Biogas in a bottle
Cow dung and water in a bottle make gas that slowly inflates a balloon. A taste of rural biogas plants.
The big idea
When plant and animal waste rots without air, tiny living things called microbes make a gas called biogas. Many villages use it to cook.
You will need
Tick things off as you gather them.
Step by step
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Step 1: Mix the slurry
Mix equal parts of fresh cow dung and water into a thick, runny paste. Stir it well.
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Step 2: Fill the bottle
Use a funnel to fill the bottle about half way. Leave the top half empty for the gas.
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Step 3: Fit the balloon
Stretch the balloon over the mouth of the bottle so it sits tight. Tape around the neck so no gas can leak out.
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Step 4: Keep it warm
Place the bottle in a warm spot, outdoors in the shade or in gentle sun. Microbes work faster when it is warm.
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Step 5: Shake gently each day
Swirl the bottle softly once a day to keep the slurry mixed. Do not open it.
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Step 6: Watch the balloon
In a few days to a week or two, the balloon starts to puff up. Measure around the balloon each day with a piece of string.
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Step 7: Plot the growth
Plot the string length on a graph. When does the balloon grow fastest?
What to look for
- Bubbles appear in the slurry after a day or two.
- The balloon inflates slowly, not all at once.
- A warmer bottle makes gas faster than a cold one.
Why it works
Microbes in the dung eat the waste. With no oxygen around, they give off a gas that is mostly methane. Methane burns well, which is why biogas can cook food. In a real biogas plant, the leftover slurry is a good fertiliser, so nothing is wasted. Our bottle makes only a tiny amount of gas, but the process is the same.
Try this next
- Try two bottles, one warm and one cool, and compare the balloons.
- Add some kitchen vegetable waste to a third bottle.
- Find out if a village near you has a biogas plant.
Stay safe
Biogas is flammable. Never light it, never keep the bottle near a flame, and keep it outdoors. Wash your hands well after touching dung or the bottle. Do not squeeze the balloon.
Link to the grid
Biomass and biogas can feed power plants as well as stoves. Burning waste gas to make electricity turns a smelly problem into a useful fuel.
Learn how the grid works