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How Reverse Osmosis Desalinators Work

"Water, water, everywhere / nor any drop to drink". See pictures of ocean currents.
©iStockphoto/Maliketh

You're setting out for a backpacking expedition and packing up the important gear you'll need for your trip. You've got your compass, a map, some comfortable hiking boots, some snacks and an army knife. Seems like you have everything you'll need, right? Well, you're missing one important item that could save your life in a pinch: a way to purify water. Without water, you're susceptible to dehydration, hypothermia or altitude sickness. A water purification system like a filter or charcoal tablets could provide you with the purified water you'll need to survive in the outdoors.

But what if you need to do more than purify the water? What if your only available water sources are saltwater? (Or as Samuel Taylor Coleridge put it in his poem "The Rime of the Ancient Mariner": "Water, water, everywhere / nor any drop to drink.") Although seawater might look tempting, its high level of salt makes it unsuitable for human consumption. Average ocean seawater contains three times the salt content found in a person's bloodstream. If you drink seawater, you'll become even more dehydrated, which could lead to seizures, kidney failure or even brain damage and death [source: Seawater Facts].

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If you're in the outdoors and the only available water is seawater, then you'll need to desalinate the water; that is, you'll need to reduce the salt content of the water. One way to desalinate water is through reverse osmosis with a reverse osmosis desalinator. This filtration process uses pressure to force water through a membrane. The solute (salt) remains on one side of the membrane, while the pure solvent (freshwater) passes to the other side. The solvent (in this case, water) moves from an area of high solute concentration to an area of low solute concentration. While osmosis was discovered as early as the 1700s, it wasn't until the 1960s that scientists were able to use the process to desalinate water [source: Water and Waste Digest]. As its name indicates, this process is the reverse of normal osmosis, in which a solvent moves with no added pressure from an area of low solute concentration to an area of high solute concentration. Not only does a reverse osmosis desalinator remove salt from water, but it also eliminates harmful bacteria and microorganisms.

On the next page, we'll take a look at the science of reverse osmosis desalinators.

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To understand the science of reverse osmosis desalinators, you should first become acquainted with a few key terms:

Desalination: Desalination is simply the process of removing salt content from water. During this separation process, the dissolved salt in water is reduced to make the water usable. Although seawater is the largest source of water on our planet, it can't be used for drinking due to its high salt content. Desalination makes seawater fit for human consumption.

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Osmosis: Osmosis is a natural phenomenon that affects a variety of biological functions in all forms of life. Osmosis does everything from allow plants to absorb nutrients from the soil to help the kidneys purify blood. An osmotic membrane, a membrane that allows water to pass through at higher levels than it does salt, allows for osmosis to occur. An osmotic membrane is semipermeable; that is, it allows some substances to pass through while others do not. Although pure water can flow freely in both directions, salt and other impurities can't pass through.

During osmosis, solvent water passes through a semi-permeable membrane toward a concentrated substance on the other side until the osmotic pressure across the membrane is equal (usually 350 pounds per square inch guage, or psig, freshwater/seawater) [source: Water and Waste Digest].

Reverse osmosis: Reverse osmosis is just like it sounds -- the exact opposite of osmosis. While in osmosis, solvent water passes through a membrane until the pressure across the membrane is equal, during reverse osmosis, a force with pressure greater than the osmotic pressure is needed to allow freshwater to pass through the membrane while salt is held back. The higher the pressure is above osmotic pressure, the more quickly freshwater will move across the membrane.

So, a reverse osmosis desalinator combines these processes to make saltwater drinkable. On the next page, we'll take a closer look at the reverse osmosis desalination process.

