How Reverse Osmosis Works


Desalination plants use reverse osmosis to turn salty water into fresh.
Desalination plants use reverse osmosis to turn salty water into fresh.
John Zoiner/Workbook Stock/Getty Images

Don't feel stupid if you've ever seen a news story about a terrible drought, then turned to your computer to see your pretty ocean beach screensaver and thought, "Why don't they just use that?"

Of course, within a moment, you probably made a few points to yourself. One, the sea is salty. Two, salty water isn't so great for drinking or growing plants. Three, you can't just take the salt out of water, just like you couldn't dissolve the sugar out of your tea. Or can you?

Reverse osmosis is one of the processes that makes desalination (or removing salt from seawater) possible. Beyond that, reverse osmosis is used for recycling, wastewater treatment, and can even produce energy.

Water issues have become an extremely pressing global threat. With climate change come unprecedented environmental impacts: torrential flooding in some areas, droughts in others, rising and falling sea levels. Add to that the threat of overpopulation -- and the demand and pollution a swelling population brings -- and water becomes one of the paramount environmental issues to watch for in the next generation.

Water treatment plants and systems are now adapting reverse osmosis to address some of these concerns. In Perth, Australia (notably dry and arid, yet surrounded by sea), nearly 17 percent of the area's drinking water is desalinated sea water that comes from a reverse osmosis plant [source: The Economist]. Worldwide, there are now over 13,000 desalination plants in the world, according to the International Desalination Association.

But while knowing that reverse osmosis can convert seawater to drinking water is useful, what we really need to understand is how the heck the process occurs. Assuming that you have a fairly good grasp on the definition of "reverse," we better start by taking a look at how osmosis works before we put the two together.

Let's learn about osmosis by filtering through to the next page.

What's Osmosis, Anyway?

Can't quite visualize osmosis in action? Here's a handy illustration to help.
Can't quite visualize osmosis in action? Here's a handy illustration to help.

Osmosis is the passage or diffusion of water or other solvents through a semipermeable membrane that blocks the passage of dissolved solutes [source: Encyclopedia Britannica].

What, you don't get it? No fear. Most of us don't, which is why there are countless explanations and analogies to clarify osmosis. We'll explore a few of those, but first let's break osmosis down to its parts to get a grasp on it.

First, we'll make our solution. We start with a boring old cup of water. To spice things up, we'll call water the "solvent" -- which is convenient, because that's what it is. To make our solvent a little tastier, we'll dissolve in some delicious sugar. The sugar is the solute. Just to keep track, we now have water (solvent) that we've dissolved sugar (solute) in, to make sugar water (our solution).

Now that we have our solution of sugar water, we'll grab a U-tube. This is not an internet video of kittens and monkeys hugging; a U-tube is a beaker, shaped in a u-shape. Right in the middle of the tube, imagine a bit of Gore-tex that cuts the U in half. Gore-tex is our "semipermeable membrane." Gore-tex is a thin plastic, dotted with a billion tiny little holes that allow water vapor to pass through, but liquid to stay out. (Saran wrap wouldn't let anything through, and a piece of cotton fabric would let just about anything.)

In one arm of the U-tube, we pour our sugar water mixture. On another we pour our plain old water. That's when the magic of osmosis begins, if you find the movement of water magical. The level of liquid in the sugar water arm will slowly rise, as the solvent (water) moves through the Gore-tex, to make both sides of the arm more equal in a sugar-to-water ratio.

But why does that happen? Simply put, because water wants to find equilibrium. And because the one side of the arm is crowded with sugar, pure water from the other side decides to move on over to make the concentration more equal or until the osmotic pressure (the pressure that happens as the molecules move) is reached.

So there you are; osmosis is when a solvent of low concentrated solute solution moves through a membrane to get to the higher concentrated solution, thus weakening it. You did it!

Now, after showing how it only makes sense for osmosis to work in one direction, let's throw that all out the window and reverse it. Walk backward to the next page to find out more.

Osmosis Down, Flip it and Reverse it

Freddie Mercury and David Bowie both recognized that being under pressure can burn a building down, split a family in two, put people on the street and also create a seriously catchy tune. One thing they left out? That pressure also makes reverse osmosis work.

