Since it was first introduced in the 1950s, reverse osmosis (RO) has most commonly been used for purifying water and removing salts and other impurities in order to improve the color, taste or properties of the fluid for potability. However, RO is finding increasing uses in industrial applications because of its effectiveness and cost-efficiency.
How it Works
RO involves separating water from a solution of dissolved solids by forcing water through a semi- permeable membrane. As pressure is applied to the solution, usually by a pump, water and other molecules with low molecular weights (less than about 200 grams per mole) pass through micropores in the membrane. Larger molecules are retained by the membrane.
Most RO technology uses a cross flow process to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane.
RO systems used in industrial and commercial applications, where large volumes of treated water are required at a high level of purity, typically operate at pressures between 100 and 1,000 psig, depending on the membranes chosen and the quality of the water being treated. Most commercial and industrial systems use multiple membranes in series. The processed water from the first stage of treatment can be passed through additional membrane modules to achieve greater levels of treatment for the finished water. The reject water also can be directed into successive membrane modules for greater efficiency, though flushing will still be required when concentrations reach a level where fouling is likely to occur.
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 waters prior to feeding into boilers for the power generation and other
industries.
- Pharmaceutical: Reverse osmosis 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 reverse osmosis system.
- Semiconductor: Reverse osmosis 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 types of copper, nickel and zinc electroplating; nickel acetate seal; and black dye.
RO in the Power Generation Industry
The use of RO has grown rapidly throughout the industrial sector, but in no industry more quickly than in the power generation sector. Globally, water treatment for power generation is estimated at more than 30 percent of all industrial water treatment sales.
Most electric power plants that use coal, gas, oil or nuclear fuel create steam that turns a turbine to produce electricity. Impurities in the steam can cause problems, reducing the amount of electricity that is produced. This costs power plants money and increases the amount of fuel that must be consumed to provide the necessary amount of electricity. In extreme cases, impurities in the process water can lead to damage and downtime that prevents a plant from producing electricity.
Historically, power producers have used a combination of coagulation, flocculation and ion exchange resin beds to create high purity water for making steam. However, these technologies require the use of hazardous chemicals, including sulfuric acid and caustic soda. As a consequence, many power plant operators are adopting RO membrane filtration as a water purification technology because it does not require the use of hazardous chemicals. RO provides additional operational and cost benefits as well.
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.
For more information, email info@severntrentservices.com