The effect of permeate recirculation on the depuration of pretreated olive mill wastewater through reverse osmosis membranes Original Research Article

Still scarce are the studies addressing the depuration of olive mill wastewater (OMW) by means of membrane technology, encountering severe fouling problems related to ineffective pretreatments. In addition, they focus only on meeting irrigation standards. The target of this investigation was to experimentally evaluate the improvement driven by recirculating a fraction of the permeate stream in the performance of a thin-film composite reverse osmosis membrane (polyamide/polysulfone) for purification of OMW previously pretreated by means of chemical oxidation based on Fentonʹs reagent, coagulation–flocculation and filtration through olive stones. For this purpose, permeate flux and rejection of pollutants under increasing permeate recirculation fraction, additionally to fouling on membrane surface, were examined and found to be improved. Similarly, influence of operating temperature was as well investigated. Operating in a diafiltration mode, instead of the batchwise common to most OMW depuration processes, attempted semicontinuous production and lower rate of increase of pollutants concentration. Upon recirculation of a fraction of the permeate stream above 10%, high and stable permeate flux was provided ensuring no significant steady-state flux decline. Under these conditions, 100% suspended solids, phenols and iron removal was achieved, in addition to around 99.4% and 98.2% overall COD and conductivity rejection efficiencies respectively.