HOTBIN Composting Science

What is the HOTBIN?

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 40-60°c breaking down all food and garden waste into compost in just 30-90 days.

The obvious difference between HOT and cold composting is the temperature, which affects the rate of compost production and the ability to compost difficult food waste such as bones and kill off weed seeds, pathogens, fly eggs and larvae.

Science between HOT and Cold Composting

HOT composting operates at a temperature between 40-60°c in comparison to less than 20°c for cold composting. HOT composting is 32 times faster than traditional cold composting methods based on the Arrhenius equation, a formula for the temperature dependence of reaction rates. In HOT composting, this equation relates to the temperature variation of thermally induced reactions, a generalisation which concludes that reaction speed doubles with every 10°c increase in temperature.

So based on an average UK temperature of 10°c an open/outdoor cold compost heap typically takes 12-24 months to compost, but when HOTBIN is operating at 60°c it will take just 12-24 days!

Arrhenius Equation

k= rate of constant chemical reaction on the absolute temperature T (in Kelvin).

A=prefactor | Ea=activation energy (per mole) | R=universal gas constant

 

How High Temperature is Reached: 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 40-60°c –but they stop working at 75°c

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).  [waste table]

Without oxygen, the production of methane (2 4 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) to maintain FAS. The bulking agent should be added each time new 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

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. http://hotbincompostingblog.com/what-makes-a-good-insulated-compost-bin/).

 

Garden Organic Review

We firmly believe in the HOTBIN which is why we had Garden Organic test it for its ease of use, effectiveness and quality of compost produced in their research field at Ryton Gardens. The tests were a huge success.

Garden Organic highlighted that during the trials there was little odour associated with the HOTBIN, virtually no flies and no problems with rodents, all major concerns when adding food waste to a compost bin. Garden Organic was satisfied that because the HOTBIN’s temperatures could be maintained, it could provide significant sanitisation of waste that includes cooked food.

Jane Griffiths, Sustainable Waste Manager at Garden Organic, Ryton who tested the HOTBIN against a leading competitor said, “The HOTBIN processed a very challenging food waste mix into compost after 90 days. During the trial we were really impressed that when checking the temperature it read 60°C when the outside temperature was around 10°C.”