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Before you buy ANY type of water purification or filtration system, you should know how each type of system performs, what they do or do not remove, relative costs and other factors. If you wish to see some comparison charts on these subjects, go to System Comparisons now. You can then return to this or other pages and continue your review of the systems.
>>Complete Solid Block Carbon & 5 Micron Sediment Filtration System with Solid Stainless-Steel(no lead) faucet and hookup equipments - $159.00.
>Note: Also Available as a "Prefilter Assembly" for All Automatic Water Distillers. $120.00 > >
>BACKGROUND TECHNICAL DISCUSSION OF CARBON FILTRATION
System
Performance
Comparison
Microbiological NO PARTIAL YES YES NO YES Organics YES YES NO YES YES YES Heavy Metals NO NO NO NO PARTIAL YES Radioactivity NO NO NO NO PARTIAL YES Inorganics NO NO NO NO PARTIAL YES Activated carbon is a sorbent used to remove a variety of organic chemicals from water.
The extent and specificity of solute adsorption varies with the source of carbon; generally, compounds with low molecular weights are adsorbed more readily than those with high molecular weights.
The fundamental carbon particle is generally less than 1 micromillimeter in diameter but is aggregated during manufacture (in the presence of a binder) to produce porous granules up to several millimeters in size.
During use, mechanical attrition can break down the granules into their fundamental particles (fines) which, because of their low density and small diameter, are diffficult to remove from water except by membrane filtration or distillation.
>Principles of Operation
The simplest way to conceptualize carbon adsorption is to think of the granular carbon particle as a porous ball. The water to be purified flows over the surface of, and between the balls. Solutes diffuse from the water stream into the pores of the ball and subsequently become adsorbed, or attached, to the surface of the pore.
Consequently, the rate of solute adsorption depends on how quickly a contaminant molecule can move from the bulk water flow to an unoccupied site within the balls' porous structures. The rate of adsorption is influenced by the number and size of the pores and temperature, but not the velocity with which the water flows over and between the balls.
The ideal particle would be small with many pores in order to minimize the distance the solute has to travel from the bulk fluid to an adsorption site. However, very small particles are difficult to contain and cause large pressure drops across the carbon bed. Solutes move more slowly at low temperatures, so that the rate of adsorption decreases as the temperature of the water decreases.
As the water enters a fresh carbon bed, contaminating solutes are swept to the surface of the initial layer of porous balls. Some of the solutes find their way into the porous structure and are adsorbed. The partially purified water flows to succeeding layers, where the process is repeated, and on through the bed.
As the porous balls at the inlet of the bed become saturated with solute, the incoming water will not be purified until it encounters a subsequent layer, where the porous balls still have adsorptive capacity. With the flow rates normally used through carbon beds, solutes are delivered to the surface of the balls much more quickly than they can be adsorbed.
As a result, some solutes may appear in the product stream, although at reduced concentrations, even though the porous balls may still have capacity to adsorb more solute.
The ratio of bed outlet concentration to bed inlet concentration is a function of the mass of solute delivered to the bed and the mass of solute adsorbed by the bed. The mass of solute delivered to the bed increases as the flow rate increases, while the mass adsorbed by the bed is largely dependent on the length of the time the water is in contact with the bed.
Consequently, for a bed of given size, outlet concentration will increase with increasing flow rate, since this both increases the mass of solute delivered to the bed and decreases the contact time.
The maximum acceptable concentration of a given solute at the outlet of a carbon bed is termed the "break-through" concentration. When the concentration at the outlet of the carbon bed reaches the break-through value, the column is said to have "broken through".
As previously described, break-through may occur even though the adsorptive capacity of the bed has not been reached. That unused capacity remains at break-through can be demonstrated by stopping the feed flow for a period to allow redistribution of the solute into the empty sites. When the flow is restarted, the outlet concentration of solute will be observed to be less than the break-through value once again.
Such a redistribution period may allow more use of the available capacity of the carbon bed. When break-through occurs, the carbon bed must be replaced in spite of the fact that adsorptive capacity remains unused.
>Types of Carbon
Activated carbon is prepared by the pyrolysis of a variety of organic materials (for example, coconut shells, bones, coal, lignite, peat, petroleum, and wood) in closely regulated atmospheres. The porosity of activated carbon and, thus, its internal surface area, varies with the type of material used and the conditions of pyrolysis. The presence of oxygen alters the surface of the carbon inside the porous structure, changing its adsorptive properties.
Depending on the material from which the carbon is prepared, various amounts and types of residual metallic contaminants, including aluminum, may be left in the activated carbon. These contaminants have the potential to leach into water as it flows through the carbon.
NOTE: A recent study has identified arsenic as a contaminant which could be leached from various types of carbon filtration elements. This study is summarized in "Impact of Proposed New Arsenic Standards on POU Carbon Filtration", Water Conditioning & Purification, July 2000. The study concludes that coconut carbon is the least likely to leach arsenic into drinking water.This study also summarizes the changes which are occurring with respect to permissible levels of arsenic and why these reduced levels now expose the carbon filtration industry to closer scrutiny with respect to arsenic residues in the carbon materials used for products.
