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.
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.