Organic fouling in pressure retarded osmosis: Experiments, mechanisms and implications

Pressure retarded osmosis (PRO) is an osmotically-driven membrane process and it has attracted increasing interest for salinity-gradient power harvesting. However, the PRO performance (both water flux and power density) can be significantly limited by membrane fouling. This study, for the first time, systematically investigated membrane fouling by organic foulants in PRO process and its effect on PRO power density. It was found that significant alginate fouling occurred 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 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 forward osmosis (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 PRO fouling control.