AP Environmental Science

## Learning Outcomes

- Define **solid waste** and explain why the term can be broader than everyday use suggests
- Distinguish between **solid waste**, **municipal solid waste (MSW)**, and **hazardous waste**
- Identify common examples included in MSW and examples that are usually excluded
- Explain why the idea of “discarded material” matters in waste regulation
- Compare **sanitary landfills** with **open dumps**
- Link key waste terms such as **leachate**, **landfill gas**, and **methane** to environmental impacts

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand solid waste definition, with a focus on the following syllabus points:

- Know that **solid waste** means discarded material, not just physically solid material
- Identify **municipal solid waste (MSW)** as everyday trash or garbage from homes, businesses, and institutions
- Explain the difference between broad **solid waste** and the narrower category of **MSW**
- Understand that **hazardous waste** is a subset of solid waste, not a separate top-level category
- Distinguish between a **sanitary landfill** and an **open dump**
- Identify how **leachate** and **landfill gas** create environmental risks
- Know that **RCRA (1976)** is the main U.S. law governing hazardous and nonhazardous solid waste
- Recognise that most MSW is managed as **nonhazardous** waste, even though some discarded items can still be dangerous

## 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 is the difference between **solid waste** and **municipal solid waste**?
2. Why is it incorrect to assume that all solid waste must be physically solid?
3. Name three materials that are commonly included in MSW.
4. How is a **sanitary landfill** different from an **open dump**?
5. What environmental problem is caused by **leachate**?

## Introduction

Solid waste sounds simple at first: it seems to mean rubbish that is solid rather than liquid or gas. That everyday meaning is useful, but for AP Environmental Science you need a more careful definition. On the exam, students often lose marks because they blur together three different ideas: the broad category of **solid waste**, the narrower category of **municipal solid waste**, and the still narrower category of **hazardous waste**.

This subtopic matters because later questions on landfills, incineration, recycling, composting, and hazardous waste all depend on accurate classification. If you do not know what kind of waste is being discussed, it is easy to choose the wrong disposal method or the wrong environmental impact.

> **Key Term: solid waste**
> Discarded material generated by homes, businesses, industry, agriculture, and other activities. In regulatory use, the term is broader than everyday language and is not limited to materials that are physically solid.

### What “solid waste” really means

In everyday speech, people use words like trash, garbage, or refuse to mean things they throw away. AP Environmental Science uses the same general idea, but the key word is **discarded**. A material counts as solid waste because society has decided it is no longer wanted and must be managed.

That is why the concept can be broader than it sounds. Under U.S. waste regulation, especially under **RCRA**, the question is not simply “Is it hard and solid?” The more useful question is “Has it been discarded?” If the answer is yes, the material may be treated as solid waste even if it is a sludge, semisolid, or contained gas.

This point matters because students often think “solid waste” excludes anything wet or semi-liquid. That is wrong. For AP purposes, solid waste commonly includes familiar discarded solids, but the legal category is wider.

> **Key Term: discarded material**
> Material that has been thrown away, abandoned, or otherwise removed from normal use and therefore enters the waste stream.

Why does this wider definition exist? Because environmental harm depends on what happens to waste after disposal, not just on its physical appearance. A sludge from treatment facilities, for example, can contaminate land or water just as seriously as a pile of broken plastic or metal cans.

A second reason is that disposal systems need categories that match environmental risk. If law only regulated hard, dry objects as solid waste, many dangerous discarded materials would escape oversight.

> **Test Tip:** If an AP question asks which statement about solid waste is correct, be careful with wording. “Solid waste is only waste that is physically solid” is usually the trap answer.

### Municipal solid waste: the everyday waste stream

Most AP questions in this area focus less on the broad legal category and more on **municipal solid waste**, usually shortened to **MSW**. MSW is the everyday waste stream produced by households and similar sources.

> **Key Term: municipal solid waste (MSW)**
> Everyday trash or garbage from homes and similar wastes from businesses, schools, hospitals, and other institutions.

Common MSW examples include:

- food scraps
- paper and paperboard
- containers and packaging
- yard trimmings
- plastics
- glass
- metals
- furniture
- clothing
- appliances

These are the ordinary materials most people picture when they think of rubbish bags, recycling bins, and landfill trucks.

