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Bulking Agents Mixed with Food Scrap | Effect on BSF Larval Propagation, Growth and Harvest

by Terry Green on 04/20/18

Propagating, growing and harvesting BSF off food scrap waste can be challenging given its complex composition and physical characteristics. Its moisture content can vary depending on the nature of the waste, generally from about 60 to 90 %, and on average around  70% (see, for example, Profiles in Garbage: Food Waste and Physical and Chemical Analysis of Canteen Wastes for Syngas Production). When compacted, its density is approximately that of water (see Waste Materials – Density Data). It is however rich in nutrients beneficial to the growth of larvae (see Black Soldier Fly Processing of Biodegradable Wastes). These attributes of food scrap fed larvae left to grow on their own without aid of bulking agents can lead to the formation of a dense semi-anaerobic sludge impenetrable to larvae, rich in nutrients but lacking an adequate oxygen supply for larvae to respire while feeding off the waste, and formation of excess carbon dioxide and putrid volatile organic acid byproducts, all detrimental to the survival and robust growth and harvest of larvae (see Farming Black Soldier Fly (BSF) Larvae | Managing Feedstock and Avoiding Colony Collapse). This blog reviews some important aspects regarding how bulking agents added to food scrap enhance larval propagation, growth and harvests.

Mixing bulking agents with food scrap waste in appropriate amounts, and managing spent waste formed as larvae grow off the food scrap under steady-state conditions (see Steady-State Farming of BSF Larvae) is extremely critical in sustaining larval harvest yields. Larvae must be able to freely burrow through waste fed to them. Mixing food scrap waste with bulking agents reduces the density of the food scrap making it much easier for larvae to crawl and navigate through the waste. The mixing of bulking agents with food scrap waste also creates voids and channels which provides for more efficient off gassing of carbon dioxide, and permeation of oxygen throughout the food scrap required by respiring larvae feeding and growing off the waste.

Aeration of food scrap additionally suppresses the growth of anaerobic microbes producing putrid volatile organic acids. Without adequate aeration, acidification of the waste occurs. This over time can lead to a sharp die off of larvae as the pH of the waste begins to drop below about 5.5.

On the other hand, too much aeration can cause food scrap waste to overheat, stressing larvae and driving them out of the waste on which they are feeding. Food scrap waste mixed with bulking agents left unchecked can easily reach temperatures of 50+ C as aerobic microorganisms set up and compete with larvae feeding off the waste.

Food scrap waste accordingly should not be allowed to overheat. A balance must be maintained in providing aeration while keeping the temperature of the waste on which the larvae are feeding and growing in check. If necessary, water can be added to waste beginning to overheat.

Operating temperatures in the maintenance of waste fed larvae should not be allowed to exceed approximately 45 C. Water has the effect of cutting off the supply of oxygen available to aerobic microorganisms, slowing down oxidative activity (e.g., combustion of the waste) which accounts for heat generated in the waste.

Long term pilot plant tests (conducted over several months) on the growth and production of viable colonies of BSF larvae demonstrate that larvae grow most vigorously and mature into viable prepupae capable of passing on into the pupa stage of their life cycle, and emerge as fertile adults, on food scrap waste maintained in a temperature range between about 35 to 40 C.   Although larvae also grow well at somewhat lower temperatures in the range of 25 to 30 C, their growth, burrowing and feeding activity on the waste is less robust than in waste maintained in the range of 35+ C. Below about 20 C larvae become immobile and stop feeding altogether on food scrap.

The mixing of bulking agents into food scrap waste may have some effect on inducing mating females to oviposit their egg clutches around the walls and upper perimeter of larval bioreactors (see Propagating BSF Using “Box in a Box” Propagation Bioreactors). This could be linked with a higher flux in off gassing of volatile odors driven off from food scrap (which attract female BSF seeking a place to deposit their egg clutches) caused by convection of air circulating through the waste as the temperature of the waste rises.

Whereas the association of mixing bulking agents with food scrap and increased ovipositing of egg clutches by mating females in propagation bioreactors (PBRs – see Propagating BSF Using “Box in a Box” Propagation Bioreactors) has not yet been definitively established because of technical complexities in carrying out these experiments while ruling out other factors, the emergence of young larvae from egg clutches deposited on and about larval bioreactors, the growth of larvae feeding on the waste, and the self-harvesting of larvae reaching the prepupae stage in their life-cycle, all are markedly accelerated and measureable in relation to the operating temperature of the waste managed inside a larval bioreactor.

