Reverse Osmosis (RO) and Water Purification

Reverse osmosis (RO) is a standard method of filtration typically used for the removal of molecules and ions present in a solution. This is done by creating a pressure gradient between the solution and a semipermeable membrane. A significant component of water purification processes, reverse osmosis involves applying pressure to the solution at hand, such that large solute molecules cannot cross the membrane. In contrast, water can readily pass through it.  

Let us now find out what is reverse osmosis and how it works.

What Does Reverse Osmosis Mean?

To understand how reverse osmosis works, it is essential to first learn what osmosis is. Diffusion, as you have learnt in previous classes, is the movement of solute or solvent molecules from an area of higher concentration to a region of low concentration. Likewise, osmosis is a special type of diffusion, wherein, water or solvent molecules pass through a semipermeable membrane, along the concentration gradient.

Note that there are several natural sources of semipermeable membranes such as goat’s bladder and sheepskin or parchment. Semipermeable membranes help allow the movement of solvent molecules, such as water, but is biased towards larger particles such as glucose and urea and do not allow them to pass. They generally contain small pores that help block contaminants in water and are commonly used in osmotic systems. 

Both diffusion and osmosis follow the laws of thermodynamics, and the processes continue until they achieve a final equilibrium. Furthermore, osmosis, as a process can be retarded, stopped, and even made to move in a reverse direction, if an ample amount of pressure is applied. 

Activity: Before you learn more about reverse osmosis, consult your mentor and find out what is osmosis and reverse osmosis. Then compile notes in your notebook.

Water purification systems usually employ the simple process of reverse osmosis to filter out impurities and undesirable components. Reverse osmosis, to put it simply, occurs when solvent molecules move from a region of lower concentration to a region of higher concentration, against the concentration gradient. To help illustrate this, let us look at the following image of a semipermeable membrane, with fresh water on one side and saltwater on the other. If regular osmosis were to take place, the water molecules would simply move from the freshwater to the salty water, with a higher concentration of solutes.  

However, in reverse osmosis, a certain amount of pressure is applied on the saltwater, such that water molecules are pushed through the membrane, and towards the freshwater, with a lower concentration of solutes.

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Figure 1: Reverse Osmosis Water Purification

Pop Quiz 1

  1. What Is Meant by Osmosis?

  1. Movement from a region of lower concentration to a region of higher concentration.

  2. Movement of solvent molecules from an area of higher concentration to a region of lower concentration.

  3. Movement of solvent molecules from an area of higher concentration to a region of lower concentration across a semipermeable membrane.

Pore Size of Semipermeable Membranes

Semipermeable membranes employed in reverse osmosis, routinely feature varying pore sizes. While smaller pores aid in more efficient reverse osmosis, bigger pores help make the process faster. Remember using strainers while making tea or coffee? That is precisely how a semipermeable membrane works. You will notice that when you use a paper towel to strain something, for instance, ground coffee liquor; you get a smoother strain, while using a strainer helps to do it quickly though the strain you obtain may not be as smooth.

Now that you know what is a reverse osmosis system, it is time to look at the various uses of reverse osmosis and what a reverse osmosis water filter system looks like.

Reverse Osmosis: Uses and Application

One of the most prominent applications of reverse osmosis lays in residential, urban and commercial water filtration systems. Seawater is desalinated, with the help of reverse osmosis. The process not only helps cut down on the amounts of salt in the water but it also efficiently filters out impurities such as pathogens, undesirable microbes, unhealthy minerals such as arsenic and other metals as well as organic impurities. Significantly, reverse osmosis may also be used to purify a specific liquid in which water is an undesirable component.

For instance, reverse osmosis is used considerably, in no small extent, in RO water purifiers.

A Brief Review of the History of Reverse Osmosis

In 1748, a scientist called Jean-Antoine Nollet first described the process of reverse osmosis using semipermeable membranes. Some years down the line, the process was eventually used to desalinate seawater, in 1950, at a laboratory in the University of California (Los Angeles).

