De-ionization (DI) is the removal of dissolved solids from water. There are several core technologies and dozens of applications of these technologies to treat water for hundreds of applications. An understanding of the basic principles will make the hundreds of applications manageable.

The uses of de-ionized water can be broken down to several broad categories including:
1. Rinsing- When dissolved solids are removed from water, the resulting water is "hungry" and will clean surfaces of oil, grease, solder, flux, dust, wax, ink, and other unwanted constituents. Every piece of metal to be painted or plated, including whole automobile chassises, is washed and then rinsed with DI water prior to painting. Your car will have spots after washing unless rinsed with de-ionized water. Computer chips with a space of only 0.2 microns between circuits must be absolutely free of any imperfection.
2. Insulating/cooling- In many manufacturing processes, particularly electronic component production electrical energy is introduced and the parts must be electrically isolated. De-ionized water can not conduct electricity and is in fact an excellent insulating medium with its resistance measured in meg ohms. In color separations (printing) DI water insulates the paper in spaces where otherwise a charged ink would be applied.
3. Non contaminating- Some applications call for the addition of water without attendant contamination. For example, many blood analyzers dilute the blood sample and conduct a series of tests. If the diluting water contains sodium, lead, sulfates, or anything other than H2O there could be some very strange test results. Research laboratories, hospitals, dentists, and chemists also use de-ionized water to clean their glassware and other instruments.
4. Power- Steam generators and power plants must be operated with de-ionized water. At high speed and temperature even the slightest contaminant adhering to heat exchangers and turbines can cause serious damage.
5. Pharmaceutical- Great care is taken in the production of drugs to assure that there are no extraneous contaminants and that the preparation is exactly the same from batch to batch.
6. Dialysis- Hemo dialysis, the removal of contaminates from a patients blood stream also requires water with low Total Dissolved Solids (TDS). The patient's blood is circulated past a membrane that has de-ionized water mixed with the dialysate on the other side. Contaminates pass from the blood through the artificial kidney membrane to the water.
7. Waste water- Non sanitary industrial waste water often contains contaminates that can not be discharged to drain for environmental reasons. Some form of de-ionization is often used to remove contaminates from this wastewater before discharge or reuse.

Most DI water applications also require additional treatment including de-chlorination, filtration, softening, de-gassification, sanitization, total organic destruction, and endotoxin removal. This discussion, while recognizing those requirements, is limited to de-ionization. De-salinization is also omitted from this paper even though it is a most striking example of de-mineralizing. Distillation is also an excellent de-ionizer but due to operating costs is almost always used as a polisher for it's sanitizing capabilities.

In practice you have three distinct de-ionization technologies that may be used along or in combination with each other. The three categories are: membranes including nano filtration (NF) and reverse osmosis (RO), ion exchange (IX), and electro de-ionization (EDI).

1) Membranes. Membrane based systems are constructed of spiral wound sheets or bundled tubes with a product and waste stream. Water, under pressure, contacts the surface of the membrane where certain ions pass through the pores and others are rejected.
a) Nano Filtration. NF is a semi permeable membrane that rejects organic compounds in the 250 to 1000 molecular weight range. It also rejects some mineral salts these are primarily multi-valiant ions. A general rule of thumb is an 85%+ rejection on total dissolved solids.
b) Reverse Osmosis. RO is a semi-permeable membrane that removes organic compounds and rejects up to 99% of all mineral salts. These are mono-valent and multivalent ions. In most applications there is a 98%+ rejection of total dissolved solids.
2) Ion Exchange Resins. IX has several variations but all work on the principle of exchanging hydrogen (H) for cations and hydroxyls (OH) for anions. The regenerant containing hydrogen is acid, muriatic or sulfuric. The hydroxyls come from regenerating anion resin with sodium hydroxide (caustic soda). The exchanged "contaminates," when combined form water (H + OH = H2O). Each resin bead is a small plastic sphere containing charged exchange sites. (see illustration #1) SAC has active sites created by the addition of arysulphonate. These sites are negatively charged and attract cations. SBA uses a quaternary ammonium to create positively charged sites that attract anions.
a) Strong Acid Cation (SAC) resin followed by Strong Base Anion (SBA) constitute the most often used single bed DI system. The cation resin has a 50% greater capacity than the SBA resin so the system capacity is determined by the SBA capacity (see chart #1).
b) Weak Base Anion (WBA) has a capacity similar to SAC (83%) but does not remove silica or carbon dioxide. In some applications silica and CO2 pose no problem. A car wash spot free rinse is a good application for SAC-WBA DI system.
c) Mixed Bed (MB) resins are a combination of cation and anion resins. This combination produces the highest quality DI water. MB systems are infinitely more complicated because the resins must be separated before regeneration and re-mixed after regeneration. The resin ratio is SAC 60% SBA 40% or SAC 50% WBA 50%.
3) Electrode-ionization (EDI) is a process that provides a continuous stream of DI water using membranes and resins. The resin is in a membrane cell filled with mixed-bed resin with a cationic and anionic membrane on either side of the resin. The cell is has a flush compartment on either side bounded by the cationic and anionic membrane. Electrical DC current drives the ions into these "flush" compartments and through hydrolysis provides the H+ and OH- ions needed to regenerates the cell. Feed water must be of RO quality.

