How A Reverse Osmosis Membrane Works.

Let’s get technical and cut open an RO membrane and see how it works.  The common RO membrane is called a Thin Film Composite or TFC. Several manufacturers make them including KOCH, Dow, DeSal and others. Save for some personalized recipes, these different brands are pretty much identical to each other. Forget for now the questions about why this process is called reverse osmosis and let’s just focus on how these crazy things work.


The membrane is really made up of two main layers of material.

  • A polysufone membrane that allows pure water to pass through while keeping contaminants behind
  • A polyamide film that is impervious to water passage. It is used to channel the pure water into a collecting tube

In the center of the membrane is a perforated collecting tube. The pure water is all channeled into this and directed to the output of the RO machine. To each side of the perforation slot a long piece of material is glued. One side has the impenetrable polyamid layer, and across from it, on the other side of the collecting perforations is glued the polysulfone material, the actual purifying material.

Once these two materials are attached on one end, they are then spun onto the collecting tube, kind of like a roll of toilet paper. The common size of a commercial RO membrane is an 8″ diameter by a 40″ length, and under adequate pressure will produce about 5 gallons per minute. At the other far end, the very ends of the two material layers are glued together. By wrapping the membrane into a circular form, a typical membrane offers about 400 square feet of surface area.

The membranes are placed into what is called a pressure vessel and the top end of the collecting tube is capped. This allows collected water to only pass through the perforations in the tube so that the pure water can be sent to a piping network, via the open end of the collecting tube. A pump keeps the water moving forward.

There is a water inlet that essentially floods the membrane, forcing water through the polysulfone filtration material. For a familiar visual, imagine a roll of toilet paper put into a perfectly fitted stainless steel housing that has a hose connection at the top of it to let feed water in.

It’s as simple as that, The water filters through all the channels and voila’ comes out as purified water. Now, there’s a lot more to it, but this is a nice place to start. Please stay tuned in the coming weeks for more RO tech talk.