February 28, 2009

Reverse Osmosis

Diffusion is the movement of molecules from a region of high concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are of water and the concentration gradient occurs across a semipermeable membrane.

Diffusion and osmosis are thermodynamically favorable and will continue till equilibrium is reached. Osmosis can be slowed, stopped or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane.

Reverse Osmosis is the movement of water molecules across the semipermeable membrane against the concentration gradient, from a region of low concentration to a region of higher concentration, by applying a pressure in excess of the osmotic pressure.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. The semipermeable membrane allows the passage of water but not ions (Na+, Ca+2, Cl-1) or larger molecules (e.g. Glucose, urea, bacteria)


- One of the methods used for desalinating water
- Purification of ethanol ( here water is present as an undesirable impurity)
- Commercial and residential water filtration

Industrial Applications:
Reverse Osmosis systems can be used to treat boiler feed water, industrial wastewater, process water and more. A few of the major uses are :

  • BOILER FEED WATER TREATMENT - RO is used to reduce the solids content of water prior to feeding into boilers for the power generation and other industries.

  • PHARMACEUTICALS - RO is an approved treatment process for the production of United States Pharmacopeia(USP) grade water for pharmaceutical applications.

  • FOOD & BEVERAGE - Water used to process food products and to produce beverages is often treated by a RO system.

  • SEMICONDUCTORS - RO is an accepted component of a treatment process to produce ultrapure water in the semiconductor industry.

  • METAL FINISHING - RO systems have been successfully applied to a variety of metal finishing operations including several type of copper, nickel and zinc electroplating; nickel acetate seal and black dye.

Cost Benefits of RO:
RO is increasingly being adopted by power producers as a treatment method for purifying boiler feed water, makeup water and in zero-liquid discharge applications. The injection of high-purity water produced by RO technology into a gas turbine can improve operating efficiency and increase energy output by 10 percent or more.

There are other cost benefits as well. For example, the prices of acid and caustic solutions continue to rise while the prices of RO units and membrane elements continue to decrease. The primary cost for operating RO systems is electricity, and since these systems consume very little energy, operating costs are relatively low.

Operating efficiency differs between ion exchange beds and RO systems. Cation and anion resin beds must be regenerated once they reach a set exchange capacity. Their efficiency is related directly to the amount of dissolved solids that pass through the system. Conversely, the operating cost for RO does not vary with the level of dissolved solids in the feed water since the operating cost is based on flow rate.

An RO system does not require significant downtime with the exception of quarterly or semi-annual routine maintenance. And RO systems are highly automated, requiring minimal operator interaction. By contrast, during regeneration, which can take up to twelve hours, ion exchange equipment cannot be used and the plant is forced to stop water production.

With such advantages, expect to see continued growth in the use of RO technology in the industrial sector, particularly for power generation applications.

By Associate Writer - Ms. Nidhi Garg

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