ABSTRACT
115The failure of conventional pre-treatment to provide acceptable feed water quality for seawater reverse osmosis (SWRO) during algal blooms underlines the significance of robust pre-treatment systems. This study investigated the effectiveness of tight ultrafiltration (10 kDa) membrane as pre-treatment in delaying the onset of organic/biological fouling in SWRO feed water during algal blooms. The proof of principle experiments were performed in laboratory and pilot plant using various MF/UF membranes and algal organic matter (AOM) produced by Chaetoceros affinis as a feed solution. The feed and permeate of MF/UF membranes were analyzed in terms of biopolymer concentration, and bacterial regrowth potential (BRP). Furthermore, biofouling experiments were also performed using membrane fouling simulators (MFS) to simulate biofouling in spacer-filled RO membrane channels.
Results illustrated that the rejection of algal biopolymer produced by Chaetocerous affinis was 3 to 4 times higher with tight UF (10 kDa) compared to the high molecular weight cut off MF/UF membranes. The lower biopolymer concentration in permeate coincide with the lower bacterial regrowth potential. The relationship between the bacterial regrowth and biopolymer concentration was found linear with R2 = 0.88. Moreover, no substantial difference was observed in measured net bacterial regrowth in permeate collected from tight UF (10 kDa) and standard UF (150 kDa) from pilot experiments. It could be attributed to the contribution of passage of low molecular weight organics from both UF membranes. The biofouling experiments performed using MFS monitor fed with permeate of 150 kDa and 10 kDa UF at a cross flow velocity of 0.2 m/s also showed no substantial increase in the feed channel pressure drop in the MFS monitor. Moreover, the result of membrane autopsy showed biomass accumulation of 860 pg ATP/cm2 in MFS fed with 10 kDa UF permeate, which was 2 times lower than in MFS fed with 150 kDa UF permeate. Overall, the results illustrated the potential of tight UF membranes towards delaying the occurrence of biofouling in SWRO membranes. Nevertheless, the non-backwashable fouling rates development after each succeeding CEB cycles were approximately 1.5 times higher for 10 kDa UF compared to 150 kDa UF. Therefore, it is still important to improve the backwashability performance of the tight UF membrane for the better future application. It is expected that improving the surface porosity of the membrane can better remove the cake/gel layerformed on the membrane surface during backwashing/CEB and improves the backwashability of the membranes.
