Olive Pomace Inclusion Alters the Microbial Community of Black Soldier Fly Larvae Frass While Maintaining Fertilizer Quality

Olive pomace (OP) is a sludge arising from the production of olive oil, generated in increasing amounts in Portugal. The management of this toxic waste stream is complex and the number of processing plants is limited. In this study, OP was incorporated as a feed component for rearing black soldier fly larvae (BSFL) under industrial conditions. Larvae were reared inside a climate-controlled room with regulated temperature and humidity. The rearing cycle lasted 13 days, after which larvae were harvested. In addition to assessing bioconversion efficiency and larval proximate composition, the resulting frass was examined for its fertilizer potential. Frass was analyzed for plant nutrient content and microbial profile in three forms: fresh, heat-treated (70°C for 1 h), and pelletized. The inclusion of OP in the diets reduced waste-to-biomass conversion efficiency (21.5%_DM to approximately 13.3%_DM) but did not affect the proximate composition of the larval biomass, which consistently contained around 43%_DM crude protein and 20%_DM crude fat. Neither the presence of OP nor the applied post-treatments altered the nutrient composition of frass, which contained on average 3.5% total N, 2.6% P₂O₅, and 5.9% K₂O. However, at the highest inclusion level (84%), the abundance of bacterial and fungal groups was significantly reduced. The predominant phyla in the frass were Actinobacteria, Bacteroidota, Firmicutes, Proteobacteria, Ascomycota and Basidiomycota, and the dynamics of microbial communities were influenced by specific micronutrients. The presence of OP led to a significant reduction of potentially pathogenic bacteria and fungi in the frass, indicating a sanitizing effect attributable to this material.