Reverse Osmosis

Reverse osmosis is a method of liquid filtration that removes larger molecules from smaller molecules by forcing the liquid through a membrane. The process usually involves high pressure and a membrane with holes that will only allow the smaller particles to pass through. Jean Antoine Nollet first described reverse osmosis in 1748. Studies led to the production of freshwater from seawater in the 1950s, and today over 10,000 desalination plants are in operation worldwide with a combined capacity of over 35 million cubic meters per day. Reverse osmosis should not be confused with filtration or straining. Reverse osmosis uses pressure, 600 to 1,000 pounds per square inch (psi), to achieve the osmotic process and therefore is known as the “reverse” of normal osmosis.

Common uses of reverse osmosis include the following:

• Water purification for drinking and industrial use, and wastewater purification for industrial, agricultural, and landscape use
• Water purification for patients undergoing kidney dialysis that will remove waste products that their diseased kidneys cannot remove
• Concentration of food juices without damage to proteins and enzymes
• Production of whey protein isolate from cheese manufacturing and concentration of milk to reduce the cost of shipping
• Removal of unwanted compounds in the wine industry
• Prevention of water spotting during the final rinse before the air blower dryers in the car wash industry
• Removal of water before the final boiling stage in maple syrup production
• Purification of tap water to remove chemicals that can kill sensitive organisms and cause algae growth in reef aquariums
• Desalination of sea or brackish water to obtain drinking water
• Household purification of home drinking water
• Military purification of water to remove nuclear, biological, and chemical agents in order to provide safe drinking and industrial water
• Hydrogen production to prevent mineral deposits on the electrodes

All methods of reverse osmosis require an energy input. Large desalination plants such as those in the Middle East rely on oil. They may be located near power plants to reduce energy transmission costs and to provide the power plant with cooling water. Pretreatment [Page 420]will remove larger solids and bacteria and adjust the pH of the water to prevent mineral scaling during the actual reverse osmosis process.

Most household reverse osmosis units are designed to go under the sink. They have low back pressure and therefore use large amounts of water, recovering only 5 to 15 percent of the water that enters the system. To produce 5 gallons of treated water, a home unit may discharge 40 to 90 gallons per day to the sewer or septic system. Under test conditions, a home unit can remove up to 92 percent of nitrate-nitrogen, 60 to 99 percent of total dissolved solids, 60 to 98 percent of sulfates, and 60 to 93 percent of sodium. A large-scale municipal or industrial system will have similar levels of efficiency for removal of nitrate-nitrogen, dissolved solids, sulfates, and sodium. The big difference between the home system and the larger system is that close to 50 percent of the water that enters the system will be recovered because the large-scale systems use higher pressures for performing reverse osmosis.

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