Each of the above listed technologies de-ionizes to a different degree. The feed water is a critical variable in determining the quality of the product water. The total dissolved solids (TDS) are a prime indicator and to a lesser degree the composition of the dissolved solids is a factor.

Dissolved solids are measured by how well water does or does not conduct electricity. In fact water does not conduct electricity. The dissolved solids conduct current and when water is free of contaminates it is an insulator rather than a conductor. Conductivity is measured in micromhos per centimeter (mho/cm or mmho/cm). As purity increases the convention is to measure the resistivity of water. Resistivity is measured in ohms per centimeter or in meg ohms per centimeter (megohms/cm). The relationship is: Conductivity = 1/Resistance (mho:ohm or mh:megohm) See chart #2.

Operation and maintenance or prime considerations in any de-ionizing plan. The simplest, from the user point of view, is to contract for an exchange tank program. Exchange tanks, or service DI (SDI) as it is know accounts for much of the high purity market especially with users of 5000 gallons per day or less. In this scenario a DI dealer brings tanks (fiberglass or stainless steel) containing carbon and various resins that are fully regenerated. The user feeds them with tap water or RO water and DI water comes out the other end. Single bed tanks consisting of a tank of cation resin and a tank of anion (either strong base or weak base) resin and will produce water of the .5 - 1 megohm resistivity. These are often followed by mixed bed tanks, containing cation and anion resins that produce up to 18-megohm water. See Chart #2. When the tanks are exhausted (in need of regeneration) the dealer brings regenerated ones and exchanges them. SDI is popular because the user does not have to handle acid and caustic and implement the OSHA safety procedures and get a discharge permit which requires a waste treatment system with monitoring, and recording devices, not to mention periodic inspections.

SDI tanks last much longer if they are fed RO water rather than ordinary tap water. However, RO membranes must be cleaned on a regular basis probably 2-4 times per year. Clean-in-place equipment is available but again a waste treatment system must exist, as the cleaning products are acids and alkalis. Dealers can visit the site and clean the membranes and leave with the used cleaning fluid or the membranes can be sent to companies specializing in membrane cleaning. A softener to lessen the scale buildup on the membranes should precede reverse osmosis units. Softeners should never be used in a DI system unless there is a RO between the softener and the DI resin. Sodium, the by-product of softening, is weakly ionized and therefore harder to exchange from resin than calcium or other hardness minerals.

When a facility has a waste treatment system for other reasons and/or the volume of water is high enough to justify the cost, then regeneration on site is a viable option. Single bed de-ionizers are simple to operate and maintain; in fact they operate very much like a softener. The major difference is that they use conductivity measurements to determine when the resin is in need of regeneration. Each time water is used there is a short cycle of rinse to drain to assure quality before water is sent to its intended use. Mixed bed equipment is much more complex due to the need to separate the resins before regeneration and then remix them after regeneration. There must be controls for inlet water feed, outlet product, caustic feed and recovery, acid feed and recovery, water or brine feed for separation, and air feed for re-mixing. These systems are not unmanageable by any means but require close study, as there is a dizzying array of valves and electronics.

Electro de-ionization (EDI) is not maintenance intensive. Like the RO membranes that precede it, the EDI stacks must be cleaned on a regular basis, about once every fourth membrane cleaning. RO/EDI is operator friendly and uses no chemicals.

When a new project is approached decisions must be made about which technology or technologies to use. One must weigh the benefits and liabilities of each technology. For instance, exchange tank service that regenerates the resins off-site relieves the end user of many issues and responsibilities but is more expensive than on-site regeneration and the user is completely dependant on the vendor delivering on time. The choice often boils down to listing the pros and cons for each technology and selecting on the basis of the weight of evidence. See Chart #3 DI Comparison.

For anyone looking at de-ionization for the first time don't be distracted by the technology. The real goal is to get reliable quality water at a reasonable cost. Often the reliability of the dealer is the most important variable. If the dealer can be counted on to maintain the equipment and/or make deliveries when needed, the most important goal has been reached. There are no cost savings if the owner's process has to shutdown for lack of DI water. The best dealers offer 24/7 service to their existing clients. Nearly every DI system uses exchange tanks as the primary or polishing de-ionizer. Dealers that do not have a regeneration plant should contact a dealer that does. Most independent regenerators welcome wholesale accounts. The local, reliable service a dealer can provide is more important than who regenerates the resin.