Biorefinery approach to olive pomace management: Digestate application as an organic fertiliser to improve soil health

Anaerobic digestion is a key technology for advancing circular bio-based models in the agro-industrial sector, especially as the final step of biorefineries. Digestate is a by-product of anaerobic digestion that may be applied to improve the soil health, which is a core element of regenerative agriculture. However, its characteristics depend on the substrate composition and anaerobic digestion conditions, and must be properly assessed before agricultural reuse. This study evaluated digestates from the anaerobic co-digestion of olive pomace with pig slurry, before and after an ionic liquid pre-treatment to recover lignin in a biorefinery approach. Both digestates exhibited suitable characteristics for agricultural application as organic fertilisers, including near-neutral pH, balanced nutrient content, and partially stabilised organic matter. Digestate from untreated pomace showed higher content of hydrophobic carbon, potentially benefiting long-term soil organic matter storage. In contrast, digestate from pre-treated pomace released more labile carbon, thus enhancing microbial activity and short-term nutrient cycling. Over a 90-day incubation period, both digestates improved soil carbon and nitrogen availability, but also increased CO₂ emissions. No phytotoxic effects were observed for concentrations ≤10 %, and pathogens and heavy metals analyses did not show critical issues, except for Hg. These limitations can be overcome by dilution or blending with other fertilisers before application, ensuring the safe return of organic matter and nutrients to soil, without compromising crop quality. These findings confirm the suitability of olive pomace-based digestate for agricultural reuse and highlight the potential of olive pomace biorefineries to achieve zero-waste circular agro-industrial systems.