Insect-mediated valorisation of anaerobically digested aquaculture waste: bioconversion performances, nutritional composition and microbial safety of black soldier fly larvae

Abstract

Recent studies have suggested that fresh aquaculture waste (ASW) could be satisfactorily treated with black soldier fly larvae (BSFL). However, pre-treatments such as drying or dewatering, which significantly modified the chemical, physical and microbiological properties of the waste, were always applied. On the other hand, industrially generated aquaculture waste might be bulk-accumulated for a long time and may not always be suitable for pre-treatments. Therefore, the present study aimed to evaluate the ability of BSFL in converting bulk-accumulated ASW from an aquaculture industrial facility, while generating high quality and safe insects. Five substrates, consisting of different mixtures of ASW (0, 25, 50, 75, and 100%) and chicken feed, were prepared and offered to the larvae. Increasing amounts of ASW in diet resulted in progressively lower larval growth, with a final larval weight reducing from 162.65 ± 4.81 g (treatment 0ASW) to 91.48 ± 3.38 g (treatment 75ASW), while no growth was observed in the substrate 100ASW. Larvae raised on substrates containing high amounts of ASW showed decreased concentration of lipids (from 35.52 ± 1.21 (treatment 0ASW) to 17.27 ± 0.52 (treatment 75ASW) % dm) and protein (from 40.62 ± 0.61 (treatment 0ASW) to 35.87 ± 0.34 (treatment 75ASW) % dm), while the amount of ash increased from 11.03 ± 0.34 (treatment 0ASW) to 31.74 ± 0.08 (treatment 75ASW) % dm). The amino acid composition and fatty acid profile of BSFL appeared to be stable. High microbial contamination (total viable count ranging between 8.39 and 9.28 log CFU g⁻¹) was always detected in the reared larvae, although no pathogens were found in any sample. It was concluded that, although the current EU legislation does not allow the use of animal manure for rearing insects, BSFL could be satisfactorily used for managing anaerobically digested ASW, reducing waste while recovering nutrients. However, the presence of an amending material capable of improving the quality of the initial waste was needed. Obtained insects might be valorised as feed or utilised to extract nutritional components for incorporation into food, veterinary, pharmaceutical or agricultural products, boosting the transition to the circular economy.