Petroleum-based carbons contain less minerals than those derived from bone, wood, or coal. The mineral content of carbon used for water purification for high purity applications has important implications on the positioning of the carbon in the purification system. In particular, since most carbon media suitable for medical and high purity applications will add metallic contaminants to the water, carbon filters should not be positioned downstream of reverse osmosis.
UNFORTUNATELY, BECAUSE OF THE LARGE AMOUNT OF WASTE WATER PRODUCED BY REVERSE OSMOSIS, REVERSE OSMOSIS MANUFACTURERS PLACE CARBON FILTER AFTER INSTEAD OF BEFORE THE MEMBRANE.
SO, IF REVERSE OSMOSIS MANUFACTURERS PRODUCE CARBON POST FILTERS WITH HIGH LEVELS OF MINERALS, INCLUDING ARSENIC, THESE CARBON FILTERS SIMPLY DUMP THESE MINERALS AND ARSENIC IN THE PRODUCT WATER---ANOTHER WELL KEPT SECRET WITHIN THE REVERSE OSMOSIS INDUSTRY.
The properties of carbon used for adsorption are often described in terms of the equilibrium amount of certain marker chemicals the carbon will absorb. This may be specified as an "X" number, where "X" is the chemical used for the test. For example, carbon used in decolorizing sugar solutions is specified by a "molasses" number and carbon used for removing phenols from water may be specified in terms of an "iodine" number.
The 'X" number indicates how much of a given species will be removed from water, if enough time is provided to reach equilibrium.
Although carbons are not characterized in terms of a "free chlorine" or "chloramines" number, the relative affinities for iodine and molasses (a large carbohydrate) provide some measure of adsorption capacity.
Carbons with high iodine numbers are more useful for removing aromatic chemicals (contaminants originating from agricultural use and manufacturing) and small solutes, such as chloramines, while carbons with high molasses numbers are generally more useful for removing lignin and humic acids (naturally occurring contaminants found in surface waters).
Applications
Carbon adsorption is used in drinking water to remove a wide variety of organic contaminants. It is not practical to attempt to list the wide range of organic chemicals versus the effectiveness of various types of carbon filter materials/products due to the wide variation in testing scenarios, concentrations of challange materials, "contact time", flow rates, etc.
Suffice it to say that carbon possesses a variety of effectiveness while exposed to these organic compounds. The slower the water flow, the more effective the adsorption process.
A wide variety of industrial applications use coal-based carbon. Since it is not used for drinking water in this application, the levels of arsenic and other minerels in the carbon do not pose a health hazard.
Even coconut carbon filters will exhibit low levels of arsenic, probably through absorption of contaminated ground water.
Rinsing or thoroughly flushing the carbon filter prior to use for drinking water purpose will substantially reduce the level of arsenic which may reside in loose carbon "fines" and thus be released in the the drinking water during normal flow conditions.
The study referenced above found that some coconut carbon filters imported from the Orient exhibited arsenic levels which not meet the current 5 part per billion(ppm) arsenic standard. In selected cases where poorer quality control is in place, these imported products could not meet a 10 ppb level.
The answer is simply one of quality control monitorying and a rigorous testing regimen to ensure compliance. When you as a consumer or dealer purchase POU products which include some type of carbon post filter, we recommend you query the supplier on this topic.
>TYPES OF CARBON ELEMENTS > >
CARBON-PLEATED: These are simply pleated cellulose or polysester cartridges that have been impregnated with powdered activated carbon. This configuration prodices sediment filtration and chlorine, taste and odor reduction at modest flow rates. Various customers experience extremely bad odors emminating from their treated water after use with this type of cartridge. Although it is fairly inexpensive, we do not recommend it for serious drinking water appliances such as reverse osmosis or steam distillation .
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GRANULAR GAC: These cartridges are essentially a canister containing granular activated carbon(GAC), a compression pad and a post filter.
The compression(or expansion) pad limits channeling and the post-filter reduces carbon fines and other suspended matter. The water enters one end and travels the length of the carbon bed exiting the other end. This configuration allows for maximum contact time and good taste and odor reduction. Low flow rates are recommended.
We find test data on this unit and the carbon block filter(both from the same manufacturer) favoring the carbon block in lifetime by factors of 6 to 8.
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CARBON BLOCK: These cartridges are either molded or extruded. In both manufacturing methods, granular or powdered activated carbon and binders are mixed. The mixture is poured into a mold or pushed through a tube(extruded) and heated unil the binders are melted.
When cooled, the mixture retains the shape of the mold or tube. Specialty cartridges are then produced by adding resin and other materials. Often an outer wrap is applied as a sediment screen or filter. End caps and cores are added to complete the product. The process can be controlled to produce various micron ratings. 0.5 microns is common for home-use applications.
