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Farming BSF Larvae |How to Build Inexpensive BSF Larval Bioreactors

by Terry Green on 03/18/18

Farming Black Soldier fly (BSF) larvae off food scrap waste requires careful planning in designing and building larval bioreactors which can be operated on installation efficiently and economically (see On the Economics of Farming BSF | Thinking Outside the Box). Bioreactors must be designed for easy setup and scalability. They must be built of inexpensive commonly available materials resistant to corrosion and/or biodegradation. The design and layout must be simple and adaptable for assembly indoors or outdoors depending upon local conditions while keeping an eye on the return on the investment. This blog describes how larval bioreactors can be built at low cost using commonly available materials on flat concrete floors or pads taking into account these designing constraints.

“Box-in-Box” Propagation (PBR) and BR1/BR2 Bioreactors are especially well-suited in achieving efficient uninterrupted steady-state propagation, growth and harvest of larvae grown off food scrap waste (see Propagating BSF Using “Box in a Box” Propagation Bioreactors and Steady-State Farming of BSF Larvae).  Furthermore, a BR1 (the primary bioreactor defining the space where young larvae grow and feed off food scrap waste) can be readily built atop a concrete floor or pad by constructing a square or rectangular perimeter wall fabricated out of polyvinyl chloride (PVC) exterior molding and trim boards (or similarly available corrosion and biodegradable resistant building materials) that defines the footprint space of the unit at a fraction of the cost of fabricating bioreactors out of fiberglass, stainless steel or concrete. An outer larval containment guide and channel can also be placed on a concrete floor or pad parallel to the walls of the BR1 unit in the same manner for collection and harvest of prepupae exiting the BR1 unit (see below, Fig. 1).

PVC and other similar plastic exterior molding and trim boards designed to stand up to the elements outdoors are widely available in the construction industry and can be obtained at low cost in all manner of shapes (see Plastic lumber). These boards can be easily cut, drilled, mitered, glued, caulked and secured in place on concrete floors or pads using anchoring bolts inserted into the concrete base of the floors or pads.  Material costs and workmanship skill levels required in working with these materials are furthermore nominal compared to that involved in fabricating large bioreactor bins built out of concrete, corrosion resistant stainless steel, or fiberglass.

Figs. 1-6 provide composite cross-sectional cutaway and top-down illustrations of the layout and design of an operating BR1/BR2 unit constructed in this manner built with PVC molding boards secured atop a concrete pad. The depth of the BR1 unit walls does not need to be very deep, typically no more than about 1.5 to 2.5 inches (3.8 to 6.35 cm). A bead of high quality silicone caulk (or other comparable water impermeable polymer capable of forming a water impenetrable barrier) can be laid down against the inside walls of the BR1 unit and at the base seam of the larval containment channel walls where the walls butt against the concrete base to contain water and food scrap leachate inside the BR1 unit, and newborn larvae, from exiting the unit where the walls butt against the concrete.

The perimeter wall of the BR1 unit, and its accompanying larval containment channel and wall is illustrated in a cross-sectional cutaway in the upper drawing, and as a top-down view in the lower drawing (see Fig. 1).
Schematic image of BR1 unit assembled on concrete floor or pad from PVC boards 
Fig. 1. Layout of PVC walls and assembly of operating BR1 unit built atop a concrete floor or pad. Upper drawing, cross-sectional cutaway view of BR1 unit showing unit base and walls fabricated out of PVC exterior boards secured to concrete floor, concrete backstop mixing blocks centered over central drain line, and stacked BR2 units nested inside BR1 base; Lower drawing, top-down view of BR1 unit assembled on concrete floor with concrete backstop mixing blocks centered over central drain line and PVC inner and outer walls anchored on concrete surface creating a channel for collection of prepupae (stacked BR2 bins not shown). Copyright © 2018 Terry Green, All rights reserved.
Image of plastic anchor bolts inserted into concrete floor or pad
Fig. 2. Image showing an example of how ordinary plastic anchors inserted into holes drilled into concrete floor or pad can be used to secure PVC exterior boards and trim flush against concrete in creating and defining the walls of BR1 unit. Screws can be driven through the PVC boards into the anchors aligned where walls are to be installed. Copyright© 2018 Terry Green, All rights reserved.
Image showing assembly of PVC board on base concrete floor or pad
Fig. 3. PVC boards can be anchored firmly against concrete by tightening screws driven through PVC exterior boards on into plastic anchors inserted into predrilled holes aligned along a concrete floor or pad defining the perimeter of the BR1 unit. Copyright © 2018 Terry Green, All rights reserved.
Image showing method of overlapping PVC boards secured to concrete floor or pad
Fig. 4. PVC exterior boards cut and secured to a concrete floor or pad can be layered atop one another and fixed in place with screws in building the depth of wall defining the perimeter of a BR1 unit. Copyright© 2018 Terry Green, All rights reserved.
Image showing how to increase depth of BR1 wall by stacking PVC on concrete floor or pad
Fig. 5. Image showing mating of PVC exterior boards in building up walls in the assembly of a BR1 unit. Upon completion of the wall, the interior seams between the concrete base and PVC board mating with the concrete can be sealed with a bead of high quality silicone (or other suitable water impenetrable polymer) to ensure that the interior base remains water tight. Copyright 2018© Terry Green, All rights reserved.
Image showing attachment of larval containment barrier on PVC wall in creating channel guide
Fig. 6. Image of a larval containment barrier projecting inward toward the larval channel where larvae exiting the BR1 unit are to be harvested attached with screws to the top of a PVC wall. The containment barrier blocks larvae from climbing over the wall. Copyright © 2018 Terry Green, All rights reserved.

