Modeling Transport Through Composite Membranes: Film Morphology, Support Properties and Their Impact On Flux, Rejection and Fouling

Composite membranes comprised of an ultra-thin coating film formed over a porous support membrane are the basis for state-of-the-art reverse osmosis (RO) and nanofiltration (NF) membranes, offering the possibility to independently optimize the support membrane and the coating film. However, limited information exists in the open literature about transport through composite membrane structures and the implications for NF and RO membrane performance in applications like desalination and water purification. In this study, 2D and 3D numerical simulations have been carried out to evaluate diffusive transport through a library of hypothetical composite membrane structures. Using the model we explore the impacts of the support membrane skin layer pore size and porosity, support membrane bulk micro-porosity, and coating film thickness and morphology (i.e., roughness) on solvent and solute transport through composite membranes. Results suggest that the flux and rejection of a composite membrane may be fine-tuned, by adjusting support membrane skin layer porosity and pore size, independent of the properties of the coating film. Also, water flux is unevenly distributed, creating  ‘hot spots’ of locally high flux that may govern initial stages of membrane fouling and scaling. The flux distribution and local ‘hot spots’ result from film roughness (variable thickness) in addition to underlying support membrane pore size and porosity. The analysis provides insight on how the non-trivial interaction of support properties and film roughness may result in widely varying transport properties of the composite structure. In particular, the simulations reveal inherent trade-offs between flux, rejection and fouling propensity (the latter due to ‘hot spots’), which are purely consequences of geometrical factors, irrespective of materials chemistry.