Steady-State Farming of BSF Larvae : The Life and Times of BSF (Black Soldier Flies)
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Steady-State Farming of BSF Larvae

by Terry Green on 01/08/18

Entrepreneurs growing BSF larvae off food scrap waste often wrestle with the question of how many grams of BSF eggs, or larvae, per square meter of bin (bioreactor) space should be seeded into the waste serving as larval feedstock to achieve an optimal output of larvae in managing a farming facility. A more germane question is however the following - “How can I maximize the growth and harvest of larvae while minimizing labor and the overall cost of operating a plant facility?” For a BSF plant facility to be economically viable, managing overall operating costs is quite important (see On the Economics of Farming BSF | Thinking Outside the Box). This blog reviews some essential elements in farming BSF on a commercial scale in achieving this latter goal.

The most important aspect in optimizing larval output while keeping costs to a minimum is to recognize the importance of farming larvae under steady-state conditions (see Modular Farming of Black Soldier Fly (BSF) | Scaling Up - Schematic OverviewScaling Up BSF Production| Integration of BSF Workstation Elements, and Propagating BSF Using “Box in a Box” Propagation Bioreactors) unencumbered by labor intensive “stop and go” processing operations associated with batch farming technologies (see Black Soldier Fly Biowaste Processing - A Step-by-Step Guide).

On fundamental grounds steady-state farming is less labor intensive, and far more flexible. To get at the central point of the question, how best to efficiently and economically optimize larval output in a farming facility, it is worth looking more closely at what are the differences in operational set up and layout of a facility carrying out a “batch” as opposed to “steady-state” farming operation.

The principles of farming larvae under steady-state conditions can best be understood by considering how BSF propagate, and how larvae grow, in their natural habitat. In Nature, mating females seek out fresh waste and deposit their egg clutches near waste having nutrient value without synchronization of the number of eggs seeded into the waste. Mating females don’t care if the waste has been seeded by other mating females. They don’t do batch farming! They simply deposit their egg clutches efficiently in and around decaying waste having nutrient value, egg clutch upon egg clutch, just as they have done through millions of years of evolution.

So how do these principles regarding how adult BSF propagate apply in managing the propagation aspects of a farming operation under steady-state farming conditions? Simple – by designing and applying farming operations with a bioreactor layout that encourages mating adult females to spontaneously lay their egg clutches in and about bioreactors housing food scrap waste in an environment set up to support young larvae hatching from the egg deposits to grow in situ off food scrap waste housed in the bioreactors while keeping human intervention in managing these steps to a minimum.

The same principle applies regarding managing larval output. In Nature, larvae, on reaching the prepupae stage in their life-cycle, spontaneously self-harvest from the waste on which they have grown in seeking a dry place to pupate. So why bother sieving larvae from waste, and the attendant extra cost in labor, material costs, and difficulties in separating larvae in this manner, when this task can be easily accomplished with less labor and expense by leaving this step in the farming operation to the larvae who are already endowed with the capability of spontaneously self-harvesting free of the waste without human intervention?

In order to achieve a steady-state output of larvae, bulking agents must be mixed in with food scrap waste provided as larval feedstock to ensure efficient drainage and aeration of the waste (see Propagating Black Soldier Fly Larvae - Mixing & Aerating Food Scrap). Adding bulking agents to food scrap waste fed to larvae also makes it much easier for larvae growing in the waste to burrow through the waste unimpeded which in turn enables them to gain much more direct access to nutrients in the waste.

Spent waste, the leftover waste depleted of nutrients that accumulates in bioreactors, processed under steady-state farming conditions, can be amended directly into soil upon removing it from bioreactors. Its NPK composition is typically in the range of approximately 2.7:0.4:1.5, a nutrient range when added to soil that is ideal for growing many agricultural crops of commercial value.

So how does steady-state farming differ from batch farming?

First, under batch farming conditions waste added to a bioreactor (bin or trough) gets seeded with larval starter trays prepared with specialty waste formulations seeded with egg clutches collected from egg clutch units built and managed for this dedicated task (see Black Soldier Fly Biowaste Processing - A Step-by-Step Guide). This method of collecting and seeding eggs into starter trays loaded with varying egg hatching formulations is labor intensive, and costly, involving a number of steps in the farming operation requiring human intervention and oversight bypassed under steady-state farming conditions.

Bioreactors infested with a new batch of larvae under batch farming conditions, hatched from the starter trays, typically get set aside for a period of a few weeks to provide time for the larvae to grow off the waste before the larvae have reached a size where they can be harvested. Larvae are then harvested by sifting the waste through wire screens, an onerous, labor intensive and difficult task not very easy to execute efficiently, especially when on trying to separate larvae from food scrap waste that is very wet and/or dense, a non-issue relative to steady-state farming technologies which rely on larvae self-harvesting free of the waste on which they are growing and therefore require no sieving or screening of waste.

