Relating intrinsic membrane water permeability and fouling propensity in forward osmosis processes

Abstract

In this study, we determine the effect of different foulants on membrane permeability in a forward osmosis (FO) process. Each model foulant, including humic acid (HA) as an organic foulant, carboxylate-modified latex particles (CML) as a colloidal foulants and Escherichia coli as a bio-foulant, was added and fouled on the membrane surface. Two membranes with different flux, a low water permeable membrane (LPM) and a high water permeable membrane (HPM), were selected to study the effect of foulants on membrane permeability. Water flux, reverse salt flux and reverse flux selectivity were investigated. Furthermore, the performances of the membrane are described in terms of the solute selectivity coefficient (K). A larger K value indicates a decrease of the FO performance. After both membranes were fouled with HA molecules, an HA fouling layer promoted the FO performance due to the increase of hydrophilicity on the membrane surface. In addition, the charge screening effect in the HA fouling layer hindered the transportation of salt ions, which decreased the reverse salt flux and increased the reverse flux selectivity. On the other hand, CML and E. coli significantly decreased the FO performance because they strongly promoted external concentration polarization and internal concentration polarization. These effects hindered the salt diffusion to the bulk feed solution and increased the salt concentration near the active layer, which decreased the osmotic pressure gradient and decreased the water flux. Moreover, the fouling also affected the different flux membranes differently. The reverse flux selectivity of HPM changed more than that of LPM after they were fouled with foulants. Thus, our study confirmed that HPM was more sensitive to fouling than LPM, and more sophisticated state for fouling mitigation would be required.