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Agro-industrial wastewaters are characterized by the presence of multiple organic and inorganic contaminants of environmental concern. The high pollutant load, the large volumes produced, and the seasonal variability makes the treatment of these wastewaters an environmental challenge. A wide range of wastewater treatment processes are available, however the continuous search for cost-effective treatment methods is necessary to comply with the legal limits of release in sewer systems and/or in natural waters. This review presents a state-of-the-art of the application of advanced oxidation processes (AOPs) to some worldwide generated agro-industrial wastewaters, such as olive mill, winery and pulp mill wastewaters. Studies carried out just with AOPs or combined with physico-chemical or biological treatments were included in this review. The main remarks and factors affecting the treatment efficiency such as chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), and total polyphenols removal are discussed. From all the studies, the combination of processes led to better treatment efficiencies, regardless the wastewater type or its physico-chemical characteristics.

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This work presents the application of Fenton's reagent process combined with anaerobic digestion to treat an olive mill wastewater (OMW). Firstly, OMW was pre-treated by chemical oxidation in a batch reactor with Fenton's reagent, using a fixed H2O2/COD ratio of 0.20, pH = 3.5 and a H2O2/Fe(2+) molar ratio of 15:1. This advanced oxidation treatment allowed reaching reductions of 17.6 and 82.5% of chemical oxygen demand (COD) and total polyphenols (TP), respectively. Secondly, OMW treatment by anaerobic digestion was performed using previously adapted microorganisms immobilized in Sepiolite. These biological tests were carried out varying the substrate concentration supplied to the reactor and COD conversions from 52 to 74% were obtained. Afterwards, Fenton's reagent followed by anaerobic digestion was applied to OMW treatment. This combined process presented a significant improvement on organic load removal, reaching COD degradations from 64 to 88%. Beyond the pollutant load removal, it was also monitored the yield of methane generated throughout anaerobic experiments. The methane produced ranged from 281 cm(3) to 322 cm(3) of CH4/g COD removed. Additionally, a methane generation kinetic study was performed using the Monod Model. The application of this model allowed observing a kinetic constant increase of the combined process (kFN = 0.036 h(-1)) when compared to the single anaerobic process (kF = 0.017 h(-1)).