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, and others cannot. Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane. "Pure water" is a relative term. Practically speaking, no water, naturally occurring or treated by man, consists solely of the H2O molecule.

Figure 1 illustrates osmosis and the selectivity of the membrane. The semipermeable 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 pressure is applied to the concentrate side of the membrane the difference of the osmotic pressure would be deducted from the net driving pressure. The water passes through the membrane leaving most of the dissolved minerals behind. This is where the term reverse osmosis is derived. The concentrate flow becomes a greater to a lesser rather than a lesser to a greater through the use of applied pressure.

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 cations, 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 cations 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.

Although Figure 2 was sufficient to illustrate the basic RO process, the feed and concentrate ports added in Figure 3 are necessary to illustrate a continuously operating RO system. It is the intent, in some applications of reverse osmosis that the concentrate would be considered the product such as concentrating sugar out of a factories waste stream.

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The most popular membrane configuration is the spiral wound, shown below in Figure 5. Different configurations of membranes have been devised CTA and TFC membranes are the most popular each offering certain advantages. CTA type membranes can be used in chlorinated water using only a 5-micron sediment prefilter. Cellulose acetate membranes are limited to a narrow pH range (2.5 to 8.5) and a lower maximum temperature, about 85F. These membranes are biologically degradable. TFC type membranes offer a higher percentage rejection of salts and a broader pH operating range from (2.5 to 11) and can tolerate a maximum temperature of about 115F.

Low-pressure RO operation generally refers to feed pressures of less than 100 PSI. This includes most of the equipment capable of being installed under the kitchen sink and those referred to as counter top modules. Figures 6 and 7 define the counter top and under the sink RO units.

For counter top R O systems and most larger permanently installed units, the storage tanks are maintained at atmospheric pressure - the majority of under-the-sink installations utilize accumulator storage vessels where as water is added to the tank, the air charge is compressed and thus the pressure in the tank rises. It is this elevated pressure that is used to displace the drinking water from the storage tank to the faucet. The accumulated pressure in the storage tank however, acts as backpressure on the membrane, and as tank pressure increases, the differential pressure across the membrane decreases. So as the tank pressure rises, the water production rate drops and yet the salt passage continues unaffected. Thus the quality of the water being delivered drops significantly if the differential pressure is allowed to become too low. Therefore, most equipment included some provision for limiting the storage tank pressure to some value less than line pressure. A ratio of two thirds is a commonly chosen limit as shown in the configuration of Figure 8. New technology has been invented for the backpressure problem. Known as a Permeate Pump this device uses the energy of the brine water running through the RO system, to pump the permeate into your pressure tank. It does not require electricity the permeate is propelled by using hydraulic energy that normally goes to the drain unused. This isolates your tank from the membrane and lets your membrane perform like in an atmospheric tank system without the backpressure.

The flow control shown in Figure 8. is known as a Split Capillary. It is a very inefficient design the average residential system will waste more than 35,000 gallons of fresh tap water a year. This configuration should be converted to a shut off type system shown in Figure 9. when found in residential applications. When the storage tank has been filled to the point at which its pressure equals two thirds of line pressure, the permeate is diverted to drain.

To conserve water consumption in reverse osmosis devices another type of control called "shutdown" is employed in the design using a shutoff valves and is illustrated in Figure 9.

At the designed-in, present ration, the storage tank pressure will close the valve and prevent further feed to the system. The valve will open again when sufficient pressure reduction is sensed at the storage tank.