What is Reverse Osmosis?

Reverse osmosis is one of the most effective, economical technologies available for removing a range of impurities from water.

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 What is Reverse Osmosis?

​Reverse osmosis (RO) is a special type of filtration that uses a semi-permeable, thin membrane with pores small enough to pass pure water through while rejecting larger molecules such as dissolved salts (ions) and other impurities such as bacteria. Reverse osmosis is used to produce highly purified water for drinking water systems, industrial boilers, food and beverage processing, cosmetics, pharmaceutical production, seawater desalination, and many other applications. It has been a recognized technology for more than a century and commercialized since the 1960’s. 

 About Ro Membranes

A reverse osmosis system is built around its individual membranes.  Each membrane is a spiral wound sheet of semi-permeable material. Membranes are available in 2-inch, 4-inch, and 8-inch diameter with the 4- and 8-inch diameter sizes most commonly used in industry. The industry has accepted a 40-inch length as a standard size so that membranes from different manufacturers are interchangeable in equipment systems. One of the primary measurements of a membrane is its square footage. Membranes are available in the range of 350-450 square feet of surface area.

Semi-permeable membranes were first constructed using cellulose acetate (CA) but later the industry switched primarily to the use of a thin film composite (TFC) being placed on top of a stronger substrate. TFC membranes are primarily used today.

*Graphic of spiral wound element courtesy of Dow.

 How does Reverse Osmosis work?

Reverse osmosis is a continuously operating treatment technology that uses pressure to pass source water through a thin membrane and thereby separate impurities from water.

RO works by reversing the principle of osmosis, the natural tendency of water with dissolved salts to flow through a membrane from lower to higher salt concentration. This process is found throughout nature. Plants use it to absorb water and nutrients from the soil. In humans and other animals, kidneys use osmosis to absorb water from blood.

In an RO system, pressure (usually from a pump) is used to overcome natural osmotic pressure, forcing feedwater with its load of dissolved salts and other impurities through a highly sophisticated semipermeable membrane that removes a high percentage of the impurities. The product of this process is highly purified water.

The rejected salts and impurities concentrate above the membrane and are passed from the system to drain or onto other processes. In a typical commercial industrial application, 75% of the feedwater is purified. In applications in which water conservation is important, 85% of the feedwater is purified.

An RO system uses cross-filtration, where the solution crosses the filter with two outlets: the filtered water goes one way and the contaminated water goes another way. To avoid buildup of contaminants, cross-flow filtration allows water to sweep away contaminant buildup and enough turbulence to keep the membrane surface clean.


 Key components of an RO system

​Individual membranes are loaded into single housing, which can hold up to six membranes in series. Housings are then grouped into parallel flow streams called a stage. A single reverse osmosis system can have up to three stages in series.

The benefit of using RO stages in series is reduction of wastewater. In an efficient RO staged system, wastewater can be reduced to 15% of the total flow, whereas reject flow from a single membrane alone can be as high as 50%.

A complete RO system will also include a pretreatment cartridge filtration, a pressuring pump, and a controls system. In a small RO system, the controls may be as simple as valves and rotameters. A larger system will contain flow, temperature, and pressure transmitters, as well as control valves operated from a HMI (human machine interface) or PLC (programmable logic controller). Larger systems also incorporate VFD (variable frequency drives) for the pressurizing pump and sometimes energy recovery devices.  Evoqua Water Technologies offers a complete line of small to large industrial reverse osmosis systems.

​Advantages of Using RO​Benefits of Industrial RO Systems
  • ​Simple to operate
  • Modular design for ease of installation 
  • ​Does not require hazardous chemicals
  • Reduces water and sewer use costs
  • ​Energy efficient, especially when used instead of distillation to produce high purity water
  • ​Can be integrated with an existing membrane filtration system or ion exchange system to achieve up to 80% rinse water recycle

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With more than 50 years dedicated to reverse osmosis systems engineering, R&D, and standard product development, as well as thousands of installations around the world, you can rely on Evoqua for your most demanding liquid separation and purification needs. Our portfolio of reverse osmosis systems includes those specifically engineered and designed for large industrial applications, medium-sized commercial applications, small laboratory applications, and trailer-mounted mobile systems for temporary and emergency applications. 

Our Vantage® series single pass and double pass reverse osmosis systems are designed for a variety of industrial applications requiring high-quality equipment with a fast delivery and competitive price. These pre-engineered, pre-assembled, and factory-tested units minimize installation and start-up time. With simple utility connections and easy setup, the unit is ready for quick on-line service.