Propagating BSF Using “Box in a Box” Propagation Bioreactors : The Life and Times of BSF (Black Soldier Flies)
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Propagating BSF Using “Box in a Box” Propagation Bioreactors

by Terry Green on 05/26/17

Propagating Black Soldier Fly (BSF) for mating, egg laying and larval replacement operations requires careful planning. This blog discusses how BR1/BR2 technology, a “box in a box” bioreactor technology (see Modular Farming of Black Soldier Fly (BSF) | Scaling Up - Schematic Overview and Scaling Up BSF Production| Integration of BSF Workstation Elements), can be used in propagating adult BSF as a means for supporting a steady-state output of BSF larvae in farming BSF off food scrap wastes.

The principle goal in propagating BSF in a farming operation is to produce replacement larvae under steady state conditions uninterrupted by the dwell time it takes in growing newly hatched larvae into prepupae.  Propagation under such steady-state conditions means that the larval population in food scrap should range in age from newborn offspring on up through larvae transitioning into the prepua stage of their life-cycle and on through pupation and emergence of new mating adults.

Providing conditions are in place for maintaining the correct environment for mating, the deposition of egg clutches, the hatching of newborn larvae from egg clutches, and larval access to nutrient waste, the continuous production of larvae replacing those harvested in the production chain can be achieved. To optimally propagate BSF plan on a design layout that exposes adults to natural light on a normal circadian cycle which encourages mating, maintain a temperature in the Propagation Workstation in the range of from about 80 to 95 F (~27 to 35 C) and keep the relative humidity in the range of 60+%.

Constructing “Box in a Box” bioreactors for the purpose of propagating BSF can be built at relatively low cost. Venting holes added to the side walls, lids, and to the base of a Propagation Bioreactor (PBR), the smaller box unit (see Examples - Conversion of Various Trash Bins and Totes Into BSF Propagation Bioreactors (PBRs) Through Addition of Vent and Drain Holes In Base Units), provides a means for continuously draining leachate out of the PBR as larvae feed off decaying waste added to the PBR, the egress of carbon dioxide out of the unit, the ingress of air into the unit, access for female adults seeking a place to lay egg clutches, and the free movement of newly hatched larvae in and out of the unit.

By providing larvae free movement in and out of PBRs as they grow off food waste added to the units, their survival under transient stress conditions, for example in situations where the waste may temporarily overheat, or where the waste compacts or becomes overcrowded with competing larvae feeding off the waste, can be markedly enhanced. Furthermore, larval propagation becomes more “hands off” in keeping with early blogs posted on this subject emphasizing the importance of minimizing transfer steps in the propagation process as much as possible in propagating young larvae (see Farming BSF | Propagating Larvae Simply and Economically and Farming BSF | General Guidelines that Work Well in Propagating BSF Larvae).

Constructing a PBR out of a plastic tote is straight forward. Start by drilling 0.75 inch (~1.9 cm) vent holes spaced evenly approximately 4 to 5 inches (~10 to 13 cm) apart around the side walls and base of a 27 gal. (~102 L) tote.  Additional holes should be added to its lid to improve the access of BSF into the unit (see Fig. 1). The rectangular shape of the unit and lid makes it easy to add food scrap to the unit as needed in attracting mating BSF to the unit, and in transferring waste and larvae in and out of the unit.
Image of stand alone BSF propagation bioreactor
Fig. 1. Conversion of a plastic 27 gallon tote and accompanying lid into a stand-alone BSF Propagation Bioreactor (PBR). Venting/drain holes must be drilled around all four side walls and at the base of the tote to provide for adequate drainage and aeration of waste added to the unit. Additional holes drilled through the lid provide female BSF seeking a place to lay egg clutches access to the underside of the lid and walls of the unit (Copyright © 2017, Terry Green, All rights reserved).

