**1. Introduction**

Olive oil production is one of the most important food sectors in the Mediterranean area as olive processing is considered a traditional industry for its countries since ancient times [1]. It is mainly produced in Spain (36% of the global production), Italy (24%), Greece (17%) followed

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by Portugal, France, Cyprus, Croatia, Turkey, Syria, and Tunisia. New producers are Argen‐ tina, Australia, and South Africa.

Olives are processed through two methods: pressing (discontinuous process) and centrifuging (continuous process, two/three phase centrifugation). The main inconvenience of these methods is the production of a polluting by-product that is a dark effluent known as olive mill wastewater (OMW) [2].

The environmental impact of olive oil production is strong due to the use of large quantities of water and the production of OMW: 1,000 kg of olives produce 0.5 m3 –1.5 m3 of OMWs [1]. They are by-products generally considered undesirable but inevitable for every olive processing.

As defined in reference [3], OMW is "a stable emulsion constituted by vegetation waters of the olives, water from the processing, olive pulp and oil." It is characterized by a particular color (intensive violet-dark brown up to black color), odor (strong specific olive oil smell), high degree of organic pollution (expressed as biological and chemical oxygen demand (BOD and COD) values), acidic pH, high electrical conductivity, high content of polyphenols, high buffer capacity, and high content of solid matter.

OMWs are generally composed of water (83%–96%) and organic fraction (3.5%–15%) com‐ posed of 1%–8% carbohydrates, 0.5%–2.4% nitrogen compounds, 0.5%–1.5% organic acids, 0.02%–1% of fatty acids such as propionic, butyric, etc., and 1%–1.5% of phenolic compounds consisting of a hydroxyl group (-OH) bound directly to an aromatic hydrocarbon group and pectins [4].

Concerning phenols, they comprise low molecular weight compounds and polyphenols. Low molecular weight compounds are represented by caffeic, cinnamic, 2,6-dihydroxybenzoic, *p*hydroxybenzoic, syringic, 3,4,5-trimethoxybenzoic, vanillic, and veratric acids; they have phytotoxic effects and antibacterial activity. Polymeric phenols (lignins, tannins, etc.) cause the typical brownish-black color of OMW and are the most recalcitrant fraction of this effluent.

The quantitative and qualitative composition of OMWs are variable due to climatic conditions, variety, ripeness of olives, and extraction processes; generally, they are produced in high quantities in a short time, thus their disposal represents an important problem.

As OMWs are rich in nutrients they could be used to remediate arid or semi-arid regions but their phytotoxicy affect plant growth [5]. OMWs have the highest polluting rate within the food industry due to the fact that they are recalcitrant to traditional biodegradative methods. The reduction of COD and BOD values represent an important goal for many industries but the high content in phenols complicate waste management; they exert an antimicrobial activity towards wastewater microflora thus biodegradation is slowed [6].

For these reasons phenols are considered as undesirable compounds; thus, physical, chemical, or biological treatments are used to reduce their pollutant load.

Waste remediation has been traditionally performed through some expensive methods (incineration, pyrolysis, landfill, etc.). In recent years, the increasing trends towards green economy and friendly approaches for the environment are the background to design alterna‐ tive ways. According to this point of view, numerous researchers proposed bioremediation, defined as "the process whereby organic wastes are biologically degraded under controlled conditions to an innocuous state or to level below the limits established by regulatory author‐ ities" [7]. According to the Environmental Protection Agency (EPA), bioremediation is a "treatment that use naturally occurring organisms to break down hazardous substances into less toxic or nontoxic substances."

Thanks to their ubiquity and metabolic pathways (aerobic, anaerobic fermentation, and cometabolism) microorganisms are able to degrade and utilize various toxic compounds as energy source. Generally, the aerobic biodegradation has a higher efficiency than anaerobic processes and it is widely used. Nevertheless, in many cases, aerobic and anaerobic processes can also be used in series to reduce the complexity or the toxicity of the contaminants.

Numerous bacteria such as *Bacillus pumilus* [8], *Pediococcus pentosaceus* [9], *Lactobacillus plantarum* [10], *Arthrobacter* sp. [11], *Azotobacter vinelandii* [12–14], *Azotobacter chroococcum* [15], *Pseudomo‐ nas putida*, and *Ralstonia* sp. [16, 17] were able to degrade and/or remove phenols from OMW.

Yeasts and molds are also able to degrade phenols, namely, *Candida tropicalis*, *Candida cylin‐ dracea* and *Yarrovia lypolitica* [18, 19, 3, 20, 4], and white-rot fungi such as *Phanerochaete chrysosporium* or the genus *Pleurotus* [21–25]. In addition, *Trametes versicolor*, *Funalia trogii*, *Lentinus edodes, Aspergillus niger*, and *Aspergillus terreus* have been also mentioned as phenoldegrading organisms [26]. The main objective of this paper was the selection of promising bacterial strains to be used as potential tools for bioremediation; namely, after the isolation of some strains from OMWs, they were studied in relation to their ability to grow in a medium containing two secondary phenols. Then, a validation on a lab scale was performed.
