## 1 Introduction  
Olive growing is spread over 10 million and 800 thousand hectares across the world, 97% of which are concentrated in the Mediterranean area, where the olive tree (_Olea europaea_\(L\)) has always occupied a central role in among its population. Olive oil is one of the oldest foods and, among European countries, Spain produces about 826 thousand tons of oil, corresponding to more than 52% of world production, and Italy holds 33% of the EU production [1]. However, olive oil extraction represents a serious environmental issue due to the generation of a high quantity of waste in a very short time. The olive mill waste, both in liquid and solid forms, includes: olive mill wastewaters (OMWW), wood and leaves, olive pomace (OP), and stones [2]. The worldwide production of OMWW is estimated around 6 \(\times\) 10\({}^{6}\) m\({}^{3}\) and 98% is produced in the Mediterranean basin. The ratio of olive oil production to OMWW is 1.0:2.5 L, reaching, in Italy, a total of 1.4 million m\({}^{3}\) of OMWW and 30 million m\({}^{3}\) in the Mediterranean basin [3, 4]. In recent years, technological innovations in olive oil extraction have affected the whole supply chain, impacting the composition of OMWW, which is primarily composed of vegetation water, and water added both during malaxation and during pressing. Specifically, three different extraction processes are commonly applied: (1) the traditional press process; (2) the two- and; (3) the three-phase decanter process. In the traditional process, olives are washed, crushed, mixed, and malaxed with the addition of a small quantity of water which can easily separate the oil from the other fractions. The resulting paste is then pressed to drain the residual oil and the liquid waste from the presses. It consists of a mixture of olive juice and added water and residual oil. Finally, the olive oil is separated from the water by vertical centrifugation or decanting. The traditional process is applied almost exclusively in small olive mills, with larger mills having been replaced by continuous systems. Through the use of an industrial decanter to separate all the phases, the discontinuous pressing process has been replaced by the continuous centrifugation, using a three-phase system, and later on a two-phase system [5,6]. The two-phase system, adopted in Spain and widespread in most countries, does not require the addition of water, other than during horizontal centrifugation, and results in olive oil and semi-solid olive cake [2]. The three-phase decanter process requires the addition of hot water, in 0.6-1.3 m\({}^{3}\)/1000 kg of processed olives [2] and results in olive oil, OMWW, and olive cake (residual solids). As a result of these differences, the three-phase extraction process presents a slightly higher yield, leading to a lower amount of olive cake but a significantly higher production of OMWW. The management of liquid wastes in olive mills has always been challenging, and extensive efforts have been carried out to find an effective strategy. Nevertheless, the disposal of OMWW in soil or waterways continues to represent a serious issue for Mediterranean countries due to its severe phytotoxicity and antimicrobial properties that can compromise the balance of ecological systems, with detrimental long-term environmental effects. In many cases, direct disposal of OMWW into lakes, rivers, and water streams has resulted in disastrous environmental consequences due to their high content of phenolic compounds, organic and long-chain fatty acids, and tannins. In addition to traditional decantation, various systems of purification and disposal have been proposed, such as chemical, agronomic, and biotechnological interventions. However, such approaches underestimate \"waste\" as a possible primary resource of high nutritional value compounds.