Bioreactors used in batch farming technologies must also be emptied of spent waste, cleaned and readied for a new load of waste before they can be used in another batch run. This task is disruptive in terms of achieving a smooth and consistent output of larvae. A new colony of larvae must be grown from a new set of egg clutches to maturity, cycle after cycle, in sequence, in accommodating each larval harvesting event. To compensate for this disruption, entrepreneurs raising larvae under batch farming conditions usually stagger the loading of bioreactors with new larvae and waste to smooth out larval output, adding yet another layer of complexity to the overall farming operation.  From an operational perspective, batch farming of BSF larvae can therefore be summed up as a “stop and go” technology which differs in many ways from the alternative much simpler, more flexible, uninterrupted method of farming larvae off waste based on steady-state technology.

Aside from the work involved in propagating and maintaining a larval colony, consider just the differences in operations involved in the harvesting of larvae.  Rather than sieving larvae from waste, with all of the attendant problems this entails, under steady-state farming conditions larvae are simply collected on spontaneously exiting from the waste in larval containment channels (see Farming Black Soldier Flies (BSF) | Managing Larval Migration). This principal difference between the two farming technologies eliminates sieving operations all together, saving time, material costs and labor in managing a farming facility.

Collecting larvae exiting bioreactors in larval channel guides has the added advantage that it is also no longer necessary to empty or clean out the bioreactors of waste and young larvae in maintaining a steady output of larvae operating under steady-state conditions. On the contrary, fresh incoming waste can be continually loaded into the bioreactors in maintaining an active larval colony indefinitely so long as conditions are maintained that are conducive to the growth and maintenance of the colony.

Under steady-state farming conditions, rather than larvae growing in batch cycles, larvae grow as a heterogeneous mix, young and old, all together, feeding off waste made available to them. Mating female adults pile fresh egg clutch deposits one on top another in and proximal to waste they are drawn to so long as environmental conditions are maintained in encouraging mating activity, and so long as there is waste having nutrient value on which their newborn larvae can thrive (see, for example, BSF Egg Clutch Deposits In and About Propagation Bioreactors in a Propagation Workstation). A semi- equilibrium balance between input and output of larvae sets up which can best be described as a steady-state condition with regard to the input of waste feedstock, newborn larvae growing off the waste feedstock fed to them, and the exit (self-harvesting) of larvae out of the waste feedstock.

Schematic layout design of BR1 and BR2 BSF larval units
Fig. 1. Cross-sectional view of a “Box-in-Box” layout of a two stage BSF larval bioreactor system used in steady-state farming of larvae. Larval containment barriers can be fabricated out of concrete or cut from exterior polyvinyl molding planks secured a plant facility’s floor with anchoring bolts. Incoming waste is added to the secondary bioreactor (BR2). Larvae self-harvest on passing into the primary bioreactor (BR1) and crawling up and over the larval containment barrier forming the walls of the BR1 unit. Variations of this design can be built at or above grade. Copyright © 2018, Terry Green, All rights reserved.

Fig. 1 is a cross-sectional schematic illustrating the general layout of a PBR (BR2) unit nested atop the base of a BR1 unit used in establishing a steady-state output of larvae growing off food scrap waste. The holes in the BR2 unit in this arrangement improve drainage, aeration and overall management of decaying food scrap waste added to units. The holes also provide a route for young larvae, and prepupae, to freely move in and out of the units in migrating about as they seek suitable environmental conditions for optimal uninterrupted growth, and through which they exit from the waste on reaching the prepupa stage in their life cycle.

In addition, adding a shallow layer of bulking agent into the base of the BR1 unit on which PBRs (BR2s) nest (Fig. 1) improves drainage of leachate processed by larvae. Processed leachate, a byproduct having significant agricultural applications (see, for example, On the Economics of Farming BSF | Thinking Outside the Box, Black Soldier Flies & Recycling | Keeping Organic Leachates in Perspective, Black Soldier Flies & Food Scrap |Putting the Leachate to Good Use, Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil, and Black Soldier Fly Processed Food Scrap | Foliant and Soil Applications), can be collected on flowing out of the central drain line of the BR1 unit.

