*4.4. Advanced Oxidation Methods*

The addition of strong oxidizing agents can influence the efficiency of wastewater treatment, mainly in terms of the breakdown of recalcitrant and toxic compounds. In this context, high mineralization levels can occur depending on the oxidizing power of the agent employed and contact time. In recent decades, the scientific community has addressed the efforts to exploit advanced oxidation processes (AOPs) to treat industrial effluents and OMWW [14,15]. In general, AOPs combine ozone (O3), light irradiation (UV, solar, visible), hydrogen peroxide (H2O2), and/or catalysts to produce unstable radical species able to degrade both organic and inorganic pollutants.

In electrolysis, the oxidation of the content of organic species directly occurs on the anode or indirectly by oxidizing agents present in the solution [81]. Over the years, several materials for anodes' production have been studied (i.e., Pt/Ir, Ti/IrO2, Pt/Ti, and borondoped diamond) [81–84]. However, this is a high-energy consuming approach. In contrast,

Fenton oxidation is based on the addition of Fenton's reagent (H2O2 and Fe(II)) into the waste [16]. In this case, the oxidation process is due to a cascade of different reactions in the solution. Although it is low energy consumption, H2O2 makes this technology quite expensive. The photo-Fenton method is very similar to the Fenton one, but the UV radiation accelerates Fe2+ regeneration, enhancing, as a consequence, the process efficiency. However, the necessity to employ UV radiation causes high energy consumption [85]. Supercritical water oxidation consists of waste oxidation in the presence or absence of catalyst above the water critical temperature and at high pressures [86–88]. It is a very efficient technology for organic content reduction, but the energy consumption is high due to the high temperatures and pressures required. Finally, ozonation employs O3 as oxidant species for waste oxidation. It is not so efficient in the organic content reduction, but that of phenols is high. Unfortunately, using O3 increases the process costs [89–91].