So we learned that in osmosis, a lower-concentrate solution will filter its solvent to the higher concentrate solution. In reverse osmosis, we are (literally) just reversing the process, by making our solvent filter out of our high concentrate into the lower concentrate solution. So instead of creating a more equal balance of solvent and solute in both solutions, it is separating out solute from solvent.

But as we've explored, that isn't something that solutions really want to do. How do we make reverse osmosis occur? Just like Bowie and Freddie, we put the solution under pressure. Let's take saltwater as an example:

In reverse osmosis, we'd have ourselves a saltwater solution on one side of a tank and pure water on the other side, separated by a semi-permeable membrane. We would apply pressure to the saltwater side of the tank--enough to counteract the natural osmotic pressure from the pure water side, and then to push the saltwater through the filter. (This takes about 50-60 bars of pressure [source: Lenntech]. But because of the size of the salt molecules, only the smaller water molecules would make it to the other side, thus adding fresh water to the water side, and leaving the salt on the other.

And voila, you've seen reverse osmosis. To distill it (ha!): reverse osmosis takes place when pressure applied to a highly concentrated solute solution causes the solvent to pass through a membrane to the lower concentrated solution, leaving a higher concentration of solute on one side, and only solvent on the other.

It's great to be able to define reverse osmosis at dinner parties, but there are surprisingly interesting uses for reverse osmosis that might make more compelling conversation. Let's push our way through to the next page to learn more about what we can do with reverse osmosis.

Where is Reverse Osmosis Used?

These nifty blue guys are reverse osmosis cells, the semipermeable membranes that take the salt out of water in desalination plants.
These nifty blue guys are reverse osmosis cells, the semipermeable membranes that take the salt out of water in desalination plants.
Andy Sotiriou/Photodisc/Getty Images

Unlike osmosis, we can't simply watch reverse osmosis happen in many everyday circumstances. It was only in the 1950s when researchers began exploring how to desalinate ocean water that reverse osmosis was brought up as a possibility. They found that applying pressure to the saltwater side could work to produce more fresh water, but the amount they created was extremely small and not useful on any practical scale. What changed?

A much more advanced filter, created by two UCLA scientists. The hand-cast membranes made from cellular acetate (a polymer used in photograph film) allowed larger quantities of water to move through much faster, and the first reverse osmosis desalination plant began running a small scale operation in Coalinga, California in 1965 [source: The Economist].

Which leads us to one of the most common uses of reverse osmosis we've already discussed: desalination of water. That includes large plants (there are over 100 countries using desalination) or smaller operations--for instance, the kind of filter you might take camping to ensure a healthy drinking supply [source: FDU].

Reverse osmosis is also one of the few ways that we can take certain minerals or chemicals out of a water supply. Some water sources have extremely high levels of natural fluoridation, which can lead to enamel fluorosis (mottled teeth), or the much more severe skeletal fluorosis (an actual bending of a person's bones or skeletal frame). Reverse osmosis can filter out fluoride, or other impurities, on a large scale in a way that a charcoal based filter (like the one most commonly found in homes) can't.

It's also used for recycling purposes; the chemicals used to treat metals for recycling creates harmful wastewater, and reverse osmosis can pull clean water out for better chemical disposal. But even more fun than recycling? Wastewater reverse-osmosis treatments, wherein wastewater goes through the process to create something drinkable. They've nicknamed it "toilet to tap" for a reason, and although it might give you pause, it's a promising ways for developing nations to produce drinkable water.

But reverse osmosis is used in other industries as well; maple syrup, in fact, is produced using osmosis to separate the sugary concentrate from water in sap. The dairy industry uses reverse osmosis filtration to concentrate whey and milk, and the wine industry has begun using it to filter out undesirable elements like some acids, smoke, or to control alcohol content. Reverse osmosis is used to create pure ethanol, free from contaminants.

