By James A. Hunt
Summary: Deionization (DI) is often overlooked as a practical water treatment technology. In effect, every technology uses DI to a certain degree. The following is a discussion of DI’s applications as well as other factors that determine its viability.
Deionization (DI) is the removal of dissolved solids from water. There are several core technologies and dozens of industries, including pharmaceutical, microelectronics, power generation and dialysis to treat water for hundreds of applications. An understanding of the basic principles will make these hundreds of applications manageable.
Uses of DI water
The uses of deionized water can be broken down into several broad categories including:
When dissolved solids are removed from water, the resulting water is aggressive or “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 deionized water. Computer chips with a space of only 0.18 microns between circuits must be absolutely free of any imperfection.
In many manufacturing processes, particularly electronic component production, electrical energy is introduced and the parts must be electrically isolated. Deionized water doesn’t readily conduct electricity and is, in fact, a good insulating medium with its resistance measured in megohms. In color separations (printing), DI water insulates the paper in spaces where otherwise a charged ink would be applied.
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 deionized water to clean their glassware and other instruments.
High-pressure steam generators and power plants must be operated with deionized water. At high speed and temperature, even the slightest contaminant adhering to heat exchangers and turbines can cause serious damage.
Great care is taken in the production of drugs to assure that there are no extraneous contaminants and that preparation is exactly the same from batch to batch.
Hemodialysis, the removal of contaminates from a patient’s blood stream as in kidney dialysis, also requires water with low total dissolved solids (TDS). The patient’s blood is circulated past a membrane that has deionized water mixed with the dialysate on the other side. Contaminates pass from the blood through the artificial kidney membrane to the water.
Non-sanitary industrial wastewater often contains contaminants that cannot be discharged to drain for environmental reasons. Some form of deionization is often used to remove contaminants 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. Desalination is also omitted from this article, even though it’s a striking example of de-mineralizing. Distillation is also an excellent deionizer but, due to operating costs, is almost always used as a polisher because of its sanitizing capabilities.
The big three
In practice, you have four distinct DI technologies that may be used alone or in combination with each other. The four categories are membranes including nanofiltration (NF) and reverse osmosis (RO); ion exchange (IX); electro-deionization (EDI), and distillation, which isn’t discussed in this article in detail.
Membrane-based systems are constructed of spiral wound sheets or bundled capillary tubes that generate both a product and waste stream. Water, under pressure, contacts the surface of the membrane where certain ions pass through the pores and a number of others are rejected.
Nanofiltration: NF is a semi-permeable membrane that rejects organic compounds in the 250-to-1,000 molecular weight range. It also rejects some mineral salts; these are primarily multivalent ions. A general rule of thumb is a greater than 85 percent rejection on TDS.
Reverse osmosis: RO is a semi-permeable membrane that removes organic compounds and rejects up to 99 percent of all mineral salts. These are monovalent and multivalent ions. In most applications, there is a greater than 98 percent TDS rejection.
Ion exchange resins
IX has several variations but all work on the principle of exchanging hydrogen (H) for cations and hydroxyls (OH) for anions. (An exception involves Weak Base Anion, or WBA, resin, which adsorbs acids and does not exchange a hydroxyl.) The regenerant containing hydrogen is acid, muriatic or sulfuric. The hydroxyls come from regenerating anion resin with sodium hydroxide (caustic soda). As contaminates are exchanged on to the resin, H and OH ions are released to form H + OH=H2O. Each resin bead is a small (typically 0.3-1.2 mm) plastic sphere containing charged exchange sites. Strong Acid Cation (SAC) resin has active sites created by the addition of a SO3-H group. These sites are negatively charged and attract cations. Strong Base Anion (SBA) resins use a quaternary amine to create positively charged sites that attract anions.
SAC/SBA: Strong Acid Cation resin followed by Strong Base Anion resin constitute the most often used dual-bed DI system. The cation resin has a 50 percent greater capacity than the SBA resin so the system capacity is determined by the SBA capacity (see Table 1).
WBA: These have a capacity similar to SAC—83 percent—but don’t 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 systems.
Mixed beds: Mixed bed (MB) resins are a combination of cation and anion resins mixed uniformly. 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 typical resin ratio is SAC-SBA (60/40 percent) or SAC-WBA (50/50 percent).
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 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 regenerate the cell. Feed water must be RO quality.
Each of the above listed technologies deionizes to a different degree. The feed water is a critical variable in determining the quality of the product water. The TDS is a prime indicator and, to a lesser degree, the composition of the dissolved solids is a factor.
All in the conduct
Dissolved solids can be estimated by how well water does or does not conduct electricity. Measuring conductivity is one manner to measure the purity of water. The lower the conductivity, the higher the water’s purity. The dissolved solids in the water conduct current and, when water is free of contaminants, it’s an insulator moreso than a conductor. Conductivity is measured in microhms per centimeter (µmho/cm or mmho/cm), also referred to as microsiemens (µS). As purity increases, the convention is to measure the resistivity of water. Resistivity is measured in ohms per centimeter or in megohms per centimeter (megohms/cm). The relationship (see Table 2) is: Conductivity = 1/Resistance (mho:ohm or mh:megohm).
Operation and maintenance are prime considerations in any deionizing plan. The simplest, from the user’s point of view, is to contract for an exchange tank program. Exchange tanks, or service DI (SDI), accounts for much of the high purity market especially with users of 5,000 gallons per day (gpd) 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 will produce water of the 20,000-to-1,000,000 ohms resistivity. On the other hand, the quality from a SAC/WBA system will typically be in the 20,000-to-50,000 ohm range. These are often followed by mixed bed tanks, containing strong acid cation and strong base anion resins that produce up to 18 megohm water (see Table 2). When the tanks are exhausted (in need of regeneration), the dealer brings freshly regenerated ones and exchanges them. SDI is popular because the user doesn’t have to handle acid and caustic or implement OSHA safety procedures. OSHA would normally require a discharge permit, which necessitates a wastewater treatment system with monitoring and recording devices, not to mention periodic inspections.
SDI tanks last much longer if they’re 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 acidic and basic. Dealers can visit the site, 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 RO units. Softeners should never be used in a DI system unless there is an RO between the softener and the DI resin. Sodium, the by-product of softening, has a low affinity for cation resin so it’s difficult to exchange on and off the resin.
When a facility has a wastewater 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. Dual-bed deionizers 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. Every time water is used, there’s 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 aren’t unmanageable by any means but require close study, as there’s a dizzying array of valves and electronics. Still, EDI isn’t 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, an 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 while the user is completely dependent 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 evidence (see Table 3).
For anyone looking at deionization for the first time, don’t be distracted by the technology. The real goal is 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 shut down for lack of DI water. The best dealers offer 24-hour service seven days a week to their existing clients. Nearly every DI system uses exchange tanks as the primary or polishing deionizer. Dealers that don’t have a regeneration plant should contact a dealer or service that does. Most independent regenerators welcome wholesale accounts. The local dealer that can provide reliable service is more important than who regenerates the resin, as long as the regeneration station is run properly.
About the author
Jim Hunt is president of DI Water Inc. and principal manager of The Water Group LLC, both of Amesbury, Mass., near Boston. He’s also a past member of the WC&P Technical Review Committee. He can be reached at (800) 840-0901 or (978) 834-3169, (978) 834-3169 (fax), email: email@example.com or website: www.diwaterinc.com