AP Environmental Science

## Learning Outcomes

- Explain how waste decomposes in a municipal solid waste landfill
- Distinguish between the initial aerobic phase and later anaerobic decomposition
- Identify the main factors that affect landfill decomposition and methane generation
- Explain why moisture, temperature, and waste composition change decay rates
- Compare conventional landfills with bioreactor landfills
- Distinguish between methane generation and methane emissions
- Apply this topic to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand landfill decomposition factors, with a focus on the following syllabus points:

- Know that most municipal solid waste is disposed of in landfills
- Explain that landfill waste typically passes through a short aerobic phase before anaerobic conditions develop
- Know that **landfill gas** is produced mainly by anaerobic decomposition of organic waste
- Identify the major controls on decomposition, including moisture, temperature, oxygen availability, and waste composition
- Understand that landfill gas is about **50% methane (CH4)** and **50% carbon dioxide (CO2)**, with small amounts of other gases
- Explain how bioreactor landfills increase decomposition by adding liquids or recirculating leachate
- Distinguish between methane produced in the landfill and methane actually emitted to the atmosphere
- Understand how gas collection systems and methane oxidation in cover soils reduce emissions

## 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. Why does methane production in a landfill not begin immediately after waste is buried?
2. How does higher moisture usually affect decomposition in a landfill?
3. Which type of waste contributes most to methane production: food waste, plastics, or glass?
4. What is the difference between methane generation and methane emissions?
5. How does a bioreactor landfill differ from a conventional landfill?

## Introduction

Landfills are the most common disposal method for municipal solid waste, so AP Environmental Science expects you to understand not just what a landfill is, but how waste behaves after burial. This topic is really about **microbial decomposition**. Once waste is buried, microorganisms begin breaking down biodegradable material, and the speed of that breakdown depends on several interacting conditions inside the landfill.

This matters because landfill decomposition affects two major environmental issues at once. First, it affects how long waste remains in the landfill. Second, it controls the production of **landfill gas**, especially methane, a greenhouse gas far more effective than carbon dioxide at trapping heat over short time periods. On the exam, you may be asked to identify one factor, explain the direction of its effect, or connect decomposition conditions to landfill management.

> **Key Term: landfill decomposition**
> The biological breakdown of biodegradable waste in a landfill, first under limited aerobic conditions and then mainly under anaerobic conditions.

### How landfill decomposition happens over time

A common mistake is to imagine a landfill as a giant compost pile. That is not accurate. Compost piles are managed to allow oxygen in, but landfills are designed to isolate waste and reduce exposure to the outside environment. As a result, decomposition in landfills is slower and becomes largely anaerobic.

Right after waste is placed in a landfill, some trapped oxygen is still present. This creates a brief **aerobic phase**. During this stage, aerobic microorganisms use oxygen to break down some organic matter, producing mainly carbon dioxide, water, and heat. Methane production is still low at this point.

> **Key Term: aerobic phase**
> The initial stage of landfill decomposition in which oxygen is present and microorganisms produce mainly carbon dioxide, water, and heat.

As oxygen is used up, the landfill shifts into oxygen-poor conditions. EPA materials commonly describe anaerobic conditions developing in less than one year after waste placement, though the exact timing depends on site conditions. Once oxygen is depleted, **anaerobic decomposition** becomes dominant. This is the stage that produces most landfill methane.

> **Key Term: anaerobic decomposition**
> The breakdown of organic matter without oxygen; in landfills, this is the main process that produces methane.

This transition matters because exam questions often ask about oxygen. If oxygen is present, decomposition may still occur, but methane production stays low. If oxygen is absent, methanogenic microorganisms can convert breakdown products into methane and carbon dioxide.

A simple sequence to remember is:

1. Waste is buried
2. Trapped oxygen supports a short aerobic phase
3. Oxygen is depleted
4. Anaerobic conditions develop
5. Methane-producing microbes generate landfill gas

Landfills may also be described in four broad stages:

- aerobic stage
- acid formation stage
- methanogenesis stage
- maturation or stabilization stage

You do not always need all four stage names for APES, but understanding the sequence helps with reasoning questions. In the acid formation stage, organic acids build up and pH can drop. In the **methanogenesis** stage, methane production becomes strong. In the maturation stage, decomposition and gas production gradually decline as easily decomposed material is used up.

