Deionization – also known simply as "DI" – is a technique used to remove a large number of contaminants from water. A DI tank contains one of two types of special resin (or a mix of the two), each of which removes positively or negatively charged particles from the water. Most non-organic contaminants in water are ionic minerals, including iron, copper, and calcium. When water containing these charged minerals passes through the deionization resin, they are attracted to the resin, which exchanges these ions for H+ and OH-. Contaminants are pulled out and pure water is left behind.
Deionized water tanks usually contain either anion media, cation media, or a mixture of the two types. Anion media is positively charged and attracts negative ions (like sulfates), while cation media is negatively charged and attracts positive ions (like iron). In many DI tank systems, a mixed media bed is used that combines anion and cation media; this type of system tends to be more efficient and easier to use. Water flows into the DI tank and through the deionizer resin media. Once it has passed through, it flows up a center tube and out of the tank. If the deionized water tanks only contain one type of resin, the water would then be sent to a tank containing the second resin.
High purity water deionization systems often include two DI tanks with a mixed bed resin in each. The water passes through the first "worker" tank, which removes the majority of ions, then is sent through the send tank to "polish" the water and eliminate any ions that remain. Once the resin in the first tank has been exhausted, it's removed and the resin replaced, with the "polishing" tank becoming the "worker" and the new media taking over the polishing role.
Periodically, deionized water tanks need to be backwashed to rinse out any particles that have collected in the resin and loosen the bed. The resin can become packed together and settle over time, so the backwash moves it around and ensures that it's working at its best. Backwashing cycles are usually scheduled for the overnight hours, when the system isn't being used.
Over time, the resins in the DI tank become exhausted, with reduced ability to attract ions from the water. The media can be regenerated, but this is a complicated process. Deionization works in a very similar way to water softeners, which also use an ion-exchange resin. Unlike water softener resin, which can be regenerated with a salt brine solution, DI media must be regenerated with a caustic sodium hydroxide solution or hydrochloric acid, depending on the type. It's not practical to perform this regeneration for an individual deionized water tank system, in most cases, so the resin is removed and sent to a specialized regeneration facility. New resin is added to the DI tank instead.