How Activated-Charcoal Media WorksActivated-charcoal water filters have been used since the mid-1950s. The media they use — activated charcoal — is a form of steam-treated charcoal. The charcoal itself can be derived from a variety of plant materials or coal. Whatever its origin, the steam processing causes the charcoal to become extremely pitted. This rugged and uneven texture greatly increases the charcoal’s surface area, which results in much greater adsorption capacity (see below) of dissolved gaseous, liquid compounds and dissolved substances. Activated charcoal does have some ability to strain out certain solids, too (called mechanical filtration). And the more compressed the activated charcoal is, the greater it’s capacity to do this.
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Adsorption causes most volatile chemical molecules (benzene, chlordane, etc.) and nearly all naturally occurring, dissolved gaseous molecules (including those of hydrogen sulfide) to adhere to the activated charcoal’s surfaces, thus removing them from the water supply.
Adsorption similarly affects radioactive radon molecules. However, it must be pointed out that the typical small filters often used in houses, because they only contain a rather limited amount of activated charcoal, can’t mechanically filter out the solid radioactive particles that this gas emits. Therefore, large granular activated charcoal radon filtering systems have been developed that hold much greater quantities of media — so much so that they’re able both to adsorb the gas and trap any radioactive particles that are produced by it.
It should be noted that activated charcoal works particularly well at adsorbing chlorine and trichloromethane (chloroform), which is a chlorine byproduct found in chlorinated utility water. It can also remove tastes and odors that are released by the glues used to assemble plastic water-supply pipes. In fact, because activated charcoal is so efficient at removing most unpleasant tastes and smells in water, activated charcoal filters are often called taste-and-odor filters. (This is particularly true for very simple, granular-media filters.)
Granulated or Solid Block?The first type developed for use in water filters was granular. However, because the loose, rough bits of activated charcoal have spaces between each of the granules, this can allow some of the water molecules to bypass the granules without ever coming in direct contact with their surface. Another problem is that sometimes channeling can occur in granulated charcoal. This happens when the water streams passing through the filtering media start following the same pathways. Because much of the activated charcoal is bypassed, and the limited amount of activated charcoal surrounding the channel gets quickly saturated with contaminants, you can see how this would provide much less efficient filtration. To counter this, some filter manufacturers have created special anti-channeling configurations.
Gaining in popularity in recent years is solid-block activated charcoal, also known as microfine carbon or carbon block. It’s increasingly being used because of its superior mechanical-filtration ability. This capacity has been considered particularly important at removing protozoan cysts from water. (Cryptosporidium parvum oocysts were responsible for a number of deaths in Milwaukee in 1993.)
Solid-block activated charcoal is tightly compressed. Because it has none of the small spaces that surround bits of granular activated charcoal, suspended solids can’t enter it and many are thus strained out. Those that do enter, tend to get trapped inside the block. In addition, solid-block activated charcoal may also be superior at adsorption, because all the water molecules are forced to travel through the compressed charcoal itself, not detour around it through channels. Apparently, some lead and asbestos can be removed in this manner.
Many activated charcoal block filters have other materials or substances incorporated in them as well. For example, sometimes plastic (such as polypropylene) is imbedded. In certain cases, it may have a positive electrostatic charge (either permanent or acquired by the movement of the water stream) to attract and hold onto certain minute, oppositely charged pollutants. In other filters, the plastic may have other purposes, perhaps to create or enhance a particular internal structural design.
Other activated charcoal-block filters have what’s termed precoat technology on their outer surfaces. (Some have both imbedded additives as well as precoat technology.) This may be in the form of a “spun-bonded polypropylene,” or some other material. These may also have an electric charge. All these additions make charcoal block filters more capable of removing minute solids from the water.
It’s very important for you to know that, with any type of activated charcoal filter, the longer the water is in contact with the media, the more effective the removal of the contaminants will be. (This is especially important with granular filters.) Therefore, manufacturers have devised two strategies to help their filters accomplish this. One is simply to use a larger amount of activated charcoal that the water must pass through. The other is to create methods that slow the water’s rate of flow through the media. Some units combine both tactics.
