**1. Introduction**

Olive oil production is a fundamental sector for several European (EU) States, especially Spain, Italy, and Greece. In particular, Spain has the largest area of olive cultivation (estimated at *ca.* 2.47 million ha), followed by Italy (*ca.* 1.16) and Greece (about 0.81 million ha) [1,2]. However, olive oil production is responsible for several environmental concerns (soil contamination, underground seepage, water-body pollution, and odor emissions) due to poor waste management practices [3]. In this scenario, concerning olive mill wastewaters (OMWW), special attention must be paid to their high phenolic content, which is responsible for their antibacterial effect, phytotoxic effect, and dark colour.

Recently, phenols, fatty acids, and volatile acids have been recognized as potentially hazardous for environmental health: the former have pronounced antimicrobial and phytotoxic properties, whereas the latter show toxicity due to their long alkyl chain.

All these components make OMWW toxic to anaerobic bacteria, thus inhibiting conventional secondary and anaerobic treatments in municipal water plants. Furthermore, the high BOD (biological oxygen demand) and COD (chemical oxygen demand) levels, which

**Citation:** Galloni, M.G.; Ferrara, E.; Falletta, E.; Bianchi, C.L. Olive Mill Wastewater Remediation: From Conventional Approaches to Photocatalytic Processes by Easily Recoverable Materials. *Catalysts* **2022**, *12*, 923. https://doi.org/10.3390/ catal12080923

Academic Editors: Ioan Balint and Monica Pavel

Received: 31 July 2022 Accepted: 19 August 2022 Published: 21 August 2022

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cannot be reduced by anaerobic digestion, represent a further threat to receivers [2,4]. Moreover, land spreading and treatment in evaporation ponds could lead to problems related to groundwater pollution. The use of olive oil waste in agriculture may also affect the acidity, salinity, N immobilization, microbial response, leaching of nutrients, and concentration of lipids, organic acids, and phenolic compounds [5].

Alternative approaches based on physical treatments, such as dilution, evaporation, centrifugation, or sedimentation guarantee a high level of OMWW purification. However, they are expensive and energy-consuming, thus leading to an exponential increase in the processing cost. The olive oil industry, in its current status, composed of small and dispersed factories, cannot bear such high costs [6–13].

In recent years, advanced oxidation processes (AOPs), including photolysis, photooxidation, Fenton, and photo-Fenton reaction, have emerged as promising alternatives for simplicity and high organic removal efficiencies [14–20]. In particular, heterogeneous photocatalysis seems to be a successful technology in water decontamination due to its non-toxicity, low cost, and mineralization efficacy. However, due to the OMWW matrices' complexity, it is not easy to develop and successively optimize efficient photocatalytic systems that are so far characterized by common limitations (i.e., difficult recovery, poor stability, low reusability, fast deactivation).

Based on these premises, in the present work, for the first time, we illustrate the conventional methods commonly used to treat OMWW along with their related advantages and limitations. Then, a critical insight on alternative strategies for developing efficient photocatalytic systems based on recoverable catalysts is proposed. The latters can be used as alternatives to conventional photocatalysts. This topic is of fundamental importance for the research community as shown by the hard work currently been done for developing novel devices with high potential in real applications, acting as a bridge between environmental protection and circular economy.
