Membrane fouling insight during reverse osmosis purification of pretreated olive mill wastewater

The chronological sequence and the relative importance of the individual fouling mechanisms were investigated on a polymeric RO membrane used for final purification of secondary-treated olive mill wastewater (OMW2ST). An insight of membrane fouling under different operating conditions is reported. The goodness of the fitting (R2 → 0.99) points for the formation of a cake or gel layer over the membrane as the dominant fouling mechanism in the transient and steady-state operation stages. A non-uniform cake was built-up, as shown in the HR-SEM analysis on the fouled membrane, principally formed by the organic matter (188.7 ± 37.9 mg L−1) and residual iron colloids carried by the OMW2ST feedstream. However, about 39.0% of the initial flux drop could be attributed to concentration polarization on the boundary region of the membrane. Moreover, it is relevant that the partial blocking model describes correctly the incipient fouling formation on the membrane, disregarding the operating conditions. This behavior was attributed to surface defects, more permeable zones of the membrane that may act as pores during the initial and intermediate operation stages, and in particular to the small organic particles (up to 63.3% between 3 and 30 kDa) remaining in the OMW2ST feedstream.