Unpacking compaction: Effect of hydraulic pressure on alginate fouling

Abstract

High hydraulic 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 proposal 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 fixedflux 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 of up to 40 bar. The same concentration of sodium chloride is used as the draw solution in all trials to eliminate possible effects of draw solution composition or 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 lowsalinity 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 fouling resistance relative to RO.

Split-feed counterflow reverse osmosis for brine concentration

“…It is unclear to what extent fouling and scaling may be accelerated in CFRO systems at the relevant operating conditions. Recent experimental studies show that high pressure per se does not affect fouling [25], and CFRO experiments are needed in order to better understand fouling and scaling for these low pressure, high salinity systems. Finally, cost modeling for CFRO systems needs to be performed to better understand how competitive this technology can be with other brine concentration technologies.…”

“…This is because CFRO's maximum recovery is not limited by the burst pressure of the RO membrane, as additional stages operating at low hydraulic pressure differences can be employed to increase the recovery ratio. Additionally, CFRO's ability to operate multiple stages at low hydraulic pressure differentials instead of a single stage at a much higher hydraulic pressure differential may reduce the propensity for fouling, which is another potential benefit of the technology [25].…”

Effect of artificial cell miniaturization on urea degradation by immobilizedE. coliDH5α (pKAU17)

“…Future efforts will encompass examining the effects of the polarization layer at the capsule membrane vicinity proposed by the Pore and Polarization Transport Model consisting of the extended Nerst-Planck equation and the partitioning of the membrane solution interfaces [52,53]. Coupled to the aforementioned model are recent findings of the use of alginate in reverse osmosis research for shedding light on the mechanisms of concentration polarization and electrolyte rejection [54][55][56].…”

Energy Savings in Desalination Technologies: Reducing Entropy Generation by Transport Processes

“…Although FO seems unlikely to offer energetic advantages over direct desalination [80], its value in pretreatment and dewatering operations is well established [81]. FO appears to have reduced fouling compared to RO, although it is a misconception that this results from from the compaction of foulants by the higher hydraulic pressures of RO relative to FO [82]. The FO exchanger can be evaluated separately from the waterrecovery process that follows it.…”