---
title: "Organic Matter — AP Environmental Science Definition"
description: "Organic matter is decomposing plant and animal material that bacteria break down. On the APES exam it links composting (8.10) and secondary sewage treatment (8.11)."
canonical: "https://fiveable.me/ap-enviro/key-terms/organic-matter"
type: "key-term"
subject: "AP Environmental Science"
unit: "Unit 8"
---

# Organic Matter — AP Environmental Science Definition

## Definition

In AP Environmental Science, organic matter is decomposed or decomposing plant and animal material, like food scraps, yard waste, and the waste in sewage, that bacteria break down. It can be composted into fertilizer or removed from wastewater during secondary treatment.

## What It Is

Organic matter is any carbon-based material that came from living things and is now decomposing. Think food scraps, dead leaves, paper, animal waste, and the solids in sewage. Because it was once alive, bacteria and other [decomposers](/ap-enviro/key-terms/decomposers "fv-autolink") can break it down, which is exactly why it matters in [Unit 8](/ap-enviro/unit-8 "fv-autolink").

The CED hits organic matter in two specific places. In [composting](/ap-enviro/unit-8/waste-reduction-methods/study-guide/P47EbL7U0etcWIggUk93 "fv-autolink") (Topic 8.10), organic matter like food scraps, paper, and yard waste decomposes into a product you can use as fertilizer (EK STB-3.M.3). In sewage treatment (Topic 8.11), secondary treatment is a biological process where bacteria break down the organic matter in wastewater into carbon dioxide and inorganic sludge, and the tank is aerated so those bacteria work faster (EK STB-3.N.2). Same material, same decomposers, two different waste-management contexts. If you can decompose it, it's organic matter.

## Why It Matters

Organic matter sits at the center of two Unit 8 learning objectives. For 8.10.A, you describe waste-reduction practices, and composting only works because organic matter decomposes naturally ([recycling](/ap-enviro/key-terms/recycling "fv-autolink") handles materials like minerals and plastics that don't). For 8.11.A, you describe best practices in [sewage treatment](/ap-enviro/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE "fv-autolink"), and the whole point of secondary treatment is getting bacteria to eat the organic matter before water gets discharged. The exam loves asking *why* a treatment step exists, and "bacteria need oxygen to break down organic matter" is the answer to one of the most common APES sewage questions. Understanding what counts as organic matter also tells you what can biodegrade, what can be composted, and what makes raw sewage a pollution problem in the first place.

## Connections

### Secondary Sewage Treatment (Unit 8)

Secondary treatment is basically a giant bacteria buffet. Aeration pumps oxygen into the tank so aerobic bacteria can break down organic matter faster, leaving [carbon dioxide](/ap-enviro/key-terms/carbon-dioxide "fv-autolink") and inorganic sludge that settles out. If an MCQ asks why the tank is aerated, the answer is to speed up bacterial decomposition of organic matter.

### Composting (Unit 8)

Composting is the same [decomposition](/ap-enviro/key-terms/decomposition "fv-autolink") process happening on land instead of in a treatment tank. Food scraps, paper, and yard waste break down into usable fertilizer, which diverts organic matter from landfills. Know the drawbacks too, since the CED names odor and rodents specifically.

### [Biodegradation (Unit 8)](/ap-enviro/key-terms/biodegradation)

[Biodegradation](/ap-enviro/key-terms/biodegradation "fv-autolink") is the process; organic matter is the stuff being processed. Materials that are organic matter biodegrade, while plastics and most e-waste don't, which is why they pile up in landfills and oceans instead of decomposing.

### [Point Source Pollution (Unit 8)](/ap-enviro/key-terms/point-source-pollution)

A sewage treatment plant's discharge pipe is a classic point source. If organic matter isn't fully removed before discharge, it enters waterways where decomposers consume it and use up dissolved oxygen, which is why treatment standards exist for what leaves the pipe.

## On the AP Exam

Organic matter shows up most often in MCQs about sewage treatment and composting. Expect stems like "what occurs during secondary treatment" (bacteria break down organic matter into CO2 and inorganic sludge), "why is the tank aerated" (oxygen speeds up bacterial decomposition), and "which of these could be composted" (pick the food scraps, yard waste, or paper, not the glass or plastic). The 2022 FRQ Q1 used the term in a pollution scenario about snapping turtle nesting sites, so be ready to apply it in unfamiliar contexts, not just recite the definition. Your main jobs: identify what counts as organic matter, explain what happens to it during decomposition, and connect it to the correct treatment step or waste-reduction method.

## organic matter vs "Organic" (as in organic food or farming)

In everyday life, "organic" means grown without synthetic pesticides or fertilizers. In APES, organic matter means carbon-based material from living things that can decompose. A conventionally grown banana peel is still 100% organic matter. Don't let the grocery-store meaning sneak into your exam answers; if it came from something alive and bacteria can break it down, it's organic matter.

## Key Takeaways

- Organic matter is decomposing material from living things, like food scraps, yard waste, paper, and the solids in sewage.
- In secondary sewage treatment, bacteria break down organic matter into carbon dioxide and inorganic sludge, and aeration speeds this up by giving the bacteria oxygen (EK STB-3.N.2).
- Composting lets organic matter decompose into usable fertilizer, but it comes with drawbacks like odor and rodents (EK STB-3.M.3).
- Organic matter biodegrades; materials like plastics, glass, and e-waste do not, which is why they need recycling or reuse instead of composting.
- If an exam question asks why a sewage tank is aerated, the answer is to increase the rate at which bacteria break down organic matter.

## FAQs

### What is organic matter in AP Environmental Science?

Organic matter is decomposing plant and animal material, like food scraps, yard waste, paper, and sewage solids. In Unit 8 it's the material bacteria break down during secondary sewage treatment and the input for composting.

### Does organic matter mean the same thing as organic food?

No. "Organic" food refers to farming without synthetic chemicals, while organic matter means any carbon-based material from living things that can decompose. On the APES exam, always use the decomposition meaning.

### What happens to organic matter during secondary sewage treatment?

Bacteria break it down into carbon dioxide and inorganic sludge, which settles to the bottom of the tank. The tank is aerated because the bacteria need oxygen to decompose the organic matter quickly.

### How is organic matter different from biodegradation?

Organic matter is the material; biodegradation is the process. Bacteria and other decomposers biodegrade organic matter, which is why food scraps break down in a compost pile but a plastic bottle doesn't.

### What can and can't be composted on the APES exam?

Composting works for organic matter such as food scraps, paper, and yard waste (EK STB-3.M.3). Glass, plastics, metals, and e-waste are not organic matter, so they belong in recycling or reuse questions instead.

## Related Study Guides

- [8.11 Sewage Treatment](/ap-enviro/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE)

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