As the name indicates, these membranes are made by forming a thin, dense, solute-rejecting surface film on top of a porous substructure. The materials of construction and the manufacturing processes for these two layers can be different and optimized for the best combination of high water flux and low solute permeability. The water flux and solute rejection characteristics are predominantly determined by the thin surface layer, whose thickness ranges from 0.01 to 0.1 pm.
Several types of thin-film composite (TFC) membranes have been developed, including aromatic polyamide, alkyl-aryl polyurea/polyamide, and polyfurane cyanurate. The supporting porous sublayer is usually made of polysulfone. Polyamide thin-film composites, like polyamide asymmetric membranes, are highly susceptible to degradation by oxidants, such as free chlorine. Consumers must be consistent in their maintenance of the TFC systems, particularly the carbon prefiltration element, which is present to remove free chlorine (and other oxidative organics) and prevent damage and premature destruction of the TFC membrane. Although the stability of these membranes to free chlorine has been improved by modifications of the polymer formulation and the processing technique, exposure to oxidants must be minimized.
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