The Life and Times of BSF (Black Soldier Flies)
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Farming BSF on a Commercial Scale | Calculating BSF Larval Outputs, Waste Inputs and Larval Replacement Requirements

by Terry Green on 11/18/16

In a series of earlier posts on this website I described varying aspects relating to the management of input waste, the output of spent waste, propagation and seeding steps in replacing harvested larvae, and the general layout and processing of bioreactors stacked vertically atop one another as a means of reducing the footprint of a modular Black Soldier Fly (BSF) plant facility (see, for example, Modular Farming of Black Soldier Fly (BSF) | Scaling Up - Schematic Overview, Commercial Black Soldier Fly (BSF) Production in 2016 | Where Are We Today?, Farming Black Soldier Fly (BSF) Larvae | Managing Feedstock and Avoiding Colony Collapse, and Farming Black Soldier Flies (BSF) | Scaling Up & Sustaining Larval Production). This blog goes into more detail on how to calculate the logistical requirements in meeting larval production goals, namely the footprint in bioreactor space required to achieve a planned larval yield, how much input waste must be supplied in supporting a steady output of larvae, and BSF mating and propagation levels needed for seeding waste with new larvae in replacing those harvested, all in keeping the operation running on a sustained basis.

Larvae grown off biodegradable wastes can be efficiently grown in bioreactors (BR), trays, bins or troughs, specifically constructed or adapted for this purpose.  On reaching the prepupae stage in their life cycle, BSF larvae grown in this manner spontaneously self-harvest from the waste which greatly simplifies this aspect of the farming operation (Fig. 1). Calculations and comments that follow are based on farming BSF along the lines described in an earlier blog outlining a modular method for farming BSF based upon integrating two types of bioreactors into the larval processing train, a primary bioreactor (BR1) built at ground level on a concrete pad atop which are nestled and stacked secondary vented commercial agricultural bins (BR2s) (see Modular Farming of Black Soldier Fly (BSF) | Scaling Up - Schematic Overview).
Image of self-harvesting Black Soldier Fly prepupae
Fig. 1. Image of harvested BSF prepupae having self-harvested free of food scrap (copyright© 2016, Terry Green, All rights reserved).

While there are many variables involved in growing larvae, there are specific variables which define production levels. The source of the waste and manner by which it is presented to larvae, its nutrient value as a feedstock on which the larvae are grown (particularly, the feed conversion ratio), its average wet weight, and the footprint space dedicated for growing larvae off the waste, all in combination with one another affect the output of larvae, spent waste and byproducts managed in a plant facility.

Other variables affecting larval output include the average temperature, humidity, the frequency of adding and removing waste from bioreactors, drainage and aeration of the waste while larvae are growing off of it, the population of adults raised from prepupae set aside and dedicated to mating and propagation of new larvae in replacing those harvested, and the efficiency of seeding new larval offspring back into waste added to the bioreactors.

Image of BSF Table 1 showing output variables

Table 1 is an example of how some of these key variables can be used to project a priori the logistics of a scaled up farming operation. Some of these variables, for example the waste to prepupae conversion ratio, and the egg viability (percent BSF eggs hatching and growing into PP), must be obtained either empirically (using data, for example, collected on growing larvae on a specific waste source in a pilot plant setting), or, in the absence of experimental data, estimated.

Table 2 summarizes calculated outcomes made by constructing a spreadsheet using the nominal values for variables listed in Table 1 on input and outputs required in managing, in this example, a 5 x 20 ft BR1 bioreactor atop which are stacked five columns of BR2 bioreactors, eight bins per column. The calculations are based upon using pre- and post-consumer food scrap waste having an average moisture content of 80%, and on operating the bioreactors and Propagation and Larval Seeding work station in the processing train at an average temperature of ~ 27 C (~80 F) and relative humidity of +60%.