Water enters the side of the cartridge and migrates to the center core. Some cartridges are configured to remove volatile organic compounds(VOC's) and pesticides in addition to chlorine and other tastes and odors.
A drinking water "filter" is intended to remove dirt, sediment, rust, bad taste and odors and what we call organic chemicals---man-made nasties concocted from a wide range of elements, but always containing the element carbon. That is all drinking water filters normally do.
Also, a filter is legally not considered a water "purifier" unless the manufacturer specifically states that it is capable of removing in at least 99.75% of all bacterial contamination.
A water filter normally fits under the kitchen sink and allows a portion of your tap water to be diverted through the filter system and be subsequently dispensed at a separate faucet mounted on your kitchen sink. Often, the refrigerator's ice maker is connected to the output of the filter system.
The filter system usually contains at least two stages of filtration. First, dirt and suspended materials are removed by a cotton or fibrous filter element. The water then passes through granular or compressed carbon, which eliminates odor, taste and organic chemicals.
Regardless of what your best friend may be telling you about what his new filter is capable of doing, do not expect a carbon filter to remove bacterial contamination, radioactivity, dissolved heavy metals(lead, mercury, cadmium, etc.), sodium or other dissolved inorganic materials.
Had these claims been true, the navy and dozens of water-starved nations world-wide would be using carbon filters to desalinate(de-mineralize) ocean water instead of using distillation or reverse osmosis.
Carbon water filters tend to be considerably oversold due to their comparatively poor performance next to systems such as reverse osmosis or steam distillation. You can't fault a filter manufacturer for trying to have his system "grow up" to be a reverse osmosis or distiller system---but you can draw the line when the advertising for the product becomes misleading to the uninformed consumer.
So, what types of questions do you ask the filter salesman when you are considering a water filter and want to sort fact from fiction?
- How many gallons of water can be used by the filter before it must be discarded? Is there test data to show that it has been tested for that number of gallons?
Many manufacturers glibly avoid this issue, saying to change the filter "once a year", or "when the water tastes bad", or "the filter clogs up".
- What specific chemicals have been tested for in the filter system? What concentrations of chemicals were used in these tests?
- Does the system use a separate, disposable sediment filter to protect the more expensive carbon element?
- Does the system use standard-sized replacement elements---or must you purchase expensive replacement elements from only one place or dealership? (Note: 10 inch elements are the industry standard today).
- Is it easy to see when the filter needs cleaning or changing? In other words, can you physically see the amount of dirt or sediment being collected by the filter without having to completely dissemble the entire unit?
- What is the manufacturer's warranty?
Do not be afraid to ask the distributor these questions. Some of these questions may evoke a torrent of evasive or defamatory rhetoric from the distributor. Time to take the kiddies by the hand and leave.
And, do not be swayed by slick brochures which simply tell you how great it will be to be drinking "filtered" or "purified" water from the device---while there is no discussion of the technical aspects of the systems---how it was tested, under what conditions, and for what chemicals.
One last caution. Avoid systems which claim to remove bacteria through the use of "bacteriostatic" materials such as silver. Although such devices carry the mandatory "registration" by the Environmental Protection Agency(EPA) because they contain a controlled, toxic pesticide(silver), even the EPA has publicly stated that they have not found such devices to be effective.
How much should you pay for a filter? It is easy to spend $30-40 and believe you have purchased a real fine piece of equipment capable of eliminating all of your drinking water woes. More likely, however, you have just bought a piece of junk.
Expect to pay between $150 and $200 for a good filtration system. If you pay more than $200 for a filtration system, you are probably just paying to grease the wheels of some large multi-level marketing program. Good hardware stores, water stores and some health-food stores have products in this price range.
How about filters for the whole house? There are a lot of companies selling filters to "do the whole house". Many times they are combined with water softeners, other times they are a stand-alone carbon system.
A "whole-house" water filtration system is designed to remove dirt and chemicals(primarily chlorine and ammonia) from all water entering your home. Recent articles indicate that in some cases skin absorption of chemicals in a hot shower is as bad if not worse than drinking several glasses of the same water daily.
"Whole-house" water systems range in price from about $300 to $3000, depending on how fast the salesperson can talk. The $300 units will give you basically the same performance as systems costing thousands more. Carbon and sediment replacement cartridges for these whole-house systems will run about $100 per year.
Finally, when the filter salesperson tells you that his system is giving you 99% pure water, remember that the impurities in water which are removed by carbon filtration constitute only 1-2% of all types of impurities found in water. Therefore, the removal of 99% of 1-2% of water impurities is still only 1-2% pure water! This is the fact which you will have the most difficulty extracting from any individual or distributor who is bent on selling you a water filter---and nothing else!
As we said last above, slick brochures are no match for an educated customer. The answer to smart buying of any type of filter or purifier is self-education.
In today's specialized retail marketplace, buying a water filter or water purifier from someone who specializes in skin care lotions, soap or gymnastic equipments makes about much sense as trying to buy a new automobile at the corner grocery store. CAVEAT EMPTOR!
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