Biodegradable and non-biodegradable waste

Two kinds of waste

Look at what goes into a dustbin in a day: vegetable peels, a newspaper, a plastic wrapper, perhaps a broken glass. If they are left in the soil, some of them disappear in a few weeks. Others stay for years.

• Substances that are broken down by biological processes are called biodegradable. Bacteria and fungi, the decomposers, break them into simpler substances that return to the soil and air.
• Substances that are not broken down in this way are called non-biodegradable.

Why decomposers cannot break everything

Decomposers break down materials with the help of enzymes. Each enzyme acts only on certain substances, the way a key fits only its own lock. Plastics are new substances, made by people from chemicals. Decomposers have no enzymes that can break them down quickly.

So non-biodegradable substances may stay unchanged for a very long time. Some are inert: they simply stay where they are. Others harm the living things around them.

How long does waste last?

Nobody can give an exact time. It depends on heat, water and air. Still, rough figures show how big the difference is.

At the starting value of one year, the peel, the newspaper and the cotton cloth are gone. The leather shoe, the can and the bottle are still there. Push the slider to the end to see how long a plastic bottle lasts.

Why the difference matters

Non-biodegradable waste does not go away by itself. It piles up and causes harm.

• On land. Plastic bags and bottles collect in heaps and in landfills, where they stay for hundreds of years.
• In water. Plastic blocks drains, so water floods the streets in the rains. In rivers and seas it harms fish and turtles.
• In animals. Cows and other animals eat plastic bags along with food scraps. The plastic cannot be digested and can block the gut.
• In food chains. Chemicals that are not broken down, such as some pesticides, pass from one organism to the next and build up along the chain.

Biodegradable waste is not harmless either if it is dumped in large amounts. Rotting heaps smell, and they draw flies and rats. But it can be turned into useful compost.

Try it in Python

The program sorts six items by their rough time to break down, and says which are gone after one year.

You should see the peel, newspaper and cotton cloth gone, and the leather shoe, can and bottle still there.

Practise with new numbers

Common mistakes

• Calling paper non-biodegradable because it goes in the dry bin. Paper is biodegradable; it goes in the dry bin so that it can be recycled.
• Thinking plastic rots if you wait a few years. It lasts for hundreds of years.
• Thinking biodegradable waste can be dumped anywhere. Large heaps of it still smell and spread disease.
• Saying decomposers "eat" everything. Their enzymes act only on certain substances.

Remember these

• Biodegradable: broken down by decomposers. Non-biodegradable: not broken down by them.
• Decomposers use enzymes, and each enzyme acts only on certain substances.
• Plastic, glass and metal cans last for a very long time.
• Non-biodegradable waste blocks drains, harms animals and can build up in food chains.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Sort these into biodegradable and non-biodegradable: a cotton rag, a glass bottle, tea leaves, a polythene bag.

2. Why can decomposers not break down plastics?

3. Give two ways in which plastic waste harms the environment.

4. A colony throws away $800$ kg of waste a day, and $60$ % is biodegradable. How much does not break down?

5. Is paper biodegradable? Why do we still put it in the dry bin?