The Olive Oil Mill as Circular Hub

Olive oil mills generate large amounts of by-products, approximately 85% of the olive weight processed, including pomace, wastewater, olive stones, and olive tree pruning, often leading to environmental and economic challenges. Additionally, the high production cost of extra-virgin olive oil compared to its market value requires medium-to-large modern mills (300 t of olives processed per campaign) to explore alternative income sources by exploiting all available by-products. This review provides a comprehensive and critical overview of olive mill by-product valorization strategies, reframing the olive mill as an integrated circular hub. A cascade approach is discussed, prioritizing the recovery of high-value compounds, such as phenolic and lignocellulosic fractions, followed by energy production through anaerobic digestion, combustion, and electrochemical processes. Particular attention is given to process scalability, technological maturity, and operational constraints, including seasonal variability and feedstock heterogeneity, to identify the most promising strategies for industrial implementation. Practical Applications: The valorization of olive mill by-products presents multiple practical applications. (i) Phenolic compounds can be extracted from olive mill wastewater, pomace, and leaves for food, feed, cosmetic, and pharmaceutical applications. (ii) Lignocellulosic materials, derived from olive pomace, stones, and tree pruning, for biomaterials, biocomposites, and packaging. (iii) Energy recovery options via biogas (wastewater and pomace), electricity (from all residues), and heat (mainly from olive stones). (iv) Biochar production from solid residues for soil amendment and carbon sequestration.