Although several scientists demonstrated the uses of reverse osmosis as a means of purification of water, it was not used on a vast scale commercially, until the beginning of the 21st Century. More efficient membranes and desalination plants were developed eventually, making reverse osmosis a profitable venture for companies.

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Water Purification

Most water in commercial and residential spaces is purified, so as to be fit for human consumption, including drinking as well as for cooking. Additionally, water may also be filtered and made free of contaminants to be used in a variety of other applications. For instance, medical, chemical, pharmacological, as well as other industrial applications are quite common. There are several types of water purification. These include:

  1. Filtration

  2. Distillation

  3. Sedimentation

  4. Flocculation

  5. Biologically Active Carbon Processes

  6. RO Water Filtration

As mentioned above, the use of reverse osmosis as a means to filter water is a widespread procedure, now used widely in industries as well as residential spaces. Water containing contaminants such as microbes and metals is filtered out, using an RO water purifier.

In an RO UV water purifier, water is made to pass through industrially-designed semipermeable membranes, under pressure, such that it is fit for drinking and cooking. An RO water purifier is also used to desalinate seawater and is a very dependable source of potable water.

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Pop Quiz 2:

  1. Which of these are commonly used in water purification systems?

  1. Filtration

  2. Sedimentation

  3. RO Purification

  4. All of the above

What is Reverse Osmosis Water?

Like the sections emphasised above, reverse osmosis works by employing high pressure pumps to drastically increase the pressure applied on the salt or highly concentrated side of the purifier. Consequently, this forces the water to move across the membrane, and in doing so, leaves out almost over 95% to 99% of the dissolved impurities on the other side.

Moreover, the amount of pressure that the pump applies depends on the concentration of salt or impurities in the feed water. The higher the concentration, the more is the pressure required to overcome the natural osmotic pressure.

  1. In reverse osmosis water filter systems, the desalinated water is usually deionised and called permeate or product water.

  2. The water that contains all the contaminants and salt is called the reject or concentrate stream.

  3. When the feed water crosses the RO membrane under pressure, the solvent molecules steadily pass through the membrane, leaving behind the rejects or the concentrates.

  4. This is how RO water is produced.

A reverse osmosis water purification system is capable of eliminating almost up to 99% of all dissolved salts, contaminates, organic material, bacteria, colloidal matter and pyrogens dissolved in the water. It is important to note, however, they cannot remove 100% of all dissolved pathogens. Membranes fitted in RO systems tend to reject undesirable components base on their charge and size. The system generally rejects components with molecular weight more than 200. 

Thus water purification through reverse osmosis has the following features:

  1. Eliminating all dissolved contaminants such as metals and harmful salts.

  2. Applications in industrial, military and wastewater purification.

  3. It is used to desalinate incoming seawater, to make fit for consumption.

  4. A typical example of a semipermeable membrane used in water purification is cellulose acetate.

Activity: Look up the Internet for more on water purification filters and the best RO purifiers. Then discuss them with your teacher and friends. 

This was all about reverse osmosis and water purification. To know more about ultraviolet water purification and related topics refer to our comprehensive range of expertly-curated notes, free study material and sample papers. Download our Vedantu app today for enhanced access to the online interactive sessions.

FAQ (Frequently Asked Questions)

1. What is the definition of osmosis?

Ans. Osmosis is defined as the movement of solvent molecules from a region of their higher concentration to a region of lower concentration through a semipermeable membrane.

2. What is RO water?

Ans. Desalinated or purified water produced as a direct result of reverse osmosis, using a semipermeable membrane is known as RO water. RO water is free of contaminants, microbes and harmful metals, and is fit for human consumption.

3. What is RO Water Purifier?

Ans. An RO water purifier employs industrially-designed semipermeable membranes, through which reverse osmosis takes place. Water moves through the membrane under high pressure applied by a pump. RO water purifiers are a very effective means to obtain potable drinking water.

4. What does reverse osmosis do?

Ans. Reverse osmosis occurs when solvent molecules move from a region of lower concentration to a region of high concentration, against the concentration gradient. In reverse osmosis, a certain amount of pressure is applied on the saltwater, such that the water molecules are pushed through the membrane, and towards the freshwater, with a lower concentration of solutes.