Filtration Methods

Methods of Filtration

• Distillation
• Ion Exchange
• Carbon Adsorption
• Reverse Osmosis
• UV (Ultraviolet)

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 at the back in the liquid phase vessel. However, there can every so often be what is called carry-overs in the water that is distilled. Organics such as herbicides and pesticides, with boiling points lower than 100 degree 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.

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
System usually takes a large space on counter


Ion Exchange:

The ion exchange process penetrates water through bead-like sphere-shaped resin materials (ion-exchange resins). Ions in the water are exchanged for other ions set to the beads. The two most common ion-exchange methods are softening and deionization. Softening is used primarily as a pretreatment method to decrease water hardness earlier than reverse osmosis (RO) processing. The softeners contain beads that replace two sodium ions for every calcium or magnesium ion detached 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 holding sulfonic acid groups, will exchange a hydrogen ion for any cations they meet (e.g., Na+, Ca++, Al+++). Likewise, the anion exchange resins, made of styrene and containing quaternary ammonium groups, will swap over a hydroxyl ion for any anions (e.g., Cl-). The hydrogen ion from the cation ex-changer unites with the hydroxyl ion of the anion ex-changer to form pure water. These resins may be packed up in separate bed ex-changers with divide units for the cation and anion exchange beds. Or, they may be packed in mixed bed ex-changers 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 revival reverses the purification process, replacing the contaminants bound to the DI resins with hydrogen and hydroxyl ions. Deionization can be a vital component of a total water purification system when used in blend with other methods discussed in this briefing such as RO, filtration and carbon adsorption. DI systems efficiently 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 consequent pyrogen generation. 

Advantages

Removes dissolved inorganic effectively

Regeneration is possible (service deionization)

Comparatively inexpensive in preliminary 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:

Carbon absorption is a widely used method of home water filter treatment because of its competence to get better water by eliminating disagreeable tastes and odors, including objectionable chlorine. Activated carbon successfully takes away a lot of chemicals and gases, and in some cases it can be useful 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 elimination of lead, asbestos, cysts, and coli-form. There are two types of carbon filter systems, each with pros & cons granular activated carbon, and solid block carbon. These two methods can also work with a reverse osmosis system. Activated carbon is formed from a variety of carbon-based materials in a high-temperature process that makes 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 capacity of an activated carbon filter to eradicate certain microorganisms and certain organic chemicals, especially pesticides, THMs (the chlorine by-product), trichloroethylene (TCE), and PCBs, depends upon quite a few 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 earlier removed.

Advantages 

Removes dissolved organics and chlorine effectively. 

Long life (high capacity) 

Disadvantages 

Does not effectively remove particles, pyrogens or bacteria 

DI beds can generate resin particles and culture bacteria 

High operating costs over long-term.

Micro Filtration:

There are three types of micro porous filtration: depth, screen and surface. Depth filters are matted fibers or materials compressed to form a matrix that retains particles by casual 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 first and foremost 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 pre-filters because they are an economical way to remove 98% of suspended solids and defend elements downstream from fouling or clogging. Surface filters eradicate 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 scheme to take away the last remaining traces of resin fragments, carbon fines, colloidal particles and microorganisms.

Advantages

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

Requires minimal maintenance 

Disadvantages

Will not remove dissolved in organics, chemicals, pyrogens or all colloidal 

Potentially high expendable costs. 

Regeneration is not possible

Ultra Filtration: 

A micro porous membrane filter removes particles according to pore size. By contrast, an 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 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.

Advantages 

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

Produces highest quality water for least amount of energy 

Regeneration is possible 

Disadvantages 

Will not remove dissolved inorganic

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). 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 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 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 methods 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 pre-treatment 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.

Advantages

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

Requires minimal maintenance

Disadvantages

Flow rates are usually limited to a certain gallons/day rating.


Ultraviolet (UV):

Ultraviolet or “UV” is a type of energy found in the electromagnetic spectrum, lying between x-rays and visible light. Although we cannot see UV light or rays, we are exposed to them every time we step out into the sun. In fact, UV light is responsible for causing sunburns. Ultraviolet systems use special lamps or bulbs that emit UV light of a particular wavelength. The Ultraviolet energy attacks the genetic core of the microorganism and rearranges the DNA /RNA eliminating the microorganism's ability to function and reproduce. If the microorganism can no longer reproduce, it cannot replicate, therefore it cannot infect other organisms with which has contact. The process is simple but effective, destroying 99.99 percent of harmful microorganisms without adding chemicals to the water.

The quality or appropriateness of both the UV light and of the 'contact ', are crucial to accomplish disinfection. It is important to properly 'size' the UV based upon the application. It is equally important to use a good pre-filter to remove any dirt or debris that may be present in the raw water supply. This dirt and debris can interfere with the effectiveness of the UV rays – virtually giving the microorganism a shield to protect them when passing the UV rays. The keyword here is quality. System manufacturers strongly recommend that any pre and post filters be replaced at specified periods and that the UV lamp is replaced on an annual basis or after 9,000 hours of use -- whichever comes first.

Advantages

There are some very important reasons why both homeowners and municipalities are choosing UV technology to treat their water.

Highly effective: For over 25 years, UV technology has been trusted as a safe, cost-effective way to purify water and eliminate harmful microorganisms. It is a proven EPA endorsed technology that is currently being used by thousands of cities, bottled water manufacturers and homeowners around the world.

Chemical free: UV provides water purification without the addition of harmful chemicals such as chlorine. It also avoids the potential of generating harmful chemical disinfection by products. Recent EPA guidelines are forcing cities across the United States to reduce or eliminate the use of chlorine for exactly this reason.

Taste & odor free: UV does not change the taste, odor or color of the water.

More effective than chlorine: Unlike chlorine, UV systems are effective against both Cryptosporidium and Giardia.

Compact and easy to maintain: UV systems are capable of treating a single faucet or an entire home in a minimal amount of space with the only maintenance being an annual lamp and filter replacement.