How Does Reverse Osmosis Work in Desalination?

Desalination was a very difficult and expensive process in the past, but now reverse osmosis makes it much easier.

Increasing water scarcity forces different Governments to use seas as water sources, especially in Caribbean countries and Latin America. That’s why the global water desalination equipment market will reach 22.79 billion by 2028 (Source: prnewswire.com). 

And the major portion of the money is hidden in reverse osmosis systems that play an essential role in desalination. 

So it’s important to know how the reverse osmosis system works in desalination. Is reverse osmosis effective for desalination? We will answer all these questions in today’s article, so keep reading.

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What Desalination Is?

What Desalination Is?

Seawater is full of dissolved salt and other concentrated minerals, and desalination removes all these minerals.

Most people think that desalination is boiling and re-condensation of seawater to remove salt and other minerals behind; this is distillation not desalination. 

Currently, desalination has completely changed; now, two processes are used for desalination, is reverse osmosis and multi-stage flash. In this article, we will talk about reverse osmosis because it’s excessively used.

Let’s Understand The Concept Of Reverse Osmosis 

Before knowing the importance of Reverse Osmosis in desalination, first of all, understand the concept of Reverse osmosis.

Reverse osmosis is used to remove large portions of contaminants. In this process, water is moved through a semipermeable membrane under pressure, and the contaminants become separate from clean water. 

Ro membrane comes in two varieties one is spiral wound, and the other is fine hollow fiber. Both are made from cellulose acetate and aromatic polyamides, and both are used for the desalination process. 

According to a report, Reverse Osmosis is a very effective way of filtration that removes 90-99% of contaminants, including salts and other minerals. In short, any contaminants which have larger molecules than water are removed in RO.

How Does Reverse Osmosis Work In Desalination?

How Does Reverse Osmosis Work In Desalination?

After understanding the pillar concept of desalination and reverse osmosis, you can easily understand how reverse osmosis works in desalination. 

In desalination, the Reverse osmosis process separates the dissolved salts from saline solution by flowing water through a permeable membrane. The water is forced to flow through the membrane by creating a pressure differential between product water and feedwater. 

Note:
The Desalination process includes three liquid streams, Saline feedwater, low-salinity water, and concentrated salinity water.

In desalination, reverse osmosis involves four major processes that you should understand.

Pretreatment: 

This process makes the feedwater compatible with the reverse osmosis process. There are many large particles and even stones present in seawater that ruin the RO membrane. So pretreatment is held to remove suspended solids, adjust the pH etc.

Pressurization:

 In this process, a pump increases the pressure of pretreated feedwater to make pressure appropriate for the membrane.

Separation: 

This is the most important and pillar part of desalination; in this process, the RO membrane blocks all the dissolved salts and permits the desalinated product water to pass through. But during this process, few dissolved salts still pass through the membrane, as no membrane is perfect in blocking dissolved minerals. 

Stabilization:

This is the last process that stabilizes the pH of water. The product water from the membrane passes through an aeration column in which pH is adjusted between 5 to a value close to 7.  

Does Reverse Osmosis In Desalination Reduce The PH?

After a reverse osmosis process, water becomes slightly acidic, but it doesn’t cause any problem because contaminants are removed. But this will only happen when it’s exposed to air.

Before, many minerals were present in water, responsible for much of alkaline water’s health benefits. Usually, the RO water has a natural pH level between 5-7 and is human-friendly. 

Mineral water 7.4
Tap Water 6 – 8
Reverse Osmosis 5 – 7
Soft Drinks 2.5 

Problems With Reverse Osmosis In Desalination

Although reverse osmosis is one of the easiest and convenient methods for desalination, there are still a lot of problems with it that require day-to-day monitoring. 

The main problem with the reverse osmosis system is fouling. This problem occurs when membrane pores are clogged with dissolved minerals and salts. It not only reduces the pressure of water permitting but also affects the filtration process. 

But this problem can be addressed by cleaning or backwashing the RO membrane every 3-4 months. In reverse osmosis, you must change the cartridge filter elements after 7- 8 months to ensure proper filtration.

Other maintenance includes pump adjustments, instrument calibration, chemical feed inspection, leak detection, structural repair etc.

Pros And Cons Of Reverse Osmosis In Desalination

Advantages:

  • RO systems are easy to install and requires low maintenance as compared to other traditional methods.
  • Requires low installation cost.
  • Use low energy (from 1 to 3 kWh per 1 0001 of product water) to process brackish water.
  • RO is very effective in removing contaminants and other organic minerals.
  • You don’t need to use chemicals in this process. That’s why it maintains the pH of the water and makes it healthy for humans.
  • RO has no or negligible environmental impact. 

Disadvantages:

  • RO membranes are sensitive and clogged with dissolved minerals and salts.
  • You have to change the RO membrane after 3-4 months.
  • The feed water needs to be filtered first to remove particulates (that damage the RO membrane).
  • RO plants need high standard equipment, materials and need for foreign assistance to design. 
  • These technologies require a reliable energy source.

Companies That Offers Reverse Osmosis Equipments For Desalination

  • Aquatech International LLC
  • Biwater Holdings Limited
  • Acciona S.A
  • Ide Technologies
  • Genesis Water Technologies
  • Xyles
  • Veolia
  • Suez

Other Methods Used For Desalination

Apart from reverse osmosis, there are a lot of methods used for desalination; some of the other common methods include.

  • Desalination by distillation (traditional method)
  • Clarification using plants and plant material
  • Disinfection by boiling and chlorination
  • Wastewater treatment technologies
  • Filtration system
  • Wastewater reuse
  • Automatic surge flow and gravitational tank irrigation systems
  • Dual water distribution

Final Words

I hope you will get a brief idea about how reverse osmosis works in desalination and its advantages. At present, most companies prefer RO plants as they are effective and easy to remove salinity from seawater and make it drinkable. 

The RO equipment market is becoming wider and wider, and now people also prefer to use Reverse osmosis systems in homes. 

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