Olive oil industry: a review of waste stream composition, environmental impacts, and energy valorization paths

The olive oil production is a key economic sector for the producing countries, mainly in the Mediterranean region. However, the worldwide increasing oil production led to the generation of huge amounts of wastes detrimental for the environment. Therefore, efficient and sustainable management of olive industry wastes has recently acquired significant interest in the scientific research community. In the actual world energy context, various studies dealt with the valorization of the solid/liquid waste streams obtained from the discontinuous/continuous extraction of olive oil for energy purposes. The application of waste-to-energy treatments to these effluents can turn them out into an important energy resource. This review article presents the main used oil extraction techniques and their related research developments. The characterization of the generated wastes and the factors behind their bad environmental impacts are highlighted. Relevant research works related to biochemical and thermochemical conversion of olive mill wastes are extensively reviewed and discussed in terms of product yields and composition. A recent update of the studies addressing olive industry waste applications for energy production is also given. This investigation revealed a lack of studies in relation to the hydrothermal processing of olive mill wastes. Despite their suitability for this process (e.g., high moisture content), few papers have investigated the hydrothermal conversion of these waste streams. This scientific gap opens a very interesting research direction, which has to be further investigated.