Study On Membrane Fouling During Osmotically Driven Processes Via Optical Coherence Tomography

Osmotically driven membrane processes(ODMPs)have been gaining more attention owing to various advantages that are not available for pressure-driven membrane processes(PDMPs),e.g.,the potential to consume less energy during desalination or water/wastewater treatment and mild operating conditions for the treatment of bio products.However,a major obstacle for the application of ODMPs is membrane fouling that could severely decrease the efficiency of forward osmosis and thereby markedly increase the cost of operation(e.g.,more frequent replacement of the membranes).It is therefore of great value to explore novel techniques for characterizing the mechanisms underlying the complex fouling phenomena,which will in turn offer powerful tools to mitigate the membrane fouling during ODMPs.This study employed optical coherence tomography(OCT)to analyze the membrane fouling during ODMPs.The OCT-based approach enabled in-situ characterization of the cake layer formed on the membrane surface;the OCT datasets were exploited to generate a series of digitalized cake layers,which were further used to assess the variations in the surface coverage and the average thickness of the cake layer.This study implemented a series of characterization experiments using nanofiltration(NF)-like membranes that were fabricated via the Layer-by-Layer(Lb L)technique,i.e.,the alternating deposition of oppositely charged polyelectrolytes onto a porous polyacrylonitrile(PAN)substrate.In order to efficiently create the osmotic pressure difference across the NF-like membrane,divalent salts were employed as the solute in the draw solution,including sodium sulfate(Na2SO4)and magnesium chloride(Mg Cl2).Moreover,both bentonite particles and whey protein were employed as the model foulants to present a comparative study.In a way of measuring the critical flux,the OCT-based characterization was incorporated with the constant pressure mode to reveal the fouling behavior during the ODMPs with various operating conditions.It was demonstrated that the critical flux could be significantly decreased in the existence of divalent cations,which were transferred from the draw solution to the feed solution,i.e.,the intrinsic back diffusion phenomenon during ODMPs.The evolution of the digitalized cake layer indicated that not only the foulant-membrane interaction but also the foulant-foulant interaction was substantially increased owing to the bridging effect of the divalent cations(i.e.,the metal-ligand complexation).In addition,the distribution of the local growth rates was determined to unravel the role of the draw solutes in varying the coupling effect between the momentum and mass transfer in the boundary layer.This study confirms that the OCT-based characterization is an effective way to exploring the fouling phenomena during ODMPs.All the characterization results provide deeper insights into the mechanisms accounting for the complex interplay between the membrane and foulants.On the other hand,more research is necessary to explore the full potential of ODMPs primarily by addressing the concerns relevant to the negative effects resulting from the use of divalent salts in the draw solution.