Demineralized water is exactly what it sounds like – it's water that has had all of the mineral impurities removed. There are several different methods that can be used to remove various minerals from water, including distillation, certain backwashing filters, and reverse osmosis (RO), but not all of these methods remove all minerals. One of the most common and effective demineralization methods is deionization (DI), which uses an ion-exchange resin to remove mineral salts. In fact, many people don't make a distinction between demineralized vs. deionized water, although they aren't necessarily exactly the same.
Although DI is a common method for demineralizing water, there is actually a difference between demineralized water vs. deionized water. Deionized water has been passed through two special types of ion-exchange resins – cation and anion – which attract positive and negative ions, respectively. Minerals that are dissolved in water (and thus cannot easily be filtered out), typically are in the form of ions, which means that they have a positive or negative charge. The anion and cation resins attract those charged particles, pulling them out of the water and exchanging them for OH- and H+ ions. When OH- and H+ are combined, they make H2O or water.
This then is how demineralized water is made with a deionizer. The water flows through the ion-exchange media and the mineral ions are removed, leaving only water behind. Although this is a very effective method for producing demineralized water, it doesn't remove other water contaminants like organic particles or any impurities that don't have a charge. It's also more expensive than some other methods of demineralization, particularly if you're not concerned about removing all of the minerals.
Distilled water is water that has been heated until it turns into a vapor, then cooled and condensed into water again. Because minerals (and many other water impurities) have a higher boiling point than H2O, they are left behind when the water turns into steam. The water that is produced by distillation is typically highly purified, especially if it's been pre-filtered first. Distilled water, then, is typically also demineralized water.
Demineralized water is not typically made via distillation, however, especially if the water doesn't need to have very high purity. The distillation process can be somewhat expensive, requiring quite a bit of time and fuel to turn the water into steam and then allow it to cool and condense. Distilled water also has to be stored carefully, because it will turn into deionized water if it's exposed to air. In cases where it's very important for water to be highly purified, however, double or triple distilled water – which has been heated and condensed several times – is often the best option.
Less expensive than deionization or distillation, oxidizing or backwashing filters can be used to remove specific minerals from water – iron, in particular. This type of backwashing filter exposes iron, sulfur, manganese, and other minerals to an oxidizer, typically greensand or another filtering media that's coated in manganese dioxide. When the minerals are exposed to the oxidizer, they become insoluble and can be filtered out by the media. This type of filter is very effective for removing minerals like iron, but they do not make completely demineralized water.
Like oxidizing filters, water softeners are very effective at removing specific types of minerals – calcium and magnesium, in particular. Softeners use ion-exchange resin, similar to how DI systems work, but instead of exchanging all charged particles for H+ or OH-, the exchange only calcium and magnesium for sodium. The resulting water is not truly demineralized, since it still contains sodium ions and possibly other minerals, but it no longer contains the "hardness" minerals that can cause limescale buildup.
A reverse osmosis (RO) system is a type of water filter that pushes pre-filtered water through a membrane with very tiny pores. Because those pores are so small, almost all contaminants are trapped and cannot pass through. While RO water is not truly demineralized, the process does remove a significant portion of mineral ions that are too large to pass through the membrane.
One of the best and most cost effective ways to produce demineralized water is to combine RO and DI systems. By passing the water first though carbon and sediment pre-filters and then a reverse osmosis membrane, very few contaminants and only a small number of mineral ions typically remain. This treated water (called permeate) can then be processed by the DI system, which will remove any remaining ions. This treatment method produces high purity water that can be used for a range of industrial and commercial purposes.
Demineralized water can be used for a range of industrial purposes, although it's available to consumers as well. It's used in car batteries and automotive cooling systems, as well as in cooling towers, boilers, and other systems where any minerals in the water could build up on surfaces and cause blockages and other problems. Demineralized water or deionized water is also required in pharmaceutical manufacturing and cosmetics industries. This type of water is used for electronics manufacture and a number of industrial processes, such as laser cutting.
Most people don't drink demineralized water because they think it tastes "flat." We're used to having some minerals in our water, giving it a specific taste, and many people notice the difference when those minerals are removed. There is some question about whether or not it's safe to drink demineralized water, but most scientists agree that it's not actually dangerous. Most people do not get a significant amount of minerals from the water they drink, and drinking water without minerals won't leech minerals out of your bones or cells. The reality is that most people prefer drinking water with some minerals in it, and it's usually less expensive than demineralized water.