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Deionization

FILMTEC Membranes

Deionization - The removal of all ionized minerals and salts (both organic and inorganic) from a solution by a two-phase ion exchange procedure: First, positively-charged ions are removed by a cation exchange resin in exchange for a chemically equivalent amount of hydrogen ions. Second, negatively-charged ions are removed by an anion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions introduced in this process unite to form water molecues. This process is also called demineralizing by ion exchange.

The terms "high quality" or "pure water" or even "ultrapure water" are sometimes used when referring to deionized water. Let us look at how deionized water is measured for mineral content - or to measure whether the DI process worked well to produce a lack of mineral content.

The effectiveness of the DI process is determined by measuring the "conductance" of the deionized water. "Conductance" is the measure of the ability of a solution to allow an electric current to flow through it. "Pure water", water totally free of minerals, will not allow electric current to pass because the current is carried by dissolved ions. If there are no dissolved minerals or other ions, less current passes through the water, so the measurement of conductance can tell us about how well the ions in water have been removed.

The unit of measure of conductance is the siemens(S)/centimeter(µS). A Siemens was formerly called a mho. However, the conductance of deionized water is so small that it is measured in microsiemens/cm (or micromho/cm). A microsiemens (µS) is a millionth of a siemens. The common convention in measuring high purity water, however, is to use resistivity in place of conductance.

Resistivity is the opposite of conductance. Mathematically speaking, it is the reciprocal of conductance. Resistivity, also called "specific resistance", refers to the capacity of a liquid to resist the flow of a current, and it is measure in ohms or megohmsºcentimeter.

The resistivity of deionized water is measured in megohms. A megohmºcm is one million ohms. The only advantage of using resistivity in megohms is that it allows the use of whole numbers, that increase as the degree of water purity increases, instead of the conductance fractions, that decrease as the water purity increases.

The following table presents a comparison of conductance, resistance, and total dissolved solids concentrations in high purity water.

In answer to our question, "How pure is pure?", there is no exact answer. There is no set number of microsiemens or megohms at which deionized water is determined to be "pure" or "high quality" or "ultrapure". It is possible to take deionized water down to under one microsiemens per centimeter or less than 0.5 parts per million of total dissolved solids (TDS). The specific use to which the DI water will be put will determine how "pure" it must be. The limit of water's resistivity at 25° Centigrade is 18.3 megohms°cm, 25° Centigrade water may be absolutely pure; it contains less than 0.03 milligrams per liter of total dissolved solids.

Comparison of Specific Conductance, Resistance and Concentration of Dissolved Minerals @ 25° Centigrade

Specific Conductance Microsiemens/cm (mhos/cm)	Specific Resistance in ohms.cm	Total Dissolved Solids in mg/L CaCO₃
0.055	18,000,000	< 0.03
0.066	15,000,000	0.03
0.1	10,000,000	0.04
1.0	1,000,000	0.40
2	500,000	0.80
4	250,000	1.5
6	166,000	1.7
8	125,000	3.1
10	100,000	4.0
20	50,000	8.0
30	33,333	13.5
40	25,000	16.5
50	20,000	20.9
100	10,000	40.5
200	5,000	80.4

If water with a conductance of less than on microSiemen is needed, a mixed-bed system is generally used. A mixed bed puts the cation resin and the anion resin together into one resin bed, usually in a volumetric ration of 40 percent cation and 60 percent anion. The cations and the anions are then removed simultaneously. Generally, a prefilter is used for the feedwater going into a mixed bed.

The mixed bed can produce higher quality water than a two-bed system, but a mixed-bed system makes regeneration more difficult since the two resins must be separated and regenerated with different types of regenerants - acid for cation resin and base for the anion resin. The regeneration process for these mixed beds will be discussed in more detail in Chapter Three.

Sometimes, a mixed-bed system is used as a "polisher" to further lower the conductance after a regular two-bed system has been used. Why does a mixed bed works so effectively? The theory is that because the cation and anion beads are all mixed together, the path taken by the water is equivalent to going through an almost infinite number of two-bed systems: cation/anion, cation/anion. So, the water is more thoroughly deionized by the mixed-bed system.

If a mixed-bed is effectively designed and maintained, the resulting deionized water can have a conductance of less than one microsiemens/cm. Water produced with a total dissolved solids (TDS) level of ten parts per billion (ppb) would be 99.999999 percent pure. Often the goal set for a mixed-bed system is to create very high-quality, mineral-free water with a specific resistance of greater than 18.0 megohmsºcm at 25 degrees Centigrade. This water would have a total dissolved solids(TDS) level of less than 30 parts per billion (ppb). "Pure water" can be produced that is so pure we cannot accurately measure low pure it really is because, as discussed previously, the autoionization of the water itself into hydrogen and hydroxide ions prevents resistivity from exceeding 18.3 megohmsºcm at an ambient temperature of 25° Centigrade.

TGI offers:

10" inline mixed-bed DI cartridge with 16.5 cubic inches of DI resin. (TGI Item #FI-DI2010)
12" inline with 27 cubic inches of DI resin. (TGI item #FI-DI2012)
10" replacement cartridge 2.5"x10" with 50.6 cubic inches of DI resin. (TGI Item #FI-DI4010)
20" replacement cartridge 2.5"x20" with 86.4 cubic inches (FI-DI4020)
20" BB replacement cartridge 4.5"x20" with 280 cubic inches (FI-DI4020B)