Integrated life-cycle environmental performance and heavy metal risk assessment of food waste valorisation via black soldier fly larvae for aquafeed and frass fertilizer substitution.

Black soldier fly larvae (BSFL) can convert food waste into protein-rich biomass for animal feed and nutrient-rich frass for fertilizer use. However, the environmental performance and heavy metal risks of integrating BSFL bioconversion with aquaculture and frass recycling remain unclear. This study evaluated four scenarios: conventional fishmeal-based aquafeed with food waste incineration, 15% fishmeal replacement with black soldier fly meal (BSFM), 25% replacement with BSFM, and 15% replacement combined with frass substitution for mineral fertilizer. Feeding trials, life cycle assessment, heavy metal analysis and economic evaluation were conducted. Replacing 15% of fishmeal with BSFM did not significantly affect juvenile grass carp growth or feed utilization (P > 0.05), whereas 25% replacement reduced feed intake and growth. The scenario combining 15% fishmeal replacement with frass substitution showed the best environmental performance, reducing climate change and acidification impacts by 8.3% and 24.0%, respectively, relative to the baseline. These benefits mainly resulted from avoided food waste incineration, fishmeal production and mineral fertilizer manufacture, although feed production remained the dominant hotspot. No notable concentration-based enrichment occurred in BSFM or edible fish tissue. Frass showed relatively higher concentrations of some metals, but all measured levels remained within relevant regulatory limits. At an annual grass carp production scale of 6.00 million tonnes, 15% fishmeal replacement could reduce feed costs by CNY 3.29-9.60 billion. Overall, combining 15% fishmeal replacement with frass recycling improved resource recovery, environmental performance and economic feasibility without compromising grass carp growth or edible-tissue safety under the tested conditions.