Direct microscopic observation of forward osmosis membrane fouling by microalgae: Critical flux and the role of operational conditions

Osmotically-driven membrane processes such as forward osmosis (FO) and pressure retarded osmosis (PRO) have interesting applications in the water and energy domains. When surface water is used as feed solution (FS) for FO and PRO, these membranes can be fouled by microalgae that are ubiquitous in rivers, lakes, and seawater. This study systematically investigates FO/PRO fouling by a model microalgae Chlorella sorokiniana. Direct microscopic observation was used to characterize FO/PRO fouling in conjunction with water flux measurements. The study demonstrated that direct microscopic observation can be highly valuable in characterizing the deposition of microalgae on FO/PRO membrane. Using a concentration stepping method, a critical flux (critical concentration) behavior was revealed. Stable flux can be achieved by operating below the critical flux (critical concentration) level. Microalgal FO/PRO fouling was more severe and less reversible when divalent Mg2+ ions were present in the FS and the draw solution (DS). The use of Mg2+-based DS can promote severe fouling due to the reverse diffusion of Mg2+ into the FS, even if Mg2+ is not present in the original FS. The use of FS spacer was beneficial in enhancing the initial flux as well as reducing the tendency of membrane fouling.