PBRs should be nested in series inside a BR1 (“outer box”) filled with a layer of coarse shredded wood chips or other cellulosic bulking agents (Fig. 2). The inclusion of coarse bulking agents in the base of the BR1 unit on which the PBRs nest helps maintain a dry environment for prepupae self-harvesting free of the PBR to pupate in situ. Adults emerge from the base of the BR1 ready to mate with this arrangement in place. Prepupae seeking a place to pupate do not crawl outside a simple containment curb built atop a flat concrete floor or pad demarking the perimeter of the BR1 unit.
Schematic layout of BSF propagation bioreactor nested in BR1 unit
Fig. 2. Layout showing top down and cutaway side views of BSF PBRs nested inside a BR1 built atop a concrete floor or pad. Leachate drains by gravity out of the PBRs and collects in the central drain line built into the base of the BR1 flowing from this point to a reservoir where it can be collected and pumped into IBC tanks using an inexpensive bilge pump assembly for further processing of byproducts recovered in the leachate fraction (Copyright© 2017, Terry Green, All rights reserved).

Incoming food waste can be shredded and stored in a fermented state pending transfer into PBRs. Shredding and fermenting the waste has certain advantages in processing the waste added to the PBRs. First, shredding markedly increases the surface area of the waste. This improves larval access to the nutrients in the waste. Second, fermentation kills off many pest insects (including house fly larvae, gnats and such) which may be in the waste at the time it is picked up for processing. Thirdly, during fermentation the acidity of the waste drops to about 3.5, a pH low enough to kill off or stop the growth of many microbes, leaving the waste in a semi-stable “pickled” state pending its transfer to the PBRs.

Fermentation occurs spontaneously upon transfer and storage of non-sterile shredded food into 55 gallon commercial drums with clamp-down lids, or in any other convenient container that can be closed off from air. Normal microbial respiratory activity occurring in waste stored in this manner causes the oxygen levels to drop precipitously. This creates conditions suitable for the growth of anaerobic organisms which in turn “pickle” the waste.

One or two small holes no larger than about 1/8th to 1/16th inch (~1 to 2 mm) in diameter should be drilled through the cover lids of the storage containers to vent carbon dioxide off gassing in stored food waste processed in this manner as it ferments. Alternatively, spring loaded or water trap vents can be attached to lids in accomplishing the same task (see Fig. 3).
Image of 55 gallon drum and fermentation of food scrap waste
Fig. 3. Processing and storing incoming food scrap waste. Left,   food scrap passed through a shredding machine and then fermented for upwards of a week or longer in a 55 gallon drum equipped with a spring-loaded venting lid; Right, the appearance of fermented food scrap before transferring it into PBRs set up for propagating BSF larvae (Copyright© 2017, Terry Green, All rights reserved).

It is important to not overload the PBRs with excess food scrap. While the load rate added to PBRs can vary depending upon larval growth conditions and the nature of the waste, keeping the load rate in the range of about 2 to 4 Kg per day per square meter of PBR space works well based on pilot plant tests on varying load rates. Overloading PBRs leads to compaction of the decaying waste, excess water retention, and a die off of newly hatched larvae unable to navigate through the sludge-like waste that forms as a result of adding waste in excess of that which the larvae can handle.

To ensure good drainage and aeration mix bulking agents with the waste added to the PBRs. Waste in the PBRs should not be allowed to compact and take on the character of sludge. Larvae will not thrive in the waste if it is left unturned and compacted in the base of the PBR. Finely shredded wood chips, straw, or dried leaves all work well as bulking agents mixed in with waste added to the PBRs (Fig. 4).  A simple test for good drainage, and adequate aeration, is to add water to the waste and to verify that the water drains freely through the waste. If the water pools or is retained in the waste, more bulking agent should be added.

A sod fork can be used to turnover waste and bulking agents added to PBRs. Larvae are not damaged by the sod fork on turning over the waste as the fork is worked through the waste.  A sod fork can also be used to lift and transfer waste seeded with young larvae into fresh incoming waste passing through the Propagation Workstation.
Image showing addition of bulking agent to food scrap waste in PBR
Fig. 4. Shredded straw added to a PBR unit. Added straw mixed in with the food waste serves to improve the drainage and aeration properties of the waste (Copyright© 2017, Terry Green, All rights reserved).

