Closing the nutrient loop: valorization of aquaponics waste through Hermetia illucens L. (Diptera: Stratiomyidae) for circular food systems

Aquaponic systems integrate aquaculture and hydroponics into a closed-loop model aimed at sustainable food production. However, these systems still generate residual waste, notably aquaculture biosolids and plant residues, that must be effectively managed. This study explores the integration of the black soldier fly ( Hermetia illucens L. (Diptera: Stratomyidae); BSF) into aquaponics as a means of transforming organic waste into high-value insect biomass and organic fertilizer, thereby closing the nutrient loop. Two trials were conducted using BSF larvae reared on wheat bran-based diets containing varying inclusion levels (0%, 25%, 50%, 75%, and 100%; calculated on a dry weight basis) of either hydroponic residue (HR) or fish feces (FF). Key performance indicators, i.e., larval survival, final individual larval weight, specific growth rate (SGR), feed conversion ratio (FCR), and efficiency of conversion of ingested food (ECI), were evaluated. Significant reductions in larval growth, feed conversion efficiency, and nutrient utilization were observed at hydroponic residue (HR) inclusion levels > 75% and fish feces (FF) inclusion levels > 50% (in both cases p < 0.001). Post hoc analysis (Dunn’s test) confirmed that the performance of larvae, in terms of larval growth and survival and feed conversion ratio, when reared on 100% FF and HR was significantly worse than that of those on lower inclusion diets, indicating the reduced suitability of these substrates for BSF larvae, probably due to a combination of nutritional limitations and unfavorable physical substrate properties. These findings demonstrate that BSF larvae can process aquaponics-derived waste at low to moderate inclusion levels, whereas higher inclusion levels negatively affect larval growth and feed utilization.