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How Reverse Osmosis Works

By David Paul


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Welcome to the March 2017 article from our archive of more than 2,100 technical articles. This article by David Paul examines how reverse osmosis systems work. The photo on the GWi | Ultrapure website is courtesy of Charles Bedford, David H. Paul Inc.

This article describes the general mechanisms of how and why water passes through a reverse osmosis or nanofiltration membrane. The intent is to make RO as common sense as possible. Equipped with this information, we are better able to understand some of the day-to-day issues seen in an operating plant.

RO is called a membrane technology because it requires a membrane in order to work. A membrane is a material that allows some water constituents to pass through while restricting other water contaminants from passing through. Because some things can permeate (pass through), the membrane and others cannot (they are rejected), the membrane is called semipermeable.

To illustrate the term semipermeable, think about a common kitchen food strainer (colander). The strainer allows water to pass freely through it because of the relatively large holes in it. Larger food solids do not pass through the strainer. We, therefore, can roughly say that the strainer is semipermeable, allowing water to pass through while restricting the passage of nearly everything else.

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