Building a compost bioreactor

Traditional home composting, as most people who have a garden will know, involves building a pile of cuttings, probably adding to it from time to time, and turning it at regular intervals.

We previously built a "composting station" for this traditional method, consisting of five separate bays, four of which we filled with grass clippings, veg peelings and vegetation cuttings, with the idea being that the contents of each bay could be turned into the empty bay next to it.

This is how the compost station looked when it was first constructed, in 2016 - it had suffered a few injuries brought about by slipping rocks and collapsing sides since then!

While we have used this method successfully (and grown vegetables in the compost it has produced), it can produce an uneven compost (some parts of the pile perfectly broken down into lovely compost, others still in their unbroken-down state), not to mention the undesirable wildlife it can attract.

In contrast, the material in the Johnson-Su bioreactor (no I had never heard of it before either) never needs turning, doesn't produce smells or attract flies or other wildlife, and in theory produces a completely even and rich compost. The material is composted aerobically, which encourages a complete biological breakdown of the compost materials.

The design for this composter comes from Dr. David Johnson, Adjunct Professor for the College of Agriculture at California State University, Chico and his wife, Hui-Chun Su (if you're interested in finding out more, you can find instructions on how to make your own here) .

The downside is that it has to be filled all in one go - you can't add your veg peelings or grass cuttings to it here and there - but we found that around half a day's work, a large pile of olive prunings and some chicken manure was plenty to fill a couple of  composters.

The construction of the composter is something like this: a circular cage of wire mesh (fencing material/chicken wire) with a series of tubes in the middle (which will create the air flow). In our case Stuart ingeniously re-purposed sections of the old flue from the wood burner.

The cage is then lined with water permeable material (we used landscape fabric - kindly donated by our friends Paul & Kathy who had some leftovers to spare).

You then fill it.

In our case this involved chipping all of our olive tree prunings (a veritable mountain of them which half-filled the car park) and emptying a barrel's worth of chicken manure  (maybe 6 months' worth of stored coop cleanings), then layering the two materials as if making a giant stinky lasagna, thoroughly dousing each layer with water as it goes in,

Making wood chip.

8 buckets of chicken manure.

Layering wood chip and manure.

Watering each layer.

Once full, irrigation pipes are added to the top of the pile, to give it a daily sprinkling of water to keep it nice and damp.

Irrigation tubes in place on top

The tubes are then removed, leaving voids in which the air can circulate, and the top of the pile is also covered with the landscape material.

24h later.

Then all you have to do is wait...!

We should have lovely usable compost in around 9-12 months.