A compost pile reaches the thermophilic range of 55-65°C only when the heat its microbes release exceeds the heat leaking out through its surface, which is why a pile under 1 cubic metre often stays cool: a 1 m cube has 6 m² of surface for each cubic metre of material and a 0.5 m cube has 12 m². The hot phase starts within 1-3 weeks in a well-managed pile and then fades, so only a thermometer, never touch or smell, shows whether it happened.
Part of Composting Toilet Systems
Thermophilic decomposition is the stage of composting in which heat-loving microbes hold a pile at roughly 55-65°C. It is the stage that kills even resistant parasite eggs within days, and it depends on pile size, moisture, air and the carbon-to-nitrogen balance.
Heat Is A Balance Between Production And Loss
Every microbe in the pile releases a little heat. The pile as a whole warms only if the total produced, which scales with the volume of active material, is larger than the heat that escapes, which scales with the surface area. For a cube of side s:
- Volume = s³ and surface = 6 s², so surface per cubic metre = 6 ÷ s.
- s = 1 m: 6 ÷ 1 = 6 m² of surface per m³.
- s = 0.5 m: 6 ÷ 0.5 = 12 m² of surface per m³, twice the loss per unit of heat made.
- s = 1.2 m (the parent page’s bin footprint is 1.2 m × 1.2 m × 1 m): about 5 m² per m³ for a true cube of that size.
This is the reason behind the parent page’s rule that a pile under 1 cubic metre will not reliably become thermophilic. Insulation lowers the loss further: straw packed against the bin walls, a lid, and a sheltered, rain-free position all help.
The Cool Shell Is Most Of A Small Pile
Heat spreads inward from the surface only slowly, so the outer layer of any pile runs cooler than the core. If, for illustration, the outer 15 cm stays cool (an assumed thickness, not a measured one), the hot core of a 1 m cube is a 0.70 m cube. Its share of the volume is 0.70³ = 0.343, so about 34% of the material is hot and about 66% sits in the cool shell. That arithmetic is the reason the parent page does not trust the 3-day figure alone and also asks for a year of ageing.
The Temperature Bands, And What To Do In Each
| Core temperature | Meaning | Action |
|---|---|---|
| Below 40°C | Too small, too dry or too much carbon | Add nitrogen (urine, green material) and moisture |
| 40-55°C | Mesophilic: composting, but kill incomplete | Keep going; do not count this as treated |
| 55-65°C | Thermophilic: the target zone | Log daily; the clock counts consecutive days |
| Above 70°C | Too hot; heat-tolerant microbes die off | Turn the pile to cool it |
All four rows are the parent page’s bands. The 70°C ceiling matters because very hot piles lose the microbes that sustain heat, and the pile then falls cold.
What Raises And Holds The Heat
Food: nitrogen-rich waste balanced by carbon cover to 25-35:1 (Carbon-to-Nitrogen Ratio).
Air: microbes in the hot phase use oxygen quickly. A pile that is packed down, flooded or buried in fine wet sawdust goes airless and cools. Loose bulky cover (straw, chopped leaves) keeps air channels open.
Moisture: Composting Basics gives 40-60% moisture. Too dry and the microbes stall; too wet and the air is displaced.
Mass: at least 1 cubic metre, kept together. Scattering a deposit thinly over a large area cools it.
A Slowly Filled Bin Is Not A Fresh Hot Pile
A family of four fills a bin only slowly. Using the cover and faeces volumes worked out in Chamber Sizing (about 2.5-4 L per day for four people), a 1 cubic metre bin takes roughly 250-400 days to fill. Each new bucket arrives as a small cold layer on an older mass. The pile may heat in bursts or never. Do not assume the parent page’s “1-3 weeks” applies to a bin filled this slowly; read the thermometer, and fall back on the time-based rule if 55°C for 3 days is not recorded.
Common Mistakes
- Counting 40-55°C as treated. It is composting, not pathogen kill.
- Judging by the hand. Touch fails above about 50°C and tells you nothing about the cool shell. Use a long-stem thermometer (Monitoring Temperature & Progress).
- Turning a cold pile and calling it hot composting. Turning adds air but not heat if the mass or the nitrogen is missing.
Safety
A hot pile of human waste is a pathogen and spore hazard when disturbed
Turning or opening a pile releases dust and fungal spores that can be breathed in. Work upwind with a cloth or respirator over nose and mouth, wear gloves and boots that are washed afterwards, and do not turn fresh humanure with bare hands. Wash with soap before eating or touching your face.
What’s Next
- Learn next: Pathogen Kill Requirements — the time and temperature each organism needs
- Combine with: Compost Bin Construction — building a bin that holds its heat