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How Does Reverse Osmosis Work?

You have probably heard the term reverse osmosis, and may even have had reverse osmosis filtered water. Most of us have reaped the benefits of reverse osmosis, even if we haven’t realized it. Reverse osmosis is used to remove contaminants from tap water, manufacture fruit juice concentrates, keep water from spotting on your car in a car wash, and even used to grow the food we eat.

But what is reverse osmosis exactly? How does reverse osmosis work? These questions have complicated answers that require gaining a better understanding of the natural process of osmosis to fully grasp. Before investing in a reverse osmosis filtration system for your home or business, it is helpful to understand what reverse osmosis is, how it works, and what the advantages and disadvantages of these systems are.

What is Reverse Osmosis?

Reverse osmosis is a process that is commonly used to remove contaminants, particulates, and other solid material from a water source. Reverse osmosis is a man-made process, unlike osmosis which occurs naturally.

The Origins of Reverse Osmosis

Reverse osmosis was first developed over two and a half centuries ago in France by Jean-Antoine Nollet, a member of the French clergy and a physicist. Over the subsequent two centuries, other scientists began to implement the reverse osmosis process in their laboratories.

The use of reverse osmosis remained isolated to laboratories until 1950 when the University of California Los Angeles began using the process in a desalination plant. The use of reverse osmosis to desalinate seawater opened the doors for the use of reverse osmosis across a wide variety of industrial and commercial applications.

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How Does it Work?

Reverse osmosis, as the name implies, is the reverse of the natural process, osmosis. To gain a good understanding of how reverse osmosis works, it is necessary to first understand how osmosis works.


Osmosis is a naturally occurring process that is necessary for life as we know it. Osmosis has a couple of moving parts that you should understand, as they will be important during a discussion of reverse osmosis. 

At a basic level, osmosis is the movement of a fluid across a semipermeable membrane from an area with a low concentration of solute to an area with a high concentration of a solute.

Let’s break down what these terms mean:

  • Semipermeable membrane – A membrane that allows only some molecules to pass through.
  • Solvent – The fluid passing through a membrane during osmosis.
  • Solute – A dissolved substance left behind after fluid passes through the membrane.
  • Solution – Both solvent and solute combined.

During osmosis, the solvent moves through the semipermeable membrane from an area with a low solute count, referred to as a hypotonic solution, to an area with a high concentration of solute, known as a hypertonic solution. This movement of the solvent across a membrane from a low concentration solution to a high concentration solution may sound odd, is easily illustrated if we look at a plant.

Plants acquire their nutrients and water through the process of osmosis. The plant’s roots go down into the soil and spread out. In the process of osmosis, the plant’s roots are the semipermeable membrane. The soil is the hypotonic solution or the solution with a low concentration of solute. The plant roots are the hypertonic solution, or the solution with the high concentration of solutes. Water is the solvent, while nutrients necessary for the plant to sustain life are the solute.

Water carrying nutrients moves from the area with a low concentration of nutrients (soil), across the semipermeable membrane (the roots), to the area with a high concentration of nutrients (the plant itself).

Osmosis isn’t confined to plants. It is a driving force behind life as we know it, and plays an important role in how the human body functions. Osmosis is the process through which our bodies maintain normal levels of salt and other minerals.

Osmosis is even the fundamental process underlying kidney dialysis, a common medical treatment. One problem that many patients who have chronic kidney failure have is a buildup of fluids. Kidney dialysis uses osmosis to remove this excess water. In kidney dialysis, the dialyzer membrane functions as the semipermeable membrane. On one side is an artificial solution that is high in sugar, while on the other side is the patient’s blood. The membrane in the middle won’t let the sugar pass through the membrane, while the extra water in the patient’s blood is diffused across the membrane into the area with a high concentration of sugar.

How Do Reverse Osmosis Systems Work?

Now that we have a firm understanding of how osmosis works, we can begin to answer the question, “how does a reverse osmosis system work?”. Reverse osmosis flips the normal process of osmosis but retains the same concepts.

Reverse osmosis is the process where a solvent moves across a semipermeable barrier from a solution with a high concentration of solute, or a hypertonic solution, to a solution with a low concentration of solute, or a hypotonic solution, using pressure.

Remember that osmosis occurs naturally, utilizing what is known as osmotic pressure to facilitate the movement of a solvent across the membrane. Reverse osmosis systems involve the unnatural movement of a solvent across a membrane from a high solute solution to a low solute solution, so the pressure required to cause that movement must be externally provided.

Perhaps the easiest way to illustrate reverse osmosis in practice is desalination. Desalination is the process of removing salt from seawater to create freshwater. Water, the solvent, is forced across a membrane using hydrostatic pressure. The solution on one side is seawater, which is rich in salt. So, seawater functions as the hypertonic solution. Water is able to move across the membrane, but the salt molecules are too large to move across as well, leaving them behind. The result on the other side is freshwater with a low concentration of salt, making it a hypotonic solution.

