What Is the Difference Between Demineralized, Deionized, and Distilled Water?


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Purified feed water and process streams can be a critical aspect of ensuring consistency in production and equipment performance. While it’s easy to imagine why purified water is essential to so many industries—including cosmetics, food and beverage, power, pharmaceutical, and microelectronics, among others—you may still be wondering “What is the difference between demineralized, deionized, and distilled water?”

This article compares and contrasts these common types of purified water, and gives an overview of the basic terminology and processes involved in demineralization, deionization, and distillation.

How are demineralized, deionized, and distilled water similar?

Demineralized, deionized, and distilled water share in that they are all types of purified water, meaning that they have undergone some treatment process to remove impurities and make the treated water suitable for a given use. While they are of similarly high quality, the distinction between the three lies in which treatment process is used to produce them.

The terms “demineralization” and “deionization” are often used interchangeably, with both referring to ion exchange (IX) processes used to remove nearly all dissolved solids from a stream. Less commonly, “demineralization” is used to refer broadly to any or all treatment processes used to remove dissolved solids from a liquid, encompassing not just IX, but filtration processes such as reverse osmosis (RO) and Nanofiltration (nano) as well. For our purposes here, we’ll stick to the more common definition and look at how demineralization and deionization collectively compare to distilled water, which is that they have similatr applications.

Demineralized/DI water and distilled water alike can be used for applications requiring high purity thresholds. Common industrial uses include:

  • Pharmaceuticals and Cosmetics: DI and/or distilled water is used as an ingredient or solvent in the manufacturing of pharmaceutical products and cosmetics in order to control product quality and safety. Different methods of producing demineralized water for different uses, rinsing, and cleaning will be RO or DI while formulations will use distilled water.
  • Microelectronics and Semiconductor: Due to their high purity, DI and/or distilled water will readily dissolve ionic contaminants on the surface of sensitive electronics components, making them useful for rinsing and cleaning purposes in the microelectronics and semiconductor industries. Typically, RO/DI water is used for microelectronics as large volumes are needed and ultra-low metals content required.
  • Food and Beverage: DI water is often used to sanitize containers and equipment, while distilled water can be used as an ingredient.
  • Power: Both distillation and DI is used to pretreat feed and makeup water for cooling towers and high-pressure boilers. The high purity of distilled and DI water ensures predictable heating and pressure, protects against scaling, and minimizes energy costs. Distillation is used typically as part of ZLD in power projects. Most projects will use RO/DI for boiler feed because of cost savings
  • Other industries: DI and distilled water are both used for cleaning and rinsing applications across a wide variety of industries, as well as solvents in applications as diverse as electrocoating, where they function as a carrier for paint solids, or in research labs, where they may be used to make a variety of solutions.

Distilled water offers slightly greater purity than DI (bacteria, organics, and particulate), though owing to its greater cost, it is generally reserved for only those applications with particularly stringent purity standards such as in pharmaceuticals. It is also used as part of an evaporation process for ZLD.

How are demineralized, deionized, and distilled water different?

Demineralized/deionized and distilled water offer similarly high purity. Still, there are a few characteristics that set them apart from one another, as detailed below.

They are produced through different processes

The chief difference between DI and distilled water is the process by which they are produced. Demineralization and deionization consist of an IX process where a stream is passed through one or more IX resin beds. The specialized resins contained in the IX unit(s) remove dissolved solids based on an electrostatic attraction between the ions in solution, and those on the resin. To learn more about the IX reaction, be sure to check out our blog post What is Water Demineralization and How Does it Work?

Another form of deionization, called electrodeionization (EDI) is similar in principle to IX DI in that it removes dissolved solids based on their electrostatic charges. It differs in that it leverages electricity and semipermeable IX membranes rather than resins. EDI is used across a variety of industries for polishing post RO.

Distillation is a fundamentally different process from deionization. In distillation, a liquid is heated in a still to boiling, then the resulting water vapor is cooled in a condenser, and the purified liquid water is then captured in a sterile container. Thus, where DI removes contaminants from the water, distillation effectively removes the water from the contaminants, which are left behind in the still after the water has evaporated away. In a high-purity system, DI or RO product water will be fed to the still as makeup.

They have different operational costs

For most industrial applications, DI is far more cost-effective than distillation. This is because distillation requires significant energy expenses for heating, circulation, and cooling, especially for the large volumes of water needed to support production on an industrial scale. Over the last few decades, innovations such as vapor compression and multiple effect distillation setups have led to greater energy efficiency, but they are still costly to run comparative to other purification technologies.

The ongoing operational costs of a DI system are comparatively quite low. DI typically requires minimal energy costs, generally only to power pumps to circulate water, as well as chemical costs for resin regeneration.

They remove different contaminants

Deionization is highly effective for near total removal of ionizable substances, however, due to the nature of the IX reaction, DI is not effective for removal of non-ionic substances such as organic or biological contaminants. For this reason, it may be necessary to employ some form of filtration to pretreat a stream prior to IX.

Distillation, on the other hand, is capable of removing nearly all contaminants, including ionic organic or inorganic substances as well as biological contaminants. While distilled water offers very high purity, it may be contaminated when volatile materials rise away with the water vapor and end up in the distillate in what is known as carry-over. Distilled water will also carry over inorganic material in small amounts so the purity regarding TDS will be higher than DI.  Additionally, distilled water is able to readily dissolve materials it is exposed to, therefore, it must be stored carefully to prevent contaminants from leaching into the treated water from the air or storage containers.

How SAMCO can help

SAMCO has over 40 years’ experience in identifying appropriate demineralization technologies to help lower costs and waste volumes while increasing product quality. For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your demineralization treatment system needs.

To learn more about SAMCO’s innovative technologies for hardness and dissolved solids removal, visit our page on demineralization and ultrapure water treatment solutions here.

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