Reverse Osmosis vs. Deionization

There are a number of processes that can be used to purify water, but two of the most common methods for industrial and commercial applications are reverse osmosis and deionization.

What is Reverse Osmosis?

The best way to explain reverse osmosis, or RI, is to first define osmosis. Using pure water and saltwater as examples, osmosis is a spontaneous process in which molecules of a low-concentrate solution (pure water) will move through a semi-permeable membrane into a higher-concentrated solution (saltwater), equalizing the solute (salt) concentration on both sides. Reverse osmosis is the opposite process in which enough pressure is applied to the higher-concentrate (salt) solution to counteract the natural osmotic pressure and push it through the semi-permeable membrane. Small water molecules pass freely through the membrane, but larger molecules (dissolved mineral salts, organics, etc.) are trapped on the other side. These contaminants are flushed from the membrane to drain as waste.

What is Deionization?

Impure water contains positive and negative contaminant ions. Deionization, or DI, is a process in which water is passed through filters containing special ion-exchange resins, which replace positive contaminant ions with positive hydrogen molecules, and negative contaminant ions with negative hydroxyl molecules, forming pure water. Eventually, all of the hydrogen and hydroxyl molecules in the resins will be displaced by contaminants, and the filter must be replaced.

RO vs. DI for Your Application

Making the best choice between reverse osmosis and deionization depends on the properties of the water to be purified and the application in which it will be used. Reverse osmosis can remove up to 99% of most contaminants, is environmentally friendly, and easy to maintain and operate. Capital and operation costs are usually lower than with DI systems. It works particularly well for water containing high amounts of dissolved solids, desalination, and removal of impurities like ethanol. However, RO usually requires some degree of water pretreatment, and has a more limited temperature range than DI.

Deionization systems can handle a wider range of water chemistries than RO systems and can be used with higher temperatures. DI produces ultra-purified water and is often used in laboratories. Systems with high flow rates can be more cost effective with DI than RO with certain water chemistries, and pre-treatment is not usually required. On the negative side, deionization is usually more expensive than reverse osmosis and requires use of hazardous acids and chemicals that can be dangerous for workers and increase insurance costs.

PJM can help you choose between reverse osmosis and deionization water purification systems for your application. For more information, contact us at 609-921-1394.

About PJM

PJM Mechanical Contractors, Inc. is a multi-faceted mechanical contractor providing construction and installation, service and maintenance, and design/build services in a comprehensive range of mechanical disciplines that include HVAC, plumbing, controls, refrigeration, custom metal fabrication, piping, fire protection, and backflow preventer testing. PJM primarily serves clients in pharmaceuticals, manufacturing, healthcare, institutional, and other various commercial and industrial sectors.

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