Distilled and deionized water are similar in that both are subjected to a process that strips the water of its minerals. The processes are different yet both yield purified water, although distilled water is considered more pure due to its natural source (a surface spring) while the source of deionized water is usually tap water, which can contain contaminants.
Deionized Water Defined
Deionized water is made by exposing tap or natural spring water to electrically charged resins, which bind to the mineral ions or salts in the water, including calcium, iron, copper, sodium, bromide and chloride, and remove them.
Distilled Water Defined
Distilled water is also demineralized water, but the minerals are removed through distillation, which involves converting spring water to steam. Since the minerals are too heavy, they remain behind while the vapor or mist is collected as it cools and condenses to create distilled water. If distilled correctly, the water should contain only oxygen and hydrogen molecules, have a PH level of 7 and contain no other minerals, contaminants or gases.
Although distilled and deionized water are both forms of purified water, because they contain no minerals, they tend to taste bitter and not be thirst-quenching. Deionized water is sometimes marketed as an antioxidant that negates the effects of aging, but this is not supported by any medical studies. Also, when drinking distilled or deionized water, your body isn't getting essential minerals found in tap or spring water, and actually rids your body of important minerals, like calcium.
Both distilled and deionized water prevents calcium deposits and rust from clogging steam irons and won't leave mineral deposits on clothes. Distilled water is the preferred coolant for engines because it prevents mineral build-up.
Deionized water is less expensive to produce and is preferred in microbiology experiments, where it is put in an autoclave, which is an apparatus that sterilizes with steam, to kill remaining bacteria and viruses. Deionized water is also added to many cosmetics and is used in lead acid batteries for trucks.
Distilled and deionized water are not recommended for everyday human consumption because it can lead to mineral deficiency and then to osteoporosis and high blood pressure. If you are going to drink one, choose distilled water. It is more pure than deionized because of the distillation process. Deionization will not eliminate any bacteria or viruses present in the water because they resist the electrical charge that demineralizes the water.