Carbon-negative treatment of anaerobic digestate via integrated cultivation of microalgae (Galdieria sulphuraria) and insects (Hermetia illucens)

Abstract

Black soldier fly larvae (BSFL) show high bioconversion efficiency to convert organic waste into lipids (feedstock for biodiesel production); however, this efficiency declines when BSFL are reared on low-nutritional organic waste. To compensate for this inferior bioconversion efficiency of BSFL for low-nutritional organic waste (such as anaerobic digestate), microalgal biomass (Galdieria sulphuraria) was added to BSFL feed. The nutritional value of the feed (particularly protein content) increased with an increase in the microalgal biomass ratio. The mean larval dry weight of BSFL that were fed a mix containing 50 wt% microalgal biomass (G50) was 33.1 mg, which was 1.7 and 2.8 times higher than that of those fed a 30 wt% (G30) and 10 wt% microalgae mix (G10). Bioconversion efficiency was enhanced in G50 compared with that in the other feeding systems, indicating more effective utilization of nutrients. This improved larval development contributed to an increase in the quantity and quality of biodiesel derived from BSFL reared in the G50 feeding system. Furthermore, the CO2 emission through BSFL respiration was offset by CO2 fixation during microalgal cultivation in our experiments, thereby opening the possibility to create a type of carbon-negative bioconversion system.