A limited-capacity cartridge designed to remove all mineral content from water.
Deionizing (DI) cartridges can remove virtually all mineral content (total dissolved solids) from water. Their capacity is limited by the TDS content of the water—for most water, including carbon-treated, softener-treated, and untreated tap, they'll last for only a few gallons. For water that's been treated with reverse osmosis or a similarly rigorous treatment method, they can last for a full year. They're useful primarily as RO polishers for those who want 0 TDS water, making them a great choice for laboratory purposes.
Further information on deionzing cartridge capacity.