Valorizing black soldier fly frass via anaerobic co-digestion: biogas potential, microbial safety, and heavy metal dynamics

Abstract

Insect frass was evaluated for its potential in biogas production, and the effect of anaerobic digestion (AD) on its microbiological quality, both alone and in co-digestion with poultry droppings and cow dung, with respect to biogas yield, process stability, and the safety of the resulting digestate. Biogas production was highest in the frass-poultry co-digestion system (8.2 mL/g VS) at pH 6.3, followed by frass alone (7.5 mL/g VS) at pH 6.7, whereas the inclusion of cow dung reduced yields. The microbiological study of frass indicated that the initial colony-forming units (CFU/ml) of frass were >3 × 10⁴ on both Salmonella-Shigella and nutrient agars, and 8.0 × 10³ on MacConkey and 9.0 × 10³ eosin methylene blue (EMB) agars. After anaerobic digestion, CFU/ml values reduced to 5 × 10⁶ on nutrient agar, with no growth found on MacConkey, Salmonella-Shigella agar, and eosin methylene blue agar. Heavy metal study showed a decrease in cobalt, nickel, copper, and lead concentrations after AD, although zinc levels increased from 2.99 to 3.01 mg/L, exceeding WHO safety guidelines (1.0 mg/L). These findings demonstrate that insect frass is a viable feedstock for biogas generation, particularly when co-digested with nitrogen-rich materials, and that AD contributes to pathogen reduction and metal immobilization for most elements. However, zinc accumulation poses a potential environmental risk, necessitating further treatment or monitoring if the digestate is to be used agriculturally.