Landfills are engineered facilities designed to contain discarded waste materials. Their primary purpose is to dispose of solid waste while minimizing environmental impact. Given the immense volumes of waste generated globally, landfills remain a necessary component of modern waste management.
Engineered Containment Systems
Modern landfills feature sophisticated multi-layered systems to prevent contaminants from reaching the surrounding environment. The base includes a composite liner system, acting as a barrier. This system begins with a compacted clay layer, two to five feet thick, providing a low-permeability foundation.
Above the clay, a geomembrane layer, commonly made of high-density polyethylene (HDPE), is installed. This synthetic liner offers exceptional impermeability and chemical resistance, preventing liquid from seeping into the soil and groundwater. A geocomposite drainage layer, consisting of a gravel or sand layer with pipes, is placed above the geomembrane. This system, known as the leachate collection system, collects liquid that drains through the waste.
The collected leachate flows by gravity through the pipe network to sumps, where pumps then transfer it to storage tanks for treatment. Landfills also incorporate stormwater management systems. These systems direct precipitation away from the waste, reducing the amount of water that can infiltrate the landfill and generate more leachate.
Decomposition and Transformation of Materials
Once waste is placed in a landfill, it undergoes significant changes. Landfills are designed as anaerobic environments, meaning they lack oxygen. This condition arises from the compaction of waste and the sealing provided by daily covers and liner systems.
In this oxygen-deprived setting, organic materials like food scraps, paper, and yard waste decompose slowly through a process called anaerobic digestion. Microorganisms break down these materials, producing landfill gas, which is a mixture of methane and carbon dioxide. The rate of decomposition can vary, with some materials taking months or even decades to break down, influenced by factors like moisture levels and temperature.
In contrast, inorganic materials such as plastics, metals, and glass decompose extremely slowly, or not at all. Plastics can persist for hundreds or thousands of years, while glass and metals do not biodegrade. The primary changes these materials undergo are physical fragmentation rather than chemical breakdown.
Management of Landfill Emissions
Landfills produce two main types of emissions that require careful management: leachate and landfill gas. Leachate, the liquid byproduct of waste decomposition and rainwater infiltration, is collected through an extensive pipe network within the landfill. This liquid is typically stored in tanks before being transported off-site for treatment at a wastewater treatment plant. Some facilities may also employ on-site treatment methods, such as biological processes or membrane filtration, to purify the leachate.
Landfill gas (LFG), composed of methane and carbon dioxide, is a natural byproduct of anaerobic decomposition. Methane is a potent greenhouse gas, making its controlled management necessary. Gas collection systems, with vertical and horizontal wells drilled into the waste, extract this gas under negative pressure.
Once collected, landfill gas can be managed in a few ways. It can be flared, burning off methane to convert it into less potent carbon dioxide. Landfill gas is also captured and converted into energy. This process involves treating the gas to remove impurities, then using it as fuel in gas engines or turbines to generate electricity or as renewable natural gas.
Long-Term Landfill Stewardship
After a landfill reaches capacity, it undergoes a closure process that involves installing a final cover system, also known as capping. This cap, often a multi-layered barrier, typically includes compacted soil layers and a geomembrane. The primary purpose of the cap is to prevent rainwater from infiltrating the waste, which would generate more leachate, and to control the release of landfill gas.
Closure does not signify abandonment; instead, it marks the beginning of a long-term stewardship phase. Landfills require continuous monitoring and maintenance for many decades, often for 30 years or more after closure. This ongoing care includes monitoring groundwater quality around the site to detect any potential contamination from leachate. Gas emissions are also continuously monitored to ensure the collection systems remain effective and to prevent uncontrolled releases into the atmosphere. Regular inspections and maintenance of the cap integrity, as well as the leachate and gas collection systems, are performed to ensure the landfill continues to protect the environment.