Fig. 5 shows the appearance of food scrap waste mixed in with cellulosic bulking agents (Fig. 5, Upper Left panel), the appearance of food scrap seeded with new generations of healthy larvae feeding and growing off the waste  (Fig. 5, Upper Middle and Upper Right  panels), mating adult BSF about the lid entry ways to the PBRs  (Fig. 5, Lower Left panel), and the appearance and harvesting of numerous egg clutches laid by females around the walls and edges of the PBRs (Fig. 5, Lower Middle and Lower Right panels). Egg clutches can be easily scraped from the walls and edges and flicked directly into the waste lying at the base of the PBRs. Egg clutches can also be harvested and transferred into food scrap wastes by running the edge of a spatula around the underside rims of the PBR lids where mating females also prefer to deposit egg clutches (Fig. 6).
Images of propagation bioreactor in varying processing states
Fig. 5. The appearance of food scrap and bulking agent deposited inside PBRs with new larvae emerging from egg clutches and feeding and growing off the waste, mating adult BSF and females atop the lids of the PBRs, and appearance of egg clutch deposits along the walls and edges of the PBRs. Top Left, waste mixed with bulking agent; Middle Top, following addition of fresh food scrap; Upper Right, the same view approximately 24 hours after addition of food scrap; Lower Left, mating BSF and female adults atop lids of PBRs; Lower Middle and Lower Right, egg clutch deposits and use of a spatula in harvesting of egg clutch deposits (Copyright© 2017, Terry Green, All rights reserved).

Image showing BSF egg clutch harvesting technique with spatula
Fig. 6. Harvesting of egg clutches from the underside of PBR lid with a painter’s spatula (Copyright© 2017, Terry Green, All rights reserved).

On a wet weight basis turnover of food scrap added to the PBRs amounts to about 95% with about 5% spent residue left unprocessed. To put this into perspective, over a one month cycle in operating 12 PBRs in series with food waste loaded every other day into the PBRs, the total input of waste dedicated toward propagation of BSF was ~360 Kg. Spent waste removed from the PBRs on average varies from about 15 to 20 Kg per month. This is the equivalent of recovering about 1 five gallon bucket of spent waste for every 18 five gallon buckets of food scrap waste loaded into the PBRs per month.

Spent waste (see Fig. 7) appears dark brown and black. It is somewhat friable, and has agricultural value spread and mixed directly into top soil or composted and used as a mulch. It contains residual BSF larval frass, and small traces of chitinous residues left behind by molting BSF larvae mixed in with polyphenolic oxidation byproducts and partially degraded fibrous bulking residues.
Image showing appearance of BSF spent waste recovered from PBR
Fig. 7. The appearance of BSF spent waste upon removal from BSF PBRs. Residue material is a combination of residual bulking agents (lignocelluloses fibrous debris), oxidized and degraded food scrap waste and BSF larval frass and chitinous residues (Copyright© 2017, Terry Green, All rights reserved).

The number of PBRs required in producing sufficient egg clutches and hatching
of new larval offspring to replace harvested prepupae in a farming operation can be expanded as needed to accommodate demand. It is best to start with a modular layout and empirically determine larval outputs based upon direct experience in working with a specific source of food scrap in the environmental setting where the Propagation Workstation is to be set up.

PBRs constructed along the layout design described here are flexible and highly adaptable for experimentation with differing waste feedstock formulations. For example PBRs can be set up with different formulations of waste feedstock added to a series of PBRs nested inside a BR1. By observing the frequency and density of egg clutches laid around the walls and undersides of the PBR walls and lids, and by observing the density of larvae and their age distribution growing in the test waste feedstock formulations, it is possible to identify which feedstock waste formulations draw more mating females in depositing egg clutches in and about the PBRs, and from this information and the growth characteristics of larvae growing inside the PBRs to select specific waste feedstock formulations that work best in a farming operation.

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