Hee Ro Chae

Preparation and Application of Patterned Membranes for Wastewater Treatment

Membrane fouling remains a critical factor limiting the widespread use of membrane processes in water and wastewater treatment. To mitigate membrane fouling, we introduced a patterned morphology on the membrane surface using a lithographic method. A modified immersion precipitation method was developed to relieve the formation of dense layer at the solvent-nonsolvent interface, that is, the opposite side of the patterned surface. Diverse patterned membranes, such as pyramid-, prism-, and embossing-patterned membranes, were prepared and compared with a flat membrane in terms of morphology, permeability, and biofouling. Patterned membrane fidelity was largely dependent on the polymer concentration in cast solution. The patterned surface augmented the water flux in proportion to the roughness factor of the patterned membrane. However, the type of pattern did not affect substantially the mean pore size on the patterned surface. Deposition of microbial cells on the patterned membrane was significantly reduced compared to that on the flat membrane in the membrane bioreactor (MBR) for wastewater treatment. This was attributed to hydraulic resistance of the apex of the patterned surface, which induced local turbulence.

A facile route to enhance the water flux of a thin-film composite reverse osmosis membrane: incorporating thickness-controlled graphene oxide into a highly porous support layer

In this study, we demonstrated that a reduction in solely the concentration of the polymer solution for preparation of the support layer effectively enhances the water flux of a thin-film composite (TFC) reverse osmosis (RO) membrane. However, a decrease in the polymer concentration caused the sub-surface structure of the support layer to become too porous, which unavoidably weakened the mechanical strength of the support layer. To overcome the problem, we prepared a highly porous support layer with improved mechanical strength by incorporating graphene oxide (GO) platelets. The thickness of the GO platelets was controlled by adjusting the mechanical energy input per volume of the precursor solution. We confirmed that well-exfoliated GO platelets (mean thickness: about 1.5 nm) are more effective in enhancing the mechanical properties of the support layer. The TFC RO membrane made of the GO composite support layer had almost 1.6 to 4 times higher water flux with comparable salt rejection compared to both the current upper bounds of the RO membranes prepared by modification of the active layer and commercial RO membranes.

Influence of the sublayer structure of thin-film composite reverse osmosis membranes on the overall water flux

We found that a support layer of a reverse osmosis (RO) membrane could have a significant effect on the membrane filtration efficiency. To be specific, we determined that the pressure drop occurring in the support layer during the RO operation can increase such that it affects the overall water flux, as a dense sponge-like structure and closed finger-like structure become predominant in the support layer. This considerable resistance was assumed to occur while more water passes through tortuous and segmented regions after being evenly discharged from the active layer. This hypothesis was supported by the estimated pressure drop using the Ergun equation and the tortuosity obtained from a forward osmosis test conducted to better reflect the influence of the sponge-like region. We attempted to factor in all of the parameters used in the Ergun equation in order to determine the main cause of the high pressure drop, and the tortuosity was found to be dominant. An interesting finding was that the tortuosity of the support layer can also significantly influence the overall water flux, even during the RO process. Moreover, the above phenomenon can become much more obvious when the active layer is highly permeable, suggesting that the support layer must be considered alongside the active layer when developing thin-film composite membranes with a highly permeable active layer. Overall, we concluded that the concentration of the polymer solution should be less than 20 wt% to ensure the best performance when preparing a support layer for brackish-water RO membranes.