Effect of operating conditions on biofouling in reverse osmosis membrane processes: Bacterial adhesion, biofilm formation, and permeate flux decrease

We systematically evaluated the effects of operating conditions on biofouling behaviors in reverse osmosis (RO) membrane processes. The biofouling experiment was performed by filtrating nutrient-containing feed water with a cross-flow cylindrical membrane cell through a circular polyamide RO membrane with pre-adhered Pseudomonas putida. The trans-membrane pressure (TMP) and stirring rates in the cell were controlled as operating conditions. The stirring rates brought about shear force and thus corresponded to cross-flow velocities on the membrane surfaces in commercial operations. Permeate flux was monitored during the filtration, and bacterial adhesion to the membrane surfaces was observed using a confocal laser scanning microscope. An increase in TMP increased both the reduction rate of permeate flux and the volume of adhered bacteria, thus facilitating biofouling. Osmolarity calculation on the membrane surface suggested that TMP affected bacterial growth by concentration polarization (CP) of nutrients. Higher stirring rate prevented reduction of permeate flux and bacterial growth on membrane surfaces. The hydrodynamic shear force generated by stirring effectively detached the adhered bacteria, while CP of nutrients was not remarkably affected by the range of stirring rates used in this study. These results suggest that lower TMP and higher stirring rates prevent biofilm formation by decreasing CP and promoting bacterial detachment.