Unpacking compaction: Effect of hydraulic pressure on alginate fouling

High pressure is often considered to be the cause of the high fouling propensity of reverse osmosis (RO) relative to forward osmosis (FO). Several experimental studies have shown that alginate fouling is more susceptible to cleaning in FO than in RO, but the theory that foulant compaction causes this disparity seems to be contradicted by the incompressibility of alginate hydrogels. In addition, the effect of hydraulic pressure on fouling in osmotic membrane desalination has never been experimentally isolated, because fixed-flux comparisons at different hydraulic pressures require different draw solution osmotic pressures. In this study, a new approach to isolating the effect of hydraulic pressure on alginate fouling and cleaning is introduced: operating FO with elevated but equal feed and draw hydraulic pressures. The same concentration of sodium chloride is used as the draw solution in all trials to eliminate possible effects of draw solution osmotic pressure on membrane fouling or cleaning. Theoretical modeling of the effect of alginate foulant compaction on flux reveals that foulant compaction should accelerate flux decline with low salinity feeds but retard flux decline at high salinity. However, in low-salinity alginate fouling trials, for which foulant compaction should accelerate flux decline, the measured flux decline rate was not affected by hydraulic pressure. Furthermore, when fouled membranes were cleaned by increasing the feed velocity and reducing the draw osmotic pressure, there was no apparent relationship between hydraulic pressure and cleaning effectiveness. Finally, in situ visualization of foulant removal during the cleaning process revealed no difference in foulant removal mechanisms between different hydraulic pressures. These findings demonstrate that alginate gel compaction by high feed hydraulic pressure does not occur and suggest that other explanations should be sought for FO's high fouling resistance relative to RO.