The accelerated rate of emergence of larvae from egg clutches deposited by female BSF around the perimeter lids of PBRs in PBRs containing food scrap waste mixed with bulking agents may be a direct result of increased rates of water evaporation given up from waste as the temperature of the waste rises. Water escaping into the gas phase tends to condense on the underside of the lids of the PBRs. This extra water condensing on the undersides of the lids, and the subsequent high humidity created proximal to egg clutches deposited on the undersides of the PBR lids under  these conditions, may account for better viability of eggs left by females by countering dessication of egg clutches known to reduce egg viability.

Bulking agents that work best in facilitating larval propagation, growth and harvest yields are those rich in cellulosic and lignocellulosic content. Larvae lack the biochemical capacity to breakdown cellulosic and lignocellulosic matter allowing such bulking agents to serve as aerating agents over a relatively extended period. Using cellulosic and lignocellulosic bulking agents furthermore renders the spent waste drawn from larval bioreactors more valuable because the same bulking agents, recycled back into soil, add to soil structure and its capacity to retain water and nutrients.

Plastic or paper bulking agents should not be substituted in place of cellulosic and lignocellulosic agents since the latter are often contaminated with inks, specialty clays, plastic filler materials and plasticizers which do not readily degrade on recycling back into soils. Moreover, there is sufficient evidence indicating the deleterious effects of cycling plastics back into the natural environment which should on its own rule out substituting the latter agents in place of cellulosic and lignocellulosic bulking agents (see, for example,  The Environmental Toll of Plastics and Pollution and Hazards from Manufacturing).

Suitable bulking agents that are in abundance in the natural environment include dried grasses, leaves, straw shredded for mixing with food scrap, wood yard debris shredded and sized for mixing with food scrap waste, etc. Figs. 1 and 2 show examples of shredded straw and woody yard debris passed through a shredder both of which work very well as bulking agents. These materials can often be obtained at very little cost, and many instances at no cost other than the cost of shredding the debris in a form suitable for mixing with food scrap. Many landscaping firms routinely chip and shred broken and damaged tree limbs and generate a very large and renewable supply of shredded debris which can often be obtained at no cost.

Images contrasting shredded straw with shredded wood chip bulking agents
Fig. 1. Contrasting characteristics of shredded straw and wood chip bulking agents. left image, shredded straw. Note diameter of straw strands relative to the size of a US quarter; right image, shredded wood – note larger wood chip fragments relative to the size of a US quarter. Copyright© 2018 Terry Green, All rights reserved.

The size and physical properties of the cellulosic bulking agent mixed with food scrap waste, itself, can have subtle effects on the aeration of the waste and corresponding heat generated in the waste which as noted earlier affects the emergence rate of larvae hatching from egg clutches, their subsequent growth rate while feeding off of food scrap waste, and harvest yields. Mixing shredded straw into food scrap waste relative to the same proportion of shredded wood chips, for example, increases the temperature of food scrap waste housed in a BR2 by approximately 5 to 10 C above ambient temperature versus approximately 7 to 15 C, respectively.

Although this difference of about 5 C may not seem very significant, this difference in the temperature of food scrap waste housed in a bioreactor markedly increases observable larval feeding activity and hatching activity of young larvae emerging from egg clutches.

Fig. 2 (left image) shows larval feeding and burrowing activity typically seen in food scrap waste housed inside a PBR approximately 10 seconds after removal of the PBRs lid. In this example the larvae can be seen crawling all over the insides of the unit and up around the walls.  Approximately 3 minutes later with the lid removed and light impinging inside the unit most of the larvae (see Fig. 2, right image) have burrowed down under the upper layer of bulking agent and small amount of food scrap waste and residual insect frass remaining as expected in a healthy robust population of larvae exhibiting photophobic behavior.

Image of BSF larvae growing in a PBR in food scrap mixed with wood chips
Fig. 2. BSF larvae growing, feeding and burrowing in food scrap waste mixed with shredded wood chip bulking agent. left image, many robust larvae actively burrowing and feeding throughout waste housed in a PBR seen within 10 seconds of removing the PBR lid from unit;  right image, view of same unit approximately 3 minutes later. Larvae actively crawling all about the food scrap waste reversed direction and burrowed into waste with admission of light into PBR unit. Ambient temperature, 25 C; temperature of the food scrap waste, 41 C. Copyright © 2018 Terry Green, All rights reserved.

The precise amount of bulking agent mixed with food scrap waste inside a larval bioreactor (PBR/BR2) must be determined empirically relative to the amount and physical attributes of food scrap waste fed larvae. Generally a good starting point in finding an optimal ratio is to start by mixing bulking agent with food scrap waste in a ratio (dry weight to wet weight food scrap) of about 1:20. Monitor the temperature of the waste as it undergoes degradation in the bioreactor, and observe the behavioral characteristics of larvae feeding and burrowing through the latter waste, and accordingly make adjustments in the ratio as needed in sustaining larval propagation and growth.

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