Upcycling food waste using black soldier fly larvae: Effects of further composting on frass quality, fertilising effect and its global warming potential

Using black soldier fly (Hermetia illucens [L.], Diptera: Stratiomyidae) larvae (BSFL) to upcycle urban solid waste into a growing medium for vegetable cultivation is a potential solution to ease cities' over-reliance on food imports and excessive waste generation. However, rapid composting by BSFL may be insufficient to remove phytotoxins from the waste and further composting may be necessary. Through a growth experiment on pak choi (Brassica rapa), this study evaluated the optimum performance of BSFL frass at three composting stages (freshly produced, composted with forced aeration and naturally composted in larval rearing chambers) in biochar-based substrate cultivation. Plants cultivated in the composted BSFL frass treatments grew larger than those in fresh frass, with 10% naturally composted BSFL frass (v/v) producing the most biomass among all treatments. A seed germination test suggested the presence of phytotoxins in the fresh frass and frass Fourier-transform infrared spectroscopy (FTIR) analysis showed that the fresh frass potentially had the highest phenol concentration. In terms of greenhouse gas emission, a life cycle analysis showed that all three types of frass had a lower global warming potential compared to incineration, with the fresh frass having the lowest emission, the frass composted using forced aeration having the second lowest and the naturally composted frass, the highest emission. Therefore, further composting of BSFL frass with forced aeration would be the most feasible method considering both its compost quality and environmental impact. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study showed that composted BSFL frass resulted in better plant growth, with 10% naturally composted BSFL frass producing the highest biomass among all treatments. Phytotoxins were found in fresh frass, and further composting reduced the greenhouse gas emissions of the final product compared to incineration.