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

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 is 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 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.

Semi permeable 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 pre-filter for the reduction of chlorine, which can damage an RO membrane; and a sediment pre-filter 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.

Two Membrane materials are available:

• Cellulose Acetate (CA or CTA) Membrane - CA/CTA membranes can tolerate chlorine. Dissolved chlorine will pass through the CA membrane into the purified water. Systems using CA membranes include pretreatment to chlorinate feed water to 0.3 - 2.0 ppm.
  
• Polyamide (PA) Membrane - PA membranes have slightly higher ionic rejection and permeate flow rates than CA membranes. They have zero chlorine tolerance, so chlorine must be removed from RO feed water when using PA membranes.
  
• CA membranes are standard. For animal drinking water, CA membranes are preferred over PA despite lower ion rejections. They pass dissolved chlorine, which tends to keep permeated water bactericidal. Very high chemical rejections are unnecessary in most laboratory animal applications, but organism content is of paramount concern.

Spiral Wound Membrane Module

Edstrom Industries uses a spiral would membrane module, which is, basically, a membrane envelope sealed to a center tube and then rolled up like a scroll. Spiral wound modules have several advantages over other designs. They have a very high membrane area per unit volume, and the flat sheet can't break and cause permeates contamination. Spacers between membrane layers promote turbulent flow to ensure low fouling and longer life .

What Does Reverse Osmosis Remove From Water?

Reverse osmosis removes 95-99% of most contaminants including microorganisms, organic compounds, dissolved inorganic compounds, microbial by-products such as end toxins and pyrogens, and many carcinogenic compounds as shown in the table below. Performance is given as percent rejection or the percentage of the contaminants removed from a given water supply.Contaminant Standard CA Membrane

The molecular weight cut-off is based on the pore size of the membrane. Organic contaminants smaller than the cut-off size can pass through the RO membrane. To remove smaller organic contaminants, carbon filtration pretreatment can be used.