## 1 Introduction  
According to the International Olive Council and official data, global olive oil production ranged between 3.13 and 3.28 million metric tons from 2017/2018 to 2020/2021, and it is known that, in the last 60 years, olive oil production has tripled worldwide. Indicatively, 95% of global olive oil is produced in the Mediterranean, mainly by Spain, Italy, Greece, and Portugal, and other countries such as Tunisia, Turkey, Morocco, and Algeria have also stood out [1, 2]. Greece ranks third in olive oil production, cultivating olive oil trees on almost 60% of the country's arable land [3] (mostly on the areas of Peloponnese, Crete, Aegean, and Ionian Islands). Additionally, Greece holds first place in black olive production, and the largest variety of oils (compared to any other country) is cultivated in Greece. An average rate of 12% of European olive oil is produced by Greece [4], 80% of which is extra virgin [5]. According to [PERSON] et al. [6], almost 46% of the total agro-industrial residues in the country come from olive processing, and more specifically, from the olive mills as solid (two-phase olive oil decanters) or liquid (three-phase olive oil decanters) waste due to olive oil extraction. As Greece's main extraction system is still the one with the three-phase decanter, at an average rate of 82%, huge amounts of liquid wastes or olive mill wastewater (OMW) are generated annually, during the period of October to February [7]. Among the hazardous industrial effluents, OMW has been indicated as one of them, mostly due to its high chemical and biological oxygen demand (COD, BOD) [7]. Such values could range from 40 to 220 g COD/L and from 35 to 110 g BOD/L, respectively,as mentioned by [PERSON] et al. [8]. Additionally, a high concentration of phenolic compounds such as oleupropein, tyrosol, hydroxytyrosol, syringe acid, p-cumaric acid, and 4-hydroxybenzoic acid (usually up to 7.5 g/L) is involved, and the pH value ranges from 3 to 6 [9; 10]. Due to their physicochemical characteristics and seasonal production, OMW can cause various environmental issues, such as surface, groundwater, and soil pollution due to the high amounts of organic compounds (including phenolics and lipids) and inorganic constituents such as potassium [11]. Consequently, all olive oil producing countries are confronting the aforementioned environmental issues due to the lack of effective, practical, and affordable solutions for OMW management, as the main option is still at their disposal (usually in big lagoons or storage ponds) [10; 11]. From a valorization/treatment point of view, several methods, including biological, physicochemical, and advanced oxidation solutions, have been widely proposed, as can be seen also in Table 1. Among these, biological processes mainly target anaerobic digestion under both mesophilic and thermophilic conditions, aerobic processes, and compost production. According to the literature, some pretreatment methods are usually implemented prior to the biological processes' applications in order to enhance efficiency and thus the yields. Such pretreatment methods include ultrasound [12], chemical oxidation with ozone, adsorption, UV irradiation [13], membrane filtration, dilution [14], drying [15], and electrocoagulation [16], among others. The aforementioned strategies may significantly affect the total cost but do so by solubilization, color removal, phenolic compounds separation, and organic load or moisture reduction. Taking into consideration some recent research findings, phenolics were first removed or dilution was required in order to achieve higher degradation yields during anaerobic digestion [7; 17; 18; 19]. On the other hand, aerobic processes have been proved inefficient for OMW treatment due to their high organic compounds content and the increased need for dilution [14]. The composting process was also evaluated by various researchers' groups. According to [PERSON] et al.