BACKGROUNDOlive mill wastewater is an important by-product obtained after olive oil extraction. In this investigation, the phenolic composition of olive mill wastewater treated with a semi-industrial membrane filtration system, including ultrafiltration and reverse osmosis modules, was studied. In particular, untreated olive mill wastewater was compared with the permeate and the concentrate fractions of each filtration step. RESULTS3,4-(dihydroxyphenyl) ethanol and p-(hydroxyphenyl) ethanol were the main compounds of all olive mill wastewater analyzed. A total of 32 compounds, detected at 240 or 280 nm by HPLC-DAD, were considered for phenol quantification. Ultrafiltration reduced phenol concentration by about 40% with respect to the initial level; in the permeate of reverse osmosis, the phenol concentration ranged from 0 to 1% of the initial content. In contrast, the content of phenolic compounds was increased about 2.6 fold in reverse osmosis concentrate. CONCLUSIONThe membranes used in this investigation produced a high phenol content fraction than might be used as functional component in food, cosmetic or feed products. Moreover, the final wastewater obtained by reverse osmosis, thanks to the low content of phenols, could be recycled and reused in the oil mill, reducing water consumption and disposal costs. (c) 2014 Society of Chemical Industry