How to Make Hot Compost.

Latest Update 5th March 2017.

You don't need a formal structure like the one above to make thermal compost, but this one has been designed to make relatively small batches with less effort. 

Go to my page "How to Build a Hot Compost Bin" for more information.
The organic waste in the photo shows a batch of compost after the initial period in the enclosure.  The enclosure has been split and is ready to be dragged to a new location for reassembly.
The 2 halves are clipped together using cabin hooks to make a new enclosure.  The compost heap is broken up and transferred into the enclosure tossing the materials into the air to mix and aerate them.
Water is sprayed onto the materials as the compost bin is being filled.  The compost must remain moist at all times, but not soaking wet (about 75% saturation).
Although the compost settles a little during the first period, aerating it has restored its volume.
Depending on the mix of materials in the compost heap, the temperature sometimes rises rapidly to over 65C on the first day.  If this happens, you will need to turn the compost heap at the end of the first day. The heap temperature should then settle down between 55C and 65C and only require turning every second day.
Its important to achieve several days within this temperature range to kill off plant pathogens and unwanted seeds in the compost.  When the temperature drops below 55C at the end of one of the 2 day cycles, the compost is ready for use.  In summer the whole process usually takes about 15 days. 
The organic waste used in a compost heap is any material which was once alive and growing, and for the purposes of hot composting, is made up of nitrogen and carbon rich wastes.  Soft green plant material and animal manures have a high nitrogen content and brown, dry and woody organic material, like straw or autumn leaves have a high carbon content.

In theory, the ideal conditions for thermophilic microbes to multiply is when there is 30 times as much carbon in the mix as nitrogen, the temperature is between 55C and 65C, and the moisture content is about 75%.  For proportions of carbon and nitrogen in different types of organic waste take a look at Deep Green Permaculture's chart in their blog  Hot Compost-Composting in 18 days.

For the amateur organic gardener, this is unnecessary detail in my opinion and you can get by quite well with a simpler formula of twice as much carbon rich waste as nitrogen rich waste.  Experience will help you get the ratio right first time if your source of materials is reasonably consistent.  For my compost heap I harvest grass clippings, vegetable garden waste, kitchen waste, and prunings from trees and shrubs. The material is a mixture of carbon and nitrogen to begin with, and I only need to add used straw mulch to increase the carbon content to reach an ideal level.

Its important when gathering organic waste for a compost heap, to keep the nitrogen rich and carbon rich components separated, and its also important to keep them as dry as possible so they don't start to decompose anaerobically.
I do this using two purpose built containers.  Their shadecloth wall inserts help the compost to dry out and avoid premature decomposition.  The sandy coloured storage unit is used mainly for nitrogen rich organic waste and the brown coloured one is for carbon rich waste.
The sandy coloured storage bin has a vermin proof lining of galvanised wire bird cage netting to keep scavengers out. 

When this bin is half full, I usually top up both bins with sugar cane straw (straw reclaimed from the garden beds if possible).  When they are both full there is enough material to fill the hot compost bin.

Before you store waste, cut it up into small pieces so the microbes can get into the materials more easily when the composting process begins.  Tree branches and large plant stalks should be put through a shredder or run over repeatedly with a lawn mower.

To easily remove stored waste from a bin remove the front panel (6 tech screws).

Composting with an insulated hot compost bin.
  • When gathering waste you should try to collect it from as many different sources as possible to maximise variability in the organic materials themselves and the microbes feeding on them.
  • Meat scraps tend to attract scavengers and are best left out, but aged animal manures are a valuable source of nitrogen when added to the compost heap.
  • As you alternate layers of green and brown waste when building your compost heap, a little good organic soil should be added to inoculate the heap with more microbes.
  • To achieve thermal composting, the compost bin should be full at the beginning of the process.
  • Water the heap as you build it to keep the compost moist but not soaking wet (75% saturation).  It should hold enough water to bind the materials together when squeezed, but not so wet that water will run or drip out of it. 
  • During the first period after you build your heap (the mesophilic stage), microbes of all types colonise it and the temperature starts to rise as they feed. 
  • Heat loving microbes become dominant as the heap gradually enters the thermophilic stage and temperatures surge initially to over the required maximum of 65C.
  • Its important to know what temperature the heap is generating and I use a stainless steel thermometer with a 500mm probe to monitor it.
  • To bring down that initial surge in temperature the heap is turned after 24 hours.
  • During the next few days the heap must be kept aerated by turning it every 2 days (see video above).  A little water is added if the compost needs it.
  • The heap will maintain 55C to 65C maximums for several of these 2 day cycles, but when the maximum temperature reading drops to below 55C at the end of a 2 day cycle, the process is finished and the compost is available for immediate use.
  • The finished compost is much more compact after the process is finished and has lost some mass to the atmosphere during the decomposition process, so don't be too surprised if you only get 250 to 300 litres out of your original 400 litres of waste.
  • In a Melbourne summer the whole process takes 15 days, but although its good to use the compost soon after its finished, it will continue to mature if it is kept moist, and can attract earthworms that have stayed away while it was hot.