Where does your trash end up after it leaves your curb?
While it may seem as though it simply disappears once on the truck, most waste ends up in landfills. In this blog post, we’ll explore the science behind landfills, including what happens inside the waste, the generation of methane gas, and the impact.
Americans, on average, generate 4.9 pounds of trash per person each day. That’s 292.4 million tons per year. It may come as no surprise, but Americans produce roughly three times the global average for garbage, and the majority ends up buried in landfills (HowStuffWorks).
What is a Landfill?
Before digging into how landfills are operated the impact, let’s take a step back. While you might think of a “landfill” and a “dump” as synonymous, a landfill is a designated site for the disposal of waste materials that cannot be recycled or reused. In contrast, open dumps — unregulated waste sites — are illegal in most countries due to their severe health and environmental risks.
Landfills are carefully engineered to safely contain waste, prevent environmental contamination, and manage byproducts like leachate (contaminated liquid) and landfill gas.
There are several types of landfills, including:
- Municipal solid waste (MSW) landfills, used for household waste
- Industrial waste landfills, used for commercial and institutional waste
- Hazardous waste landfills, used for toxic materials (EPA)
Many times, these landfills are designed as “sanitary landfills,” operating with liners and covers in place to isolate waste from the surrounding environment.
Why Do Landfills Matter?
Landfills are an essential part of modern waste management, and if not managed correctly, they can release harmful chemicals and pollutants into our soil and drinking water, threatening our health. A landfill’s proper design, operation, and continuous monitoring are vital to minimizing their impact on surrounding communities, employees, and the air we breathe (Enviroliteracy).
When organic waste in the landfill — including food, yard clippings, paper, and more — breaks down without oxygen, landfill gas is produced. Approximately half of this landfill gas generated is methane, while the other half is carbon dioxide and non-methane organic compounds, such as trichlorethylene, benzene, and vinyl chloride. According to the EPA, MSW landfills are the third-largest source of human-related methane emissions in the US.
As greenhouse gases go, methane is 28 times more potent than carbon dioxide at trapping heat in the atmosphere over a 100-year period (EPA). Methane is a short-lived climate pollutant (SLCP) — over just a 20-year period, its warming effect jumps to 86x that of carbon dioxide (CCAC).
The Anatomy of a Landfill
Landfills consist of several key components (HowALandfillWorks):
- Bottom liner system: Separates trash and leachate from groundwater
- Cells: Areas where trash is stored within the landfill
- Storm water drainage system: Collects rainwater that falls on the landfill
- Leachate collection system: Gathers water that has percolated through the landfill
- Methane collection system: Collects methane gas formed during waste breakdown
- Covering (or cap): Seals off the top of the landfill
Source: ResearchGate, 2017, https://www.researchgate.net/publication/329957631_Recent_Technologies_for_Treatment_and_Recycling_of_Used_Disposable_Baby_Diapers.
What Happens Inside a Landfill?
As waste decomposes, it goes through several phases. This process, mainly driven by microbial activity and chemical reactions, is at the heart of landfill science (Waste360).
There are four phases in which bacteria decompose landfill waste. In these phases, the composition of the gas produced changes. Because most landfills accept waste over a 20- to 30-year period, waste in one area may be going through a different phase of decomposition than more recently buried waste in another part of the same landfill.
Phase 1, sometimes referred to as the “initial adjustment phase,” is characterized by aerobic bacterial activity and can last days or months, continuing until all available oxygen is depleted (ATSDR). Initial oxygen levels can depend on how loose or compacted the waste was when buried, and high compaction can therefore slow down the waste breakdown (Compact&Bale).
Once the oxygen is depleted, phase 2 begins, and a new group of bacteria takes over the decomposition process. These anaerobic microorganisms transform the compounds produced by their aerobic predecessors into various acids (acetic, lactic, and formic) and alcohols (including methanol and ethanol). This results in a highly acidic landfill environment which, combined with the moisture present in the waste, leads to the dissolution of certain nutrients, making nitrogen and phosphorus available to the bacteria.
Phase 3 begins when bacteria consume the previously produced organic acids and convert them into acetate. This neutralizes the acidic environment and allows methane-producing bacteria to thrive. A symbiotic relationship forms between the two types of bacteria: the acid-producing bacteria generate compounds that the methane-producing bacteria consume, while the methane-producing bacteria help prevent toxic accumulation of carbon dioxide and acetate — fostering a balanced ecosystem. This phase establishes conditions that promote methane production.
Phase 4 is characterized by steady-state conditions, in which the landfill gas contains 45-60% methane. A landfill’s gas production typically persists for approximately 20 years, but in some cases, can continue for more than 50. Landfills with higher organic content may experience those extended gas production periods.
Production Phases of Typical Landfill Gas
Source: Agency for Toxic Substances and Disease Registry. Landfill Gas: A Primer for Understanding the Basics. U.S. Department of Health and Human Services, 2001, figure 2-1, "Production Phases of Typical Landfill Gas." https://www.atsdr.cdc.gov/hac/landfill/pdfs/landfill_2001_ch2mod.pdf.
The Need for Continuous & Real-Time Landfill Management
Recognizing these phases is essential for efficient waste management, as different sections of a landfill can decompose at varying stages simultaneously. Since landfill decomposition can continue for more than half a century, consistent monitoring and prompt management to reduce emissions and increase methane recovery are vital to minimizing environmental and health impacts.