Is it possible to make a successful small hot compost pile? Keep reading to find out the answer.
One of the main impediments to achieving a typical hot compost pile is the sheer amount of material required. The reason for this is that in order to maintain the consistently high temperatures necessary to promote the growth of thermophilic bacteria, the heat inside the pile must be allowed to increase faster than heat is lost to the environment. The usual method for achieving this is simply to build a big enough pile with enough mass that the heat in the center of the pile stays constant. Almost all the successful hot piles I have constructed to date have been HUGE.
The usual minimum size for a pile is usually quoted as three cubic feet. Here in the Pacific Northwest, I have found the lower average ambient temperature means you usually need four cubic feet. This is a LOT of material. Generating and collecting this amount of material is time consuming and moving it from place to place is a pain.
The average home garden only produces three to four cubic feet of material once or maybe twice a season. Not only is it difficult for many gardeners to generate the volume of waste material needed to build a hot pile, but the sheer weight of wet compost is too much for many people to deal with. To ensure the pile is oxygenated it is necessary to turn the pile often. As I age, I find myself less and less inclined to turn my huge hot compost piles as often as I should.
But what if you could find a way to make a hot compost pile that doesn’t require a huge mass of material?
The “four in the morning” brain fart is:
1. Build a small container that is heavily insulated to hold the heat in.
2. Structure it in such a way as to passively pull air through it.
3. Add a small amount of super moist waste material and see if it heats up.
In diagram the idea looks like this:
As I have been experimenting with biochar kilns for some time I happen to have a double walled barrel with about a 1 1/2” void space between the two walls.
By stuffing this void space with rock wool I created an insulated inner barrel about 25 gallons in volume. The barrel was open on both ends. So I covered one end with screening material. The other end I covered with a lid and 5’ length of 6” stovepipe. The chimney stack effect from heat rising off the compost passively keeps air moving through the chamber ensuring aerobic decomposition.
Amazingly enough, my preliminary results are quite encouraging. After 12 hours the temperature had risen to 100 degrees. After 24 hours the temp was 120 – 130. 36 hours and the temp was up to 135!
I will be monitoring and refining this design idea in the weeks to come. The chief unknown at this point is whether high temps can be maintained when it is really cold outside. Ambient air temperature the past few days has been in the sixties. If the temp is in the forties will the compost continue to cook or will further refinements be necessary?
Stay tuned as the phantom tinkerer continues to prowl the night.