The Science Behind HOTBIN Composting

The Science Behind HOTBIN Composting

The HOTBIN is the compost bin for people serious about composting and reducing the amount of waste they send to landfill. HOTBIN is a hot aerobic composting bin, reaching temperatures of 104-140°F, breaking down food and garden waste into compost in just 30-90 days

The Science behind HOTBIN - Different from COLD composters, HOT composters' temperature affect the compost production rate, and have the ability to compost difficult food waste such as bones, kill-off weed seeds, pathogens, fly eggs and larvae.

Whereas cold composters operates at less than 68°F, HOTBIN operates between 104-140°F or 32 times faster than traditional cold composting methods based on the Arrhenius equation, commonly known as the Q10 formula. Based on the temperature coefficient Q10 formula, the factor rate increases when the temperature is raised by ten degrees.

That is, in an average temperature of 50°F, an outside open/outdoor cold compost heap typically takes 12-24 months to compost. In contrast, a HOTBIN operating at temperatures between 120-140°F,  can produce compost in 12-24 days. 

Understanding Bacteria -  Aerobic composting is reliant upon supplying plenty of oxygen to the decomposition process and promoting the necessary bacterial activity responsible for the breakdown of waste into compost. For bacteria to perform effectively, they require: 

  • Food - the activity of bacteria digesting waste produces the required heat. 
  • Water – required for bacteria to grow and aid digestion.
  • Oxygen – prevents excess water and waste becoming a putrid, sloppy mess.
  • Warmth – bacteria operates happiest at 104-140°F –but they stop working at 167°F.

The bacterial speed of heat release depends on the individual digestibility of the waste. This is affected by what goes in, what size, and how much. For example, digestion and release of heat can be fast, e.g. from plant material or slower for lignin (wood). 

Without oxygen, the production of methane (25 times more potent than Green House Gases) will occur, and compost will not be produced effectively. The most efficient way of ensuring sufficient oxygenation to the waste is by creating Free Airspaces.

So What Are Free Airspaces? - Free Airspace (FAS) is the sum of all the gaps around and between particles though which air can circulate. When particles are able to create a ‘self-supporting structure’, the gaps and spaces are maintained for long periods. Aerobic compost heaps such as the HOTBIN rely on aeration via buoyancy air flow which in turn is reliant on retaining around 20-30% 'free air space’ (FAS).

Turning a heap will produce bacterial growth for short periods of time however if there are no self-supporting particles in the turned waste the heap will quickly collapse restricting airflow once again. Conversely, if the heap has self-supporting particles, no turning is needed.

The HOTBIN achieves this using a bulking agent (composted wood chip/mulch) to maintain FAS. The bulking agent should be added each time new chopped waste is added to the HOTBIN to ensure buoyant air flow through the waste, which in turn allows the bacteria to digest the material effectively without the need for turning or tumbling.

HOTBIN – Insulation by Expanded Polypropylene (EPP)
The HOTBIN is manufactured from expanded polypropylene, a durable material chosen for its unbeatable insulation properties which when the bin is running at its optimum temperature retains heat for longer and keeps the bacteria working effectively.

Polypropylene is the same plastic material used to make garden chairs and tables however the key difference is that the material comes in bead form which is then expanded, so the HOTBIN is actually about 98% air! 

To put the insulation properties of the HOTBIN into perspective, consider for example that you would need 5 railway sleepers in thickness, or 600m thick walls of polyethylene to achieve the same insulation benefits (measured in U value) as the HOTBIN.

Leave a comment

Please note, comments must be approved before they are published