> **Key Term: landfill gas (LFG)**
> Gas produced by decomposition in a landfill, usually about 50% methane and 50% carbon dioxide, with small amounts of other gases.
>
> **Test Tip:** If an FRQ asks for a factor affecting landfill methane, name one factor and state the direction clearly, such as “higher moisture increases anaerobic decomposition and methane production.”

### The main factors that control decomposition and methane generation

The APES scoring guidance accepts several landfill decomposition factors, but some are much more important than others. The clearest high-yield factors are moisture, temperature, oxygen conditions, and waste composition.

#### Moisture

Moisture is one of the strongest controls on landfill decomposition. Microorganisms need water for metabolic activity. If waste stays dry, microbial activity slows and decomposition takes much longer. If moisture is higher, microbes function more effectively and organic matter breaks down faster.

This is why wetter landfills usually produce methane earlier and at higher rates than drier ones. The key word is **usually**. It is a strong trend, not an absolute rule.

Think through the chain:

- more moisture
- more microbial activity
- faster decay of organic waste
- faster development of anaerobic by-products
- greater methane generation rate

Dry conditions create the opposite pattern. Waste persists longer, decay slows, and methane production is delayed or reduced.

#### Temperature

Temperature also affects microbial activity. In general, warmer conditions increase biological reaction rates, so decomposition tends to be faster in warm waste than in cold waste. Cold conditions slow microbial metabolism and delay decomposition.

This does not mean you need a precise optimum temperature for APES. The sources here do not give a simple landfill-specific value suitable for exam use. Safer wording is: **warmer conditions generally increase decomposition, while colder conditions slow it**.

Temperature also interacts with moisture. Warm and moist conditions together usually speed decay more than either factor alone. That is why AP-style answers often combine them: decomposer microorganisms break down organic waste faster in warm, moist, anaerobic conditions.

#### Oxygen availability

Oxygen changes the type of decomposition rather than simply speeding or slowing it. Early oxygen supports aerobic decomposition, but methane is mainly linked to anaerobic conditions. So if a question asks specifically about methane, the best oxygen-related answer is that **lower oxygen or anaerobic conditions favor methane production**.

Students sometimes say “more oxygen means more decomposition,” which can be partly true at first, but it misses the landfill-specific point. In landfills, the exam usually wants you to connect **lack of oxygen** with **methane production**.

#### Waste composition

Not all trash decomposes equally. **Organic waste** drives methane production because it contains biodegradable carbon compounds that microorganisms can break down. Food scraps, yard waste, and paper decompose much more than plastics, glass, and metals.

This leads to one of the most important distinctions in the topic:

- more biodegradable organic waste = greater methane potential
- more inert or nondegradable waste = little or no methane production

Food waste and yard waste tend to break down relatively quickly. Wood and some paper products may break down more slowly. Glass, metals, and most plastics do not significantly decompose and contribute little to landfill gas.

> **Key Term: waste composition**
> The proportion of biodegradable organic material compared with nondegradable material in waste; it strongly affects how much methane a landfill can produce.

You may see this phrased as the amount of organic vs. nondegradable waste. Both are acceptable. The important idea is that methane comes mainly from biodegradable organics.

### Why landfill design and management affect the outcome

Decomposition factors do not act in isolation. Landfill design changes these conditions, which is why management decisions can increase or reduce gas production and emissions.

A good example is the **bioreactor landfill**. In a conventional landfill, moisture may stay relatively low, so decomposition is slower. In a bioreactor landfill, operators add liquids or recirculate leachate to increase moisture. This speeds microbial activity and can cause methane production to begin earlier and at higher rates.

> **Key Term: bioreactor landfill**
> A landfill designed to increase decomposition by adding liquids or recirculating leachate to raise moisture levels in the waste.

