Water contains a variety of impurities that can generally be classified into five major groups:

Particulate matter.

Microorganisms.

Pyrogens, endotoxins, DNase and RNase.

Dissolved non-ionized solids and gases.

Dissolved ionized solids and gases.

The first 3 groups of contaminants are all essentially "particulate matter" in one form or another, and can be "filtered" out by passing the water through a sieve that has a pore size that is smaller than the matter to be removed.

Dissolved non-ionized solids and gasses include natural organic remains, man-made organic chemicals, and oxygen, resulting from exposure of the water to ecological contaminants.

Dissolved ionized solids and gasses come from exposure to rock and minerals in the earth, such as sodium chloride, calcium carbonate (limestone), calcium magnesium carbonate (dolomite), and other soluable chemicals that occur either naturally or as a result of man-kind's contamination of the water supply. The main ionized gas is carbon dioxide. These contaminants contribute to the hardness, conductivity, alkalinity and pH of the water.

There are five major technologies that can be used to purify water:

1. Filtration: Filtration can be further divided into 5 sub-categories:

1.1  Particle filtration: This can include anything from a coarse sand filter, with an effective pore size greater than 1,000 microns, to a cartridge filter with a pore size of 1 micron or greater. Note that a human hair is about is about 100 microns in diameter, while the smallest particle visible to the naked eye is about 30 microns in diameter.

1.2  Microfiltration: Also known as sub-micron filtration, microfiltration includes filter devices with pore sizes ranging from just under 1 micron to about 0.05 micron. Bacteria, which ranges from about 0.2 to 30 microns in diameter, can actually be removed from water and other liquids via microfiltration at the 0.2 micron level. Essentially, this is how beer is "cold sterilized".

1.3  Ultrafiltration: An ultrafilter is essentially a membrane filter or molecular seive, that can remove molecules from water that have a diameter larger than about 0.003 micron. Virus, pyrogen, endotoxin, R-Nase and D-Nase can be removed from water via ultrafiltration.

1.4  Nanofiltration: Nanofiltration fills the gap between ultrafiltration and reverse osmosis, with an effective pore size of 0.001 to 0.01 micron. It serves no useful purpose in the field of water purification.

1.5  Reverse Osmosis: Reverse osmosis or (RO) membrane filters have a pore size less than 0.001 micron, giving them the ability to separate individual ions (i.e., dissolved solids) from a solution.

2. Adsorption by activated carbon: In this process, impurities such as organic compounds and chlorine are actually attracted to activated carbon filters, and essentially "cling" to them, much as water clings to skin.

3. Ultraviolet radiation: UV radiation of water at certain wavelengths at known concentrations can either sterilize the microorganisms that are present, or actually reduce the amount of organic carbon compounds that are present, by breaking them down into their less harmful constituents.

4. Distillation: This, the oldest technology for producing laboratory grade purified water, involves heating water to its boiling point and condensing and collecting the water vapor. In theory, only "pure" water would vaporize. In actual fact, contaminants that have a boiling point less than or near that of water, also vaporize and come over with the pure water.

5. Deionization: Also known as ion exchange, this technology involves passing the water through resin beds (i.e., like sand beds) that have an affinity for dissolved, ionized salts. Cationic resins remove positively charged ions, giving up an H+ in the process, while anionic resins remove negatively charged ions, giving up an OH- in the process. The 2 kinds of resins can either be in separate containers, or mixed together. When mixed together in a single container, they essentially perform an almost infinite series of positive and negative ion exchanges, resulting in the purest possible water for that particular type of resin.

Note that new (i.e., virgin) ion exchange resin has more ion exchange capacity
than regenerated resin, and produces the purest water.

A modern laboratory water purification system will combine all of the above technologies (except distillation) to produce reagent grade water. Reagent grade water is defined by the CAP (College of American Pathologists), ASTM (American Society of Testing and Materials), and NCCLS (National Committee for Clinical Laboratory Standards) as being either Type I, Type II, Type III or Type IV, in terms of its:

In addition, each discipline specifies several other criteria for each Type (i.e., I, II, III, or IV). Needless to say, the three disciplines DO NOT AGREE on exactly what constitutes each Type!

For practical purposes:

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