What is reverse osmosis and how is it best used?

March 11, 2015

In wastewater treatment, reverse osmosis is a water purification technology that, like ultrafiltration, uses a semipermeable membrane to remove larger particles from drinking water. But …

In wastewater treatment, reverse osmosis is a water purification technology that, like ultrafiltration, uses a semipermeable membrane to remove larger particles from drinking water. But in reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential, a thermodynamic parameter.

Reverse osmosis removes many type molecules and ions from solutions, including bacteria, and is used in both industry and potable-water production.

In use, solute is retained on the pressurized side of the membrane and the pure solvent passes through it. The membrane “selectively" allows smaller component solutions, such as the solvent, to pass freely, while not allowing large molecules or ions through the pores.

When forcing water through a semi-permeable membrane, pressure is applied to the solution, usually by a pump, allowing water and other molecules with low molecular weights (less than about 200 grams per mole) to pass through micro-pores in the membrane.

Most reverse-osmosis installations use a cross-flow to allow the membrane to continually clean itself. As fluid passes through the membrane the rejected species is swept away from the membrane.

Reverse osmosis in industrial and commercial applications, where large volumes of treated water are required at a high level of purity, typically operates at pressures between 100 psig and 1,000 psig, depending on the membranes chosen and the quality of water treated. Most commercial and industrial applications use multiple membranes in series. Processed water from the first treatment stage can pass 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..

Applications & industries

Reverse osmosis systems find frequent use in the following:

Boiler feed-water treatment: used to reduce solids content of waters prior to feeding into boilers for power generation or otherwise.

Pharmaceutical:an approved treatment for producing U.S. Pharmacopeia (USP) grade water used in this industry.

Food & beverage:used for both solids and liquids.

Semiconductor:an accepted component of treatment in producing ultrapure water.

Metal Finishing:successfully applied to these operations, including several types of copper, nickel and zinc electroplating; nickel acetate seal; and black dye.

Reverse Osmosis use is growing rapidly, but nowhere so fast as in power generation. Globally, water treatment for power generation is estimated at more than 30 percent of all industrial water treatment sales.

Most electric power plants using coal, gas, oil or nuclear fuel create steam that turns a turbine to produce electricity. Steam impurities cause problems and reduce the electricity produced. This costs power plants money and increases the fuel consumed to produce a given amount of electricity. In extreme cases, process water impurities can lead to damage and downtime.

Historically, power producers use a combination of coagulation, flocculation and ion exchange resin beds to process high-purity water to make steam. However, these technologies require hazardous chemicals use, 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.

Other benefits

RO is increasingly adopted by power producers for purifying boiler feed water, makeup water and in zero-liquid discharge applications. Injection of high purity water produced by reverse osmosis into a gas turbine can improve operating efficiency and increase energy output by 10 percent or more.

Other cost benefits include, for example, that prices for acid and caustic solutions continue to rise while the costs of using reverse osmosis and membrane elements is decreasing. Primary cost for reverse-osmosis systems is electricity, and since these systems consume little energy, operating costs are relatively low.

In a final note, operating efficiency runs differently for ion exchange beds and reverse-osmosis 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 passing through the system. Conversely, reverse-osmosis operating costs don’t vary with the level of dissolved solids in the feed water since operating cost is flow-rate based.

The only downtime is usually for of quarterly or semi-annual routine maintenance. Reverse-osmosis 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 reverse-osmosis in the industrial sector, particularly for power generation applications.

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