system relies heavily on the strategic placement and depth of the extraction wells, ensuring sufficient coverage across the gas-generating areas of the landfill.
Active systems are required to achieve significant capture rates, as passive systems rely only on the natural pressure gradient within the landfill. Operators must continuously monitor and tune the vacuum pressure to maximize gas extraction while preventing the excessive draw of outside air into the system. High collection efficiency is paramount, as it directly reduces the amount of methane that escapes as fugitive emissions.
Converting Captured Methane into Usable Energy
Once the landfill gas is successfully extracted by the collection system, the captured methane must be either destroyed or put to productive use. The simplest and most common form of treatment is flaring, where the gas is combusted in a controlled device. Flaring converts the potent methane into the less-potent greenhouse gas, carbon dioxide, significantly reducing the overall warming impact of the emissions.
The most beneficial pathway is through Landfill Gas to Energy (LFGTE) projects, where the captured gas is processed and utilized as a fuel source. The raw landfill gas can be combusted in turbines or reciprocating engines to generate electricity for the grid or on-site use. Alternatively, the gas can be piped directly to nearby industrial facilities to provide thermal heat for manufacturing processes.
A more advanced utilization method involves upgrading the landfill gas into Renewable Natural Gas (RNG). This process involves extensive cleaning and filtration to remove impurities, such as carbon dioxide, nitrogen, siloxanes, and hydrogen sulfide, to increase the methane concentration to pipeline quality, often 90% or higher. The resulting RNG can then be injected into existing natural gas pipelines or used as compressed or liquefied vehicle fuel.
Site Management Techniques to Minimize Leakage
Even with an active collection system in place, operational management techniques are necessary to prevent methane from escaping through the landfill surface, known as fugitive emissions. The integrity of the landfill cover system is a primary defense against these leaks. Intermediate cover, which is a layer of soil applied daily or frequently over waste, helps to temporarily suppress gas migration.
The final cover, or cap, is a more robust, long-term barrier, often incorporating geomembranes, synthetic liners, and clay barriers to form an impermeable seal. Some advanced sites utilize a bioreactor cap, which is a layer of material designed to promote the biological oxidation of methane into carbon dioxide by methanotrophic bacteria before it reaches the atmosphere.
To ensure the system’s effectiveness and minimize leakage, landfill operators conduct regular surface monitoring using handheld detectors or flux boxes to identify localized emission hot spots. Maintaining proper pressure within the gas collection system is also important; the vacuum must be strong enough to capture the gas but not so aggressive that it pulls air into the waste mass, which can decrease the methane quality and damage the biological activity.