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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. Image 1 • Black soldier fly larvae (BSFL) frass tested as a sustainable compost. • Rapid composting by BSFL resulted in immature compost. • Further composting increased available nutrients and reduced phytotoxin content. • Pak choi grown in 10–30% BSFL frass (v/v) achieved the highest yields. • Compared to incineration, composted BSFL frass had a lower GWP. [ABSTRACT FROM AUTHOR]