Sequential coagulation–flocculation, solvent extraction and photo-Fenton oxidation for the valorization and treatment of olive mill effluent

An innovative process battery comprising coagulation–flocculation, extraction of phenolic compounds and photo-Fenton post-oxidation has been developed for the valorization and treatment of olive mill effluents (OMEs). Pre-conditioning by coagulation–flocculation using FeSO4⋅7H2O as the coagulant, and an anionic polyelectrolyte (FLOCAN 23) as the flocculant was performed to remove the solid content of the effluent. The addition of 6.67 g/L of FeSO4⋅7H2O and 0.287 g/L of FLOCAN 23 led to the optimal removal of total suspended solids (TSS) (97 ± 1.3%), of Chemical Oxygen Demand (COD) (72 ± 1.5%), and of Total Phenols (TPs) (40 ± 1.3%). Solvent extraction was then applied to recover a fraction of the remaining phenolic compounds; for instance, extraction for 15 min with ethyl acetate at a solvent to sample ratio of 2:1 (v/v) led to 36% TP recovery post-coagulation–flocculation. Finally, photo-Fenton was applied as a post-treatment method; oxidation for 240 min at 0.2 g/L Fe2+, 5 g/L H2O2 and pH = 3 reduced the remaining COD and TP by 73 ± 2.3% and 87 ± 3.1%, respectively. Toxicity assays to Daphnia magna as well as phytotoxicity tests to three plant species to untreated OME and oxidized samples were also performed, indicating the evolution of more biologically potent products during the oxidation.

► OME was treated by coagulation–flocculation, solvent extraction and solar-Fenton.
► The solid particles were successfully removed (97%) by coagulation–flocculation.
► Solvent extraction was applied to recover a fraction (36%) of the remaining TPs.
► Solar-Fenton was able to reduce the residual COD and TP, at 73% and 87%, respectively.