I’m testing a strategy for improving the conditions in a BSF composter based on the addition of corn cob pieces sold as hamster bedding.* Before putting any waste into the unit (in this case a BioPod) I added a 3 inch layer of the corn cob bedding which I had soaked overnight in water. The cob was placed directly on top of the drainage pad which is the material used to filter effluent (liquid waste).Possible benefits of corn cob (general)
One function I’m considering is that of a moisture buffer. When there is an abundance of liquid I picture the cob soaking it up and then retaining that moisture to help keep the compost above the 70% moisture level that I’ve read is the minimum for black soldier fly larvae (BSFL or “grubs”).
I think the sponge-like structure of corn cob may make it a good substrate for beneficial bacteria. I have fed fresh whole corn cobs to BSFL in the past and I noticed that larvae could be seen in the crevices of the cobs well after any visible corn kernel was gone. In some cases I saw this activity on cobs that had been added several months earlier. The larvae appeared to be feeding, and most of the time it was the smaller larvae. I’m guessing that they were feeding on bacteria since no food was visible. Maybe only the smaller larvae could fit into the small crevices of the cobs. I’m thinking that the sponge-like structure of corn cob promotes the presence of oxygen and therefore if bacteria were present they would be the aerobic type (thrives in the presence of oxygen). This relates to the idea that BSFL consume and derive nutrition from bacteria; something I’m not sure of. If the cob pieces promote aerobic bacteria them maybe it will help maintain overall aerobic conditions in the compost.
Drainage is always a critical issue in BSF composters, and corn cob may facilitate better drainage. Given its porous structure the cob may allow liquids to pass through the otherwise dense residue created by BSFL. This is also related to anaerobic bacteria (thrive in the absence of oxygen) which are associated with foul odors and BSF system crashes. Flooded conditions limit the oxygen present in the compost which promotes undesirable anaerobic bacteria. Therefore, good drainage is key to keeping a BSF unit balanced and corn cob may help with this.
Possible benefits (for system start up)
In addition to the concepts above I began this test with the idea that in the start up phase the cob would work as a pre-filter and keep the initial waste off of the filter material. To be honest I wasn’t exactly sure if there was any benefit in that, it was more of a hunch or a guess. The concept of a pre-filter morphed into something else, more on that below.
Maintaining a high moisture level is always important in a BSF unit, but it’s usually automatic in an established system which receives regular large additions of waste. In a new system with little or no larvae present the waste has a tendency to dry out, especially in dry climates. Keeping the beginning waste moist is important for two reasons; any newly hatched larvae are at risk of dehydration, and moist waste is better at giving off the odor which serves as a beacon for egg laying BSF females. The three inch layer of moistened cob was designed to increase the bulk in the new unit with something that would absorb and retain liquid without overloading the unit with food waste. After adding the soaked corn cob I observed that it too was drying out after a few days. As a result I began adding more liquid to the unit which led me to the idea of flushing the system with liquid several times per day.
Flushing a new BSF unit with liquids
When I saw the need to rehydrate the cob pieces I decided to wet it with the liquid produced when I fermented dried corn as a BSF attractant. (A favorite BSF attractant of mine is dry cracked corn covered with water and allowed to ferment. Sugar can be added to speed up the fermentation, but I have more often not used sugar.) I slowly poured approximately one gallon of the corn liquid into the unit, making an effort to distribute it as evenly as possible. As the liquid evaporated from the cob it gave off a good volume of the scent that BSF females followed back to the unit. The effect was strongest just after pouring in the liquid so I began draining the corn liquid into a bucket and re-introducing it to the unit as described above. Cycling liquid through the unit several times a day has kept the contents moist and I believe it has enhanced the attractiveness of the unit to BSF.
After 3-4 days of flushing the unit with the corn liquid I noticed that the pleasant sour smell was developing a less pleasant odor that I think indicated growth of anaerobic bacteria. At that point I stopped recycling the liquid and began flushing the system with fresh water. 4-6 times everyday I’m draining any liquid in the unit and adding it to my tomato plants, and then adding 4-6 quarts of fresh water evenly over the corn cob pieces and waste. I don’t drain the system immediately, I have been leaving the newly added liquid in the bottom of the unit as I believe it will help keep the ambient humidity within the unit higher and increase the attractive odor which helps attract BSF.
I anticipate that as the volume of waste increases, the churning action of the BSFL will mix the layer of cob evenly throughout. My hope is that the cob will continue to help keep the unit aerobic and improve drainage. I plan to continue flushing the waste with fresh water until I see some indication that it is having either no effect or a negative effect. I’m very curious if it might make for a more stable system. At the least it would serve as a constant indicator of how well the system is draining.
I’ll add photos and videos soon.