It should be noted in Table 2 that the calculation of the "number of viable females required per d = 25" represents an overly optimistic assumption that 100% of female adult BSF mate, each laying on average 500 eggs, and assuming that all eggs hatch and new larvae grow and self-harvest as prepupae. Not all female adults successfully mate, and not all eggs laid by mating females hatch successfully. Some eggs are lost to dessication, some to microbes and predators, and for a variety of reasons the success rate in reaching the prepupae stage is significantly less. To correct for this problem, the "number of viable matings required per d = 494" factors in a 90% loss of viable eggs and includes the obvious requirement that an equivalent number of male adults must also be produced in affecting successful mating giving a more realistic projection of replacement needs in sustaining prepupae harvested from the bioreactors. These adjustments in the calculation aim to approximate realistic conditions but ultimately require validation. Nevertheless, this latter calculation is useful principally for planning purposes in scaling up an operation in mapping out the logistical needs in sustaining the farming operation in replacing prepupae drawn from the bioreactors.

Image of Table 2 showing BSF larval outputs
These variables are interdependent upon one another. Changing the value of any one affects the outcome. Consequently, projections in scaling up a plant facility can be framed in varying ways with a spreadsheet depending upon what needs to be calculated. One might, for example, want to ask how much food scrap waste would need to be delivered on a daily basis to a scaled up bioreactor having a fixed footprint (specified in the spreadsheet) from which one could then calculate the amount of daily input waste required in operating the plant facility at capacity, and from which the corresponding daily larval output and propagation requirements needed in maintaining the plant would likewise be calculated. Alternatively, given a specific daily mass input of waste, one might be interested in predicting how much of a bioreactor footprint would need to be built into the plant facility from which one would be able to obtain the daily larval output, and mass of prepupae needed for propagation purposes in keeping the operation running on a sustained basis.

Setting up a spreadsheet in general form to calculate inputs and outputs is relatively straightforward. The formulas for each variable, for example, in calculating inputs and outputs for a specific bioreactor footprint incorporating the BR1/BR2 layout scheme are as follows:

Bioreactor Footprint & Prepupa Yield:
Footprint (total)* = (BR1 L)/ (BR2 L) x (BR2 WxL) x (the number BR2 bins stacked in columns atop BR1)
*Note that the width of BR2 defines the width of BR1 which must be as wide, or slightly wider, than BR2 so that BR2’s can nestle atop BR1. The length of BR1 must accommodate all BR2’s aligned in a single row and can be lengthened in increments corresponding to the length of each BR2 positioned side by side atop BR1.

Prepupa Yield (MT/Yr)** = (PP, Kg-ww/sq meter-day) x (Total Footprint in square meters) x 0.360
**assuming  an average 360 day operation per year, converting Kg into metric tons (MT), and expressing the total footprint in metric units.

Food Scrap (FS) Waste Requirement for Growing Larvae in Bioreactors:

FS (MT/Yr) = (conversion ratio, FS/PP) x BSF Yield (MT/Yr)

Prepupae (PP) & Food Scrap Needed to Sustain PP Output:

Number PP removed/d = (PP harvested, Kg-ww/d) x (average number PP/Kg-ww)*
*The average number of PP (ww)/Kg is ~500/Kg.

Number viable female adults mating/d = (Number PP removed/d)/(average number eggs laid/female)

Total Number Viable Matings = 2 x (Number viable female adults mating/d) x (egg viability)**
**egg viability is defined as the percent BSF eggs hatching and newborn larvae growing into prepupae off waste added to BR2 bins. The assigned value for this variable can at best be an educated estimate (reliable data on this variable in not available and in the present example was estimated to be 10% of the total eggs laid by females).

PP Needed for Propagation Purposes (Kg-ww/d) = (Number Viable Matings)/500

Whereas the examples discussed in this blog apply principally to scaling up operations using food scrap waste as larval feedstock, a similar approach in projecting inputs and outputs should be applicable with regard to growing BSF off other biodegradable wastes, for example manure.

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