Trash Materials Investigation: What Will Biodegrade?

Students investigate what types of materials biodegrade in the soil, and learn what happens to their trash after they throw it away. They learn about the concepts behind landfills and compost piles. In an associated activity, students create their own miniature landfills—a hands-on way to learn the difference between organic and inorganic waste.

Waste management engineers use the process of biodegredation to minimize the amount of space that landfills take up. By burying layers of trash with other layers of soil, all existing waste is compressed under weight from above. For organic matter to decompose, however, aerobic bacteria, which needs to be exposed to air, is required. Thus, large machines are used to churn up and aerate top layers, allowing the organic matter that is present to decompose.

After this lesson, students should be able to:

Explain what the term "biodegrade" means, and generally describe how the process works.
Identify whether certain objects can biodegrade or not.
Explain the difference between organic and inorganic materials.
Explain how engineers apply what they know to help with waste management.

Biodegradable materials, or pieces of organic matter, are those that can break down from larger molecules to smaller ones, much like in the process of rotting. Usually living organisms, in the form of bacteria, break apart these molecules. Biodegradable materials ultimately return nutrients and useful organic matter to the soil. Many products are made with organic materials with the intent that their disposal is more ecologically friendly, such as various wrappers, bags and containers. Example organic matter: natural foods, food peels, egg shells, coffee grounds, paper, hair, leaves and grass clippings.

Materials that degrade can do so aerobically, with oxygen, or anaerobically, without oxygen. Once trash is thrown away, it is taken to a dump where it just sits and becomes buried with other trash. Thus, anaerobic biodegradation is accountable for some of the degrading that takes place at landfills. Without oxygen, these buried pieces of organic material are slowly eaten up by bacteria known as anaerobic digesters, which break down what they eat and produce several useful products, such as methane gas. For this process to occur, it needs to be quite warm inside the landfill, approximately 40-50 °Celsius (approximately 105-120 °Fahrenheit). These temperatures are generally quite easily achieved, however, because of the heat generated by the many living organisms working to decompose the waste. In fact, many waste management sites employ the use of these digesters in separate large tanks for sewage treatment, in order to capture the large amounts of heat and methane, as well as other by-products that are used to produce fertilizer.

When organic materials are mixed with ample air and heat, specialized bacteria growth is quickly encouraged. These bacteria, known as aerobic organisms, help break down the larger compounds containing nitrogen, oxygen, hydrogen and carbon into smaller compounds. These processes are done much quicker than the anaerobic processes, and produce on average 19 times more heat. Landfills generally employ the use of large tractors and other specialized machinery to churn up the waste to achieve this quickened biodegradation. By encouraging oxygen to flow through the mounds, aerobic bacteria are able to remain alive and aerobic biodegradation is facilitated.

Most biodegradable materials are able to turn back into nutrient-rich soil, which can be used for other purposes. At the dump, however, the newly formed soil is generally not re-used, since it is mixed in with other pieces of trash that have not yet or will never decompose. Compost piles are designed for the purpose of re-using soil. They are essentially small dumps in which only organic materials are deposited, and in which aerobic biodegradation is facilitated. As noted before, aerobic biodegradation produces a large amount of heat, so the inside of a compost pile is capable of very high temperatures.

Two types of compost piles: an enclosed plastic bin with a lid (left and center) and an open wire cage (right).Copyright 2012 Denise W. Carlson, ITL Program, College of Engineering, University of Colorado Boulder

Most of the biodegradation that occurs in the associated activity Bury Your Trash Experiment (the student experiment) is caused by aerobic organisms because the trash is buried quite close to the surface of the soil. Furthermore, the soil that compost piles create is great to use for gardening, to amend existing soil. After the experiment, when the inorganic materials that did not degrade are removed, the soil your class creates is suitable to be added to a classroom garden plot.

Three photos: 1) looking into a kitchen trash can that contains item such as energy bar wrappers, cellophane, twist ties, plastic lids, plastic bags, napkins, 2) looking into a kitchen recycling bin that contains items made of glass, cardboard, paper, metal, plastic and glass, and 3) looking into a metal container that contains kitchen scraps such as vegetable peelings and a tea bag. Many different items compose what we call "trash." What is biodegradable? What can be recycled? Which are organic? How can we find out?Copyright 2012 Denise W. Carlson, ITL Program, College of Engineering, University of Colorado Boulder

What do you think happens to your trash after you throw it in a trash can? Where do you think it goes after it leaves your kitchen, house or school? Most of your trash ends up at a local waste management site, or a dump or a landfill. A waste management site is a specific plot of land, usually located near your town or city, that is designated to hold your community's garbage. Your trash usually gets there via a large garbage truck, which you may have seen coming to your house on certain days of the week. Now, what do you think happens to the trash while it is at the dump? Does it sit there unchanged forever, or do some items slowly rot? What happens to the trash that rots?

Does anyone know what we mean when we say that something biodegrades? What is the difference between organic and inorganic materials? (Introduce to the class these concepts; refer to the Vocabulary/Definitions section.)

Today, at our very own test waste management site, we will bury various pieces of trash and leftover food from all of our lunches in diferent patches of ground. Then we will let all the experimental sites "sit" for two to three months to allow biodegradation to occur (or not). Finally, we will dig into these sites to see what remains of our trash.

The definition of "biodegrade: is to decay and become absorbed by the surrounding environment. Have you heard of the term "decay," as in tooth decay? Or in the rotting of dead plants or animals. Perhaps you have even seen the decay or fruits or vegetables in your kitchen. Another example of decay occurs outside, when dead trees and leaves decay back into soil. The same is true for animals and insects that have died. These organic, or once living, materials can turn back into soil because they are made of the same tiny molecules and elements as soil. Almost any material that was once a part of a living organism is capable of decay. For example, the pencils that you write with and the paper you write on were once a part of living trees, and are capable of decaying.

Let's discuss one more important concept before we begin our work. Have you ever seen or used a compost pile? If so, what did it look like? (Listen to student responses.) Compost piles are mounds comprised completely of organic waste and trash, and they are set up so that bacteria helps the material biodegrade back into soil as quickly as possible. Compost piles are left outside so that heat from the sun is able to encourage bacteria growth and designed to allow some air to enter the pile. If anyone has any questions about any of these related processes, such as the difference between a landfill and a compost pile, feel free to ask before we begin the activity.

Summary

Engineering Connection

Learning Objectives

Educational Standards

International Technology and Engineering Educators Association - Technology

Next Generation Science Standards - Science

North Carolina - Science

Pre-Requisite Knowledge

Background

Introduction / Motivation

Lesson Closure

Vocabulary

Assessment

Extensions

References

Curriculum in this Unit

Copyright & Attributions

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