• Plumbpedia™ - How Does Reverse Osmosis Work?

Anyone who has been through a high school science class will likely be familiar with the term osmosis. The process was first described by a French Scientist in 1748, who noted that water spontaneously diffused through a pig bladder membrane into alcohol. Over 200 years later, a modification of this process known as reverse osmosis allows people throughout the world to affordably convert undesirable water into water that is virtually free of health or aesthetic contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence to installations used in manned spacecraft.

How Reverse Osmosis Works
A semipermeable membrane, like the membrane of a cell wall or a bladder, is selective about what it allows to pass through, and what it prevents from passing. These membranes in general pass water very easily because of its small molecular size; but also prevent many other contaminants from passing by trapping them. Water will typically be present on both sides of the membrane, with each side having a different concentration of dissolved minerals. Since the water i the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane from the lower concentration side to the greater concentration side. Eventually, osmotic pressure (seen in the diagram below as the pressure created by the difference in water levels) will counter the diffusion process exactly, and an equilibrium will form.


The process of reverse osmosis forces water with a greater concentration of contaminants (the source water) into a tank containing water with an extremely low concentration of contaminants (the processed water). High water pressure on the source side is used to "reverse" the natural osmotic process, with the semi-permeable membrane still permitting the passage of water while rejecting most of the other contaminants. The specific process through which this occurs is called ion exclusion, in which a concentration of ions at the membrane surface from a barrier that allows other water molecules to pass through while excluding other substances.


Semipermeable membranes have come a long way from the natural pig bladders used in the earlier osmosis experiments. Before the 1960's, these membranes were too inefficient, expensive, and unreliable for practical applications outside the laboratory. Modern advances in synthetic materials have generally solved these problems, allowing membranes to become highly efficient at rejecting contaminants, and making them tough enough to withstand the greater pressures necessary for efficient operation.
Even with these advances, the "reject" water on the source side of a Reverse Osmosis (RO) system must be periodically flushed in order to keep it from becoming so concentrated that it forms a scale on the membrane itself. RO systems also typically require a carbon prefilter for the reduction of chlorine, which can damage an RO membrane; and a sediment prefilter is always required to ensure that fine suspended materials in the source water do not permanently clog the membrane. Hardness reduction, either through the use of water softening for residential units or chemical softening for industrial use, may also be desirable in hard water areas.


Low Pressure (Residential) Systems
Low pressure RO systems generally refer to those systems with a water feed pressure of less than 100 psig. These are the typical countertop or undersink residential systems that rely primarily on the natural water pressure to make the reverse osmosis process function; a typical system is shown schematically below.

Typical Point of Use Reverse Osmosis System
Countertop units typically have an unpressurized storage tank; Undersink units typically have a pressurized accumulator storage tank where the water pressure tends to increase as the tank fills. This pressurized system provides sufficient pressure to move the water from the undersink storage tank to the faucet. Unfortunately, this also creates a back pressure against the membrane, which can decrease its efficiency. Some units overcome this by using unpressurized tanks with a pump to get the treated water where it is needed.
Low pressure units typically provide between 24 and 35 gallons per day of water, Water purity can be as high as 95 percent of rejection. These systems can be highly affordableand can produce water for a cost as low as five cents per gallon once maintenance and water costs are factored in. Maintenance usually requires replacing any pre- or postfilters (typically one to four times per year); and the reverse osmosis cartridge once every two to three years, depending on usage.

What Reverse Osmosis Treats
Reverse osmosis can treat for a wide variety of health and aesthetic contaminants. Effectively designed, RO equipment can treat for a wide variety of aesthetic contaminants that cause unpleasant taste, color, and odor problems like a salty or soda taste caused by chlorides or sulfates.

Conclusion
Reverse osmosis is a relatively new, but very effective, application of an established scientific process. Whether it is used to meet the needs of a typical family of four, or the needs of an industrial operation requiring thousands of gallons per day, it can be a cost effective to provide the required quantity of highly treated water. With continual advances in system and membrane design that boost efficiency and reliability, RO can be expected to play a major role in water treatment for years to come.
Typical Reverse Osmosis System


Stage 1
When water first enters the R.O. system, it flows through a sediment prefilter that protects the automatic shut-off and the membrane from clogging with debris. The job of the prefilter is to filter out larger particles such as sand, silt, rust or scale, extending the life of your R.O. membrane and allowing it to tackle the smaller contaminants. In a higher output TFC unit, the prefilter also has activated carbon in it. Not only does the porous activated carbon remove chlorine particles, (which is necessary to protect the refined TFC membrane), it serves to filter out other contaminants as well.

Stages 2 & 3
Water then travels to the carbon block filters where primarily odor and taste are extracted.

Stage 4
Water then travels to the operational center of the system - the TFC membrane. Here, most particles too small to be trapped by the prefilter are removed from the water stream and rinsed to the drain. The membrane's microscopic pores allow Hydrogen and Oxygen molecules through, (and water is H2O). The majority of the dissolved solids and other contaminants are flushed into the drain's water stream and exit the system. After the membrane, the R.O. water is routed to the storage tank. The automatic shut-off tells the system when it's time to shut off or make more water.

Stage 5
When you turn on the faucet and draw water from the storage tank, it then goes through its final stage of filtration, a carbon postfilter, to remove any remaining tastes and odors before reaching your glass.

What A Reverse Osmosis System Treats:   Typical Nominal Rejection Characteristics of a Reverse Osmosis Membrane Material % Rejection   Material % Rejection   Material % Rejection Aluminum 98-99 Ammonium 86-92 Pentavalent Arsenic 94-96 Barium 96-98 Bicarbonate 90-95 Bromide 87-93 Cadmium 96-98 Calcium 84-97 Chloride 93-97 Chromate 86-92 Copper 98-99 Cyanide 86-92 Ferro cyanide 98-99 Fluoride 87-93 Iron 96-98 Lead 96-98 Magnesium 96-98 Mercury 96-98 Manganese 95-98 Nitrate 60-75 Nickle 60-75 Phosphate 98-99 Potassium 98-99 Selenium 94-96 Silicate 85-90 Silver 93-96 Sodium 87-93 Strontium 96-98 Sulfate 98-99 Sulfite 96-98 Thiosulfate 98-99 Zinc 98-99