Purified water

Purified water is created through mechanical filtration or other processing methods that remove impurities, making it suitable for various applications. While distilled water was historically the most common form, purification now frequently involves techniques such as capacitive deionization, reverse osmosis, carbon filtering, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodeionization. Combinations of these processes are used to produce ultrapure water, where trace contaminants are measured in parts per billion or parts per trillion.

Purified water is available in different purity levels and is widely used in pharmaceutical production, scientific and engineering laboratories, and industrial settings. It also serves as the primary ingredient in commercial beverage formulas to ensure product consistency. It can be produced on-site for immediate use or purchased in containers. In everyday language, "purified water" may also refer to water treated to neutralize harmful contaminants, though not necessarily removing them entirely.

The purity of water is assessed by monitoring several categories of impurities. These include inorganic ions (measured by electrical conductivity, resistivity, or specific tests), organic compounds (tracked via total organic carbon or specific tests), bacteria (monitored through total viable counts or epifluorescence), endotoxins and nucleases (detected using LAL or specific enzyme tests), particulates (controlled by filtration), and gases (managed by degassing when needed).

Distillation produces purified water by boiling water and condensing the vapor into a clean container, leaving solid contaminants behind. This method yields very pure water, but a white or yellowish mineral scale builds up in the apparatus and requires regular cleaning. Distilled water must be stored in a sterilized container to prevent bacterial contamination. For many procedures, more economical alternatives like deionized water are used instead.

Double-distilled water, abbreviated as ddH2O, Bidest. water, or DDW, is made by slowly boiling the condensed vapor from a prior slow boiling. It was historically the standard for highly purified laboratory water in biochemistry and trace analysis, until combination purification methods became widespread.

Deionized water, also called DI water or demineralized water, has had nearly all mineral ions removed, including cations like sodium, calcium, iron, and copper, and anions like chloride and sulfate. Deionization is a chemical process using ion-exchange resins that exchange hydrogen and hydroxide ions for dissolved minerals, which then recombine to form water. Since most non-particulate impurities are dissolved salts, deionization produces highly pure water similar to distilled water, but faster and without scale buildup. However, it does not significantly remove uncharged organic molecules, viruses, or bacteria, except by incidental trapping in the resin. Specially made strong base anion resins can remove Gram-negative bacteria. Deionization can be done continuously and inexpensively using electrodeionization. There are three types: co-current, counter-current, and mixed bed.

In co-current deionization, both input water and regeneration chemicals enter at the top of an ion-exchange column and exit at the bottom. Operating costs are higher than counter-current deionization due to greater use of regenerants. Because regenerant chemicals are dilute when they reach the bottom resins, product quality is lower than a similarly sized counter-flow column. This process is still used and can be optimized by fine-tuning regenerant flow.

Counter-current deionization has two forms, both requiring engineered internals: upflow columns (water enters from the bottom, regenerants from the top) and upflow regeneration (water enters from the top, regenerants from the bottom). In both cases, separate distribution headers must be tuned to input water quality and flow, operation time between regenerations, and desired product water analysis. This method is more attractive because chemicals flow opposite to the service flow, requiring less regeneration time. Product quality can be as low as 0.5 parts per million, and operating costs are low due to minimal regenerant use.

Mixed bed deionization uses a 40/60 mixture of cation and anion resin in a single column. With proper pretreatment, a single pass through a mixed bed column produces the purest water possible. Mixed bed demineralizers are most commonly used for final water polishing to remove the last ions before use.