Technical and economic feasibility of composting recalcitrant olive oil industry wastes for the production of organic amendments and bio-based fertilisers

Olive oil production generates large volumes of organic residues with high recalcitrance and phytotoxicity, posing significant environmental and management challenges. This study evaluated the co-composting of recalcitrant "orujillo" (RO), bottom ash from "orujillo" combustion (BAOC), urban tree pruning (UTP), and broiler litter (BL) to transform these materials into agronomically valuable products. Three composting piles were formulated with increasing RO proportions and monitored for over 12 months. The composting process exhibited extended thermophilic phases, largely influenced by the high RO content. Organic matter degradation followed first-order kinetics, with losses up to 69.5%. Labile compounds such as water-soluble carbon, carbohydrates, polyphenols, and fats decreased significantly, while humification indices, particularly polyphenolic-associated humic acids, exceeded 80% in all final composts. Phytotoxicity, evaluated via the Zucconi germination index, was effectively reduced, though delayed in RO-rich piles. The final composts were characterised by high organic matter content, alkaline pH, moderate salinity, and relevant nutrient levels. All samples complied with safety criteria for pathogens and heavy metals, and exhibited no phytotoxicity. Additionally, a Liquid Organo-mineral Fertilizer was developed from the most mature compost using BAOC as an alkaline extractant. The resulting product contained high concentrations of soluble organic carbon and nutrients (N, P, K), meeting the criteria for commercialization under EU regulations. Economic evaluation showed that composting is a cost-effective valorisation strategy, with nutrient recovery values partially offsetting production costs. These findings support composting as a viable circular bioeconomy approach for managing recalcitrant olive oil industry wastes in Mediterranean regions.