Green Environment — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science. Kinds of waste, reducing what we throw away, and turning kitchen waste into compost.
1. Two kinds of waste
Throw a banana peel and a plastic wrapper into the same corner of a field and come back a month later. The peel has gone. The wrapper has not.
That difference is the whole chapter.
Biodegradable waste is broken down by bacteria and fungi into simple, harmless substances that mix back into the soil. It takes days to months.
Examples: vegetable peels, leftover food, paper, cotton cloth, dry leaves, wood, cow dung.
Non-biodegradable waste is not broken down by living things. It stays in the environment for years, and some of it effectively forever.
Examples: plastic, glass, metal, thermocol and e-waste.
| Item | How long it lasts |
|---|---|
| Vegetable peel | 1 to 2 weeks |
| Paper | 2 to 5 months |
| Cotton cloth | 2 to 5 months |
| Plastic bag | Hundreds of years |
| Glass bottle | Effectively forever |
2. Why it matters
Plastic thrown on the road blocks drains and causes flooding. Cattle swallow plastic bags and die. Plastic that reaches the sea breaks into tiny pieces which fish eat, and which come back to us in our own food.
Never burn plastic waste. Burning it releases poisonous gases called dioxins and furans, which cause cancer and lung disease. The right answer is to use less plastic and to recycle what we do use.
3. The three Rs
Reduce — use less in the first place. Carry a cloth bag to the shop instead of taking a plastic one. Refuse a straw. Buy the bigger packet rather than several small ones.
Reuse — use it again instead of throwing it away. Wash and refill glass bottles. Turn old clothes into dusting cloths. Use one side of used paper for rough work.
Recycle — turn waste into something new. Paper, glass, metal and some plastics can be recycled into fresh products.
They come in that order deliberately. Reducing is best, because waste that is never made needs no handling at all. Recycling is last, because it still costs energy.
4. Segregating waste
Recycling only works if waste is separated at the source — that is, in your own home.
- Green bin — biodegradable: kitchen waste, food scraps, garden leaves
- Blue bin — non-biodegradable that can be recycled: paper, plastic, glass, metal
- Kept apart — hazardous waste such as batteries, medicines and electronics
Once wet kitchen waste is mixed with dry paper and plastic, the paper is soiled and can no longer be recycled. That single mistake is why so much recyclable waste ends up in landfill.
5. Composting
Composting turns biodegradable waste into food for plants.
- Collect kitchen waste — vegetable peels, fruit waste, used tea leaves, eggshells.
- Add dry leaves or shredded paper, and some soil.
- Layer them in a pit or a bin, and keep it moist but not wet.
- Microorganisms do the work. In two to three months you have dark, crumbly compost.
- Mix it into the garden. It is rich in nutrients and free.
Compost returns to the soil what the plants took out of it — the same idea as the water cycle, applied to nutrients.
6. Worked examples
Example 1. Classify: banana peel, plastic bottle, cotton cloth, glass jar.
Solution: Biodegradable — banana peel, cotton cloth. Non-biodegradable — plastic bottle, glass jar.
Example 2. Why should plastic never be burnt?
Solution: Burning plastic releases poisonous gases such as dioxins and furans, which cause cancer and lung disease.
Example 3. Name the three Rs in the correct order and say why that order matters.
Solution: Reduce, Reuse, Recycle. Reducing comes first because waste never created needs no handling; recycling comes last because it still uses energy.
Example 4. Which bin does a vegetable peel go into?
Solution: The green bin, for biodegradable waste.
Example 5. How long does composting kitchen waste take?
Solution: About two to three months.
Example 6. Why does mixing wet and dry waste cause a problem?
Solution: Wet waste soils the paper and cardboard, so they can no longer be recycled and go to landfill instead.
7. Practice
- What is biodegradable waste? Give three examples.
- What is non-biodegradable waste? Give three examples.
- About how long does a plastic bag last in the environment?
- Name the three Rs.
- Give one example each of reduce, reuse and recycle.
- Which colour bin is used for biodegradable waste?
- Name four things that can go into a compost pit.
- Why should batteries and old electronics not go into the ordinary bin?
8. Answers
- Waste broken down naturally by bacteria and fungi — vegetable peels, paper, dry leaves.
- Waste not broken down naturally — plastic, glass, metal.
- Hundreds of years.
- Reduce, Reuse, Recycle.
- Reduce: carry a cloth bag. Reuse: refill a glass bottle. Recycle: send paper for recycling.
- The green bin.
- Vegetable peels, fruit waste, used tea leaves and eggshells.
- They are hazardous and leak harmful chemicals; they need separate, safe disposal.
9. Summary
- Biodegradable waste is broken down by bacteria and fungi in days to months.
- Non-biodegradable waste such as plastic, glass and metal stays for years or forever.
- Plastic blocks drains, kills cattle and sea life, and must never be burnt.
- The three Rs are Reduce, Reuse and Recycle, best applied in that order.
- Waste must be separated at home: green bin for wet, blue for dry recyclables, hazardous kept apart.
- Mixing wet and dry waste ruins recyclable paper.
- Composting turns kitchen waste into rich compost in two to three months.