The important exam distinction is this:

- **All MSW is solid waste**
- **Not all solid waste is MSW**

That second line is often tested. Industrial process wastes, some mining wastes, and many hazardous wastes are solid waste, but they are not part of ordinary municipal trash.

The textbook treatment of MSW also stresses how mixed this waste stream is. Some items decompose quickly, such as food scraps and many plant materials. Others last for decades, centuries, or even far longer, such as many metals, glass, and plastics. This is one reason landfill management matters: different materials persist for very different lengths of time.

Here is a useful included-versus-excluded comparison.

Included in MSW:

- food waste
- packaging
- newspapers
- yard trimmings
- bottles and cans
- clothing
- furniture

Usually excluded from MSW:

- municipal sludge
- combustion ash
- industrial nonhazardous process waste
- many mining wastes

Students often assume that anything discarded by a city must count as municipal solid waste. That is not always true. Sludges and ash are commonly managed as separate waste streams.

> **Key Term: waste stream**
> The total flow of waste materials from their source through collection, processing, disposal, or recovery.

This distinction matters because disposal options differ. A sanitary landfill designed for household waste is not automatically suitable for every other discarded material.

### Hazardous waste is part of solid waste, but not the same as MSW

Another common confusion is between hazardous waste and solid waste. Many students think these are two completely separate categories. For AP purposes, that is not the best way to think about them.

**Hazardous waste** is better understood as a subset of solid waste. First, a material is discarded. Then, if it also has dangerous characteristics, it may be regulated as hazardous waste.

> **Key Term: hazardous waste**
> A subset of solid waste that poses a danger to human health or the environment, often because it is toxic, corrosive, ignitable, or reactive.

Examples include paints, solvents, pesticides, and some industrial chemicals. These need different disposal methods because they can poison organisms, burn, corrode, react violently, or contaminate soil and water.

The AP-friendly comparison looks like this:

| Category              | Main idea                            | Typical examples                       |
| --------------------- | ------------------------------------ | -------------------------------------- |
| Solid waste           | Broad category of discarded material | trash, sludge, some industrial wastes  |
| Municipal solid waste | Everyday community trash             | food scraps, paper, bottles, packaging |
| Hazardous waste       | Dangerous subset of solid waste      | solvents, pesticides, toxic chemicals  |

This helps with multiple-choice questions. If you see an item like food scraps, it belongs in MSW. If you see an item like industrial pesticide residue, it is likely hazardous waste. If you see a statement saying hazardous waste is not solid waste, that statement is inaccurate.

RCRA, enacted in **1976**, is the major federal law behind these distinctions. For exam purposes, the key facts are simple:

- it governs hazardous and nonhazardous solid waste
- it encouraged systematic waste management
- it set standards for disposal facilities, including landfills

You do not need deep legal detail, but you should know that waste classification affects regulation.

> **Key Term: Resource Conservation and Recovery Act (RCRA)**
> A U.S. law passed in **1976** that established the framework for managing hazardous and nonhazardous solid waste.

### Included and excluded examples students mix up

A strong AP answer does more than repeat definitions. It classifies examples correctly.

Consider these likely examples:

Food scraps
These are MSW. They are everyday discarded material from homes, schools, or businesses. They may later be composted, landfilled, or digested anaerobically.

Packaging
Also MSW. Packaging makes up a large share of the municipal waste stream and is one of the main targets for reduction and recycling.

Yard trimmings
These are usually MSW as well, especially in residential waste collection systems.

Municipal sludge
This is where many students make mistakes. Sludge is waste, but it is usually treated separately from ordinary MSW.

Combustion ash
Again, waste, but not typical MSW. It belongs to a different waste stream.

Industrial nonhazardous process waste
This is solid waste in the broad sense, but not municipal solid waste.

Paint and some batteries
These can appear in household waste, but they raise classification issues because they may contain hazardous components.

This last point is especially useful in FRQs. Real waste streams are messy. Household waste can contain materials that are potentially hazardous, but AP questions usually still want you to recognise that the main municipal waste stream is mostly **nonhazardous**.

> **Test Tip:** When classifying examples, first ask “Is this ordinary community trash?” If yes, think MSW. If it comes from industrial processing or has clear toxic/reactive properties, think beyond MSW.

### Disposal terms linked to the definition

Once waste is defined, AP Environmental Science usually connects the definition to disposal systems. The biggest distinction here is between an **open dump** and a **sanitary landfill**.

