Adapting Johnson-Su Composting Bioreactors for Home Use

posted in: Regenerative Ag | 15

By Pat DeSimio, founder and coordinator of the Seeding Regenerative Agriculture Project,
a cooperative network of farmers, ranchers, and support organizations who work together to share knowledge, complete projects, conduct research, develop new technologies, and in general make sustainable and regenerative agriculture easier to practice.

The Johnson-Su Composting Bioreactor produces a diverse, fungally dominant microbial inoculant that interacts symbiotically with plants to sequester carbon in soils, increase soil water infiltration and water retention, fix nitrogen, and increase plant growth and fruit production, among other benefits.

Like many people, as soon as I learned about this system–which was designed to work at a commercial, agricultural scale–my next thought was, “How do I do this at home?” Since Dr. David Johnson and Hui-Chun Su-Johnson have been friends and colleagues since 2015, I’ve had the benefit of repeatedly picking their brains about how a home version of the Johnson-Su Composting Bioreactor might work. With their input, I’ve developed a home version of the Johnson-Su Composting Bioreactor. In this article, I invite you to benefit from what I’ve learned, use it to make your own system, and try your own experiments to make a version that works even better.

If you’re not already familiar with how a Johnson-Su Composting Bioreactor works, now would be a great time to check out a short booklet that you can find HERE

At the farm or ranch scale, the original design works marvelously. For home use, though, there are a few barriers: the original design requires more than 2,000 pounds of feed materials, which is . . . a lot; the bioreactor needs to be filled all at once, which doesn’t align well with the small amounts of compost produced daily in a home kitchen; and the feed materials can’t have internal moisture, or the bioreactor winds up with wet, anaerobic pockets that stink, draw pests, and don’t break down properly. 

Fortunately, these barriers proved manageable with just two modifications to the original Johnson-Su design: 

1. Shrinking the height of the bioreactor by half, from 5’ to 2.5’, to reduce the amount of feed materials required (an even smaller 2’ tall x 2’ diameter bioreactor should also work, but results from that scale are pending), and

2. Incorporating several fully aerobic holding containers where domestic food waste is simultaneously stored and dried.

The first modification, a half-height bioreactor, follows all the same steps in the CSU-Chico booklet except for the height of the bioreactor, which I reduced by half. The resulting bioreactor was much easier to fill than the full-scale version (no ladder required!), and over the course of a year, finding enough feed material was easy enough between my own food waste, yard waste, and some free, composted mulch from the City of Las Cruces. Figure 1 shows the half-scale bioreactor.

The second modification, the holding containers to store and dry food waste, took some more work to solve. The first truly effective system is shown in Figure 2.

Yep, the holding containers are that simple! The basic materials for the holding containers are the same as in the larger-scale composting bioreactors: landscaping fabric and remesh. The difference is the dimensions. Unlike in the full-scale bioreactors, which need every piece of compost in them to be within 12” of air, the wetter, denser nature of unprocessed food waste meant that the raw food waste in the holding containers went anaerobic unless it was within 4-6” of air. A 10” diameter holding container proved narrow enough to stay aerobic and let kitchen waste dry out completely. The holding containers also needed to be relatively short: holding containers between 1’ and 2.5’ in height all worked just fine, while a 3’ holding container retained some moisture at the very bottom. 

Also, at the rate my family produces food waste, two holding containers were better than one. A single holding container always stayed a little wet because of moisture from new food waste additions, and although a 70% moisture level is great in an actual Johnson-Su Composting Bioreactor, moisture is not a good thing in a holding container—the food waste needs to dry out quickly to minimize odor and pests. Alternating food waste deposits between two holding containers let each one dry thoroughly between deposits, keeping odors to a minimum and pests to a few flies, similar to a conventional compost pile. Depending on your own levels of food waste production, you might want different numbers of holding containers.

