Distilled water is produced by a process of distillation and has an electrical conductivity of not more than 11nano seconds /cm and total dissolved solids of less than 10 mg/ liter. Distillation involves boiling the water and then condensing the vapor into a clean container, leaving solid contaminants behind. Distillation produces very pure water. A white or yellowish mineral scale is left in the distillation apparatus, which requires regular cleaning. Distilled water, like all purified water, must be stored in a sterilized container to guarantee the absence of bacteria. For many procedures, more economical alternatives are available, such as deionized water, and are used in place of distilled water.
Double-distilled water (abbreviated “ddH2O”, “Bidest. water” or “DDW”) is prepared by slow boiling the uncontaminated condensed water vapor from a prior slow boiling. Historically, it was the de facto standard for highly purified laboratory water for biochemistry and used in laboratory trace analysis until combination purification methods of water purification became widespread.
Deionized water (DI water, DIW or de-ionized water), often confused with demineralized water / DM water, is water that has had almost all of its mineral ions removed, such as cations like sodium, calcium, iron, and copper, and anions such as chloride and sulphate. Deionization is a chemical process that uses specially-manufactured ion exchange resins, which exchange hydrogen and hydroxide ions for dissolved minerals, and then recombine to form water. Because most non-particulate water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup. However, deionization 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 electro deionization.
Three types of deionization exist: co-current, counter-current, and mixed bed.
Co-current deionization refers to the original downflow process where both input water and regeneration chemicals enter at the top of an ion exchange column and exit at the bottom. Co-current operating costs are comparatively higher than counter-current deionization because of the additional usage of regenerates. Because regenerate chemicals are dilute when they encounter the bottom or finishing resins in an ion exchange column, the product quality is lower than a similarly sized counter-flow column.
The process is still used and can be maximized with the fine tuning of the flow of regenerates within the ion exchange column.
Counter-current deionization comes in two forms, each requiring engineered internals:
- Upflow columns where input water enters from the bottom and regenerates enter from the top of the ion exchange column.
- Upflow regeneration where water enters from the top and regenerates enter from the bottom.
In both cases, separate distribution headers (input water, input regenerate, exit water, and exit regenerate) must be tuned to the input water quality and flow, the time of operation between regenerations, and the desired product water analysis.
Counter-current deionization is the more attractive method of ion exchange. Chemicals (regenerates) flow in the opposite direction to the service flow. Less time for regeneration is required when compared to concurrent columns. The quality of the finished product can be as low as .5 parts per million. The main advantage of counter-current deionization is the low operating cost, due to a low usage of regenerates during the regeneration process.
Mixed bed deionization
Mixed bed deionization is a 50/50 mixture of cation and anion resin combined in a single ion exchange column. With proper pretreatment, product water purified from a single pass through a mixed bed ion exchange column is the purest that can be made. Most commonly, mixed bed de mineralizers are used for final water polishing to clean the last few ions within water prior to use. Small mixed bed deionization units have no regeneration capability. Commercial mixed bed deionization units have elaborate internal water and regenerate distribution systems for regeneration. A control system operates pumps and valves for the regenerate of spent anions and cations resins within the ion exchange column. Each is regenerated separately, then remixed during the regeneration process. Because of the high quality of product water achieved, and because of the expense and difficulty of regeneration, mixed bed de mineralizers are used only when the highest purity water is required.
Demineralization is often a term used interchangeably with deionization. Demineralization is essentially removing all the minerals that can be found in natural water. This process is usually done when the water will be used for chemical processes and the minerals present may interfere with the other chemicals. All chemists and beauty products have to be made with demineralized water for this reason. With the demineralization process, the water is “softened” replacing the undesired minerals with different salts (NaCl). Demineralized water has a higher conductivity than deionized water.
Other processes are also used to purify water, including reverse osmosis, carbon filtration, microporous filtration, ultrafiltration, ultraviolet oxidation, or electrodialysis. These are used in place of, or in addition to, the processes listed above. Processes rendering water potable but not necessarily closer to being pure H2O / hydroxide+ hydronium ions include the use of dilute sodium hypo chloride, ozone, mixed-oxidants (electro-catalyzed H2O + NaCl), and iodine.