Reverse osmosis (RO)

Reverse osmosis (RO) is a filtration process through which various foreign contaminants, solid substances, and large molecules and minerals can be removed from water through the application of pressure and the use of a special membrane. As its name implies, reverse osmosis is essentially the reverse effect of normal osmosis. Reverse osmosis is most commonly used in the desalination of saltwater but is also used for activities such as recycling, wastewater treatment, and energy production. In practice, reverse osmosis allows for the purification of water for drinking, cooking, and other purposes. While reverse osmosis systems are primarily used at water treatment plants and other similar locations, smaller systems can even be installed in private homes. Although reverse osmosis can be used to produce clean, purified water, some critics point out that actually drinking such water could potentially be dangerous because reverse osmosis tends to remove necessary, health-promoting minerals along with substances that are unfit for consumption.

Background

To properly understand the process of reverse osmosis, it is first necessary to understand the concept of osmosis. Osmosis is a naturally occurring process that is of critical importance to all living things. In short, osmosis is a process in which a fluid passes through a semipermeable membrane from an area where a solute is present in low concentrations to an area where the same solute is present in high concentrations.

There are a number of important terms to know when discussing osmosis and reverse osmosis. A semipermeable membrane is any sort of membrane that will allow some atoms or molecules to pass through it, but not others. The fluid that passes through a semipermeable membrane during osmosis is called a solvent. On the other hand, the dissolved substance in the fluid is called the solute. Together, the solvent and solute make up a solution. A solution with low solute levels is said to be hypotonic, while a solution with high solute levels is said to be hypertonic. When osmosis occurs, solvent molecules move through a semipermeable membrane from a hypotonic solution into a hypertonic solution. While water is the solvent most commonly associated with osmosis, other liquids or even gases can undergo osmosis as a solvent.

Perhaps the most common example of osmosis is the interaction between red blood cells and water. The cell membrane of red blood cells acts as a semipermeable membrane. When red blood cells are placed in freshwater, the concentration of ions and various other solute molecules is greater inside the cell than outside. This circumstance allows water to pass into each cell through osmosis and causes each cell to swell. If a cell absorbs more water than its membrane can sustain, it will burst.

Osmosis also plays a key role in life processes of plants. Plants rely on osmosis to absorb water and nutrients from soil. The solution found within plant roots is hypertonic, and the surrounding soil is hypotonic. Plant roots also function as semipermeable membranes. This all means that plant roots can draw in water and nutrients from the soil through osmosis.

Overview

At the most basic level, reverse osmosis is simply a variation of normal osmosis that happens in reverse. Unlike normal osmosis, however, reverse osmosis is not a naturally occurring process. Rather, it requires the application of an external force in the form of pressure. When pressure is applied to a highly concentrated solute solution, the solvent is able to pass through a semipermeable membrane to a lower concentrate solution. Reverse osmosis is most often thought of in terms of its ability to desalinate saltwater. When saltwater is put in a reverse osmosis system and pressure is applied, the water itself is pulled through the semipermeable membrane, while the larger salt particles are left behind. At the end of the process, the highly concentrated solute—salt in this case—is left on one side of the membrane while the solvent—desalinated freshwater—is left on the other.

The history of reverse osmosis dates back to the discovery of normal osmosis by French clergymen and physicist Jean-Antoine Nollet in 1748. Over the next two centuries, scientists developed and used reverse osmosis systems in their laboratories. It was not until 1950, however, that reverse osmosis began to be adapted for use outside the laboratory. That year, researchers at the University of California in Los Angeles started using reverse osmosis to desalinate seawater. Since that time, the commercial use of reverse osmosis has become increasingly common.

Reverse osmosis is used for a variety of practical purposes. It is most commonly used as a means of purifying water. Water purified through reverse osmosis is used for direct consumption and cooking purposes. Reverse osmosis purified water is also used for many industrial, agricultural, and landscaping purposes. Patients undergoing kidney dialysis are often urged to drink water purified through reverse osmosis because the process removes waste products that their kidneys may not be able to filter out. Reverse osmosis is also used to desalinate seawater or brackish water so that it can be used as drinking water. Food producers use reverse osmosis to make fruit juice concentrate, isolate whey protein from cheese, and more. In the wine industry, reverse osmosis is used to remove water from wine must and to remove excess alcohol from finished wine. Winemakers also use reverse osmosis to remove certain compounds that would otherwise cause finished wine to have unpleasant flavors. Many automatic car washes employ reverse osmosis systems as a way of preventing water spotting during the final rinse before cars pass through air-blower dryers. Military organizations sometimes use reverse osmosis systems to remove dangerous nuclear, biological, and chemical agents from contaminated drinking water or industrial water supplies.

While reverse osmosis is a very effective means of purifying water, there may be some potential drawbacks to this approach if the purified water is to be used for drinking. Essentially, reserve osmosis can actually be too good at filtering drinking water. In addition to removing dangerous contaminants, reverse osmosis can also remove certain minerals that are necessary for maintaining good health. Some of these key minerals include iron and manganese. People often rely on getting iron and manganese through their drinking water because they typically do not get enough of either from food alone. Drinking water purified through reverse osmosis for an extended period can lead to dangerous mineral deficiencies. Such deficiencies can, in turn, result in serious health consequences such as anemia or hormonal imbalances.

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