Suitable bulking agents are principally cellulosic or lingo-cellulosic in composition, for example, shredded wood chips, coarse dried leaves, small stems and twigs passed through a shredder, coarse dry grass, shredded straw, etc. (Fig. 2). Mixing bulking agents with decaying food scrap housed in the bioreactors markedly reduces the presence of obnoxious odors emanating from the waste, possibly through establishment of a biofilter system of sufficient efficiency under these conditions to scrub gases circulating through the bulking agents of noxious odors (see, for example, Development of a Low Cost Biofilter  on Swine Production Facilities).

Image showing addition of straw bulking agent inside larval PBR
Fig. 2. An example showing the layering of a suitable bulking agent (shredded straw) atop food scrap waste housed inside a PBR (BR2) bioreactor (Upper image), and the appearance of the waste in the bioreactor after mixing the bulking agent into the waste on which larvae feed and grow (Lower image).  Copyright © 2018, Terry Green, All rights reserved.

Layering incoming food scrap waste in the units (Fig. 3) on a regular loading schedule draws mating females to the units and markedly enhances the quantity of new egg clutches laid in and around the units. Waste in the units should be aerated just before layering fresh waste into the units (see Propagating Black Soldier Fly Larvae - Mixing & Aerating Food Scrap). This alleviates compaction of the waste in making it much easier for larvae growing in the waste to burrow through and feed off the waste housed in the bioreactors.

Image showing appearance of food scrap layered on waste inside larval PBR
Fig. 3. The appearance of food scrap waste layered atop BSF larval infested waste housed inside a BR2 unit. The layering of fresh waste, whether seeded with young larvae, or not, atop larval infested waste attracts mating females, markedly increasing the deposition of egg clutches in and near the decaying waste. Copyright © 2018, Terry Green, All rights reserved.

Incoming loading rates should not exceed the turnover rate of the waste added to the bioreactors to avoid excess buildup of unprocessed waste. In a properly managed bioreactor residual waste previously loaded into bioreactors should have the appearance of almost finished compost (Fig. 4). A good rule of thumb in a well-managed bioreactor is to plan on adding on average around 5 Kg wet weight of fresh incoming food scrap waste per square meter of bioreactor surface area per day. This loading rate can be adjusted upwards or downward depending upon the composition and turnover rate of the waste processed (see Scaling Up BSF Production | Theoretical and Practical Effect of BSF Bin Space Surface Area and Food Scrap Load Rate on Larval Yield). Loading cycles can also be expanded from once a day to longer intervals of once every second or even third day without any negative consequences.

Image showing appearance of residual food scrap and larvae in PBR before layering of input of more food scrap to PBR
Fig. 4. The appearance of food scrap waste processed by BSF larvae under steady-state farming conditions before layering  fresh a load of incoming food scrap waste inside a BR2 unit. Copyright © 2018, Terry Green, All rights reserved.

Depending upon the depth of bulking agent added to the base of the BR1 unit, it is furthermore possible to have prepupae exiting BR2s to pupate in the base of the BR1 unit on which the BR2s nest without having exited the BR1 units. If a BR1 housed in the Propagation Workstation drains well, for example, and bulking agents are maintained at a level where prepupae exiting the PBRs (BR2s) are able to find dry places to pupate, many prepupae exiting the PBRs (BR2s) will pupate in situ around the base of the BR1 unit, emerge from their puparia as adults, fly directly out of the base of the BR1 unit, then mate and repopulate the PBR (BR2) units with new egg clutches in a round-robin manner.

Fig. 5 (left image) shows how pupae and empty puparia shells appear left behind at the base of BR1 units in a Propagation Workstation with bulking agents filling the base of the unit. Compare this image with the appearance of prepupae that have self-harvested from the BR1 unit (Fig. 5, right image). Immature larvae can also be found in wetter sections of the BR1 unit, especially beneath PBRs (BR2s) (Fig. 6).

Images of pupa and prepupa including puparia inside base of BR1 unit and washed prepupae harvested free of unit
Fig 5.  Left image, BSF pupae and puparia (white arrows) can be seen accumulating at the base of a BR1 unit having bulking agents added at its base.  Right image, washed prepupae having self-harvested free of food scrap waste on which they have grown after crawling up and over the larval containment barrier of a BR1 unit.

Image of BSF larvae feeding in base of BR1 unit beneath PBR
Fig. 6. Actively growing BSF larvae, having fed on food scrap waste housed inside a Propagation Bioreactor (PBR/BR2), processing nutrients that have passed into the base of the BR1 unit on which the PBR/BR2 nests. Copyright © 2018, Terry Green, All right reserved.

