Production of a nutrient-rich organic amendment via bioconversion of coffee husk by black soldier fly (Hermetia illucens L.) larvae

The bioconversion of organic waste using black soldier fly larvae (BSFL, Hermetia illucens L.) offers a sustainable pathway for nutrient recovery and organic amendment production within a circular economy framework. This study investigated the effect of time-dependent bioconversion processes on frass quality and the potential of phytotoxicity reduction using a mixed organic substrate consisting of coffee husk (CH) and cow manure (CM) at a 1:1 ratio. Changes in physicochemical properties, nutrient composition, trace elements, phytotoxicity, and microbial activity were monitored throughout the bioconversion process. BSFL treatment resulted in a progressive increase in substrate pH from mildly acidic to alkaline conditions (7.9), accompanied by a decrease in the C/N to values below 15 in comparison with raw materials, indicating substrate stabilization. The resulting frass exhibited elevated concentrations of essential macronutrients (NPK), confirming its potential as a nutrient-rich organic amendment. Water-extractable trace element concentrations (Ni, Pb, Zn, and Cu) slightly decreased during bioconversion, suggesting a potentially lower environmental risk. Phytotoxicity reduction was indicated by an increase in germination index (GI), reaching values above 50%. Microbial biomass carbon and fluorescein diacetate hydrolysis decreased over time, consistent with progressive organic matter stabilization. Overall, BSFL bioconversion of CH and CM improved substrate stabilization, nutrient availability, and phytotoxicity reduction potential.