## 1 Introduction  
Vegetable oils are a staple food for all countries worldwide. They are extracted from multiple sources such as olive oil, palm oil, coconut oil, sunflower oil, canola oil, soybean oil, peanut oil, cottonseed oil, corn oil, argon oil and many more. Particularly, olive oil is considered one of the most versatile and healthy oils in terms of consumption as a food product. Olive oil extraction occurs in either two- or three-phase systems. The two-phaseextraction process generates the olive oil and olive mill pomace (OMP), also called 'alperujo' in Spanish. This method helps in saving water discharge [1; 2]. The three-phase extraction process produces the olive oil and two by-products; a liquid olive mill wastewater (OMW) and a solid olive mill pomace (OMP) [3; 4].  
Olive oil production is centered in the Mediterranean region including Spain, Italy, Greece and Tunisia, which are the major olive oil producers worldwide, accounting for 97% of the global market [5; 6]. Particularly, Tunisia provides 6% of the world's olive oil production, it occupies the 4 th position worldwide and the 1 st position in the north African countries with 60% production [6]. [PERSON] et al. [7] mentioned that Tunisia produces 770.000 m\({}^{3}\)/year OMW and 550,000 tons/year of OMP by itself. OMW is a liquid discharge fraction containing water (\(\geq\)90%) and multiple organic and inorganic compounds. OMP is the solid discharge fraction which is mainly composed of olive pulp, seed and skin [8]. These by-products constitute a potential danger to the environment, plants and living organisms in all olive oil-producing countries [7; 9; 10].  
The Mediterranean olive oil industries generate huge amounts of OMW and OMP attaining about 30 million m\({}^{3}\) discharges per year [9; 11], with an average of 0.5 to 1.5 m\({}^{3}\)/ton of olive [12; 13]. OMW characteristics vary according to the geographic and climatic change, the olive cultivar, the fruit quality, the extraction process and the use of chemical fertilizers [13; 14]. Overall, OMW features acidic pH, high-sodium content, blackish-purple color, and distinct smell due to its high polyphenols, polyalcohols, and carbohydrates content [15; 16; 17; 18].  
Olive mill effluents are frequently discharged in the environment into uncontrolled evaporation ponds. Several studies have proved their high concentrations in toxic waste matters (heavy metals, volatile acids, polyphenols and long-chain fatty acids) leading to foul odors release and several harmful effects to the ecosystem such as soil, water, plants, animals, insects and microorganisms [19; 20; 21]. The long term disposal of OMW in the open evaporation ponds cause the transformation of the disposed by-products from a liquid to a solid more toxic form. Recalcitrant compounds either concentrate in a dry sludge or infiltrate inside the soil [22]. For that reason, several treatments have been implemented such as the biological, physicochemical, thermochemical methods or the integration of the above-mentioned methods to ensure better results [18; 23; 24]. The role of the indigenous microbial communities in the bioremediation of OMW has been extensively studied [25; 26]. These microbes could alleviate, remove or bio-transform toxic compounds from the OMW by-products [25; 27].  
This paper reports the environmental threat of uncontrolled OMW and OMP disposal in evaporation ponds, as well as an in-depth study of the OMW and OMP heavy metals and secondary metabolites composition. Lastly, the indigenous microbial diversity in the OMW was extensively monitored to investigate its role in alleviating the toxicity of the evaporation ponds.