## 1 Introduction  
The Mediterranean region is the main producer of olive oil, concentrating more than 95% of the world's olive trees. Within this area, Spain represents around half of total world manufacturing, considering this industry as one of the most important agri-food sectors for this country [1, 2]. Presently, the main industrial process to obtain olive oil is continuous extraction by two-phase or three-phase systems. Depending on the oil extraction system used, considerable amounts of solid and liquid waste as final products are generated. Although the three-phase extraction process mainly generates albenim and pomace as final residues, the main waste stream from two-phase extraction system is the alperujo (AL) [3]. The pomace is a solid waste composed of the pulp and pits of the olive, commonly employed as fertilizer, biofuel production, or animals feed. The albenim corresponds with the liquid effluent, composed of water and minerals and characterized by a high organic matter load. This waste stream is considered, together with the AL, a highly polluting residue, for which reuse is not an easy task so its revalorization is still being investigated [4, 5]. Due to the global increase of olive oil demand, the excessive amount of waste streams generated throughout the olive oil industry is a growing problem that poses an environmental challenge [6]. In Spain, the two-phase olive oil extraction system is used in approximately 90% of olive mills [4]. The application of this extraction system generates about 800 kg of AL per ton of processed olive, which represents an annual production of around four million tons for the Spanish oil industry [3]. The AL obtained is a semi-solid waste stream composed of vegetable water and olive pomace with high moisture content (60%) that still contains a certain amount of oil [7]. The AL is subjected to a second centrifugation, to obtain a pomace oil. The resulting residue is usually dried in rotary heat dryers at high temperatures and the by-product is subjected to an extraction with hexane to recover more oil, which requires a large amount of energy and incurs high costs [8; 9]. Dried or wet AL can also be used in composting processes. However, due to its low porosity, the addition of bulking agents such as bark chips or cotton gin is necessary [9; 10]. In addition, its high content of lignocellulosic compounds and polyphenols, which are toxic to animal cells, plants, insects, and microorganisms, is an important drawback [11]. Discharging these wastes without treatment would cause serious damage to aquatic systems, such as the reduction of soluble oxygen. Furthermore, its strong odour would also cause serious problems for the population living near the discharge area [12; 13]. Therefore, the removal of the pollutant compounds of the AL, and therefore favouring of the subsequent biological/physical treatment, is one of the main problems that the olive oil industry must confront. The literature has been mainly focused on the treatment of olive mill wastes coming from the three-phase extraction system, with the aim of removing the organic and phenolic compounds and improving the biodegradability of the effluent. Regarding biological methods, anaerobic digestion or aerobic activated sludge processes have mainly been applied for the treatment of olive mill wastewater (OMW). However, these methods are not usually applied directly to the effluent due to the presence of recalcitrant molecules, polyphenols, the low nutrient load, and the acidic pH of the waste, which make treatment difficult [14]. Previous studies have reported an improvement in the efficiency of these processes reducing the acidity, adding nutrients such as cobalt, or extracting polyphenols before biodegradation [15]. Due to the high level of antimicrobial compounds present in OMW, the acclimatization of biomass or the use of physical CaCO\({}_{3}\) supports has also been required to improve the biodegradation and methanization process [16].