The example of seawater highlights the importance of the semipermeable membrane in the reverse osmosis process. If you have ever wondered, “how does a reverse osmosis filter work?”, the answer here lies in the size of the molecules that it allows to pass to the other side. In a desalination plant, the filter membrane allows smaller molecules like water to pass through, while larger molecules like salt are too big to pass through the membrane.

What are the Practical Applications of Reverse Osmosis?

Reverse osmosis is used primarily to remove contaminants from water, but it also has a wide variety of other uses. Although the solvent involved in reverse osmosis is most commonly water, other liquids and even gasses can undergo the reverse osmosis filtration process to remove unwanted particles and molecules.

Here are some of the top uses of reverse osmosis today:

  • DesalinationDesalination plants are used to convert seawater into clean water that’s drinkable. In the past, desalination plants lacked viability due to the high costs associated with them and the availability of cheaper alternatives to getting fresh water, technological advances have made large-scale desalination economically viable. Modern desalination is an important water treatment resource in many arid places. The most notable example of this is Israel, which gets roughly 55% of its water from desalination.
  • Pharmaceuticals – The production of pharmaceutical products requires the highest degree of control to maximize purity and eliminate potentially harmful substances. Water is a critical component of this, and reverse osmosis offers the most effective method of removing unwanted contaminants, bacteria, dissolved solids, and other molecules.
  • Power Plants – Power plants create steam to turn a turbine using a variety of fuel sources, including nuclear, oil, coal, or gas. One challenge that power plants faced in the past were contaminants in the steam reducing the operational efficiency of the boiler over time. The solution was to remove contaminants from the water through reverse osmosis prior to converting it to steam.
  • Winemaking – You may be surprised to find out that reverse osmosis plays an important role in winemaking. Reverse osmosis is used to remove excess alcohol from finished wines. This allows winemakers to create a consistent product, while still retaining the colors, flavors, and tannins.
  • Agriculture – Reverse osmosis systems are commonly used to help grow the food you eat. The availability of PH balanced water for agriculture has become an increasingly pressing issue over the past few decades. In agricultural applications, reverse osmosis systems are used to control the PH of the water that is used for crops. Water that contains a high concentration of carbonates, otherwise known as hard water, is too basic for agriculture applications. Farmers use reverse osmosis systems to remove these dissolved solids, giving them water that acts as a more effective vehicle for nutrient delivery to their crops.
  • Food Production – Reverse osmosis is an important process in food production for many products you use. Fruit juice concentrate is manufactured using reverse osmosis. Using reverse osmosis allows producers to retain the sugars, aroma molecules, and other components of the fruit they want while reducing the amount of water in the solution. This results in a purer product than older techniques that used evaporation. Whey protein isolate is also extracted from cheese using reverse osmosis.
  • Car Washes – Have you ever wondered why you get spots on your car when you wash it at home, but when you wash it at the car wash there aren’t any? It isn’t because you are bad at washing your car, but rather because car washes use water filtered with reverse osmosis to remove the dissolved solids that create those spots. Similarly, if you were to wash the dishes in your dishwasher using reverse osmosis water you wouldn’t have unsightly spots left on them after they dried.

Are There Any Disadvantages to Reverse Osmosis?

While reverse osmosis is a crucial process for many commercial and industrial applications and is increasingly relied on for cost-efficient residential drinking water, there are some downsides worth touching on.

The first downside is that reverse osmosis is sometimes seen as too effective at filtration by removing all of those dissolved solids from water. Reverse osmosis water has reduced mineral content. In certain parts of the world, such as developing countries, these minerals can provide a health benefit. 

In developed countries like the United States, the vast majority of people get their required minerals from their diet, so this is less of a concern.  

Closing Thoughts

Reverse osmosis is the process where a solvent, typically water, is pushed across a membrane using externally provided pressure to remove contaminants, particles, and other molecules that are too large to fit through the membrane. Desalination, or the process of turning seawater into freshwater, is a prime example of reverse osmosis. 

Reverse osmosis is not a natural process but is rather the reversal of the natural process of osmosis. Reverse osmosis requires some external force to increase the pressure of a solution to pass across a membrane that will remove unwanted molecules and dissolved solids.

Reverse osmosis is a highly effective filtration process for removing dissolved solids from water. Reverse osmosis removes not only minerals and salts from water, but it can remove bacteria, fluoride, and other dissolved solids that other filtration methods don’t. This makes it an ideal method for producing potable water in both a commercial and residential setting. 

To learn more about residential reverse osmosis drinking water systems, please contact Rayne today. We carry both residential and commercial reverse osmosis systems and have locations in both Arizona and California! Everything from reverse osmosis systems in Phoenix to water softener systems in Orange County! Check out the location nearest you!

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  1. Lasky, Jack. 2019. “Reverse Osmosis (RO).” Salem Press Encyclopedia of Science.
  2. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 15.8, Osmosis, Water Channels, and the Regulation of Cell Volume. Available from: