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Overview of Purification Technologies

» Distillation» Ultrafiltration
» Ion Exchange» Reverse Osmosis
» Activated Carbon» Elix Continuous Deionization
» Microporous Filters» Ultraviolet (UV) Radiation
 
The major water purification technologies are described below. Each technology has its benefits and limitations. Some are able to remove a large fraction of several contaminants, while others excel at removing one specific type of impurity down to very low levels. As a result, in order to remove all contaminants to the levels required for critical applications, it is necessary to use a combination of technologies.
Distillation
Distillation is probably the oldest method of water purification. Water is first heated to the boiling point. The water vapor rises to a condenser where cooling water lowers the temperature so the vapor is condensed, collected and stored.


Distillation


Many contaminants remain behind in the boiling vessel. However, the process has several limitations:

  • Inorganic contaminants are able to migrate along the thin water film that forms on the inner walls of the still. This explains why ions can be found in the distillate, whose resistivity is therefore usually between 0.5 and 1 MΩ•cm @ 25 °C (i.e., about 500 ppb total ionic contamination in water). Contaminants are also extracted from the glass or metal boiling pot used to heat the water (silica, sodium, tin, copper).
  • Organics with boiling points lower than 100 °C will automatically be transferred to the distillate, and even organics with a boiling point superior to 100 °C can dissolve in the water vapor and also pass into the distillate. In addition, new organochlorine compounds may be generated during the distillation process, which provides the energy required for chlorine in the tap water (added for sanitization purposes) to react with the natural organic substances also present in this water. This explains why the TOC level of distilled water is typically around 100 ppb.
  • Distillation is a slow process that requires storage of water for long periods. During this time, recontamination occurs from the ambient air (inorganic and organic volatile substances, bacteria, particulates and algae) and the container (organics from plastic tanks or ions from glass reservoirs).
  • Distillation requires large amounts of energy and water, and therefore is expensive to operate. In addition, a still requires regular cleaning of the boiling pot with HCl, a brush and sand paper to remove the contaminants accumulated during the process.

Benefits
  • Removes a broad range of contaminants and therefore useful as a first purification step.
  • Reusable.
 
Limitations
  • Contaminants are carried to some extent into the condensate.
  • Requires careful maintenance to ensure purity.
  • Consumes large amounts of tap water (for cooling) and electrical energy (for heating).
  • Not environmental friendly.

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Ion Exchange
The ion-exchange process percolates water through spherical, porous bead 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 “softened” water.


Deionization

Deionization (DI) beads exchange either hydrogen ions for cations, or hydroxyl ions for anions. The cation-exchange resins, which are made of polystyrene chains cross-linked by divinylbenzene with covalently bound sulfonic acid groups, will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly, the anion-exchange resins, which are made of polystyrene polymer chains with covalently bound quaternary ammonium groups, will exchange a hydroxyl for any anions (e.g., Cl-, NO3-, SO4--). 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 packaged in mixed bed exchangers containing a mixture of both types of resins. This last configuration enables more efficient ion removal and provides higher water resistivity values.


Jetspore ion-exchange resin beads		   The resin may be “regenerated” by strong acid and bases 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. However, this is a harsh chemical process that may damage the polymer chains constituting the beads, leading to contamination of the resin by organics and particulates, and creating an issue in the production of high purity water.

For the production of high purity water, two solutions exist:

  • Use “virgin” mixed bed ion-exchange resin packs containing monosphere beads with low TOC (such as Millipore Jetpore ion-exchange resin) only once and discard the pack after usage. This is an economically acceptable process provided that these packs are fed by pretreated water of good quality to limit the replacement frequency. A good pretreatment should remove not only the bulk of ions to limit the burden of ionic contaminants reaching the resin pack, but also organics, particulates and colloids. This is required to prevent the build-up on the resin beads of a coating preventing the ions from accessing the ionic binding sites located mainly inside the beads’ porous structure.
  • Perform the regeneration of the ion-exchange resins using a gentle and continuous procedure such as electrodeionization, in order to avoid damaging the ion-exchange resin beads and consequently generating contaminants. This process was developed by Millipore in the 1980s.

Deionization can be an important component of a total water purification system when used in combination with other methods such as RO, filtration and carbon adsorption. DI systems effectively remove ions, but they do not effectively remove most organics and microorganisms. Microorganisms can attach to the resins, providing a culture media for bacteria growth and subsequent pyrogen generation over the long run. The benefits and limitations of this technology are summarized below.

