Purification Technologies


Distillation

Distillation is probably the oldest method of water purification. Water is first heated to boiling. The water vapor rises to a condenser where cooling water lowers the temperature so the vapor is condensed, collected and stored. Most contaminants remain behind in the liquid phase vessel. However, organics with boiling points lower than 100 °C cannot be removed efficiently and can actually become concentrated in the product water. Another disadvantage is cost. Distillation requires large amounts of energy and water.

Table 1. Distillation
Advantages

Removes a broad range of contaminants

Reusable

Disadvantages

Some contaminants can be carried into the condensate

Requires careful maintenance to ensure purity

Consumes large amounts of energy


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.

Table 3. Deionization
Advantages

Removes dissolved inorganics effectively.

Regenerable (service deionization).

Relatively inexpensive initial capital investment.

Disadvantages

Does not effectively remove particles, pyrogens or bacteria.

DI beds can generate resin particles and culture bacteria.

High operating costs over long-term.


Carbon Adsorption

Organics can be cationic, anionic or nonionic. Ion-exchange resins remove some soluble organic acids and bases (anions and cations) from raw water, but some nonionic organics coat the resin. This process, known as resin "fouling," decreases the life of the resin and diminishes its capacity. To protect the ion-exchange resin, carbon filters can be placed upstream to remove nonionic organics.

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.

Table 3. Carbon Adsorption
Advantages

Removes dissolved organics and chlorine effectively.

Long life (high capacity).

Disadvantages

Can generate carbon fines.


Microporous Membrane Filtration

There are three types of microporous filtration: depth, screen and surface. Depth filters are matted fibers or materials compressed to form a matrix that retains particles by random adsorption or entrapment. Screen filters are inherently uniform structures which, like a sieve, retain all particles larger than the precisely controlled pore size on their surface. 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.

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.

Surface filters remove 99.99% of suspended solids and may be used as either prefilters or clarifying filters. Microporous 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.

Table 4. Microporous Membrane Filtration
Advantages

Absolute filters remove all particles and microorganisms greater than the pore size.

Requires minimal maintenance.

Disadvantages

Will not remove dissolved inorganics, pyrogens or all colloidals.

Potentially high expendable costs.

Not regenerable.


Ultrafiltration

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 size 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 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.

Table 5. Ultrafiltration
Advantages

Effectively removes most particles, pyrogens, microorganisms, and colloids above their rated size.

Produces highest quality water for least amount of energy.

Regenerable.

Disadvantages

Will not remove dissolved inorganics.


Reverse Osmosis

Reverse osmosis (RO) is the most economical method of removing 90% 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.

Table 6. Reverse Osmosis
Advantages

Effectively removes all types of contaminants to some extent (particles, pyrogens, microorganisms, colloids and dissolved inorganics).

Requires minimal maintenance.

Disadvantages

Limited flow rates.


Electrodeionization

This new 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.

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 filed 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.

Table 7. Electrodeionization
Advantages

Removes dissolved inorganics effectively

Non-polluting and safe:
• No chemical regeneration
• No chemical disposal
• No resin disposal

Inexpensive to operate

Disadvantages

Needs pre-purification.


Ultraviolet (UV) Radiation

Ultraviolet radiation has widely been used as a germicidal treatment for water. Mercury low pressure lamps generating 254 nm UV light are an effective means of sanitizing water. The adsorption of UV light by the DNA and proteins in the microbial cell results in the inactivation of the microorganism.

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.

Table 8. Ultraviolet Radiation
Advantages

Effective sanitizing treatment.

Oxidation of organic compounds (185 nm and 254 nm) to < 5 ppb TOC

Disadvantages

Decreases resistivity.

Will not remove particles, colloids, or ions

Pulling It All Together

Water Purification Systems

Because each purification technology removes a specific type of contaminant, none can be relied upon to remove all contaminants to the levels required for critical applications. A well-designed water purification system uses a combination of purification technologies to achieve final water quality.

Each of the purification technologies must be used in an appropriate sequence to optimize their particular removal capabilities. The schematic below shows a central laboratory water purification system designed to produce water for critical applications.

The first step is pretreatment equipment specifically designed to remove contaminants in the feed water. Pretreatment removes contaminants that may affect purification equipment located downstream, especially reverse osmosis (RO) systems. Examples of pretreatment are: carbon filters (or tanks) for chlorine removal, particulate filters for sediment/silt/particulate removal, and softening agents to remove minerals that cause "hard" water.

The next purification step is RO. RO removes 90 to 99% of all the contaminants found in water. It is the heart of any well designed water purification system because it effectively removes a broad range of contaminants.

However, the tight porosity of the RO membrane limits its flow rate. Therefore, a storage container is used to collect water from the system and distribute it to other points-of-use such as polishing systems.

Polishing systems purify pretreated water, such as RO water, by removing trace levels of any residual contaminants. Polishing elevates the quality of pretreated water to "Type I" or "ultrapure" water.

A polishing system is designed to remove residual traces of impurities from water already pretreated by some other means (such as reverse osmosis or deionization). Treating raw tap water using such a system would quickly exhaust its capacity and affect final quality.

A typical polishing system may consist of activated carbon, mixed-bed deionization, organic scavenging mixtures and 0.22 µm final filtration. Systems can also be enhanced with ultrafiltration, ultraviolet oxidation or other features for use in specific applications.

This combination of purification technologies, combined with proper pretreatment, will produce water that is virtually free of ionic, organic and microbial contamination.