Influence of membrane morphology on the flux decline during dead-end ultrafiltration of refinery and petrochemical waste water

Reuse of refinery and petrochemical effluent, high in turbidity and salinity, as cooling water was proposed by a treatment scheme comprising ultrafiltration (UF) and reverse osmosis (RO). Dead-end UF was investigated in this study to reduce the turbidity of the waste water prior to RO, which was necessary to decrease the salinity to an acceptable level for reuse as cooling water and/or boiler feed water. UF tests were performed at constant transmembrane pressure (0.2 bar) using hollow fiber polyethersulphone membranes coated with polyvinylpirrolidone. UF membranes with two different molecular weight cut-offs (50 and 150 kDa) were compared. The two membranes performed very differently, i.e. the 150 kDa membrane showed a very fast flux decline (i.e. 20% in 2 min) requiring frequent backwashing (BW), whereas in the case of the 50 kDa membrane, 20% flux decline was reached in 20 min. An analysis of the blocking mechanisms could not explain the rapid drop in flux for the 150 kDa membrane as compared with the 50 kDa membrane, since a gradual change from complete to intermediate blocking and cake filtration was observed in both cases. However, a field emission scanning electron microscopy (FESEM) analysis of both UF membranes suggested that the superior filtration performance may be due to the highly interconnected pore system of the 50 kDa membrane, resulting in a ‘3D-bridge-type’ surface morphology.