Evaluating heavy metal risks to biomass valorization during black soldier fly larva bioconversion of municipal sewage sludge: speciation, bioaccumulation, and safety assessment

Municipal sewage sludge can potentially be valorized using black soldier fly larvae (BSFL), but heavy metal contamination may pose risks to the safety and circularity of the resulting biomass and residue. This study evaluated the distribution, operational fractionation, and larval accumulation of Cu, Zn, Mn, Ni, Cd, Pb, and Cr during BSFL treatment of an artificial-diet control (S0) and sludge-incorporated substrates prepared at wet-weight ratios of 1:1 (S50) and 1:3 (S75). BSFL processed all substrates, with S50 providing the best overall balance between larval growth and substrate reduction. Metal-loading analysis identified the endpoint residue as the principal sink for most metals. BCR (Community Bureau of Reference) sequential extraction showed that Zn, Mn, Cd, and Ni occurred largely in the more readily extractable F1-F2 fractions, whereas Cu, Pb, and Cr were distributed mainly in the less extractable F3-F4 fractions. Increased F4 proportions of Pb and Cr in sludge-incorporated residue indicated enhanced stabilization. Larval accumulation was assessed using the bioaccumulation factor (BAF), background-corrected bioaccumulation factor (BAF net ), and net metal retention efficiency (NRE). Cd showed the clearest accumulation pattern, with BAF values above unity under all treatments and consistently positive NRE. Regulatory comparison identified Cd as the principal constraint on using sludge-reared larvae as feed, while Cu, Zn, and Ni constrained residue use as fertilizer. Integrating metal loading, fractionation, accumulation indices, and regulatory thresholds provides an environmental risk assessment of BSFL-based sludge valorization.