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Reverse Osmosis 101

What Is Reverse Osmosis?

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.

Reverse Osmosis is a technology that is found virtually anywhere pure water is needed; common uses include:

  • Drinking Water
  • Humidification
  • Ice-Making
  • Car Wash Water Reclamation
  • Rinse Waters
  • Biomedical Applications
  • Laboratory Applications
  • Photography
  • Pharmaceutical Production
  • Kidney Dialysis
  • Water used in chemical processes
  • Cosmetics
  • Animal Feed
  • Hatcheries
  • Restaurants
  • Greenhouses
  • Metal Plating Applications
  • Wastewater Treatment
  • Boiler Water
  • Battery Water
  • Semiconductor production
  • Hemodialysis

How Does Reverse Osmosis Work?

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 in 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 form 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 discarded 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. 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.

Inside A RO Membrane

Water enters the RO membrane under pressure and travels from the feed end to the reject end. Some of this feed water passes through the membrane and becomes purified. This water is called the RO product water. The contaminants that were previously in the product water exit the membrane in the reject water.

Several layers of membrane material are sandwiched between spacer material to form leaves with a feed / reject channel and a product channel. These leaves are then rolled around a central product collection tube. This assembly is referred to as a spiral wound membrane element.

The spiral wound membrane element is installed in a pressure vessel. A seal between the outside of the membrane and the inside of the pressure vessel prevents the feed water from flowing between the membrane and pressure vessel. This is called a brine seal. Membranes should always be installed with the brine seal on the feed end of the vessel.

The reject water exits the vessel and feed the next vessel or is sent to drain. The product water exits the vessel and is sent to a storage tank or point of use. An o-ring seal prevents the reject water from mixing with the product water.

Low Pressure (Residential) Systems

Low pressure RO systems generally refer to those systems with a feed pressure of less than 100 psig. These are the typical countertop or under sink residential systems that rely primarily on the available line pressure to make the reverse osmosis process function; a typical system is shown schematically below.

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 5 and 50 gallons per day of water, with an efficiency of 2-4 gallons of reject water per gallon of treated water. These systems typically remove greater than 90% of the dissolved solids found in the feed water. These units produce water for a cost as low as ten cents per gallon once maintenance and water costs are factored in. Maintenance usually requires replacing any pre or post filters (typically one to four times per year); and the reverse osmosis cartridge once every two to three years, depending on the feed water and usage.

High Pressure (Commercial/Industrial) Systems

High pressure systems typically operate at pressures between 100 and 1000 psig, depending on the membranes chosen and the water being treated. These systems are usually used in industrial or commercial applications where large volumes of treated water are required at a high level of purity.

Commercial / industrial systems use a pump to provide the pressure necessary to drive the reverse osmosis process. These systems typically use multiple membranes modules arranged in parallel to provide the required quantity of water. The reject water from one module can be directed into successive membrane modules for greater efficiency (see diagram below). An adequate reject flow must be maintained to prevent membrane fouling and scaling from occurring.

High pressure commercial / industrial units typically provide from 2 gallons to 400 gallons per minute of water with an efficiency of 0.3 - 6 gallons of reject water per gallon of product water produced. These systems typically remove greater than 95% of the dissolved solids found in the feed water. These systems tend to be much larger and more complicated than low pressure systems.