Once the holding containers were full, I stopped adding compost and let them dry out for an additional two weeks, making sure that any remaining internal moisture was gone. I wound up with about 6 cubic feet of dried bioreactor feedstock, which I complemented with about 30 cubic feet of dried yard waste and about 30 cubic feet of composted mulch from the City of Las Cruces Green Waste site. Following the standard instructions for filling a Johnson-Su composting bioreactor, I chopped up the feed materials (for lack of a chipper, using the edge of a shovel), ran the feed materials through a water bath, drained the feed materials using a propped-up wheelbarrow, and loaded the moistened feed materials into the half-height bioreactor. After following the standard instructions, the results a year later (Figure 3) looked great.

Figure 3. Mature compost, complete with worms

By the way, since I knew from conversations with David that the bioreactors could handle at least some meat and dairy, I decided to push the system a bit a lot. Any organic matter that came through the kitchen and wasn’t eaten by my family went into the holding containers: with apologies to Shel Silverstein, I’d like to point out that the beautiful compost in Figure 3 is made of, among other things:

Brown bananas, rotten peas,
Chunks of sour cottage cheese  . . .
Bacon rinds and chicken bones,
Drippy ends of ice cream cones,
Prune pits, peach pits, orange peel,
Gloppy glumps of cold oatmeal,
Pizza crusts and withered greens,
Soggy beans and tangerines,
Crusts of black burned buttered toast,
Gristly bits of beefy roasts. . .

Out of due caution regarding the meat and dairy, I applied the produced compost to trees rather than to a vegetable garden (and I’d advise you to do the same if you compost meats and/or dairy), but the finished compost looked as it should and had no odor at any time during the Johnson-Su composting process. It seems the approach worked!

For context, this system was set up in Las Cruces, NM, a very dry, sunny city with a high desert climate. Successfully drying food waste in a wetter environment may require a different approach than the holding containers I used. Even in Las Cruces, there are doubtless more improvements to make, and I’ll keep experimenting. May what you’ve learned here leave you better positioned to do the same!

Some final notes:

  • Even with the incorporation of (dried) meat and dairy, the actual Johnson-Su composting system had no smell, consistent with full-scale systems. Also consistent with full-scale Johnson-Su systems, it drew no flies or other pests (though a few toads, appreciating the moisture and maybe the red wriggler worms, did make little burrows at the top of the pile)
  • Almost everything, even bones, broke down over the course of a year. Things that made it through the composting process included a few avocado and peach pits that I’d missed splitting with the shovel, several chicken thigh bones (hollow) that I’d also missed with the shovel, and a T-bone.  
  • The holding containers did have a modest smell (noticeable within 5 feet or so after a new compost addition) and drew some flies, especially in the day or two after a new compost addition. This struck me as acceptable and consistent with other composting approaches. If you want to minimize smell and pests, avoid adding meat and dairy, and minimize the size of any given addition to a holding container: in general, the smaller each compost addition was and the less meat and dairy it contained, the less it smelled, the more briefly it smelled, and the less it drew flies. 
  • For the sake of all that’s holy, don’t try using 5-gallon buckets as holding containers. I tried a lot of different approaches to holding containers; I warn you about only this one. 
  • Unlike the final Johnson-Su Composting Bioreactors, the holding containers shouldn’t be watered. The sooner and more completely the holding containers dry out, the better.

15 Responses

  1. Francine Lindberg

    Thank you for this very informative and helpful article! I’m eager to try this composting method for my home garden, fruit trees, and native grasses field. It’s interesting to note your success with incorporating animal products! I will probably forego that, at least at first. Too many skunks in my neighborhood already!

  2. Jenny

    It’s kind of interesting how the feedstock needs to be dry, but then you wet it again before piling it into the reactor. Would it be possible to simply layer wetter and very dry materials together and skip the wetting process altogether?

    • admin

      Jenny, it’s important to get a consistent moisture throughout the bioreactor. If you layer wet and dry you likely end up with dry pockets which would not be good. The wetting process when building the bioreactor is essential!