Benefits
  • Removes dissolved inorganics (ions) effectively, allowing resistivity levels above 18.0 MΩ•cm @ 25 °C to be reached (corresponding roughly to less than 1 ppb total ionic contamination in water).
  • Regenerable (by acid and bases in “service deionization” or by electrodeionization).
  • Relatively inexpensive initial capital investment.
 
Limitations
  • Limited capacity: once all ion binding sites are occupied, ions are no longer retained (except when operating in an electodeionization process).
  • Does not effectively remove organics, particles, pyrogens or bacteria.
  • Chemically regenerated DI beds can generate organics and particles.
  • Single use, “virgin” resins require good pretreated water quality to be economically efficient.

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Activated Carbon

Synthetic activated carbon		   Activated carbon is made of organic material porous particulates containing a maze of small pores, which account for the substance’s highly developed surface. One gram of activated carbon has a surface of up to 1 000 m2. Organic molecules dissolved in water may enter the pores and bind to their walls due to van der Waals forces. The 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 molecular weight and the molecular size of the organics.

Activated carbon used in water purification is available in two forms:
  • Natural activated carbon produced by treating vegetal products such as coconut shells at high temperature. The result of this process is a fine powder made of irregularly shaped grains. Natural activated carbon contains a high concentration of ionic contaminants and is therefore used only as a pretreatment step to remove excess chlorine from tap water by a reduction reaction and, to some extent, to reduce organic contamination.
  • Synthetic activated carbon is made by the controlled pyrolysis of polystyrene spherical beads. This cleaner material is used for the removal of trace organics of low molecular weight.

Activated 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.


Carbon Adsorption

Benefits

  • Removes dissolved organics and chlorine effectively.
  • Long life due to high binding capacity.
 
Limitations
  • Does not efficiently remove ions and particulates.
  • Limited capacity due to a high, but limited, number of binding sites.
  • Can generate carbon fines.


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Microporous Filters
Microporous filters can be classified in three categories: depth, surface and screen. Depth filters are matted fibers or materials compressed to form a matrix that retains particles by random adsorption or entrapment. Surface filters are made from multiple layers of media. When fluid passes through the filter, particles larger than the spaces within the filter matrix are retained, accumulating primarily on the surface of the filter. Screen filters (also called membrane filters) are inherently uniform structures which, like a sieve, retain all particles larger than the precisely controlled pore size on their surface.


Depth Filter


Screen Filter		      The distinction between filters is important because the three serve very different functions. Depth filters are usually used as prefilters because they are an economical way to remove ≥ 98 % of suspended solids and protect elements downstream from fouling or clogging. They owe their high capacity to the fact that contaminants are trapped and retained within the whole filter depth. Surface filters remove 99.99 % of suspended solids and may be used either as prefilters or clarifying filters. Screen (microporous membrane) filters are 100 % efficient at retaining contaminants larger than their pore size. These filters are placed at the furthest 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.


Benefits

  • Screen filters are absolute filters that remove all particles and microorganisms greater than their pore size.
  • Efficient operation throughout their lifetime, unless they are damaged.
  • Maintenance is limited to replacement.

 
Limitations
  • They will clog when the surface is covered by contaminants. Therefore, they should be used as a last purification step, as a type of insurance.
  • Will not remove dissolved inorganics, organics or pyrogens.
  • Not regenerable.

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Ultrafiltration

Ultrafilter		      A microporous membrane filter removes particles according to pore size. By contrast, an ultrafiltration (UF) membrane functions as a molecular sieve. It separates dissolved molecules on the basis of their size–often reported as the “molecular weight“ (both parameters are related, but not always directly)—by passing a solution through an infinitesimally fine filter.

The ultrafilter is a tough, thin, selectively permeable membrane that retains most macromolecules above a certain size (Nominal Molecular Weight Limit, or NMWL) 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.

Ultrafilters are available in several selective ranges. In all cases, the membranes will retain most, but not necessarily all, molecules above their rated size. In water purification, ultrafilters are routinely used to provide pyrogen-free and nuclease-free water for critical cell culture or molecular biology experimentation. The key point here is the validation process, which ensures that the ultrafilter, when challenged by pyrogens, RNases or DNases at levels far above those likely to occur during regular operation, will be able to reliably deliver water within specification.

