DIFFERENT WATER FILTRATION METHODS EXPLAINED
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, there can sometimes be what is called carry-overs in the water that is distilled. Organics such as herbicides and pesticides, with boiling points lower than 100 Â°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.
Distilled water can also be very acidic, having a low pH, thus should be contained in glass. Since there is not much left, distilled water is often called “hungry” water. It lacks oxygen and minerals and has a flat taste, which is why it is mostly used in industrial processes.
Table 1. Distillation
|Advantages Removes a broad range of contaminants
|Disadvantages Some contaminants can be carried into the condensate
Requires careful maintenance to ensure purity
Consumes large amounts of energy
System usually takes a large space on counter
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.
|DisadvantagesDoes not effectively remove particles, pyrogens or bacteria. DI beds can generate resin particles and culture bacteria.
High operating costs over long-term.
Carbon absorption is a widely used method of home water filter treatment because of its ability to improve water by removing disagreeable tastes and odors, including objectionable chlorine. Activated carbon effectively removes many chemicals and gases, and in some cases it can be effective against microorganisms. However, generally it will not affect total dissolved solids, hardness, or heavy metals. Only a few carbon filter systems have been certified for the removal of lead, asbestos, cysts, and coliform. There are two types of carbon filter systems, each with advantages and disadvantages: granular activated carbon, and solid block carbon. These two methods can also work along with a reverse osmosis system, which can be read about below.
Activated carbon is created from a variety of carbon-based materials in a high-temperature process that creates a matrix of millions of microscopic pores and crevices. One pound of activated carbon provides anywhere from 60 to 150 acres of surface area. The pores trap microscopic particles and large organic molecules, while the activated surface areas cling to, or adsorb, small organic molecules.
The ability of an activated carbon filter to remove certain microorganisms and certain organic chemicals, especially pesticides, THMs (the chlorine by-product), trichloroethylene (TCE), and PCBs, depends upon several factors, such as the type of carbon and the amount used, the design of the filter and the rate of water flow, how long the filter has been in use, and the types of impurities the filter has previously removed.
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 Basic 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.
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, chemicals, pyrogens or all colloidals.
Potentially high expendable costs.
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.
|Disadvantages Will not remove dissolved inorganics.|
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). In fact, reverse osmosis technology is used by most leading water bottling plants.
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. Pak water is driven from the concentrated solution and collected downstream of the membrane.
Because RO membranes are very restrictive, they yield 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.
Reverse osmosis is highly effective in removing several impurities from water such as total dissolved solids (TDS), turbidity, asbestos, lead and other toxic heavy metals, radium, and many dissolved organics. The process will also remove chlorinated pesticides and most heavier-weight VOCs. Reverse osmosis and activated carbon filtration are complementary processes. Combining them results in the most effective treatment against the broadest range of water impurities and contaminants.
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.
In addition, Reverse osmosis treatment is an insurance policy against nuclear radiation such as radioactive plutonium or strontium in the drinking water. If one lives near a nuclear power plant, this is a key way to ensure the household is drinking the best water for their health.
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 Flow rates are usually limited to a certain gallons/day rating.|
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 Âµm 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.