How Are Reverse Osmosis Membranes Classified in Industrial Water Treatment?

Currently, reverse osmosis membranes are mainly classified into two categories based on their chemical composition: cellulose membranes and non-cellulose membranes. Based on their physical structure, they can be broadly classified into asymmetric membranes and composite membranes.

Among cellulose membranes, the most widely used is the cellulose acetate membrane (CA membrane). This membrane has a total thickness of approximately 100 μm, with its skin layer being approximately 0.25 μm thick. The skin layer is filled with micropores with a pore size of approximately 0.5-1.0 nm, thus filtering out extremely fine particles. The porous support layer contains much larger particles, approximately several hundred nm in diameter. Therefore, this type of membrane with an asymmetric structure is also called an asymmetric membrane. In reverse osmosis operation, the cellulose acetate membrane only achieves the desired desalination effect when its skin layer is in contact with the high-pressure raw water; the contact process must never be reversed.

Non-cellulose membranes are mainly composed of aromatic polyamides, with others including piperazineamide, polybenzamide, polysulfoneamide, polytetrafluoroethylene grafted membranes, and polyethyleneimine membranes. Polyamide composite membranes, which have been developed in recent years, consist of a layer of polyester nonwoven fabric as the support layer. Because polyester nonwoven fabric is very irregular and too porous, it is unsuitable as the bottom layer of a salt barrier. Therefore, microporous engineering plastic polysulfone is cast onto the surface of the nonwoven fabric. The pore size on the polysulfone layer is controlled to approximately 15 nm. The barrier layer uses highly cross-linked aromatic polyamide, with a pore size of approximately 200 nm. The highly cross-linked aromatic polyamide is polymerized from benzotriacyl chloride and phenylenediamine. Because this membrane is composed of three different materials, it is called a composite membrane.