What is Desalination by Reverse Osmosis?


What is desalination by reverse osmosis? In answering this question it is important to begin by understanding the different elements that are involved in this process. First is the water that has the salt in it, commonly called saline water. In simplest terms, desalinization is the process that saline water is put through in order to make it usable. In most circumstances, water that has had its saline levels reduced to levels of 500 mg/1 dissolved solids is considered safe for most domestic, agricultural, and industrial uses.

In order to get saline water to this level of usability, it must undergo one or several (depending on the saline levels) processes of desalination. One of the most popular treatment methods is called reverse osmosis. Reverse osmosis consists of four different stages. These stages consist of pretreatment, pressurization, separation, and stabilization.

Pretreatment: The water to be treated, called feedwater, is brought into a plant where it is compatible with membranes, and as a result solids are removed. This process adjusts the pH of the water, and adds a threshold inhibitor in order to control the scaling that is caused by other elements that are part of the water. This includes such materials as calcium sulphate, which is an inorganic compound. The resulting water is then pressurized.

Pressurization: The pretreated feedwater is then sent to a pump, which brings it to a pressure that is appropriate for the feedwater to pass through the membrane.

Separation: It is at this point that the separation of the feedwater and the salts that have been dissolved occurs, allowing the water that has been desalinated to pass through the membrane. This process results in two products, the new freshwater product stream and the brine, a concentrated salt solution. Obviously, this is an imperfect system that allows trace elements of salt to go through, but for all practical purposes, the water is now salt-free.

You would get a general idea of this process by taking a screen and placing it over a jar, then running saltwater through the screen. Depending on the size of the screen being used, there would be greater or smaller levels of salt remaining on the screen while a greater portion of the water went through to the jar below. Obviously, there would still be salt in the jar of water but, theoretically, there would be less salt present than before it was sent through the screen.

Obviously, this process is dependent on the reverse osmosis membranes, which are available in several different configurations, the most popular being spiral wound and a hollow fine fiber types. Both of these are used in today’s reverse osmosis desalinization processes. Both are also noted for doing a good job at what they are designed to do.

Stabilization: It is important to note that even after the treated water is completed with the separation process, it must usually still go through a process of stabilization, which is another adjustment of the pH in the neighborhood of approximately five to a value of close to seven, and removal of gasses in the product water. It is only after this is complete that the water is considered safe for use in residential, industrial and agricultural applications.

Over the years the reverse osmosis process has proven to be extremely effective at removing salt from water in order to make it available to those who use if for a wide variety of purposes. But not only has it proven to be effective in creating water that is usable, but it has proven to be an extremely cost-effective method of preparing water for use that otherwise would have been wasted.