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Water Purification Systems: Reverse Osmosis versus Deionized

Both methods have their uses, but one is typically preferred over the other

Aimee O’Driscoll

Aimee O’Driscoll, BSc, MBA, has a decade of experience as a development chemist and is a seasoned science writer. She can be reached at

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Using purified water in the lab is important to ensure repeatable and predictable results, prevent contamination, and protect equipment. Bacteria in tap water can contaminate samples such as cell cultures and ions present in facility water can erode stainless steel equipment or cause deposits, impacting the function of heating elements.

Two important methods of water purification are reverse osmosis (RO) and deionized (DI) water. Both methods remove impurities from water, but they do so in different ways. Also different is the level of purity they provide, with the reverse osmosis process removing relatively large contaminants while deionization removes ions such as mineral impurities and dissolved gases.

Reverse osmosis water purification systems

RO is a process that uses pressure to force water molecules through a semipermeable membrane. This removes some impurities including bacteria and certain chemicals from the water. Reverse osmosis water can provide 90 to 99 percent purity and offers an inexpensive option because the membrane can last years without being replaced. However, the purity level is too low for many lab applications.

RO purification can generate Type III or Type IV water by ASTM International standards. This classification system considers the water’s resistivity, conductivity, and total organic content (TOC). Type III water is usually used for tasks such as media preparation, glassware rinsing, water baths, and other non-critical applications. RO is also typically used as a feed method for higher-purity systems. For example, Type I (ultrapure) water purification systems combine multiple processes including RO and DI to achieve maximum purity.

Deionized water purification systems

Charged ions present in facility water can come from a variety of sources, including minerals, pollutants, and dissolved gases. In certain cases, these ions can cause inaccurate results, raise safety concerns, and damage equipment. Deionized water purification removes these ions from the water, making it suitable for use for a range of purposes.

There are several different methods of DI water purification, but all of them involve passing the water through a special filter that removes the charged ions. DI water is purer than RO water but more expensive. Ion-exchange resins are used to remove the ions, and these must be replaced once all the ions have been exchanged. The deionization process only removes ions, so water should be pretreated with RO to remove microorganisms and organic compounds. 

DI water is usually classified as Type II water unless it has undergone additional treatment to make it ultrapure—DI water is another option for feeding Type I systems. Type II water can be used for a range of laboratory applications including sample dilution, media preparation, chromatography, electrochemistry, and radioimmunoassay, although in certain cases some of these applications may call for Type I water.

Both RO and DI water purification systems play a key role in the laboratory. RO systems offer an inexpensive method to filter out contaminants to produce water suitable for general tasks and feeding higher-purity systems. Water can then be further treated by deionization to ensure its suitability for more technical applications.