Today, you can purchase a filter with activated charcoal as a whole-house unit, or as a point-of-use device (for under your sink, above your sink, or to replace your regular shower head). Filters designed for whole-house use, as well as most of the under-sink models, will generally require some special plumbing installation work. Although a few activated charcoal filtering units have stainless-steel housings, most of them are made of plastic, as are many of the granular activated cartridge refills. Fortunately, however, the activated charcoal will most likely adsorb any plastic contaminants, odors and tastes that happen to get into the water from these particular components. However, some extremely chemically sensitive individuals may want to purchase a stainless-steel model that doesn’t use plastic cartridges.
As you might expect, the cost of activated charcoal filters varies. However, as a rule, many of the simpler granular cartridge units are relatively low priced. The actual cost will depend on the filter’s size, its casing material (plastic or stainless steel), whether it requires professional installation, the replacement schedule and the cost of the replacement media. While most units use replaceable cartridges, there are some that require the entire filtering unit to be replaced.
Activated Charcoal Media ConcernsIt must be emphasized that activated charcoal filters are not the answer to every water-quality problem. It must be understood that granular activated charcoal filters can’t purify your water by killing any bacteria (or other microorganisms) or by removing bacteria effectively from your water. Even some solid activated charcoal blocks aren’t able to strain out everything. You should also be aware that research has found that bacteria can build up and eventually thrive on activated charcoal media, particularly if the water supply hasn’t been chlorinated.
Even activated charcoal filters with bacteriostatic additives (compounds that inhibit the growth of bacteria), such as silver, can apparently become contaminated if given enough time. Silver (it may be in the form of silver nitrate) is used because, as the water passes it, minute quantities of positively charged ions are released which, theoretically, negatively react and ultimately kill bacteria. While silver compounds are the most commonly utilized, other anti-microbial compounds have also been tried. Unfortunately, the U.S Federal Trade Commission (FTC) reported that filters with bacteriostatic additives “have shown unpromising results.”
As another anti-bacteria-buildup strategy, some activated charcoal filters have been designed to have a backwashing cycle to periodically rinse off the media. But, while backwashing will certainly help, it may not be completely effective in preventing contamination problems. In addition, backwashing tends to “waste” water because the water used is flushed down the drain.
Because of activated charcoal’s very real capacity to support bacterial growth, some water experts believe that such filters should only be used with chlorinated water, or where some other purification treatment such as ultraviolet-light, ozonation, etc. has been previously used on the water. The thinking behind this is that, if chlorine is present in the in-coming water supply, it’s oxidizing disinfectant action will kill many types of bacteria, and the activated charcoal itself will then adsorb the chlorine before the homeowner actually uses the water.
Charcoal Filter CareWith all activated charcoal filters, the media must be replaced from time to time — not just because of possible microbial growth — but also because it’ll eventually be unable to adsorb effectively any more contaminants. In fact, if the activated charcoal becomes saturated, it may release some of these pollutants back into your “filtered” water supply.
The actual life expectancy for a particular cartridge (or filtering unit) will differ with the type of unit, the brand, the quality of your water and the volume of water used daily. Generally, a filter manufacturer will give a suggested replacement schedule — perhaps once a month, several times a year, or once a year. However, sometimes these recommendations are somewhat exaggerated. One thing is for certain, if you start tasting or smelling chlorine, for example, it is time to replace the media — no matter what the timetable says.
You should also know that hot water reduces activated charcoal’s adsorption ability, so odorless, tasteless trihalomethanes (chlorine by-products) may be released without you knowing it. Apparently, these filters must be used with cold water to effectively remove the most toxic, chlorinated-water by-products. Therefore, it’s best to use activated charcoal media before the water is heated.
It should be mentioned that, after installing a replacement activated charcoal filter (or filtering cartridge), you should run the cold water from the nearest tap. This is to remove any excess powdery charcoal particles from the water stream. Generally, it only takes a few minutes for the water to run clearly. (Some filter manufacturers may suggest a longer flushing period.)
As a final note, activated charcoal filters are commonly used in conjunction with other types of water-treatment strategies, such as reverse osmosis and distillation, as prefilters or postfilters.
From Creating a Healthy Household: The Ultimate Guide For Healthier, Safer, Less-Toxic Living, © 2000 by Lynn Marie Bower. Used by permission.
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