An open dump is simply an uncontrolled disposal site. It lacks the engineering controls needed to reduce pollution. A sanitary landfill is an engineered system designed to isolate waste from the environment as much as possible.

> **Key Term: sanitary landfill**
> An engineered waste disposal site designed to reduce environmental contamination using liners, cover material, leachate collection, and gas management.

Modern sanitary landfills differ from open dumps in several ways:

- they are lined to reduce seepage
- waste is covered regularly with soil or other material
- groundwater is monitored
- **leachate** is collected and managed
- gas buildup is monitored or captured

This is where the definition of solid waste connects directly to environmental impacts. Once large amounts of discarded material are placed together, water moving through that waste can dissolve or carry pollutants.

> **Key Term: leachate**
> Contaminated liquid formed when water moves through waste and picks up dissolved or suspended pollutants.

Leachate is a major reason landfills can threaten groundwater. If liners fail or systems are absent, pollutants can migrate into aquifers.

Waste also decomposes, especially when oxygen is limited. In landfills, anaerobic decomposition produces gases.

> **Key Term: landfill gas**
> Gas produced by decomposing waste in landfills, mainly **methane** and carbon dioxide.

Methane matters for two reasons. First, it is a strong greenhouse gas. Second, gas accumulation can create fire or explosion hazards if unmanaged.

> **Key Term: methane**
> A greenhouse gas produced during anaerobic decomposition of organic matter, including waste in landfills.

That is why landfill gas may be captured and flared or used for energy generation. Capturing methane reduces one environmental risk, although it does not remove all waste-related impacts.

### Why these definitions matter for exam reasoning

Students sometimes treat waste definitions as pure vocabulary. On the AP exam, they are often the starting point for cause-and-effect reasoning.

Here is a typical chain:

1. A material is discarded
2. It enters the solid waste stream
3. If it is everyday community trash, it is classified as MSW
4. It may be sent to a sanitary landfill or incinerator, or diverted by recycling/composting
5. Poor disposal can produce leachate, methane, or toxic exposure
6. Those impacts can affect soil, water, air, climate, or human health

If the waste is hazardous, the chain changes because handling and disposal require stricter controls.

This means that precise definition supports correct environmental analysis. If you misclassify the waste, you may also misidentify the disposal method and the likely pollutant pathway.

A second reason these definitions matter is that APES often blends environmental science with policy. A question may ask which law applies, or why certain facilities are regulated differently. Knowing that solid waste is a broad category, and that hazardous waste is one subset, helps you answer such questions without needing to memorise every legal detail.

### The high-yield summary distinction

The single most important distinction for this subtopic is:

- **Solid waste** = the broad category of discarded material
- **Municipal solid waste** = the everyday community waste stream
- **Hazardous waste** = a dangerous subset of solid waste
- **Sanitary landfill** = the engineered facility used for much MSW
- **Open dump** = the uncontrolled version that lacks those protections

If you can hold that structure clearly, later topics such as waste reduction methods, landfill design, incineration, and e-waste become much easier.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- **Solid waste** means discarded material and is broader than the everyday meaning of physically solid rubbish
- A material can count as solid waste because it is discarded, even if it is sludge or semisolid
- **Municipal solid waste (MSW)** is the everyday trash or garbage produced by homes and similar sources
- Common MSW examples include food scraps, packaging, paper, yard trimmings, plastics, glass, and metals
- Municipal sludge, combustion ash, and industrial nonhazardous process waste are generally not classified as MSW
- **Hazardous waste** is a subset of solid waste, not a completely separate top-level category
- **RCRA**, passed in **1976**, is the main U.S. law governing hazardous and nonhazardous solid waste
- A **sanitary landfill** is an engineered disposal site with liners, cover, leachate control, and gas management
- **Leachate** is contaminated liquid formed when water moves through waste
- **Landfill gas** is mainly methane and carbon dioxide from decomposition in landfills
- **Methane** matters because it is a greenhouse gas and can also create explosion risk
- All MSW is solid waste, but not all solid waste is MSW

## Key Terms and Concepts

- solid waste
- discarded material
- municipal solid waste (MSW)
- waste stream
- hazardous waste
- Resource Conservation and Recovery Act (RCRA)
- sanitary landfill
- leachate
- landfill gas
- methane


Solid Waste Disposal - Solid waste definition