Permeate recirculation impact on concentration polarization and fouling on RO purification of olive mill wastewater

• Impacts of permeate recirculation on membrane polarization and fouling were studied.
• Permeate recirculation reduces polarization and fouling build-up on both membranes.
• Severe fouling on low-pressure membrane stabilized upon 30% permeate recirculation.
• Irreversible fouling minimized permitting satisfactory restoration of permeability.
• 15.8–42.1% fouling build-up reduction for SC membrane whereas 22.9–55.2% for AK

Analysis of permeate recirculation impact on membrane concentration polarization and fouling was studied upon well-controlled reverse osmosis (RO) experiments for final purification of secondary-treated olive mill wastewater (OMW). Two commercial RO membranes presenting different characteristics, that is, a thin-film composite (TFC) and a low-pressure one, were selected for comparison purposes. Incrementing the permeate recirculation ratio to the feed tank (10–30%) reduced the concentration polarization and fouling build-up, that is 15.8–42.1% for the TFC membrane whereas 22.9–55.2% for the low-pressure membrane. This fact is especially relevant for the low-pressure membrane on which fouling triggers rapidly causing severe flux loss, but tends to stabilize upon 30% permeate recirculation. Moreover, irreversible fouling was satisfactorily minimized, reducing scaling formation and permitting restoration of the initial permeability. The fouling index can be minimized by 57.1% for the TFC membrane by recirculating 30% of the permeate stream (measured at 15 °C), and also 47.1% reduction of the fouling index was observed for the low-pressure membrane upon the same recirculation ratio. The data obtained in this study highlight permeate recirculation as an important operating variable for membrane fouling control in the final RO purification of OMW.