  3. Boyd Myers

    Thanks will have to re-read the above…but am a home own with a no till garden and need advise. I built the 5 footer SJ Reactor and collected maple > oak leaves in fall of 2020, and ground to fine structure, pilled up and covered to keep dry during the winter when I got the bioreactor all the components of the bioreactor as to build to specs. This spring I loaded to the top in the manner shown by Johnson-Su’s video…wetting prior etc….I did layer in some powdered alfalfa (that I have used in other piles to add nitrogen source to the carbon) every 12-15 inches. Periodically I layered in ‘starter’. from my other matured compost piles. The loading was done over 5 days…single bucket …~4 hours was right. In the past from my usual compost piles, new piles reach thermophilic level of 120-140* in 3-5 days then wain to near ambient temp, unless re-aerated, gave it two more days then pulled the 6 PVC pipes. I have seen NO temperature rise in in the composter. I fully expected to hit 120-140 like my other piles. Would love expert opinion, for experienced input into what I should expect or what is amiss.

    • admin

      Hi Boyd, thanks for your interest and questions. Pat DeSimio sent this reply below -hope this helps!

      Based on the filling process that Boyd describes, my bet is that the bioreactor didn’t enter the thermophilic phase because of the longer filling period (five days). For the thermophilic phase to really kick off, the bioreactor needs to be filled all at once.

      In some good news for Boyd, it’s still possible to get good compost even if the bioreactor doesn’t go thermophilic. The thermophilic phase hastens the breakdown of some materials and definitely helps kill weed seeds, but as long as Boyd has added worms, kept the pile moist, and kept it from freezing, he should still get good compost.

      • Jennifer

        You mention keeping it from freezing, why is that? I thought the johnson – su bioreactor was tested in multiple locations (including freezing climates) and the only difference is they take longer to fully break down.

        • admin

          Simply put, soil microbes die at freezing temperatures. The BEAM compost is a microbial rich compost, that is its super power, so you want to avoid freezing temperatures.

  4. Jean

    Why dry out your food waste and then wet it again before putting in the bioreactor? Can you explain, please?

    • admin

      Food waste is dried in order to be able to store it until the bioreactor is being filled. Source materials are being added all at once, not on a continuous basis as is commonly done in home composting systems. Hope this helps!

  5. Joanne

    Hi there.

    Thank you for this information.

    The dried food waste I am now storing but have been slightly wet at some stage and blue mould is growing.

    What do you think I should do?

    p.s. my process is drying food in the botton oven of my Aga until enough material is used. Then collecting in plastic bins.

    Many thanks,

    • admin

      It’s best to dry food scraps in the sun to prevent mold!

  6. Gary Rippel

    I’m excited to try this methodology for my home garden/yard. I have heavy clay soil and want to amend it and improve permeability. I was thrilled to find this small version of the bioreactor process.

    My concern before getting started is drying out the kitchen scraps. I live in Kansas and we get plenty of rain and don’t always have the sunny skies so prevalent n NM! Short of spreading out my garbage all over the backyard in thin layers, how would you recommend drying it out with minimal attraction of flies and critters? My food scraps are minimal since there are only two of us! I may need to ask my neighbors or local restaurant for help with the supply!

    • admin

      HI Gary,
      If you have a gas oven with a pilot light, that’s a good way to dry out kitchen scraps as well. Or a dehydrator works, too.
      When you scale up, working with local restaurants, there’s another option. We just learned about an exciting new project in Silver City: school cafeteria food waste gets processed into a slurry, which then gets added to a Johnon-Su bioreactor. This only works if you fill up a bioreactor every time you have slurry, as it doesn’t keep. You also need a steady supply of carbon-rich material, such as wood chips. We will post a video of this exciting new project soon!
      Hope this helps and keep up the good work!

Leave a Reply

Your email address will not be published. Required fields are marked *