AP Environmental Science

## Learning Outcomes

- Explain how landfill gas forms from anaerobic decomposition in municipal solid waste landfills
- Identify the main steps in landfill gas-to-electricity systems, including collection, treatment, and combustion
- Distinguish between flaring, landfill gas-to-electricity, and waste incineration
- Apply methane emissions accounting using recovery and oxidation concepts
- Evaluate the environmental benefits and limits of landfill gas energy projects
- Use this topic in AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand landfill gas to electricity as a waste-management and pollution-control strategy, with a focus on the following syllabus points:

- Know that landfill gas is produced by anaerobic decomposition of organic waste and is typically about **50% methane** and **50% carbon dioxide**
- Identify the sequence of landfill gas use: waste burial, anaerobic decay, gas collection, treatment, and either flaring or energy recovery
- Explain why capturing and burning methane reduces climate impact compared with uncontrolled release
- Understand the difference between landfill gas-to-electricity, flaring, and incineration of municipal solid waste
- Recognise common electricity-generation technologies, including internal combustion engines, gas turbines, and microturbines
- Understand that landfill gas projects can continue after landfill closure because methane generation continues for years or decades
- Apply the distinction between methane recovered by engineering systems and methane oxidized in landfill cover soils
- Evaluate landfill gas-to-electricity against other waste strategies such as source reduction, recycling, and composting

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What causes methane to form inside a landfill?
2. How is flaring different from landfill gas-to-electricity?
3. Why can burning methane still reduce climate impact even though it produces carbon dioxide?
4. Name two technologies used to generate electricity from landfill gas.
5. What is the difference between methane recovery and methane oxidation in landfill emissions accounting?

## Introduction

Landfill gas to electricity sits at the meeting point of waste management, air pollution, and climate change. It is often grouped with waste reduction methods in AP Environmental Science, but you need to be careful with the wording: this is not true **source reduction**. It does not prevent waste from being created. Instead, it deals with pollution after waste has already been buried.

This makes it a very exam-friendly topic. It links Unit 8 waste disposal to Unit 9 climate change. It also rewards students who can explain process chains clearly: organic waste is buried, oxygen is limited, microbes break it down, methane forms, gas is captured, then burned for either emissions control or electricity generation.

> **Key Term: landfill gas (LFG)**
> Gas produced when organic waste decomposes anaerobically in a landfill; it is typically about half methane and half carbon dioxide, with trace gases.

### How landfill gas forms and why it matters

Modern sanitary landfills isolate waste from the surrounding environment with liners, leachate controls, and daily cover, but they do not stop decomposition. When food scraps, paper, yard waste, and other organic materials are buried, they are eventually broken down by **anaerobic bacteria** because oxygen is limited deep inside the landfill.

That detail matters. If oxygen were abundant, decomposition would produce more carbon dioxide directly. In a landfill, low-oxygen conditions shift decomposition toward methane formation. This is why landfills are major human-related methane sources.

> **Key Term: anaerobic decomposition**
> Breakdown of organic matter in the absence of oxygen, often producing methane and carbon dioxide.

Methane is the main reason landfill gas is environmentally significant. It is a strong greenhouse gas, so letting it escape directly into the atmosphere has a larger warming effect than releasing the same carbon as carbon dioxide. That is the central logic behind capture systems.

The AP exam often tests this as cause and effect:

- Organic waste is buried
- Oxygen becomes limited
- Anaerobic microbes decompose the waste
- Landfill gas forms
- Methane can escape, be flared, or be used for energy

A common mistake is to think methane production stops when the landfill is closed. It does not. Gas production usually rises, peaks, and then declines over time, and this can continue for years or decades after closure. So a closed landfill can still be an active gas source.

> **Key Term: municipal solid waste (MSW) landfill**
> A landfill used mainly for everyday discarded materials such as paper, food waste, packaging, and other household or commercial waste.

Landfill gas also creates local safety issues, not just global climate issues. Methane is combustible. Gas moving through cracks, pipes, or surrounding soils can build up and create explosion hazards. Other gases can also produce odor and health concerns. So gas capture has both climate and safety value.

The United States context is also useful. The EPA reported that MSW landfills were the **third-largest human-related methane source in the United States in 2022**, accounting for about **14.4%** of those emissions. That makes landfills much more than a small local waste issue.

> **Test Tip:** If a question asks why landfill methane is a pollution problem, do not stop at “it smells bad.” The stronger answer links **anaerobic decomposition**, **greenhouse effect**, and **explosion risk**.

