Olive mill wastewater concentration by two-stage reverse osmosis in tubular configuration, in a scheme combining open and tight membranes

A novel concept for olive mill wastewater (OMWW) concentration, involving the combination of open and tight reverse osmosis (RO) membranes in tubular configuration, was studied. The OMWW, prior to RO concentration, was clarified in two successive steps: passing through a coarse rotating finisher and by ceramic microfiltration, in order to remove suspended matter and facilitate the RO concentration. The clarified raw material was then concentrated using the tubular PCI UK AFC40 RO membrane. A parametric study was conducted to investigate the effect of main process parameters on permeation flux. The permeation flux of AFC40 RO membrane was found to reach consistently high values without substantial membrane fouling problems. Furthermore, the parametric study proved that the flux was linearly increased by increasing the transmembrane pressure, and exponentially decreased by increasing the OMWW mass concentration factor. In most cases, the permeation flux increased with increasing flow velocity. The effect of temperature on permeation flux was depended on both transmembrane pressure and flow velocity. The AFC40 permeate content of total dissolved solids (TDS) was approximately 0.6%, which accounts for roughly 15% of the TDS in the initial OMWW solution. The AFC40 permeate was, therefore, further concentrated using the AFC99 PCI UK tight RO membrane, aiming to obtain a permeate suitable for disposal to natural resources, whilst simultaneously reclaiming valuable antioxidants by mixing the two retentates of AFC40 and AFC99 membranes. The results of this study show that the proposed two-stage RO scheme may be potentially applied in a commercial scale and contribute towards processing of OMWW to produce liquid antioxidant and water suitable for disposal to natural resources.