Electrodialysis, Reverse Osmosis & Nanofiltration Technologies for Desalination

What is Electrodialysis and Electrodialysis Reversal? | What is Reverse Osmosis and Nanofiltration? | Further Information | References |

Desalination refers to any of several processes that remove the excess salt and other minerals from water in order to obtain water suitable for human and animal consumption or irrigation. Desalination provides a means of upgrading poor quality, and refers to a suite of technologies to produce freshwater from seawater or brackish water. There are two common types of desalination processes that make use of membranes that allow passage of water molecules but exclude salts and other contaminants: Reverse osmosis and electro-dialysis technologies (Wikipedia, 2006).

 

What is Electrodialysis and Electrodialysis Reversal?

Electrodialysis (ED) is a voltage-driven membrane process, where an electrical potential is used to move salts through a membrane, leaving fresh water behind. ED was commercially introduced in the 1960s, about 10 years before reverse osmosis (RO). ED has generally been used for brackish water desalination and the technology is dependent on the following principles:

·        Most salts dissolved in water are ions, and are either positively charged (cations), or negatively charged (anions). As like poles repel each other and unlike poles attract, the ions migrate toward the electrodes with opposite charge.

·        Suitable membranes can be constructed to permit selective passage of either anions or cations. In a saline solution, dissolved ions pass through selected membranes that either allow cations or anions to pass through, but not both. During this process, the salt content of the water channel is diluted, while concentrated solutions are formed at the electrodes (Krishna, 1989).

In the early 1970s, the Electrodialysis Reversal (EDR) process was introduced (Buros, 2000). An EDR unit operates on the same general principle as an ED unit, except that both the product and concentrate channels are identical in construction. At intervals of several times an hour, the polarity of the electrodes is reversed, causing ions to be attracted in the opposite direction across the membranes. Immediately following reversal, the product water is removed until the lines are flushed out and desired water quality restored. The flush takes just a few minutes before resuming water production. Flushing helps in reducing the problem of membrane fouling. This technology is usually used to desalinate brackish water, rather than high salinity water such as seawater (Krishna, 1989).

 

What is Reverse Osmosis and Nanofiltration?

Reverse Osmosis (RO) is a process that was commercialised in the 1970s (Buros, 2000). Osmosis is a natural phenomenon by which water from a low salt concentration passes into a more concentrated solution through a semi-permeable membrane. When pressure is applied to the solution with the higher salt concentration solution, the water will flow in a reverse direction through the semi-permeable membrane, leaving the salt behind. This is known as the Reverse Osmosis process or RO process. The RO process uses pressure to push saline water through a semi-permeable membrane, creating fresh water and separate concentrated brine. Nanofiltration (NF) is also a membrane process that is used for removal of divalent salt ions, such as Calcium, Magnesium, and Sulphate. RO, on the other hand, is used for removal of Sodium and Chloride. RO processes are used for desalinating brackish water, and seawater (Krishna, 1989). The NF pressure driven separation process takes place on a separation layer formed by an organic semi-permeable membrane. This technology is not as effective at filtering as RO, because the membrane's pores are typically larger than RO membranes. However NF does not require the amount of energy that RO requires.

An RO desalination plant essentially consists of four major systems:

A Pre-treatment system.

Pre-treatment is very important in RO because the membrane surfaces must remain clean. All suspended solids must be first removed and the water pre-treated so that salt precipitation or microbial growth does not occur on the membranes. This may involve conventional methods such as a chemical feed followed by coagulation/flocculation/sedimentation, and sand filtration, or pre-treatment may involve membrane processes such as microfiltration (MF) and ultrafiltration (UF) (Krishna, 1989).  

High-pressure pumps.

High pressure is needed to enable the water to pass through the membrane and have the salt rejected. The pressures generated by such pumps range from about 150 psi for slightly brackish water, to 800 - 1,000 psi for seawater (Krishna, 1989).

Membrane systems.

The membrane assembly consists of a pressure vessel with a semi-permeable membrane inside that permits the feed water to pass through it. RO membranes for desalination generally come in two types, spiral wound and hollow fibre. In the spiral wound design, the membrane envelope is wrapped around a central collecting tube. The feed water flows in a spiral path within the membrane envelope under pressure and fresh water is collected in the central tube. The amount of feed water that is discharged as concentrated salty water ranges from about 20 percent for brackish water to about 50 percent for seawater. The hollow fibre design is basically a large number of hollow membranes in a pressurised vessel. Under pressure, fresh water passes through the fibre walls, and flows in the hollow fibers for collection. However this technology is not as widely used as spiral wound technologies for desalination (Krishna, 1989).

Post-treatment.

Post-Treatment consists of stabilising the water and preparing it for distribution. The post-treatment can consist of adjusting the pH and sanitising the desalinated water and combining it with other sources of water (Krishna, 1989).

 

Further Information

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Answers.com - Desalination of Water

Texas Water Development Board - An Introduction to Thermal and Membrane Methods of Desalination

Texas Water Development Board has many excellent resources on their Desalination Activities Page

Western Australia’s Water Corporations Perth Seawater Desalination Project 

 

 

References

Buros, O.K., 2000. “The ABCs of Desalting”, International Desalination Association.

Krishna, H. J., 1989. “Introduction to Desalination Technologies”, Texas Water Development Board.

Wikipedia, 2006. “Desalination” (Online) http://en.wikipedia.org/wiki/Desalination (Accessed February 21 2007).