Reverse osmosis, sometimes called hyper filtration, is the most thorough form of water filtration available. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the color, taste or properties of the fluid. Reverse osmosis can be used to produce water that meets the most demanding specifications for clean water, which is useful for both drinking and industrial purposes.
What is Osmosis? Osmosis is a natural process, known for over 200 years, on which reverse osmosis systems are based. The walls of living cells are natural membranes. This means that the membrane is selective, some materials can pass through, others cannot. The general operation of all RO modules is the same. The feed stream is supplied to the membrane and split into the permeate which has diffused through the membrane, and the concentrate which passes over the membrane, carrying away the minerals to waste.
Figure 1 illustrates osmosis and the selectivity of the membrane. The semi-permeable nature of the membrane allows the water to pass much more readily than the dissolved minerals. Since the water in the less concentrated solution seeks to dilute the more concentrated solution, the water passage through the membrane generates a noticeable head difference between the two solutions. This head difference is a measure of the concentration difference of the two solutions and is referred to as the osmotic pressure difference. This head pressure, converted to the familiar pressure units of pounds per square inch (2.31 feet of water head equals 1 psi), allows the observation of a valuable rule of thumb. That is, that each 100 mg/L total dissolved difference is equal to approximately 1 psi osmotic pressure difference.
When a pressure is applied to the concentrated solution which is great that the osmotic pressure difference, the direction of water passage through the membrane is reversed and the process that we refer to as reverse osmosis is established. That is, the membrane's ability to selectively pass water is unchanged, only the direction of the water flow is changed. Thus, as shown in Figure 2, a water treatment technique in which the water is being separated from the dissolved minerals is demonstrated.
Were the membrane to act as a perfect separator, the permeate would contain 0-mg/L total dissolved solids, no matter what the concentration on the feed side of the system. This is not the case, however. And, in fact, let us consider, for the sake of illustration, 90% rejection to be an average operating condition. By considering the mechanism of salt and water passage through the membrane, it will be clear why complete salt elimination is not possible and how operating conditions can effect permeate quality and quantity.
The membrane's ability to hold back salts while allowing water to pass is based on the fact that the salts are in solution as ions, that is, charged particles. The dissolved salts are in solution as continuous, with a positive charge, and as anions, with a negative charge. A descriptive analogy of what is happening is to consider the membrane to be a mirror. As the charged particles, ions, approach the membrane, they are repelled by a reflection of their own charge. That is, similar charges repel, just as similar magnetic poles repel each other. Therefore, the layer of water immediately adjacent to the membrane is void of charged particle, and it is this water which will subsequently diffuse through the pores and be delivered as permeate. Since the anions and citations are constantly moving around in solution, sometimes they are near enough to each other to be attracted to one another, thus canceling their individual charges. Without a net charge, these particles are free to pass through the membrane.