Black Soldier Fly composting of fecal sludge cake and its effects on frass quality

Abstract The increasing global demand for sustainable waste valorization and organic soil amendments has driven interest in Black Soldier Fly Larvae (BSFL) mediated bioconversion. This study investigated co-composting of fecal sludge cake (FSC) with cattle manure (CM) or fruit/vegetable waste (FVW). Seven treatments were tested: T1 (100% FSC, control); T2 (75% FSC + 25% CM); T3 (65% FSC + 35% CM); T4 (55% FSC + 45% CM); T5 (75% FSC + 25% FVW); T6 (65% FSC + 35% FVW); and T7 (55% FSC + 45% FVW). BSFL composting significantly altered substrate properties: frass pH increased from 7.08–7.63 to 7.49–7.96, moisture content declined by 24–32%, and organic carbon decreased 14–46%. Nitrogen increased most in FVW-amended treatments (14–22%), phosphorus was highest in the FSC control (20–21 g/kg), and potassium peaked in T7 (16–20 g/kg). Among all treatments, T7, with the highest FVW proportion, achieved the most balanced nutrient enrichment and the most effective organic matter stabilization. Essential elements exhibited substrate-dependent trends: magnesium and silicon increased, while calcium, sulfur, and micronutrients (zinc, molybdenum, copper, iron, manganese, and chlorine) decreased by 16–44%. These results demonstrate that BSFL co-composting efficiently converts high-organic wastes into nutrient-dense soil amendments, with feedstock composition critically influencing agronomic quality. The process offers a viable circular bio-economy strategy for integrated organic waste management and fertilizer production.