Initial steps for developing large-scale rearing and processing of black soldier flies, Hermetia illucens L., in a start-up bioconversion facility

Abstract

The aim of this project was to establish large-scale rearing and processing methods for Hermetia illucens L. larvae reared on organic waste streams for use in animal feed. This work builds on existing knowledge of the effects of organic waste streams as larval substrates, and waste mixing to optimise larval development on substrates with variable nutrient content. The experiments were conducted at an industrial scale using thousands of larvae fed many kilograms of feed, rather than at a lab scale (hundreds of larvae fed grams of feed) in a newly established bioconversion facility that was still optimising its processes. As such, the results are more applicable to large-scale, commercial bioconversion operations. The results also provide insights into the processing methods and safety of insects reared on organic waste as animal feed supplements, and provide some guidance on the optimal substrate composition for improved performance of larvae, bioconversion and waste reduction. Following an in depth literature review (Chapter 1), four different experiments were conducted. For all experiments, freshly hatched H. illucens neonates were collected from an established colony held at Aegis Environmental Bioconversion Facility, Centurion, South Africa. Preconsumer fruit and vegetable waste was sourced from a local fresh produce market and poultry manure was sourced from a local chicken layer hen farm. The first three experiments were conducted with 12 kg of waste and 90 mg (approximately 6000) neonates in large plastic meat trays. All trays were held at 28 ± 0.5°C with 65% relative humidity. Measurements of the larvae were taken to determine the development time, size, biomass production, survival, bioconversion, efficiency of conversion of digested feed, and waste reduction, collectively referred to as larval performance. The first experiment (Chapter 2) focused on the nutritional differences between larvae and prepupae (harvested at 15 and 20 days, respectively) that had been fed fruit and vegetable waste. Prepupae had slightly higher fiber content than larvae, and no other nutritional differences were found. Larvae had higher total biomass and higher survival rates. Based on the results of this experiment, larvae should be harvested in preference to prepupae to maximise waste reduction and biomass production. However, harvesting a mixture of larvae and prepupae should not impact the nutritional quality of the final product due to the minimal nutritional differences between the two stages. The second experiment (Chapter 3) investigated the effects of three different drying methods on the nutritional composition and microbiological contamination present in H. illucens larvae. A sample of 280 g of 15-day old larvae, fed on fruit and vegetable waste and euthanized using boiling water, was used. There were no nutritional or microbiological differences between hot air, oven or microwave drying. A cost benefit analysis of each drying method was completed, and it was determined that hot air drying, followed by oven drying were the most cost-effective while microwave drying was expensive and unprofitable. If there is no effect on the nutritional or microbiological content of H. illucens larvae, the best drying method is the one that is most cost effective and time efficient. The third experiment (Chapter 4) investigated the performance of larvae fed varying mixes of fruit and vegetable waste and poultry manure, and it was determined that larvae performed best on fruit and vegetable waste only but tolerated up to 60% fruit and vegetable waste mixed with 40% poultry manure before the performance of the larvae was severely reduced. In terms of using poultry manure as a substrate for H. illucens larvae, it should be added only in low quantities, up to 40% total wet mass of the substrate. The final experiment (Chapter 5) used large, specifically designed metal trays with 60 kg of waste and 450 mg (approximately 30 000) neonates. The effect of poultry manure inclusion, from 5 to 40%, with fruit and vegetable waste was assessed and temperature and pH of the trays were recorded throughout development of the larvae. There was no effect of mixing poultry manure with fruit and vegetable waste at different inclusion percentages on any measure of larval performance. However, temperature and pH of the substrate were key factors in development, survival and waste reduction potential of H. illucens larvae. This study showed that a substrate temperature of 28°C, an initial substrate pH of 6.3, and a final pH of around 9.6 is ideal for larval performance and waste reduction when larvae are fed a mixture of fruit and vegetable waste with a small portion of poultry manure. Comparing the results between Chapter 4 and Chapter 5, specifically trays that contained 40% poultry manure, larvae performed better overall in medium-sized trays, although individual larval masses were similar between both tray types. This study indicates that medium-sized experiments do not scale up in a predictable manner, making it more difficult for knowledge gained from especially small-scale experiments to be applicable to large-scale applications. Comparing the results from the larger and medium-sized trays, the use of medium-sized trays would be recommended in a facility over the use of larger trays that are more exposed to the ambient air. In Chapter 6, an overall discussion of each experiment, implications of the results, limitations experienced, future directions and a final conclusion are presented. This work has added to and expanded on the scientific knowledge that is now available about H. illucens larvae and their use in bioconversion. This work builds on the current knowledge base of organic waste streams, and mixing of waste streams, with variable nutrient content and the experiments were conducted at an industrial scale, in a bioconversion facility, rather than a lab-scale study. As such, the results are more applicable to large-scale, commercial bioconversion operations and will provide some guidance on future large-scale rearing and processing experiments and application of that knowledge in industry. There are several very complex interactions that occur during larval rearing; including substrate conditions like nutrients, microbes, moisture, pH and temperature, the ambient temperature and rates of substrate evaporation, larval density and age, and size and shape of the tray. Future work should consider these complex interactions to untangle and understand these complex interactions. Doing so will provide industry with recommendations for future best practices