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. © 2015 Elsevier B.V. All rights reserved.

Fig. 8. Effect of stirring rates on bacterial adhesion to the membrane surface after 24 h of biofouling experiments: (A to C) Analyzed CLSM images of the membrane surface at different stirring rates of 100 rpm (A), 200 rpm (B), and 300 rpm (C). (D) Relation of the calculated volume of the bacteria adhered to the membrane surface (●) with J/J0 (○) after the biofouling experiments. The TMP was 0.75 MPa. increasing stirring rates was bacterial detachment from the biofilm formed on the membrane surface. Fig. 10 shows the calculated osmolarity on the membrane surface at different stirring rates. The osmolarity on the membrane surface was higher than that of the feed water and slightly decreased with increased stirring rates. The increased stirring rate prevents CP due to decreasing thickness of the boundary layer on the membrane surface [28]. In this study, the range of stirring rates analyzed was relatively small and therefore stirring rates did not affect the CP remarkably. The theoretical study by Qiu and Davies showed that cross-flow velocity slightly decreased the degree of CP [19]. In this study, inhibition of biofouling with increasing stirring rates was mainly caused by bacterial The effects of operating conditions on biofouling in RO membrane processes were systematically investigated. The biofouling of the RO membranes was evaluated by filtrating bacterial medium through a circular polyamide RO membrane with pre-adhered bacteria. The TMP and stirring rate affecting the membrane surface in a cylindrical membrane cell were changed as the operating conditions. The application of TMP facilitated decreased rates of the permeate flux and the increased bacterial adhesion on the membrane surface. The increase in calculated osmolarity on the membrane surface suggested that the TMP caused CP of bacterial nutrients and accelerated the biofouling. An increase in the stirring rate prevented the decrease of permeate flux as well as bacterial growth on the membrane surface. This biofouling inhibition was mainly caused by reduction in bacterial volume due to hydrodynamic shear force and not the CP. These results show that lower TMP and higher stirring rates are favorable to prevent the biofouling by reducing CP reduction and promoting bacterial detachment. Supplementary data An AFM image of the unused polyamide RO membrane and methods of AFM observation (Fig. S1), effect of TMP on J0 (Fig. S2), and effect of the hydraulic pressure on the bacterial growth (Fig. S3). Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j. desal.2015.09.020.