Co-Digestion of Olive Pomace and Goat Manure for Hermetia illucens Growth and Potential Coupling with Biogas Production

Olive pomace is an abundant agro-industrial residue whose recalcitrant composition limits its efficient valorisation, highlighting the need for sustainable recovery strategies. This study investigated the use of Hermetia illucens larvae for the bioconversion of olive pomace and its integration with anaerobic digestion to evaluate the simultaneous recovery of biomass and energy. Larvae were initially reared on olive pomace, goat manure, and their mixture to assess growth performance, survival, and substrate suitability. Subsequently, olive pomace was subjected to thermal pre-treatment and anaerobic digestion with goat manure, and the resulting digestates were reused as substrates for larval rearing. Their phytotoxicity was also evaluated through germination tests on alfalfa. Larval performance was higher on non-digested substrates, confirming the suitability of raw mixtures for insect rearing. Thermal pre-treatment did not result in a significant increase in methane production. In contrast, digestates, particularly those obtained under more severe thermal treatments, resulted in reduced larval growth and survival under the tested conditions and showed variable phytotoxic effects. Overall, although anaerobic digestion enabled energy recovery, its integration with insect-based bioconversion resulted in reduced larval performance under the tested conditions. These findings highlight the importance of optimising substrate treatment to ensure effective integration of insect rearing within circular bioeconomy systems.