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During reverse osmosis, saltwater is forced through a semipermeable membrane that allows water molecules to pass through while all other impurities, including salt, are held back. Here's a look at the step-by-step process of reverse osmosis desalination:

  1. To set up a reverse osmosis desalinator, you first need an intake pump at the source of the seawater.
  2. Next, you need to create flow through the membrane. This will cause water to pass through the salted side of the membrane to the unsalted side. Pressure comes from a water column on the salted side of the membrane. This will both remove the natural osmotic pressure and create additional pressure on the water column, which will push the water through the membrane. Generally, to desalinate saltwater, you need to get the pressure up to about 50 to 60 bars [source: Lentech].
  3. Feed water is then pumped into a closed container. As the water passes through the membrane, the remaining feed water and salt solution become more concentrated. To reduce the concentration of the remaining dissolved salts, some of the feed water and salt solution is taken out of the container because the dissolved salts in the feed water would continue to increase and thus require more energy to overwhelm the natural osmotic pressure.
  4. Once water is flowing through the membrane, and the pressure is equal on both sides, the desalination process begins. After reverse osmosis has occurred, the water level will be higher on the side where salt was added. The difference in water level is caused by the addition of the salt and is called osmotic pressure; generally, the osmotic pressure of seawater is 26 bars. The quality of water is determined by the pressure, the concentration of salts in the feed water, and the salt permeation constant of the semi-permeable membrane. To improve the quality of the water, you can do a second pass of membrane.

Once the freshwater and saltwater are separated, the freshwater should be stabilized; that is, the pH should be tested to make sure it's fit for consumption.

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On the next page, we'll take a look at how reverse osmosis desalinators are used.

 

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Reverse osmosis desalinators make seawater into delicious freshwater.
Reverse osmosis desalinators make seawater into delicious freshwater.
©iStockphoto.com/romakoshel

Reverse osmosis desalinators can operate on both large and small scales. A backpacker or boating enthusiasts can purchase a reverse osmosis desalination system for personal use, or they can be acquired by larger industrial or community groups in need of freshwater.

Many communities in equatorial zones, arid environments and coastal areas are good candidates for reverse osmosis desalination systems because they generally have available seawater but lack freshwater. For example, places like California, Florida, the Caribbean, Central and South America, the Mediterranean, the Middle East and the Pacific Rim are areas in which reverse osmosis desalination could be a viable option for the production of freshwater on a large scale.

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In comparison to two other desalination processes, distillation and freeze-thawing, reverse osmosis is the most cost effective and energy efficient. For example, while distillation require 30-186 horsepower of mechanical energy to remove one gallon (3.7 liters) of water from saline solution, reverse osmosis desalination only needs about 0.5-1.4 horsepower [source: Desalination: FAQ].

In addition to being energy efficient, reverse osmosis desalinators are also smaller in size than other desalination units. On a larger scale, they are also less costly to purchase and operate. Most desalinators are run by electricity; however, if electricity is not available or too expensive, you can also use a diesel or solar-powered desalinator. Although solar powered desalinators are initially more expensive to purchase, the energy savings may pay off in the end.

To take care of your reverse osmosis desalinator, make sure to keep an eye on the day-to-day operation of the system. Make sure to adjust the calibration and pumps for leaks or structural damage. The main problem that you can run into with reverse osmosis desalinators is fouling, when membrane pours become clogged. To prevent fouling, clean the unit every four months or so and replace filter elements about once every eight weeks.

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Sources

  • "Desalination: FAQ." Blue Spring Corporation.http://www.bluspr.com/desalination_faq.html
  • "Desalination by reverse osmosis." Organization of American States.http://www.oas.org/dsd/publications/Unit/oea59e/ch20.htm
  • "Desalination with Reverse Osmosis modules." Lentech.http://www.lenntech.com/desalination-ro-modules.htm
  • "Emergency Water, Desalination and Filtration." Seapackhttp://www.sea-pack.com/emergency-backpacking-water-backpacking-gear-backpacking-equipment.html
  • "Seawater Desalination in California." California Coastal Commission.http://www.coastal.ca.gov/desalrpt/dchap1.html
  • "Seawater Desalination With Reverse Osmosis." Water and Waste Digest.http://www.wwdmag.com/Seawater-Desalination-With-Reverse-Osmosis-article2207
  • Seawater: Factshttp://www.absoluteastronomy.com/topics/Seawater
  • "Water Desalination Using Novel Method Of Reverse Osmosis Promises High Recovery Levels." Science Daily. August 23, 2009.http://www.sciencedaily.com/releases/2009/08/090819135931.htm

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