Building a compost bioreactor

Updated: Dec 2, 2020

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

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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 Theoretically every 19 days according to Will Bonsall in his fabulous book 'Radical self reliance'. Another fool proof way is to have a compost thermometer, turning the pile each time the temperature drops below the 'active' range should reheat the pile again for another cycle, an inexpensive but incredibly useful piece of kit as without you could tun your pile while it was still 'cooking' and cool it down thereby slowing the whole process.

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In reality it was rarely turned this frequently with so much else to do on the farm, but as Will also says in his book, composting will happen all by itself, in fact you can't stop it, it's just a question of the time it takes.

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.

Most of the piles we tried to build were 'hot piles' rather than 'cold piles', the difference being a hot pile is built in one moment to a minimum size of 1 cubic metre while a cold pile is simply a pile or heap added to as and when you have material. The benefits of a hot pile is the size allows it to heat up to temperatures that largely steralise the resulting compost of soil borne disease and weed seeds, although some seeds can survive this, the resulting 'black gold' is generally much better. You can go into quite some depth about carbon to nitrogen ratios when building your piles as the heap needs both to compost effectively, a great bible on the subject is 'Secrets to great soil' by Elizabeth Stell.

In trawling around compost groups on social media over recent years I chanced upon the Johnson-Su bioreactor (no I had never heard of it before either) which by contrast to other methods 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 once the pile is fully built and tubes removed). In our case Stuart ingeniously re-purposed sections of the old flue from the wood burner and used materials we already had as this was a pandemic lock-down project.

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
Making wood chip.
Chicken manure for compost
Eight buckets of chicken manure.

Making compost
Layering wood chip and manure.
Making compost
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. 24 hours later, the pile will have settled enough to remove the tubes allowing air into the middle of the pile the fabric folded over the outside of the cage can be folded over to cover the top leaving the irrigation 'ring' sitting atop the heap. Other than a short daily watering, which we will automate next year, you need do nothing more until it's ready for sieving and spreading.

Irrigation of compost pile
Irrigation pipes in place.

24 hours later, tubes removed...
Hot compost thermometer
...and already cooking!

Covered up until ready to empty and use.

We should have lovely usable compost in around 6-9 months.

Update: After 7 months we decided to open up the compost reactors to see what was inside. We were not disappointed!

While on the outside not much had changed, peeking beneath the outer layer of wood chips revealed a soft and rich compost.

Cage removed.
Revealing a pile that didn't look too different from how it started.

We began digging into the pile and sifting the compost using a frame with a piece of small mesh fencing material pinned to it, held over a collection bucket.

Compost sifting.
Compost sifting.

And soon we were rewarded with bucket after bucket of lovely compost - which went straight onto the veg beds.


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