Effects of membrane fouling on solute rejection during membrane filtration of activated sludge

To assess the relationship between solute rejection and membrane fouling in a MBR system, membrane filtrations of activated sludge in different physiological states were carried out with ultrafiltration membranes. Regardless of the physiological states of the activated sludge (foaming, bulking, pin-point hoc, exponential growth. endogenous phase, normal state sludge), the hydrophobic membrane (PM30) always showed greater solute rejection than the hydrophilic membrane (YM30). To investigate the key factors affecting solute rejection, the cake layer resistances (R-c) and the fouling resistances (R-f) were measured. The R-c and R-f values for the PM30 were always higher than for the YM30 and the R-c prevailed over the R-f in all cases. The solutes rejection by the adsorption onto/into the membrane was relatively small. This suggests that the cake layer deposited on the membrane surfaces played an important role in the solute rejection, i.e. dominant solute removals were attributed to the adsorption and/or sieving onto the cakes. Consequently, the difference in solute rejection efficiency between hydrophilic and hydrophobic membranes was mainly due to the degree of sieving and/or adsorption onto the cakes deposited on the membrane, and partly due to adsorption into membrane pores and the surfaces. (C) 2001 Elsevier Science Ltd. All rights reserved.