One more fun thing about reverse osmosis is that the high pressure that makes reverse osmosis effective can actually recycle itself. High pressure pumps force water through, and the remaining salty water is shot out at an extremely high rate. If this off-shoot is put through a turbine or motor, the pressure can be reused to the pumps that initially force the water through, thus re-harvesting energy.

All this industrial jazz is great, but how does reverse osmosis technology affect you, the consumer, on a smaller scale? Find out on the next page.

Smaller-scale Applications of Reverse Osmosis

At some retail stores, you can buy jugs of reverse osmosis-treated water from vending machines.
At some retail stores, you can buy jugs of reverse osmosis-treated water from vending machines.
Cary Herz/News/Getty Images

Maybe you've decided that you'd like to get your hands on some delicious reverse osmosis water. Why don't you just pour some water into a reverse osmosis pitcher and enjoy a long, cool drink?

Well, it's not quite that simple. Because reverse osmosis requires a certain amount of pressure, you won't find a reverse osmosis filter pitcher. And if you do want reverse osmosis water running through your entire house, you are essentially committing to buying an entirely new water system. But if you just want reverse osmosis water for drinking or cooking, that doesn't mean you've committed to converting your basement to a mini-industrial reverse osmosis plant.

Your first smaller-scale option is an "under the counter" system. A reverse osmosis system is connected to the water supply under your sink, where the water passes through three to five filters to achieve purity. The filtered water is then stored in a storage tank (also under the sink). An entirely separate faucet is then installed on your sink, fed from the storage tank below. Expect to pay an average of $200-500 for a system like this. And remember that you're probably doing the installation yourself, so you might want to be fairly confident in your fix-it skills.

Maybe you're little nervous about installing an entire faucet and water system (or perhaps nervous that your landlord might not be thrilled with your DIY resourcefulness). Renters and not-so-handy folks, rejoice. There are also reverse osmosis countertop filters, which allow you to hook up a small filtration system directly from your sink. Simply attach the "feed" line to the faucet, turn the faucet on, and the water is filtered through a small system that's small enough to cram next to the microwave. The purified water line can then be placed in a pitcher for easy, accessible purified water.

But they might not be ideal for everyone; keep in mind that the countertop systems can be quite slow due to lower-flow water faucets, and they'll cost around $150 at least -- not to mention the cost of changing the filters (about $30) every few months.

Let's shoot through to the next page to see some more of the drawbacks of reverse osmosis.

Disadvantages of Reverse Osmosis

So now we've seen some of the ways we can harness reverse osmosis to work for us. But does asking nature to reverse itself necessarily a good idea? There are a few issues that arise from using reverse osmosis, and we'll start with checking out what happens in desalination reverse osmosis.

After the water is filtered, you're left with lovely drinking water. But on the other hand, you have a whole mess of salt left to deal with. What do you do with the brine, which usually contains twice the amount of salt as seawater [source: The Economist]? Is it a problem to dump that brine back in the ocean? According to the Australian Centre for Water research, salinity seems to return to normal around 500 meters (about 1,600 feet) from the source [source: The Economist]. However, no one has yet gotten clear answers about if the metals and chemicals also trapped in the brine can cause an environmental impact.

Reverse osmosis systems, in general, are also not entirely self-sustaining. Water must be pretreated with chemicals, for instance, so nothing will clog the fine membrane. And the membrane itself is not entirely easy to deal with; it must be cleaned often, and can trap bacteria. A concern unique to the desalination plants is that small fish or marine life can be sucked into the system; adjusting intake pressures and velocities can usually prevent harm.

The biggest impediment of reverse osmosis filtration systems is the cost. For a developing nation, installing reverse osmosis systems is a fairly impractical possibility. Organizations like the WHO and UNICEF consider building reverse osmosis water treatment plans -- to remove toxins or provide a clean water supply -- part of their mission.

As for individual use, reverse osmosis systems can produce frustratingly little yield. A typical system will only be able to reuse about 5 to 15 percent of the water that's being pumped in, thus leaving up to 85 percent wastewater [source: NDSU].

Reverse osmosis -- and the ways it works and doesn't work -- can be a bit daunting. But if you're thirsty for more reverse osmosis information, go to the next page where you can find a lot more information.


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