Influence of Sludge and Feed Mixtures on Metal Retention, Pathogen Reduction, and Nutritional Value in Black Soldier Fly (BSF) ( Hermetia illucens ) Larval Substrates

Black soldier fly (BSF) larvae are increasingly used in sustainable waste management, offering potential for the bioconversion of organic waste into insect-derived fertilizer and animal feed. This study investigates the impact of varied substrate mixtures percentages of sludge and chicken feed on heavy metal accumulation, pathogen reduction, and nutrient composition in BSF frass. Methods: The experiment was conducted with four substrate treatments (100% sludge, 75% sludge + 25% chicken feed, 25% sludge + 75% chicken feed, and 100% chicken feed) over a 20-day period. Chemical and microbiological analyses were performed on the feed mixture before adding larvae and on the frass produced in each treatment. Heavy metal concentrations, including cobalt (Co), chromium (Cr), nickel (Ni), and lead (Pb), pathogen levels (Escherichia coli, total coliform, and fecal coliform), and nutrient composition, including moisture content, pH, ash, nitrogen, phosphorus, calcium, potassium, sodium, magnesium, and chlorine, were assessed. Statistical analysis was used to determine significant differences across treatments. Results: Heavy metal levels in frass varied with substrate composition, with significantly higher concentrations of cobalt (Co), chromium (Cr), nickel (Ni), and lead (Pb) in sludge-dominant treatments ( p < 0.05). Treatments with higher chicken feed content were associated with lower metal levels, indicating organic feed’s potential in limiting heavy metal accumulation ( p < 0.001). Pathogen analysis showed high microbial levels in sludge-based treatments, while the 100% chicken feed treatment exhibited minimal contamination, highlighting its safety profile ( p < 0.05). Nutrient characterization revealed that chicken feed-enhanced treatments produced frass with higher nitrogen and potassium levels, suggesting improved nutrient density and potential for agricultural use. Conclusions: Tailoring BSF substrates by combining sludge with organic feed can enhance the nutritional quality of frass while reducing environmental risks associated with heavy metal and pathogen presence. This study supports the potential of BSF as a sustainable bioconversion tool, promoting circular agriculture.