Membrane Fouling in Forward Osmosis and Pressure Retarded Osmosis: Similarities and Differences

Forward osmosis (FO) and pressure retarded osmosis (PRO) are osmotically driven membrane processes. FO utilizes the osmotic pressure difference between (Dπ) a more concentrated draw solution (DS) and a less concentrated feed solution (FS) to drive the permeation of water from FS through the selective membrane into the DS. In PRO, a hydraulic pressure less than Dπ is applied in DS to retard the osmotic permeation of the feed water, which converts the mixing energy (osmotic energy) into the mechanical energy in the DS. Both FO and PRO are attracting increasing interest in research and applications in environment and energy related fields. However, membrane fouling is an inevitable and problematic phenomenon that can reduce the efficiency of water recovery and osmotic power production in FO and PRO processes. The membrane fouling in FO has been received numerous studies in recent years, while there is few studies reporting PRO fouling.

The current study systematically investigated the membrane fouling by alginate in FO and PRO. In addition, the similarities and differences between FO fouling and PRO fouling were compared. It was found that significant alginate fouling occurred in both FO and PRO when the draw solution (DS) contained large quantities of divalent cations (i.e., Ca²⁺ and/or Mg²⁺). This is attributed to the reverse solute diffusion enhanced organic fouling, the effect of which is related to (1) the type of draw solute and the rate of its diffusion into the feed solution (FS), and (2) its ability to interact with feed foulant. It was also found that the increase of DS concentration exacerbated the FO and PRO fouling due to the synergistic effects of increased reverse solute diffusion and increased initial water flux level. However, the increase of applied hydraulic pressure mitigated the alginate fouling for NaCl draw solution but exacerbated the alginate fouling for CaCl₂ draw solution due to the competing effects between the increased reverse solute diffusion and the reduced initial water flux. It was further found that the alginate fouling was more severe under PRO operation than that under FO operation at an identical initial water flux level using seawater-based DSs due to the faster reverse solute diffusion under PRO operation. Our results provide significant implications for FO/PRO fouling control.