BSF Metrics & Yields| Scale Up Production of Black Soldier Fliesby Terry Green on 01/31/14
A major barrier hindering large scale continuous year round production of Black Soldier fly larvae (BSFL) is the absence of practical knowledge on how to design, build and economically operate a BSFL production facility. This blog looks at (i) food scrap loading rates relative to BSFL processing space needed; (ii) cyclic fluctuations and average yields in BSFL production accompanying continuous operation of a BSFL facility; and (iii) the average feed conversion ratio associated with raising BSFL on food scrap waste.
We collected our data from field tests at our pilot BSFL production facility (see Scaling Up Black Soldier Fly Food Scrap Processing | Phase III). Total bin space dedicated to these measurements in our pilot plant is 6 square meters. Here are our findings:
- Propagation (mating, egg laying, and the hatching of new larvae), larval processing of food scrap and harvesting of larvae works best (with but a exceptions) when all operations are done in the same enclosed environment;
- Larval exit ramps are unnecessary (Fig. 1);
- The installment of passive continuously operating drainage systems in bins reduces time and labor in dealing with leachate as larvae feed and grow off of the waste added to bins. The volume of leachate (in L) varies from about 3% to 5% of the initial wet weight of food scrap (in Kg) placed in bins;
- Bins without thermostatic controls run on average about 5 to 10 C above the ambient temperature outside the bins;
- Cyclic fluctuations from day to day in BSFL harvest yields (peaks and troughs) are normal and to be expected (see Fig. 2); and
- There is an optimum food scrap loading rate in bins (~ 5 Kg per day per square meter) which produces the best yield of harvested BSFL while operating bins continuously year round. Loading bins with food scrap in excess of what larvae are able to consume results in accumulation of sludge and a reduction in BSFL yield. Loading bins at too slow of a rate results in slower growth, avoids build up of sludge, but also results in a sharp reduction in the yield of harvested BSFL.
Fig. 1. Image of BSFL prepupae climbing with ease vertically up and over the top walls of bins. Larvae have no difficulty climbing vertically up and over vertical walls as they egress from the bins. Copyright © 2014, Terry Green, All rights reserved.
We consistently and repeatedly observed dynamic oscillating changes in the BSFL population density inside the bins driving cyclic fluctuations in larval egress rates (Fig. 2). Very young larvae (newly hatched from egg clutches) are so small that the density of young larvae per unit volume reaches very high levels (>15 per ml). As the young larvae grow, the volume they occupy per unit volume of bin space increases sharply. With heterogeneous populations of larvae of varying size co-existing in the confined spaces of the food scrap processing bins, population pressures (larval neighbor to neighbor interactions) appear to trigger cyclic intervals of mass egress linked with the growth cycle of the new larvae competing for feed and space with older larvae.
Fig. 2. BSFL harvest yields (Kg per day per square meter) measured on food scrap processing bins beginning at the time the bins received food scrap and an inoculum of BSFL (day 1) and progressing over an approximate four month interval at an optimal average food scrap loading rate of 5 Kg per day per square meter. Temperatures in the bin held relatively constant between 30 and 40 C throughout the interval monitored. Copyright 2014, Terry Green, All rights reserved.
Table 1 summarizes the metrics of BSFL production while holding the food scrap loading rate of bins constant. We calculated the cumulative average yield of BSFL (Kg wet weight per day per square meter bin space) while aiming for a target optimal loading rate of ~ 4.8 Kg food scrap (wet weight) per day per square meter bin space based on prior work designed to find an optimal food scrap load rate. The optimal food scrap loading rate was determined by measuring BSFL yields taking into account loading rates ranging from less than 5 to as much as 30 Kg per day per square meter. At loading rates in excess of ~ 5 Kg per day per square meter we observed that the larvae fell behind in their ability to keep up with the food scrap loaded into the bins. They handle all of the food scrap presented to the bins at loading rates of ~5 Kg per day per square meter or less.
Table 1 provides information that can help those interested in scaling up BSFL production to predict the total footprint size of processing bins and troughs needed to achieve a specific annual yield, and how much food scrap would be needed in meeting a set production rate. Using the FCR in Table 1, for example, it takes on average about 9.35 MT of food scrap (wet weight) to produce 1 MT (wet weight) of harvested BSFL (~ 18.7 MT to produce 1 MT of dried BSFL). To calculate the footprint in bin space needed to produce a target amount of BSFL, simply divide the cumulative average BSFL yield into the target production rate desired. For example, a plant designed to produce 100 MT of BSFL (dry weight) will require 100/0.09, or ~1100 square meters of bin space.
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