How To Build A Woodchip Bioreactor

My Woodchip Bioreactor Is Based On The Johnson-Su Bioreactor Design & This Article Discusses Woodchip Bioreactor Variations.

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The Johnson-Su Bioreactor Design, aka Johnson-Su Biodigester Design Intrigues Me. So much that I wanted to use the principle to make a Woodchip Biodigester.

What Is A Johnson-Su Bioreactor?

The Johnson-Su bioreactor is an aerobic composting system that uses aerated static piles to produce fungal rich, high-quality, compost. It was developed by Dr. David Johnson and his wife, Hui-Chun Su Johnson. Dr. Johnson is a molecular biologist and research scientist at New Mexico State University, and Hui-Chun Su Johnson is a soil scientist and research associate at the same university.

Dr. David Johnson’s background is a as a molecular biologist who has been studying soil microbes for over 20 years. He is the author of several books and articles on soil health and sustainable agriculture.

Hui-Chun Su Johnson is a soil scientist who has been working on the development of the Johnson-Su bioreactor for over 10 years. She is a passionate advocate for sustainable agriculture and has a deep understanding of the importance of soil health.

Johnson-Su Bioreactor Principles

Aeration: The piles are aerated by using vertically installed perforated pipes or tubes to form airways through the heaps. This helps to keep the compost aerobic, which is necessary for the growth of beneficial bacteria and fungi. The pipes can be removed after 4-5 days as the fungal hyphae will maintain the integrity of the heap and the airways.
Moisture: The piles are kept moist, but not soggy. The ideal moisture content is around 50-60%.
Temperature: The piles tend to heat up quite quickly to a temperature of 55-65C for the first 3-5 days. This helps to kill pathogens and weed seeds. They then start to cool and develop a wide number of microbes that prefer lower temperatures.
Worms: worms can be added once the pile cools. Too hot and the worms die but once the temperature is lower they soon breed and a small handful can soon breed sufficiently to colonise the whole heap.
Turning: Unlike many composting systems the piles are NOT turned. This helps to preserve the beneficial microbes in the compost and maintain the airways and aerobic conditions.
Time: The heaps can take anything up to two years to mature where woodchip is composted. It will of course depend on the type of wood and size of the chip. Other materials will compost much quicker.
When composting woodchip it’s important to note that the Carbon:Nitrogen ratio is very carbon rich, or perhaps it’s better to say nitrogen depleted. In conventional composting systems we are told to go for a 30:1 mix. This is closer 400:1 but still works. It just takes longer.

The Johnson-Su bioreactor is a simple and effective way to produce high-quality compost. It is a good option for farmers, gardeners, and anyone else who wants to improve the quality of their soil.

How Can A CN 400:1 Ratio Work?

It seems that unlike normal composting this system boosts fungal growth far more than other systems. And it seems that some nitrogen fixation occurs, which supplements the low levels that occur at the start of the process. At first I found this incredible, but if plants can use bacteria to fix nitrogen, why can’t a similar process based on fungi also exist.

My Version Of The Johnson-Su Bioreactor

I built my system specifically to compost woodchip. The heap is made from some recycled plastic coated netting and some lengths of plastic down pipe.

Many advocates start their heap on a pallet, so that air gets under the heap, and use six vertical pipes to form the air holes. A few days after making the heap they remove the pipes and air then percolates through the holes which have stabilised due to fungal growth holding the heap together.

The idea is that no part of the heap is more than 6 inches from fresh air.

In my case I placed my heap direct on the soil and used one central pipe to form the airway. My logic was that the woodchip contained large particles, hence offering high levels of porosity, and this enabled good airflow with one pipe. My concern was that too many airways would dry the woodchip too much.

The other issue is that many people say the pipes make filling the heap very difficult as they get int he way. I found one central pipe was no problem when filling.

Basic Principles Can be Ignored .. Sometimes!

When following basic principles in horticulture and gardening I’ve found it sensible to apply common sense and on occasion to modify the technique.

In all the videos I’ve seen of people practising and explaining the technique they all seem to use a permeable woven fabric inside the netting. They explain this is necessary to stop the contents from falling out of the retaining wire.

I didn’t bother with the fabric. It seems a lot of faff to me. And when I filled the wire netting cages very little fell out. Much less than 1%. It really wasn’t a problem.

I made one other modification. Then I added a thick layer of grass cuttings to the heap. I wanted to see if the additional nitrogen speeded the process up. Time will tell.

Johnson Su Woodchip Composting Results

A lot of people have tried variations on the basic Johnson Su process. The results vary depending on the type of material processed, it ranges from fresh green material to very large woodchip; the moisture content, and especially the length of time left to mature the heap.

It’s clear that the heaps need to be kept reasonably moist, and that’s a reason I didn’t use a pallet. And it’s clear the the outside of the heap barely decomposes. But go a few inches into the heap and decomposition certainly takes place. After a year it is very noticeable. But in many cases it looks like a further year produces even better compost.

Of course it depends on the grade of compost you want, and what you want to use if for. In many cases, if sieved, the one year old compost is fine. Just put the coarser stuff back in another heap to finish off.

The Benefits of the Johnson-Su Bioreactor

Here are some of the benefits of using the Johnson-Su bioreactor:

It produces high-quality compost in a short period of time.
It is a simple and easy-to-use system.
The system can be built using recycled materials such as wire mesh and plaice drain pipes.
It requires little maintenance.
There are no moving parts
It is a low-cost system.
It can be used to compost a variety of materials.
It does not produce odors or attract pests.

If you are looking for a way to compost woody material and improve the quality of your soil, the Johnson-Su bioreactor looks a great option. It is a simple, effective, and affordable way to produce high-quality compost.

How To Use Johnson-Su Producers Compost

The principle behind this compost is a little different to traditional compost. This one is often used as an inoculum at quite low levels. The research has focused on agriculture where the idea is to boost fungal populations and the research on this is very interesting. High fungi to bacteria ratios seem to favour growth and is an area I’ll be investigating further. There is more information in this article which originally appeared in Direct Driller magazine.

Do the principles apply to gardens and allotments? The plant doesn’t know if it is in a garden or on a farm, so why not? Certainly the scale of operation is different but that shouldn’t alter the basic principle.

I’m going to try using the Johnson-Su compost as a dressing on top of my existing No Dig beds to see if they boost growth even further in my biointensive growing system.

A Soil Requirement For Johnson-Su Compost?

If using the compost from a bioreactor as an inoculum I suspect it will work far better on solid with a high organic content which the fungi can work on. My logic is that the fungi need something to work on and without organic matter they will be short of material on which to feed.