How to Build a Hot Compost Bin.


Latest Update 5th March 2017.
 
This is my homemade hot compost bin.  Its purpose is to make relatively small batches of compost using high temperature microbial breakdown.  Usually this method needs a compost heap containing at least a cubic metre of material.  The outer layers of which provide insulation so the centre of the heap can reach the required 55 to 65 degrees centigrade.

The bin has a capacity of 400 litres and is better suited to my small suburban garden where organic waste is limited.  The bins walls and roof are insulated and hold the heat in easily raising the average temperature in the whole bin to the required level.  Note the current roof is a prototype, and will be replaced by a much lighter design for easy lifting, shortly.

Its important to supply thermophilic microbes in the compost with plenty of oxygen, so aeration is a regular chore.  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 is left behind, but is then tossed through the air back into the bin in its new location aerating and re-mixing it at the same time.

Its important to regulate the temperature of the compost during this process.  It is an important indicator of how the break down process is progressing.  There is usually an initial surge in temperature during the first day after building the heap.  It depends to some extent on the mix of materials and the water content, but in my heap it will usually rise to 72C.  For technical reasons beyond the scope of this article, this is too high, and I will usually turn the heap after the first 24 hours to reduce the maximum temperature to 65C.

After that, I turn the heap every 2 days until the maximum temperature has dropped below 55C.  In my garden this takes from 15 to 18 days depending on outside temperatures, after which time the compost is ready for use.

I use a stainless steel thermometer with a 500mm probe to monitor progress.

The unit's external dimensions are 950 x 950 x 950mm (lid on) and it is assembled from 4 identical wall panels plus a roof panel.  Recycled timber is used to make a simple exterior frame for each panel, and 2 layers of 60mm rigid polystyrene foam are glued together to make a single 120mm thick panel which is then bonded into position within the frame.  Once the glue is set, both sides of the wall are covered with heavy duty shadecloth stapled to the timber frame to protect the foam from damage.
Each sub-assembly is made by joined 2 panels together overlapping them at their corners.  They are joined top and bottom using galvanised angle brackets on the inside and galvanised flat joining plates on the outside.
Two saddle clamps are screwed into position as shown.  They help to handle the sub-assemblies when they are being separated so the compost heap can be aerated.
The two halves of the enclosure are held together with cabin hooks at the top and bottom of the walls where the two halves join.  In this photo a freshly made compost heap has started to heat up and a thermometer has been put in place to keep a check on the core temperature.
I leave the thermometer in situ at all times except when the heap is being turned.  It keeps me informed about the temperature in the middle of the heap.