Current State of Technological Applications and Novel Approaches to Value-Adding Methods for By-Products from Oil Mills

The olive cultivation and oil extraction processes generate wastewaters and solid wastes that have a large impact on ecosystems due to their high organic matter content and phytotoxicity. Olive mill wastewater (OMW) has a generation ratio between 0.4–0.6 and 0.8–1.2 liters of OMW per kg of olive fruits for batch pressing and continuous centrifugation system using two-outlet decanters, respectively. In addition, the semisolid waste obtained is usually dried and deoiled, resulting in a by-product known as “extracted pomace” (by-product from extraction process), which constitutes a very interesting solid biofuel with a higher heating value in the range of 13.8–15.8 MJ/kg. It is estimated that olive pomace (OP) is one of the major by-products, with a generation ratio of 0.3–0.4 kg-OP/kg of olive fruits. Olive leaves are generated at the first stage of the olive oil processing and can weigh up to 10% of the raw material that arrives at the mill. Finally, olive stones and seeds (16–25% of the total olive fruit weight) are recovered after oil separation in olive mills or from the olive table industry. Taking into account the huge amount of wastes produced during the olive oil extraction, it is not surprising the search for strategies to transform these olive oil wastes into resources for olive production. In addition to this, another approach is the promotion of diversified agroecosystems with a broader biodiversity and ecosystem services, through the design and implementation of mixed farming systems (i.e., integration of livestock and olive cultivation). This chapter briefly reviews well-established and emerging valorization technologies applied mainly to by-products and OMW. Among others, there are biological-based processes such as composting or biogas production, but also thermochemical technologies such combustion, gasification, and pyrolysis (olive stones and seeds have been mainly used as an energy source), or concentration techniques such as spray-drying and the production of biosorbents. In some cases, these practices are integrated with extractive processes, using supercritical fluids or pressurized liquids and ultrasound-assisted or microwave-assisted extractions, to separate specific high-value compounds as fatty acids, antioxidants, enzymes, dietary animal supplements, and functional ingredients for food, cosmetic, and pharmaceutical industries.