Aside from managing the harvest of prepupae, and the overall operations of the larval colony, two other elements of the farming operation must be addressed, namely spent waste and BSF processed leachate.  The harvesting of leachate is easily addressed through incorporation of a central drain line at the base of the BR1 unit (Fig. 1). Leachate passing into the drain line flows to a receiving reservoir set below grade, and gets pumped into storage reservoirs placed at grade such as IBC tanks using either a bilge or sump pump housed in the below grade reservoir holding tank.

The volume of processed leachate recovered will vary widely depending upon the water content of the food scrap waste, the type of food scrap waste added to bioreactors, the operating temperatures in the plant facility, the efficiency of the drainage system capturing this byproduct, the experience of the operator managing the plant facility, the layout design of the bioreactors, and many other variables. Pilot plant field tests under varying operating conditions and with different types of food scrap waste  indicate that once can expect recovering anywhere from about 50 to about 200 L leachate per metric ton wet weight food scrap waste processed through a well-managed farming facility. This volume can increased somewhat if a plant facility chooses to also harvest processed leachate carryover recovered in the spent waste, and that carried over with larvae self-harvesting fee of the waste (Fig. 7).

Image showing appearance of BSF processed food scrap leachate recovered from washings of spent waste and larvae
Fig. 7.  BSF processed leachate, a byproduct in farming BSF larvae off food scrap waste, recovered under steady-state larval farming conditions obtained by combining washings of self-harvesting larvae and spent waste. Copyright © 2018, Terry Green, All rights reserved.

Managing spent waste under steady-state conditions requires a more nuanced strategy relative to the collection of BSF processed leachate. Without a strategy in place for removing spent waste, a farming operation cannot be sustained. Spent waste must be removed without interrupting the input of fresh waste in supporting and maintaining the larval colony on an ongoing basis.

In an eight month field tests set up to evaluate how best to process spent waste, the efficiency of handling spent waste was tested while farming larvae under BR1/BR2 steady-state conditions. Daily larval output held constant without any falloff in larval production levels day in and day out throughout the trial period. The loading rate of incoming fresh food scrap waste into BR2 units was kept constant at an average 5 Kg (wet weight) per square meter bioreactor footprint space per day.  Food scrap was layered into the BR2s on alternate days throughout the trial study.

To address the accumulation of spent waste in the BR2 units, portions of the waste were transferred to separate BR2 units dedicated to processing only spent waste. No fresh waste was added to the latter units set aside for this purpose.

Spent waste left behind in the dedicated BR2 units on a wet weight basis averaged 10 % of the input wet weight of fresh food scrap passing through the BR2 units over the eight month study. None of the BR2s were emptied of all waste over the duration of the test study. Mating females continuously seeded the waste loaded into the BR2 units with new egg clutches producing and sustaining as a result of their activities a dense heterogeneous population of larvae growing and feeding off the waste throughout the eight period.

Spent waste held for about three weeks in the dedicated BR2 units set aside for this purpose took on the appearance of finished compost (Fig. 8). It was devoid of offensive odors. Most larvae entrapped in the waste at the time of its transfer to the dedicated BR2 units set aside for processing the spent waste had exited the waste retained in the BR2 units during the three week holding interval and were observed migrating on into the BR1 unit on which the dedicated units nested.

Image showing appearance of larval processed spent food scrap waste recovered from BR2 units
Fig. 8. The appearance of food scrap spent waste removed from BR2 units managed using BSF larval steady-state farming technology. Copyright © 2018, Terry Green, All rights reserved.

The spent waste recovered in this manner is principally made up of residual bulking agents in the form of cellulose and lignocelluloses fibers, polyphenolic and humic acid-like byproducts, chitinous degradation byproducts left over from larvae that have shed their exoskeletons in passing through the varying growth stages of their life-cycle, soil microbes, and mineralized nutrients (NPK).

While there is still much to be learned on how best to farm larvae efficiently and economically, based on reviewing the basic elements and strategies of farming larvae off food scrap waste as described in this blog under steady-state farming conditions, I believe there is reason to be optimistic that with more insights and further development of potential markets for both larvae and the byproducts created in farming larvae, that this technology can become economically viable.

Lastly, getting back to the opening question posed in this blog, namely what’s the right number of eggs or larvae to be a farming facility, the simple answer is that there is no clear cut answer given the number of variables that weigh in on what constitutes an optimal output of larvae in a farming operation. What matters is optimizing the layout of the facility with the goal of optimizing and maintaining conditions conducive to the growth and harvest of larvae with special attention on keeping labor and operating costs as simple and low as possible. Attend to the biological needs of the larvae and build up the input of waste in supporting larval growth with the goal of attaining a balance between input and turnover of spent waste. With these goals in mind, leave the rest to the professionals (you know – the BSF!). Take care of the larvae. They’ll in turn take care of your farming operations.

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