2. Ion-Exchange

The ion-exchange process percolates water through bead-like spherical resin materials (ion-exchange resins). Ions in the water are exchanged for other ions fixed to the beads. The two most common ion-exchange methods are softening and deionization.

Softening is used primarily as a pretreatment method to reduce water hardness prior to reverse osmosis (RO) processing. The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the "softened" water.

Deionization (DI) beads exchange either hydrogen ions for cations or hydroxyl ions for anions. The cation exchange resins, made of styrene and divinylbenzene containing sulfonic acid groups, will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly, the anion exchange resins, made of styrene and containing quaternary ammonium groups, will exchange a hydroxyl ion for any anions (e.g., Cl-). The hydrogen ion from the cation exchanger unites with the hydroxyl ion of the anion exchanger to form pure water.

These resins may be packaged in separate bed exchangers with separate units for the cation and anion exchange beds. Or, they may be packed in mixed bed exchangers containing a mixture of both types of resins. In either case, the resin must be "regenerated" once it has exchanged all its hydrogen and/or hydroxyl ions for charged contaminants in the water. This regeneration reverses the purification process, replacing the contaminants bound to the DI resins with hydrogen and hydroxyl ions.

Deionization can be an important component of a total water purification system when used in combination with other methods discussed in this primer such as RO, filtration and carbon adsorption. DI systems effectively remove ions, but they do not effectively remove most organics or microorganisms. Microorganisms can attach to the resins, providing a culture media for rapid bacterial growth and subsequent pyrogen generation. The advantages and disadvantages of this technology are summarized below. 
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3. Carbon Adsorption

The carbon adsorption process is controlled by the diameter of the pores in the carbon filter and by the diffusion rate of organic molecules through the pores. The rate of adsorption is a function of the molecular weight and the molecular size of the organics. Certain granular carbons effectively remove chloramines. Carbon also removes free chlorine and protects other purification media in the system that may be sensitive to an oxidant such as chlorine.

Carbon is usually used in combination with other treatment processes. The placement of carbon in relation to other components is an important consideration in the design of a water purification system.

The distinction between filters is important because the three serve very different functions. Depth filters are usually used as pre filters because they are an economical way to remove 98% of suspended solids and protect elements downstream from fouling or clogging.

Surface filters remove 99.99% of suspended solids and may be used as either pre filters or clarifying filters. Micro porous membrane (screen) filters are placed at the last possible point in a system to remove the last remaining traces of resin fragments, carbon fines, colloidal particles and microorganisms. For example, 0.22 µm Millipore membrane filters, which retain all bacteria, are routinely used to sterilize intravenous solutions, serums and antibiotics. 
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4. Ultra filtration

Ultra filtration (UF) membrane functions as a molecular sieve. It separates dissolved molecules on the basis of size by passing a solution through an infinitesimally fine filter.

The ultra filter is a tough, thin, selectively permeable membrane that retains most macromolecules above a certain size including colloids, microorganisms and pyrogens. Smaller molecules, such as solvents and ionized contaminants, are allowed to pass into the filtrate. Thus, UF provides a retained fraction (retentate) that is rich in large molecules and a filtrate that contains few, if any, of these molecules.

Ultra filters are available in several selective ranges. In all cases, the membranes will retain most, but not necessarily all, molecules above their rated size. 
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5. Reverse Osmosis

Reverse osmosis (RO) is the most economical method of removing 95% to 99% of all contaminants. The pore structure of RO membranes is much tighter than UF membranes. RO membranes are capable of rejecting practically all particles, bacteria and organics >300 daltons molecular weight (including pyrogens).

Natural osmosis occurs when solutions with two different concentrations are separated by a semi-permeable membrane. Osmotic pressure drives water through the membrane; the water dilutes the more concentrated solution; and the end result is an equilibrium.

In water purification systems, hydraulic pressure is applied to the concentrated solution to counteract the osmotic pressure. Pure water is driven from the concentrated solution and collected downstream of the membrane.

Because RO membranes are very restrictive, they yield very slow flow rates. Storage tanks are required to produce an adequate volume in a reasonable amount of time.

RO also involves an ionic exclusion process. Only solvent is allowed to pass through the semi-permeable RO membrane, while virtually all ions and dissolved molecules are retained (including salts and sugars). The semi-permeable membrane rejects salts (ions) by a charge phenomena action: the greater the charge, the greater the rejection. Therefore, the membrane rejects nearly all (>99%) strongly ionized polyvalent ions but only 95% of the weakly ionized monovalent ions like sodium.

Different feed water may require different types of RO membranes. Membranes are manufactured from cellulose acetate or thin-film composites of polyamide on a polysulfone substrate.

RO is the most economical and efficient method for purifying tap water if the system is properly designed for the feed water conditions and the intended use of the product water. RO is also the optimum pretreatment for reagent-grade water polishing systems. 
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6. Deionization

The ion-exchange resins capture dissolved ions in the feed water at the top of the cell. Electric current applied across the module pulls those ions through the ion-selective membrane towards the electrodes. Cations are pulled through the cation-permeable membrane towards the cathode, and anions through the anion-selective membrane towards the anode. These ions, however, are unable to travel all the way to their respective electrodes since they come to the adjacent ion-selective membrane which is of the opposite charge. This prevents further migrations of ions, which are then forced to concentrate in the space between the cells. This space is known as the "concentrate" channel, and the ions concentrated in this area are flushed out of the system to the drain.

The channel running through the resin bed in the center of the cell is known as the "dilute" channel. As water passes down this channel, it is progressively deionized. At the lower end of the dilute channel, where water is free of ions, splitting of H2O occurs in the electric field. This generates H+ and OH- which regenerate the ion exchange resins, effectively eliminating chemical regeneration. 
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7. Ultraviolet Radiation

Recent advances in UV lamp technology have resulted in the production of special lamps which generate both 185 nm and 254 nm UV light. This combination of wavelengths is necessary for the photooxidation of organic compounds. With these special lamps, Total Organic Carbon (TOC) levels in high purity water can be reduced to 5 ppb. 
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Don't Make High Purity Water Decisions With out Talking to us.
Call on The Water Doctors

There will be times when isolation is a pleasant alternative to everyday life, like walking in the woods in autumn or cross-country skiing on a sunny day. But not when you've got to choose a high purity water system for your business, home or  laboratory. That's when you should call on the Certified Water Specialists at The Water Doctors. With over 14 years of experience and a large database of successful applications history, our Applications Specialists and Technical Service representatives have a wealth of information you won't find anywhere else. They can answer questions about purity, sensitivity, and contamination issues that may affect the outcome of your decision. And, once you've purchased your system, our professionally-trained staff of dedicated Field Service Engineers will keep your system running at optimal levels. Sure, feeling isolated is normal when you're miles from civilization, but not when you have questions about high purity water for your Home or Business.