What it shows
Microbes breaking down garden waste release heat, so a compost heap warms up. A daily log of temperature shows the stages of composting and the effect of turning and moisture.
The science
Bacteria and fungi digest organic matter using oxygen, and some of the energy is released as heat. Temperature rises over a few days, peaks, and then falls as the easy food runs out. Turning the heap brings in air and restarts the heat; too dry or too wet slows the microbes.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two identical buckets or drums with holes for air
- Dry leaves and green kitchen waste
- Two long stem thermometers (0 to 110 °C), one for each bin
- A trowel
- Water in a spray bottle
- A notebook for daily readings
Safety
For this project: Wear gloves and a mask when turning compost, and wash hands afterwards. Keep the bin away from food, and do not use meat, dairy or oily food scraps.
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.
Step 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.
Fill two identical bins, A and B, each with three parts dry leaves to one part green kitchen waste.
Sprinkle water on both until the mix feels like a squeezed sponge.
Push a thermometer 15 cm into the middle of each pile and leave it there.
Read both temperatures at the same time every day and record them with the weather.
Turn bin A every seven days and note its reading before and after; never turn bin B, so it is the comparison.
Keep both bins in the same shaded spot and add a little water if either one dries out.
Note smell and appearance weekly.
Graph temperature against day for both bins.
What you should see
The graph usually shows temperature rising above room temperature in the first week, peaking, then falling, and bin A warming again after turning. A small bin heats only a few degrees; a large heap gets much hotter.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Where does the heat come from?
From microbes respiring as they break down waste.
02Why does turning help?
It adds oxygen.
03Why mix dry and green waste?
It balances carbon and nitrogen for the microbes.
04What is compost used for?
To enrich soil.
Take it further
Add finely chopped waste to one bin and coarse pieces to another to see which heats faster.
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
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