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Fouling remains a prevalent and serious problem in industries using membrane processes. Efforts to mitigate fouling are improving, however, membrane fouling cannot be completely eliminated. Therefore fouling control via development of sustainable cleaning methods are crucial. Despite osmotic backwashing showing promise, little is understood about this cleaning method for removal of fouling from reverse osmosis (RO) membranes. This paper systematically examines how organic fouling characteristics and osmotic backwashing parameters influence cleaning efficiency. Alginic acid was used as a model foulant and numerous microscopy techniques, including confocal microscopy, scanning electron microscopy and atomic force microscopy were used to examine the membrane fouling before and after cleaning to gain a clearer understanding of the mechanisms involved. Increasing CaCl2 concentration in the fouling solution resulted in an increase in fouling layer thickness from 37 to 179 μm, due to the complexation of Ca2+ and the carboxyl groups in the alginate. Osmotic backwashing efficiency with 0.7 M NaCl decreased as the fouling layer became thicker and the pure water flux (PWF) recovery decreased from 92% to 81%. Osmotic backwashing efficiency also decreased with increasing initial permeate flux, as less fouling was removed: the fouling generated at higher initial fluxes is largely irreversible, resulting in a denser and more compact fouling layer. In an effort to increase osmotic backwashing flux, a CaCl2 draw solution was used, however, the Ca2+ ions were found to interact with the alginate in the fouling layer, rendering this method inefficient, when compared to NaCl draw solutions which originated similar osmotic backwashing fluxes. Interestingly, the fouling layer was found to swell from 16 μm to 141 μm, when osmotic backwashing was carried out with a NaCl draw solution, followed by contact with a low ionic strength solution used for PWF testing. This phenomenon does not occur to the same extent after backwashing with CaCl2. The same trends were obtained for bovine serum albumin (BSA) fouling, whilst humic acid (HA) did not display any swelling phenomena. However, it showed the same cleaning inefficiency when using CaCl2 as a draw solution.
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