This does not automatically make the landfill worse for climate. Faster methane generation may actually make gas capture easier if a collection system is in place. That leads to another distinction students often miss: **methane generation is not the same as methane emissions**.

A landfill can generate large amounts of methane inside the waste, but if gas collection pipes capture much of it, the amount released to the atmosphere can be much smaller. Some of the captured methane may be flared, which burns methane to carbon dioxide, or used as fuel for electricity generation.

The simplified climate logic is:

- methane generated inside landfill
- minus methane captured by gas collection
- minus methane oxidized in cover soil
- equals methane emitted to atmosphere

Cover soil matters because some bacteria in the cover can convert methane to carbon dioxide before it escapes. This process is called methane oxidation.

> **Key Term: methane oxidation**
> The conversion of methane to carbon dioxide by bacteria, often in landfill cover soils, which reduces methane emissions.

This is why a question about surface emissions should not be answered exactly the same way as a question about decomposition rate. Faster decomposition usually increases methane generation, but actual emissions also depend on capture and oxidation.

Sanitary landfills are also lined to reduce leachate contamination and are covered regularly with soil to reduce odors, pests, and gas escape. When the landfill is full, it is capped and monitored. Those features do not stop decomposition entirely, but they affect moisture entry, gas movement, and environmental risk.

> **Test Tip:** In MCQs, watch for whether the question asks about methane “produced,” “generated,” or “emitted.” Produced/generated refers to decomposition inside the landfill. Emitted means what actually escapes to the atmosphere.

### Applying the factors in AP exam questions

On APES FRQs, you are often rewarded for short, direct causal statements. For landfill decomposition factors, the best answers usually follow this pattern:

- identify one factor
- state whether it increases or decreases decomposition
- explain why in one clause

Examples of scoring-friendly responses:

- Higher moisture increases decomposition because decomposers need water.
- Warmer temperatures increase microbial activity, which speeds decomposition.
- A greater proportion of organic waste increases methane generation because biodegradable material can be broken down anaerobically.
- Lower oxygen favors methane production because methanogenic microbes function under anaerobic conditions.

You might also be asked to compare a conventional landfill and a bioreactor landfill. A strong answer would say that bioreactor landfills add moisture, often through leachate recirculation, so decomposition happens faster and methane may be produced earlier than in conventional landfills.

Here are common student errors:

- saying all waste produces methane
- confusing landfill gas composition with atmospheric air composition
- assuming methane forms immediately after burial
- confusing faster methane generation with greater methane emissions
- saying oxygen-rich conditions produce the most methane

Correct these by remembering:

- mainly biodegradable organic waste produces methane
- landfill gas is about 50% methane and 50% carbon dioxide
- there is a short aerobic phase first
- emissions depend on capture and oxidation, not just production
- methane is mainly associated with anaerobic conditions

A final subtle point: pH is recognized as a factor in technical landfill models, but for APES it is usually not the best first factor to choose unless the question specifically points you there. Moisture, temperature, oxygen, and waste composition are safer and clearer.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Landfill decomposition begins with a short aerobic phase and then shifts to anaerobic decomposition
- Methane production is mainly associated with anaerobic conditions, not the initial aerobic stage
- Landfill gas is typically about 50% methane and 50% carbon dioxide
- Moisture is one of the most important factors controlling decomposition rate
- Higher moisture usually increases microbial activity and speeds waste decay
- Warmer temperatures generally increase decomposition, while colder conditions slow it
- A greater proportion of biodegradable organic waste increases methane potential
- Plastics, glass, and metals contribute little or no methane because they do not significantly decompose
- Bioreactor landfills increase moisture to speed decomposition and methane generation
- Methane generation inside a landfill is not the same as methane emissions to the atmosphere
- Gas collection systems can capture methane, and cover soils can reduce emissions through methane oxidation
- Strong APES answers clearly name one factor and explain its direction of effect

## Key Terms and Concepts

- landfill decomposition
- aerobic phase
- anaerobic decomposition
- landfill gas (LFG)
- waste composition
- bioreactor landfill
- methane oxidation


Solid Waste Disposal - Landfill decomposition factors