Rearing black soldier fly to convert spent grain to high value biomass : a thesis presented in partial fulfilment of the requirements for the Degree of Master of Engineering at Massey University, Manawatū, New Zealand
The main objective of the study was to provide the engineering expertise to upscale the BSF production of Prescient Nutrition. This study was novel as no published work has been found that attempted designing a full-scale plant to produce BSF biomass was done before. However, some facilities may have tried designing and running a full-scale plant. The project focused on rearing BSF larvae to BSF prepupae and was undertaken in partnership with Plant and Food Research (PFR) and Prescient Nutrition. Values of key parameters were obtained from literature and used in the mass and energy balance of the system. The sensitivity of the balances from each parameter was tested and it was deduced that the larvae, prepupae and spent grain must be characterized nutritionally and the time requirement of the larvae to reach the prepupae stage must be confirmed. Several factors were also optimized for the design.
The feed load was optimized to 18 g dry weight of spent grain per 1 g dry weight of 1-week-old larvae introduced in the system. The ideal feeding regime for the tested conditions was found to be weekly feeding without frass removal. This regime was selected for the design since this feeding regime resulted in comparable results to the regime with frass removal and is more practical. The feed height of 18 cm was found to not have an adverse effect on the growing patterns of the BSF larvae and hence was used in the design of the system. Using this feeding regime (18:1 ratio, 18 cm depth, weekly feeding without frass removal), the biomass yield was estimated to 3.33 kg dry BSF output per 1 kg dry larvae input. Methods for harvesting larvae from the feed were also explored. It was found that force harvesting through heat (32°C to 39°C) is the most cost effective and simplest way of harvesting larvae from the feed.
A pilot study was initially planned with a pilot cabinet that can handle 5 kg wet weight of 1-week-old (1.08 kg dry) larvae and a total of 80 kg wet weight of spent grain (19.84 kg dry, yielding a feeding ratio of approx. 18:1 kg/kg); this pilot unit was fabricated. The larvae were to be grown over 2 weeks to produce 13 kg wet weight of BSF prepupae. However, PFR was not able to provide the necessary larvae for the pilot and hence it was not done. The initial design for the full-scale production was a modular unit that can be placed in the customers’ premises. However, the economics and logistics of this design is expensive, with the transportation costs alone reaching $62 000 per year, and hence a centralized plant operated by 1 operator that can produce a weekly biomass output of 1200 kg dry weight was chosen. This plant will require a land area of about 2000 m² and 26 wet tonnes of spent grain weekly at a feeding ratio of 18:1. The design will have an operational cost of $270 000 a year and a gross revenue of $670 000 a year. The capital for the plant was estimated to be $1 540 000 and the NPV of the plant was estimated to be $2 100 000. The internal rate of return (IRR) for the project was calculated to be 31% which is above the average minimum acceptable rate of return (MARR) for new plants which is 15% (Bizfluent, 2018).