Back to Basics

Part 1: Keys to Understanding the Fundamentals of RO Systems

By Jane Kucera


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Reverse osmosis (RO) is a desalination technique developed and first applied commercially in the early 1960s. Today, RO is used for everything from drinking water and boiler feedwater to generating ultrapure water for pharmaceutical and microelectronics industries, and for advanced wastewater recovery and recycle. Despite the commonplace nature of RO, the technology is still a bit of a mystery to many operators and their teams. Understanding the technology is critical to keeping an RO system operating well at minimal cost. This article goes back to the basics of RO technology, as a refresher for some readers and as an introduction for others.

Principles of RO

Imagine a beaker divided into two equally-sized compartments by a semi-permeable membrane. The membrane allows water to pass through it but does not allow the bulk of dissolved solids to pass. A high concentration solution of dissolved solids is introduced into one compartment and a low concentration solution is added to the other compartment. Water will move in a natural process from the low concentration compartment through the membrane into the high concentration compartment. 

This process is called osmosis. Osmosis continues until the concentration of dissolved solids is the same in each compartment; this condition is called equilibrium. At equilibrium, there is a difference in height (or volume) between the two compartments, with the once highly concentrated compartment now having the same concentration but with a greater height than the other compartment. This difference in height is related to the osmotic pressure of the solution. Osmotic pressure is a function of the concentration of dissolved solids or total dissolved solids with 100 parts per million of total dissolved solids generating about 0.07 bar (1 pounds per square inch gauge) of osmotic pressure. If a pressure is applied to one of the compartments and that pressure is greater than the osmotic pressure, water will be forced to move in the direction reverse to osmosis; this is reverse osmosis. During RO, the pressurized compartment becomes more concentrated in dissolved solids, while the other compartment becomes more dilute.

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