### From buried waste to electricity: the process step by step

Landfill gas-to-electricity works through a chain of engineering steps. You should know the order clearly because AP questions often test the sequence.

First, landfill gas must be collected. This is done with a **gas collection system** that usually includes wells drilled into the landfill, piping, and pumps or blowers. These create a controlled path for gas movement rather than allowing uncontrolled escape.

> **Key Term: gas collection system**
> A network of wells, pipes, and pumps or blowers used to capture landfill gas from within a landfill.

Second, the collected gas often needs treatment. Raw landfill gas contains moisture, condensate, particulates, and trace impurities. If these are not removed, they can damage engines or turbines and reduce efficiency. Treatment needs vary by gas quality and intended use.

Third, the collected gas has two main possible fates:

- **Flaring**
- **Energy recovery**

Flaring means burning the gas without making useful electricity. This destroys methane and reduces its climate effect, but it wastes the fuel value.

> **Key Term: flaring**
> Burning collected landfill gas to destroy methane without generating useful electricity or heat.

Energy recovery means using landfill gas as a fuel to generate electricity, heat, or both. For electricity generation, the most common technologies named by the EPA are:

- internal combustion engines
- gas turbines
- microturbines

Internal combustion engines are the most common because many landfill projects fit their scale and cost. Gas turbines are more suitable where gas flow is larger and conditions support turbine operation. Microturbines are smaller-scale systems that may work better for smaller landfills.

A further option is **combined heat and power**, where the same fuel stream produces electricity and useful heat. This can raise overall energy efficiency because waste heat is captured rather than discarded.

> **Key Term: combined heat and power (CHP)**
> A system that uses one fuel source to generate electricity and useful heat at the same time.

The final step is distribution. Once electricity is generated, it is delivered to the local electrical grid or used on-site.

Students often confuse landfill gas projects with incineration. These are not the same. Incineration burns the solid waste itself. Landfill gas-to-electricity uses gas produced after the waste has already been buried and decomposed.

A side-by-side distinction helps:

- **Landfill gas-to-electricity**: captures methane produced after disposal
- **Flaring**: burns methane after disposal, but with no energy use
- **Incineration/WTE**: burns solid waste directly before or instead of landfill disposal

This distinction matters because the pollution profile, timing, and energy source differ.

> **Test Tip:** If an MCQ contrasts landfill gas-to-electricity with incineration, look for whether the fuel is the **gas from decomposition** or the **solid waste itself**. That is usually the key difference.

### Why burning methane can reduce climate impact

At first, landfill gas-to-electricity can sound contradictory. If methane is burned, carbon dioxide is still released. So how can this help climate change?

The answer is about comparing alternatives, not chasing a perfect zero-emission outcome.

If methane escapes directly, it enters the atmosphere as methane. If it is captured and burned, most of that methane is converted to carbon dioxide and water. Carbon dioxide is still a greenhouse gas, but the direct release of methane is generally worse from a warming perspective. So combustion reduces the climate harm relative to uncontrolled release.

This is why landfill gas-to-electricity is often described as a better option than venting and often a better option than flaring alone. It reduces methane emissions and also provides usable electricity that can offset some fossil fuel use elsewhere.

However, balance is important. Landfill gas-to-electricity is not carbon-free.

Its limits include:

- gas collection is not 100% efficient
- some methane leaks before capture
- combustion still releases carbon dioxide
- combustion can also produce air pollutants such as nitrogen oxides and particulates
- gas production declines as the landfill ages
- installation and maintenance are costly

Because of these limits, landfill gas-to-electricity should be seen as a **partial pollution-control strategy**, not the best overall waste solution.

Upstream methods are usually preferred:

- source reduction
- reuse
- recycling
- composting of organic waste

These approaches reduce the amount of organic waste entering the landfill in the first place, so they cut methane generation at the source.

That is why AP questions may ask whether landfill gas-to-electricity is “renewable” or “sustainable.” The strongest answer is cautious: it is often treated as a biogenic or alternative energy source because it uses methane produced from organic waste, but it still depends on continued waste disposal and should not be confused with waste prevention.

### Methane accounting, oxidation, and exam-style reasoning

This topic becomes stronger when you can apply a simple accounting model. The IPCC framework describes landfill methane emissions conceptually as:

CH4 emissions = (CH4 generated − CH4 recovered) × (1 − oxidation factor)

This equation is useful because it forces you to separate two ideas students often mix up.

