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How does a reverse osmosis (RO) system work? First you need to understand diffusion and osmosis. Diffusion is the quality water has where it always tries to spread out evenly in all directions. Osmosis is when it’s kept from being able to do that by solutes, such as minerals or other dissolved materials, whose charges pull water together on the more concentrated side of a semipermeable membrane. Reverse osmosis, then, is forcing the water back through the membrane, leaving behind solutes that include contaminants like lead, mercury, volatile organic compounds, and more. You don’t want to be drinking those—which is what makes an RO system so valuable for purifying your water.

Reverse Osmosis—The Way to Cleaner Water!

If you’re in the market for a drinking water system to provide your family with clean, safe, healthy drinking water, chances are you’ve seen a lot of water treatment jargon on various product specification pages. And in the process, you’ve probably hear the term “RO system” thrown around a lot. But what does it mean?

Maybe you know that RO stands for reverse osmosis, but you’re not entirely clear on what osmosis is, why and how it’s reversed, or how that reversal creates filtered water.

To help you understand what a reverse osmosis system is and how it works, here’s our breakdown of the science behind it all.

What Is Diffusion?

Infographic of how a reverse osmosis system helps clean your drinking water.

Osmosis is a word you might remember from science class back in school, but not in very great detail. So before we get into the mechanics of what a reverse osmosis system does, it may be useful to start by reviewing what osmosis is. But to understand what osmosis is, you first need to understand diffusion.

Diffusion is a process in nature that you’ve probably seen more times than you could ever count. It’s the natural tendency of gasses and liquids to spread evenly over time in a given space. When a gas is released in a room, it spreads out until the concentration is the same throughout.

You can see this most clearly with visible gasses like smoke—when there’s a fire, the smoke doesn’t just stay lingering around it, but rather spreads across whatever space it’s in.

The reason for this is that the molecules of this substance, whether gas or liquid, are constantly moving around, bumping into each other, and flying apart. Logically, the more clustered together the molecules are, or the more concentrated, the more likely they are to bump into each other and get flung apart. Over time, as the more concentrated areas grow less concentrated because of that, this results in even concentrations across the substance.

Think of water: no matter what, it’s evenly concentrated throughout. There aren’t chunks of more dense water amid areas of less dense water.

The basic rule of diffusion is that if have two areas of different concentration of one substance within a closed system, the molecules always tend toward moving down the concentration gradient until there’s balance.

So, for example, if you have two areas of water, one where the water is a few feet high and one where the water is a few inches high, and they’re separated by a dam, what happens when you remove the dam? The water moves from where there’s a lot to where there’s less, until the water level is equal.

What Is Osmosis?

Now that we understand diffusion, we can discuss osmosis.

Osmosis occurs when there are two areas of solution with different concentrations of solute and water, separated by a semipermeable membrane. What do we mean by a semipermeable membrane though?

A semipermeable membrane is a barrier that lets some molecules pass through but not others. Sometimes, it has openings in it that are big enough for some molecules to get through, but too small for other molecules. Other times, the membrane can have a charge, which prevents molecules with certain charges from passing through as well.

Water molecules are very small compared to many other molecules, so typically water will be able to pass through a semi-permeable membrane but some solutes will be unable. A solute is any substance that has been dissolved in a solvent to create a solution. For example, salt water is a solution where water is the solvent and salt is the solute.

In addition to not being able to pass through the membrane themselves, the solute molecules can often keep water molecules from passing through to the other side as well. Sometimes these big molecules simply get in the way and block the openings, and other times they attract the water molecules to themselves with charge.

Some solutes may have a negative charge, and some may have a positive charge. Sodium ions are positive for example, and chloride ions are negative. As we know, opposites attract, but regardless of the charge of the molecule, they can always attract water because each water molecule has a slight positive charge at one end and a slight negative charge at the other. Because the solute molecules have a stronger charge than the water, the water molecules are more attracted to the solute than to each other, and stick to it.

The end result is rather than diffusing evenly, more water will move into the area with a higher concentration of solutes.

In nature, many living things use osmosis to survive. Saltwater fish have membranes in their body to keep the water from all rushing out into the high-solute saltwater, and freshwater fish have membranes to keep water from filling them to the point of bursting. Plants and other organisms use it to regulate water in their bodies as well.

For many, osmosis is easier to understand when you can see it rather than read about it. This video from Khan Academy offers an easy and clear visual demonstration of diffusion and osmosis that’s very helpful if you’re having trouble imagining the process in your head.

What Is Reverse Osmosis?

Reverse osmosis, then, is when osmosis is reversed. Seems pretty obvious, but let’s look at what that means.

The various contaminants that are in your water can be thought of as solutes. So naturally, when you use a membrane to separate water with no contaminants and water with lots of contaminants, the water largely flows to the contaminants. The opposite of what you want.

What reverse osmosis systems do is use a lot of pressure to force the water through the semipermeable membrane in the opposite direction to the side with no solutes. The water gets pushed through the membrane, and the solutes— calcium, arsenic, and various other particles in your water—can’t follow.

The clean water that this produces, called “permeate water,” flows onward to your appliances and fixtures, and a bit of it is used to wash away the left-behind concentrates.

The benefit of this is that reverse osmosis systems remove a far more complete range of contaminants than other filtration methods. Reverse osmosis removes heavy metals like lead or mercury, volatile organic compounds from industrial or agricultural runoff, and more.

If you’re looking for the highest quality clean, safe, pure water, reverse osmosis is the way to go. And if you want to get the benefits of reverse osmosis in Barrington, IL or the greater Chicago area, the systems from Angel Water are the best value you can get. They’re certified by the internationally-respected NSF International (formerly the National Sanitation Foundation) to remove the contaminants found in local water supplies. Not only that, but they’re installed and maintained by our trained professionals who have years of hands-on experience working with reverse osmosis systems.

So simply call Angel Water at (847) 382-7800 if you have questions. We’re always happy to make this information clear for you—just like your water.

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