Black Soldier Fly-based bioconversion of biosolids creates high-value products with low heavy metal concentrations

Annually, over 30 million dry tonnes of biosolids are produced worldwide, most of which are disposed into landfills or discharged into waterbodies, exacerbating eutrophication and water-borne diseases. Bioconversion of biosolids using Black Soldier Fly larvae (BSFL) can produce high-value biomass (protein, lipids, and chitin) while reducing the volume of waste that requires disposal. We aimed to determine the bioconversion performance of BSFL on different types of biosolids and biosolids blends and analysed the bioaccumulation of heavy metals in the larvae. BSFL converted all substrates into larvae biomass of similar protein content (31–37%). However, larvae growth and substrate reduction were significantly lower for unblended biosolids compared to wheat bran. Blending of biosolids with other organic waste such as food waste or wheat bran improved larvae performance (< 40% substrate reduction after 20 days; < 149 kg larvae/ tonne dry substrate) and fat content (< 31%). Despite initial high concentrations in biosolids (< 8700 mg/ kg), heavy metals were largely partitioned into the residues instead of the mature BSFL, resulting in low bioaccumulation of those elements in BSFL (< 180 mg/ kg). These concentrations were even below limits of international guidelines for animal feed. Therefore, this study demonstrated that BSF-based bioconversion can be an innovative and sustainable waste management and resource recovery technology to rapidly reduce the volumes of biosolids while transforming it into high-value biomass of low heavy metal concentrations.