Making Compost in 12 days.

Latest Update 17th July 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.

Usually this method needs a compost heap containing at least a cubic metre of material so the outer layers provide enough insulation to heat the heap up to the required 55 to 65 degrees centigrade.

My new bin has a capacity of 418 litres and is better suited to my small suburban garden where space and organic waste are in short supply.  The bins walls, floor and roof are insulated and hold the heat in, easily raising the temperature to the required levels.

Its important to supply the composting (aerobic) microbes with plenty of oxygen, so the bin's contents must be exposed to air periodically during the course of this process.  The hot compost bin is designed to be split into 2 halves so it can easily be moved to a suitable new space and re-assembled.  The compost, which was left behind, is tossed through the air back into the bin in its new location aerating and re-mixing it at the same time.

The temperature of the compost must be regulated during each cycle so we know how the break down process is progressing.  I use a stainless steel thermometer with a 500mm probe to monitor this temperature.  Its long enough to reach into the centre of the heap and the dial gauge is easy to read.

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 above photograph was taken before the new insulated base became part of the design.  The following 3 photos are the same.  I will rectify this with new photographs when I make my next batch of compost.
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 and aerate the heap. The heap temperature should then settle down between 55C and 65C and only require turning every second day.

The compost's temperature must be maintained within this range for several days 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. 

The finished compost is compacted during the process and loses volume as it decomposes, so don't be too surprised if you only get 300 litres of compost out of your original 418 litres of waste.
 
In a Melbourne summer the whole process takes 10 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.  The process takes a couple of days longer in winter.
This photograph shows finished compost with the new insulated base incorporated in the design.

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 three times 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 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 only.
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. 

It's easy to remove stored waste from these bins.  Just unscrew the 6 Tek screws holding each front panel in place.