Benefits
  • Effectively removes most particles, pyrogens, enzymes, microorganisms and colloids above their rated size, retaining them above the ultrafilter surface.
  • Efficient operation throughout their lifetime, unless they are damaged.
  • Their lifetime can be extended by a regular water flush at high speed.
 
Limitations
  • Will not remove dissolved inorganics or organic substances.
  • May clog when challenged by an excessive level of high-molecular-weight contaminants.

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Reverse Osmosis

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 that of UF membranes. RO membranes are capable of rejecting practically all particles, bacteria and organics > 200 Dalton molecular weight (including pyrogens) at a rate close to 99 %. 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 at a flow rate proportional to applied pressure and collected downstream of the membrane.

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

RO also involves an ionic exclusion process. Only solvent (i.e., water molecules) is allowed to pass through the semi-permeable RO membrane, while virtually all ions and dissolved molecules are retained (including salts and organic molecules such as sugars). The semi-permeable membrane rejects salts (ions) by a charge phenomenon 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. Salt rejection increases significantly with applied pressure up to 5 bar.

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.

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

Benefits

  • Effectively removes all types of contaminants to some extent (particles, pyrogens, microorganisms, colloids and dissolved inorganics), and is therefore useful as a first purification step.
  • Requires minimal maintenance.
  • Operation parameters (pressure, temperature, flow rate, ionic rejection) are easy to monitor.
 
Limitations
  • Limited flow rates per surface unit require either large membrane surfaces or an intermediate storage device to satisfy user demand.
  • Requires good pretreatment to avoid rapid membrane damage by water contaminants: scaling (CaCO3 deposits on the surface), fouling (deposits of organics or colloids on the surface) or piercing (RO membrane cut by hard particulates).


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Elix Continuous Deionization
This technology is a combination of electrodialysis and ion exchange, resulting in a process which effectively deionizes water, while the ion-exchange resins are continuously regenerated by the electric current in the unit. This electrochemical regeneration replaces the chemical regeneration of conventional ion-exchange systems.



ELIX Module

The Elix module consists of a number of “cells” sandwiched between two electrodes. Each cell consists of a polypropylene frame onto which are bonded a cation-permeable membrane on one side, and an anion-permeable membrane on the other.

The space in the center of the cell, between the ion-selective membranes, is filled with a thin bed of ion-exchange resins. The cells are separated from one another by a screen separator.

The feed water entering the module is split into three parts. A small percentage flows over the electrodes, 65-75 % of the feed passes through the resin beds in the cell, and the remainder passes along the screen separator between the cells.

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.

Benefits

  • Removes dissolved inorganics effectively, allowing resistivity above 5 MΩ•cm @ 25 °C to be reached (which corresponds to a total ionic contamination level in water of approximately 50 ppb).
  • Environmental friendly:
    • No chemical regeneration.
    • No chemical disposal.
    • No resin disposal.
  • Inexpensive to operate.
  • Safe: no heating element.
 
Limitations
  • Removes only a limited number of charged organics.
  • Requires feed by good quality water (for instance, reverse osmosis-treated water) for economically efficient operation.


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Ultraviolet (UV) Radiation
Ultraviolet radiation has been widely used as a germicidal treatment for water. Mercury low pressure UV lamps generate light at different wavelengths, including 185 and 254 nm. UV lamps with a regular quartz sleeve allow passage of 254 nm light. These lamps are an effective means of sanitizing water. The adsorption of UV light by the DNA in the microbial cells results in the inactivation of the microorganism.

UV lamps with a very pure quartz sleeve allow passage of both 185 and 254 nm UV light.This combination of wavelengths is necessary for the photooxidation of organic compounds, which ultimately allows conversion of dissolved organic substances into carbon dioxide. With these special lamps, Total Oxidizable Carbon (TOC) levels in high purity water can be reduced to ≤ 5 ppb.



Ultraviolet Radiation

Benefits

  • Effective sanitizing treatment.
  • Oxidation of organic compounds (185 nm and 254 nm) to reach water TOC levels below 5 ppb.

Limitations
  • Photooxidation of organics is a polishing step, able to decrease the TOC level only by a limited value.
  • The CO2 produced during photooxidation decreases the water’s resistivity.
  • UV light will not affect ions, particles or colloids.


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