Desalination with Reverse Osmosis modules

Membrane technology

Osmosis is a natural phenomenon, without which life would be made impossible. Osmotic processes enable plants to absorb nutrients from the ground. Our kidneys purify the blood in our bodies by means of osmosis.
Although osmosis has been discovered and studied as early as 1850, it has taken us until 1960 to be able to apply the process for water desalination.

Membrane filtration has been approached as a futuristic, expensive and complex process for a very long time. However, in the past fifteen years this process has developed to a mature and reliable technique, which is very usable for (drinking) water purification.

When we are dealing with a system, consisting of two fluids that are separated by a semi permeable membrane (permeable, just not for salts) and we add salt on one side of the system, pure water will start flowing through the membrane. This flow will continue until the pressure is equal on both sides of the membrane. Afterwards the water level will be higher where the salt was added. The difference in water level, caused by the addition of a specific amount of salt, is called osmotic pressure. The osmotic pressure of seawater is around 26 bar.

We can explain the term “reverse osmosis” as follows:
To desalinate water, we must create a flow through a membrane, causing the water to leave the salty side of the membrane, flowing into the unsalted side. To achieve this, pressure must be created upon the water column on the salt side of the membrane; firstly, to remove the natural osmotic pressure and secondly, to create extra pressure on the water column, in order to push the water through the membrane. For the desalination of seawater, the pressure must be about 50-60 bars.

There are several different techniques that can be applied for water desalination. Examples are reverse osmosis, electro dialysis, distillation and ion exchange (see image).
Reverse osmosis is the most economic process for the desalination of brackish water and seawater. When we compare this process to the traditional thermic process of distillation, the capital investments and the energy use are much lower.

Reverse Osmosis modules

There are four different types of reverse osmosis modules, which are used for reverse osmosis processes, mainly desalination processes. These are the tubular, plate & frame, spiral, and hollow fiber modules.

System costs:
Tubular, plate & frame >> hollow fiber, spiral

Design flexibility:
Spiral > hollow fiber > plate & frame > tubular

Required system space:
Tubular >> plate & frame > spiral > hollow fiber

Susceptibility to fouling:
Hollow fiber >> spiral > plate & frame > tubular

Energy use:
Tubular > plate & frame > hollow fiber > spiral

The system costs for the treatment of mineral water are the same for spiral modules and hollow fiber modules. Pre-treatment costs for the purification of surface water are higher when hollow fiber membranes are used. These membranes require a more specific pre-treatment, because they are more susceptible to fouling.
The use of tubular modules and plate & frame modules may be more expensive than that of hollow fiber modules or spiral modules. The costs of the use of tubular modules and plate & frame modules are approximately equal.
The required system space for the modules is very high when hollow fiber modules or spiral modules are used. Tubular modules take up much less space.
It has been said that for reverse osmosis systems for the desalination of seawater, spiral membranes are used most often.

· Reverse Osmosis desalination installation (seawater)
· System design
· Factors that influence prestations

Go to the osmotic pressure calculator

Desalination

Desalination pre-treatment

Desalination installations

Desalination installations system design

Membrane technology

Performance influence factors

Scaling and antiscalants

Sea water desalination

If you have any questions considering this or other filtration systems, feel free to contact us







Lenntech BV

Rotterdamseweg 402 M
2629 HH Delft
The Netherlands

tel: +31 152 610 900

fax: +31 15 261 62 89

e-mail: info@lenntech.com











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