In Fig. 1, the drawing of the BR1 unit in the upper schematic shows the presence of a main drain line spanning the long axis of the unit set below grade which serves as a means of draining and collecting food scrap leachate processed by larvae feeding on waste inside the bioreactor (see Propagating BSF Using “Box in a Box” Propagation Bioreactors and Steady-State Farming of BSF Larvae). Prepupae exit the BR1 unit by simply climbing over the first wall that defines the base of the BR1 unit. They are unable to climb over the second wall running parallel with the first wall around the BR1 unit due to the presence of a larval containment collar secured on top of the second wall and become trapped on exiting the BR1 unit in the larval containment channel pending harvest from this site (see Farming Black Soldier Flies (BSF) | Managing Larval Migration).

Concrete barrier blocks can also be installed along the center section of the unit atop the drain line channel spanning the center of the unit along its long axis as illustrated in Fig. 1. These concrete blocks can be obtained at low cost from concrete manufacturers. They serve as backstops in mixing food scrap and larvae added to the unit using a forklift shovel adapter or front loader to pick up and turnover the waste and larvae as a means of aerating and managing food scrap and larvae processed in the unit.

If stacking bins (BR2 units) are nested inside the BR1 to increase the footprint of the operating unit, their larval-food scrap content can also be dumped back into the base of the BR1 unit and remixed in situ with incoming food scrap waste using the concrete blocks as a backstop Mixing Workstation (see Scaling Up BSF Production| Integration of BSF Workstation Elements).

Although the placement of the concrete blocks inside the BR1 unit is not a necessary element of a working BR1 unit, the schematic in Fig. 1 illustrates the flexibility of adapting assembly of the BR1 unit using PVC molding boards in defining the perimeter of the BR1 unit in meeting varying space limitations in the layout of an operating plant facility. In this particular instance, rather than building a separate Mixing Workstation separate from the BR1 unit, Fig. 1 shows how the two operations can be merged together in a single unit in setting up a farming facility.

It is possible to also manage a BR1 unit without inclusion of a central drain line in the unit. In this situation more bulking agents must be added to the base of the BR1 unit to help aerate food scrap waste added to the unit. Larval processed food scrap leachate in this instance concentrates on the bulking agents and spent residues left behind as water evaporates into the air rather than draining free from the unit.

The bulking agent and spent waste remaining at the base of the BR1 unit should be removed and replaced with fresh bulking agent on a regular basis to avoid building up too much salinity in residues residing at the base of the BR1 unit as water vaporizes free of the unit’s base.

Whether a drain line is incorporated into the base of the BR1 unit, or omitted, strong and obnoxious odors including excess ammonia accumulation are generally not a problem in a well-managed operating unit. This is likely due to the presence of microbial colonies growing on the surfaces of the bulking agents mixed into the unit scrubbing much of the volatiles and ammonia byproducts from the waste and frass as occurs in biofiltration systems designed for scrubbing volatile odors free from polluted air (see, for example, Development of a Low Cost Biofilter  on Swine Production Facilities and Biofilters in Animal Agriculture).

The versatility and flexibility in using exterior molding and trim boards fabricated out of PVC or other recycled plastics in defining the perimeter of a BR1 unit laid out on a concrete floor or pad makes it much easier to scale up a farming operation. Boards can be easily laid out and configured on concrete in targeting a specific output of prepupae based upon the footprint and logistical metrics in farming larvae off food scrap waste (see Farming BSF on a Commercial Scale | Calculating BSF Larval Outputs, Waste Inputs and Larval Replacement Requirements). The choice of BR2 bins can also be factored into the layout and projected metrics in assembling BR1 units by taking into account the width and length of the BR2 bins.

For example, a 8 m x 2 m BR1 unit set up on a concrete floor or pad based on logistical calculations derived from prior metrics in farming larvae off food scrap should be of sufficient size to process about 29 MT of food scrap per year and produce approximately 2.9 MT prepupae (wet weight) per year. If perforated stacking bins (BR2s) having outside dimensions in meters of 0.8 (L) x 0.6 (W) x 0.235 (H) are nested inside the BR1 unit in two rows of ten columns, and in stacks of ten bins per column (total column height, approximately 2.4 meters), the processing capacity of the BR1 unit can be increased to handle approximately 180 MT food scrap per year with a corresponding increase in projected annual yield of prepupae of approximately 18 MT (wet weight).

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