What it shows
Microbes break down organic waste without air and release biogas, mostly methane and carbon dioxide. Measuring balloon size over days compares how much gas different feedstocks give.
The science
Anaerobic bacteria digest organic matter in stages and release methane and carbon dioxide. They work best in warm, wet, airless conditions around 30 to 37 degrees Celsius. A feed that already contains these bacteria, like dung, starts faster than plain peels, which need time for the right microbes to grow.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three 500 ml plastic bottles with caps
- Three balloons
- Fresh cow dung or buffalo dung slurry
- Chopped vegetable peels and cooked rice
- Warm water and a measuring cup
- A tape measure and a notebook
- A warm shaded shelf
Safety
For this project: Wear gloves and wash hands after handling dung or waste. Keep the bottles outdoors or in a ventilated area, away from flames, because biogas is flammable. Never light the gas, and an adult must dispose of the contents.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Label the bottles A, B and C and put 150 ml warm water in each.
Add 100 g dung slurry to bottle A, 100 g chopped peels to bottle B and 50 g of each to bottle C.
Mix well, then fit a balloon over the neck of each bottle, flat and empty.
Place all three bottles on a warm shaded shelf away from direct sun.
Each day measure the girth of each balloon with a thread and a ruler.
Shake the bottles gently once a day to mix the contents.
Continue for two weeks and note the smell and any foam.
Graph the balloon girth against days for the three bottles.
What you should see
Balloons swell at different rates; the dung bottle usually inflates first, and the mix often gives the biggest balloon later. The graph compares the feedstocks.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What gas is mainly produced?
Methane with carbon dioxide.
2Why is oxygen kept out?
The microbes that make methane work only without air.
3Why is biogas renewable?
It is made from waste that can be replaced.
4What is left after digestion?
A nutrient-rich slurry used as manure.
Take it further
Keep one bottle in a warm place and another in a cool place to see the effect of temperature.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology