Deionized water and distilled water are both types of high purity water, but they are produced in different ways. Depending on the source water, distilled water can be more pure than deionized water – but that doesn't necessarily mean that it's better. There are pros and cons to using deionized water vs. distilled water for particular processes, particularly when it comes to cost and efficiency.
Before you decide which is better, distilled vs. deionized water, you need to understand the differences. Deionized (DI) water is water that has been treated to remove all ions – typically, that means all of the dissolved mineral salts. Distilled water has been boiled so that it evaporates and then re-condensed, leaving most impurities behind.
Organic materials and inorganic minerals are the most common impurities found in water. The organics can typically be removed via filtering methods, including physical filters, carbon filters, and reverse osmosis (RO) membranes. After this pre-treatment, the water can be sent through a DI system, which contains two types of resin: cation and anion. These two resins attract positive and negative ions, respectively, replacing them with H+ and OH-. H+ combined with OH- becomes H2O – water. The combination of filters and DI resins can remove nearly all contaminants.
Distillation is one of the oldest methods of purifying water. Filtered water is heated until it has evaporated, turning into steam. This steam is collected in a sterile container, where it condenses and becomes water again. Because water has a lower boiling point than most contaminants (including minerals), they are left behind when the water turns into steam. The resulting water is, therefore, very pure. In addition, some water is double or triple distilled, with the condensed water being boiled and condensed a second or third time.
When it comes to distilled water vs. deionized water, both are very pure. In each case, however, the purity of the water before it goes through the water treatment makes a difference. The deionization process, for example, only removes ions – charged non-organic particles – from the water. The water should be filtered first to remove organic material, and additional filtering with a reverse osmosis (RO) system will remove a significant number of additional contaminants. This leaves only a small amount of ionized minerals for the DI system to remove.
Water distillation, on the other hand, can remove more impurities than just ions. This process removes nearly all minerals, many chemicals, and most bacteria. That doesn't mean that it removes everything, however, especially if the water contains volatile organics and certain other contaminants. These impurities will evaporate and stay in the distilled water. As with deionized water, pre-treatment filtering is an important step.
Since both treatment methods produce high purity water, choosing between deionized water vs. distilled water often depends on how you're using it. Distilled water is often more pure, especially if it's been filtered first, and it should not contain any bacteria or other pathogens which could, in theory, be left in DI water. Distilled water, especially if it's been double or triple distilled, can be used for nearly all laboratory applications, including those in which DI water might not be pure enough.
That said, deionized water is a good option for many uses, including cooling applications, many laboratory uses, pharmaceutical industries, and more. Unless very high purity water is required, deionized water is often a better alternative because it can be made more quickly and for less money.
When extremely high purity water (double or triple distilled) is not required, many people look to the cost of deionized water vs. distilled water when choosing between the two. The distillation process can take a relatively long time, especially when large amounts of water need to be boiled, cooled, and collected. In addition, this process requires fuel to heat the water and a sterile container to store it in. When distilled water is exposed to the air over time, it essentially becomes deionized water.
Deionization, on the other hand, can be performed relatively quickly – especially if a mixed bed resin is used, so the water only needs to pass through one time. Many systems use two mixed bed cartridges or tanks, helping to ensure that all ions have been removed, but it's still a relatively fast process when compared to distillation. In addition, deionization is a chemical process, so energy is only typically needed to monitor the process and move the water through the system. If the DI resin is regenerated on site, this can add both time and expense to the process, however.