First, **recovered methane** is methane captured intentionally by engineering systems such as wells and pipes.

Second, **oxidized methane** is methane that was not recovered but is biologically converted, often in landfill cover soils, before it reaches the atmosphere.

> **Key Term: oxidation factor**
> The fraction of methane oxidized in landfill cover soils before release to the atmosphere.

The order matters. Recovery happens first in the accounting framework. Oxidation applies to the remaining methane, not to the full amount originally generated.

Here is a worked example.

Suppose a landfill generates **100 units** of methane.
It recovers **60 units** using wells and piping.
Of the remaining methane, **10%** is oxidized in cover soils.

Step 1: Subtract recovered methane  
100 − 60 = **40 units remaining**

Step 2: Apply oxidation factor  
40 × (1 − 0.10) = 40 × 0.90 = **36 units emitted**

So the final methane emissions are **36 units**.

Notice what students often get wrong:

- applying oxidation before subtracting recovery
- forgetting that recovered methane includes gas sent to either flaring or energy recovery
- assuming oxidation is always the same at every landfill

You may also see a simplified AP-style version:

Methane emitted = methane generated × (1 − recovery efficiency) × (1 − oxidation factor)

This is fine for basic reasoning, but the conceptual mass-balance idea is the same.

A landfill cover can reduce emissions in two ways:

- physically slowing gas escape
- supporting microbes that oxidize methane

But neither process captures all methane. That is why even good landfills still have fugitive emissions.

Another useful exam angle is comparing flaring and energy recovery with numbers only in words:

- If methane is recovered and flared, methane emissions fall but no electricity is generated.
- If methane is recovered and used for power, methane emissions fall and electricity is produced.
- If methane escapes, neither climate control nor energy recovery happens.

This helps with FRQ evaluation because you can compare trade-offs rather than just describing the system.

### Evaluating landfill gas-to-electricity as a waste strategy

AP Environmental Science rewards balanced evaluation. Landfill gas-to-electricity is worth neither blind praise nor blanket criticism.

Its main advantages are:

- reduces methane emissions compared with uncontrolled release
- lowers explosion and odor risks
- produces useful electricity from waste already in place
- may offset some fossil fuel use
- can continue operating after landfill closure

Its main disadvantages are:

- it is not true waste prevention
- gas capture is imperfect
- combustion still emits carbon dioxide and other pollutants
- output declines over time as the landfill ages
- infrastructure and monitoring are expensive
- it may reduce pressure to move toward better upstream waste strategies

One subtle evaluation point is reliability. Landfill gas can be more steady than solar or wind because it is produced continuously from decomposition, not only when the weather cooperates. That can make it useful as a dispatchable fuel. But this steadiness is temporary on a long timeline because gas generation eventually declines.

Real-world scale also matters. EPA’s Landfill Methane Outreach Program reported **542 operational landfill gas energy projects in the United States as of September 2024**. That tells you the technology is established and not just theoretical.

Still, the best judgment is usually:

- better than letting methane escape
- often better than flare-only if energy recovery is feasible
- less sustainable than reducing landfill inputs in the first place

That structure is strong in FRQs because it shows environmental reasoning rather than memorized slogans.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Landfill gas forms when organic waste decomposes anaerobically in a landfill
- Landfill gas is typically about **50% methane** and **50% carbon dioxide**
- Methane matters because it is a strong greenhouse gas and also creates explosion risks
- Landfill gas can continue to be produced for years or decades after landfill closure
- A gas collection system usually uses wells, pipes, and pumps or blowers
- Collected landfill gas may be flared or used for energy recovery
- Flaring destroys methane but does not generate useful electricity
- Landfill gas-to-electricity commonly uses internal combustion engines, gas turbines, or microturbines
- Combined heat and power systems produce electricity and useful heat from the same fuel stream
- Landfill gas-to-electricity is different from incineration because it uses decomposition gas, not solid waste itself
- Methane recovery and methane oxidation are different parts of emissions accounting
- Landfill gas-to-electricity is a pollution-control strategy, but source reduction, recycling, and composting are still better upstream options

## Key Terms and Concepts

- landfill gas (LFG)
- anaerobic decomposition
- municipal solid waste (MSW) landfill
- gas collection system
- flaring
- combined heat and power (CHP)
- oxidation factor


Waste Reduction Methods - Landfill gas to electricity