Reverse Osmosis – a Popular Water Purifying Technology

In simple terms, reverse osmosis is a water purification technology. It involves the use of a semipermeable membrane to remove ions, molecules, salts, and other impurities from drinking water.

In order to understand the concept of ‘Reverse Osmosis’, one should be conversant with the process of ‘Osmosis’.

What is Osmosis?

Osmosis is a natural phenomenon where a solution of lower concentration tends to move towards a higher concentration through a semipermeable membrane. One of the best examples of osmosis is the absorption of water from the blood by the kidneys. Plant roots absorbing water from the soil is also a good example of osmosis. This is a natural process that does not involve the application of any type of external pressure.

What is Reverse Osmosis?

As the name suggests, RO is the opposite of Osmosis. This concept involves movement of solution from a higher concentration towards a lower concentration, again through a semipermeable membrane. In this case, the process involves an application of external pressure. Thus, this is not a natural process. The simplest example that comes to mind is the desalination of water. You have the RO water purifiers at home that work on this principle.

Role of a semipermeable membrane

The semipermeable membrane is a membrane that allows the passage of certain ions and molecules while restricting others depending on the size of the molecule. The screen door is one simple example that comes to mind. It allows the air to flow but restricts the passage of pests and other impurities larger in size than the pores on the door. The Gore-Tex clothing fabric is another example of a semipermeable membrane.

How does Reverse Osmosis work?

As explained earlier, RO is not a natural process. It involves an application of pressure to force water across the semipermeable membrane. The result is that you have pure water bereft of salts and other impurities in one stream. The reject stream contains dissolved salts (95% to 99%). This stream either goes down into the drain or is recycled into the RO system.

The amount of pressure depends on the concentration of the feed. The higher the concentration, the greater is the pressure you apply. You need to apply this pressure to counter the effects of osmotic pressure.

The RO system works on the principles of cross filtration rather than standard filtration. Let us see the difference between the two. In the standard filtration method, there is a build-up of the contaminants within the filter media. This can clog the filters and require frequent maintenance. The cross filtration method has two outlets. One outlet caters to the filtered water whereas the other one takes care of the contaminated water. This process avoids the build-up of contaminants. This system allows enough turbulence to keep the membrane clean.

Which contaminants does RO eliminate?

An RO system of good quality can remove 99% of dissolved salts, ions, molecules, and other impurities like bacteria and pyrogens (harmful elements). As a thumb rule, the RO membrane does not allow any contaminant having a molecular weight of 200 to pass through it. Water has a molecular weight of 18. Similarly, it does not allow ions having higher ‘valency’ to pass through. Sodium ion is monovalent. Hence, the RO membrane allows the sodium ions to go through it.

This quality of the RO membrane is a disadvantage because it does not remove gases like carbon dioxide (CO₂). This explains why the resultant RO water is slightly acidic. It has a pH level less than 7. However, this process is effective in treating brackish surface and groundwater.

What are the uses of the RO system?

Reverse Osmosis has great use in commercial, industrial, and domestic applications. One of the simplest applications of RO is the water purifiers available in the market today. It has other uses like,

• Boiler feed-water treatment – This is an industrial use of the RO system. It reduces the solid content in water before feeding it to the boilers for generating power.
• Pharmaceutical – This technology has use in the pharmaceutical industry to provide pure water for preparing medicines.
• Food and beverage industry.
• Semiconductors.
• Metal finishing – Electroplating of metals like zinc, etc.
• Medical systems – Dialysis, etc.

Advantages of Reverse Osmosis

• It is an environmentally-friendly process as it does not use any harmful chemicals.
• It has great domestic use in home filtration systems.
• The use of RO improves the taste of water. This process involves the eliminations of dissolved minerals. These minerals and contaminants are the main causes of unpleasant smell in ground and surface water.
• The use of RO water improves the life of plumbing system because it does not contain dissolved salts and metals. It takes care of the scaling and corrosion factor.

Disadvantages of Reverse Osmosis

• The biggest disadvantage of RO systems is that they waste a large quantity of water. There is a process by which you can recycle the reject water back into the RO system. However, this increases the concentration of the feed water thereby increasing the load on the system.
• This wastewater has an effect on the home septic systems.
• The RO system requires constant maintenance like changing filters frequently.

Shortcomings of Reverse Osmosis – The solutions to overcome them

Reverse Osmosis removes dissolved salts and other contaminants from drinking water; however, it does not remove all microorganisms. This is why you have multi-filtration systems in your domestic water purifiers.

The domestic water purifiers have additional filters as listed below

• Sediment filters to remove the solid and visible impurities.
• Activated Carbon filters to remove the unpleasant smell.
• RO membrane that works on the principles of Reverse Osmosis to remove dissolved salts and other contaminants.
• Ultra-violet filters to remove the bacteria and microorganisms that manage to sneak through the RO filters.
• Ultrafiltration to remove all traces of contaminants.

Conclusion

Reverse Osmosis is an effective water purification method to remove dissolved salts and contaminants from ground and surface water. The desalination plants in the advanced countries work on this principle. It is an effective method to purify water.