Filter Carbon: What It Does and What It Doesn't

by Gene Franks

If you've wondered what makes water filters effective, this article will give some answers. Water filters aren't magic. Except for rare specialty items, what we call a water filter works because of a single ingredient--activated carbon. The present article is a slightly truncated and revised version of one that's been on Pure Water Products' main commercial website for a number of years.-- Hardly Waite, Editor.

The largest single section in the “EPA Regulated Water Contaminants” list is the section on Organics (including VOCs, or “Volatile Organics”). In this category the EPA lists numerous very nasty organic chemical contaminants—many with familiar names like benzene, 1,1 dichlorethylene, carbon tetrachloride, dioxin, styrene, toluene, chloroform, and vinyl chloride. To give an idea of the extensiveness of this list, a single item,  “Total Trihalomethanes,” consists of hundreds of chemicals, some  still undiscovered or unstudied, that are formed as by-products of the chlorination process. The EPA's maximum allowable level for trihalomethanes, many of which are suspected or known cancer causers and are present in virtually all chlorinated tap water, is less than 1/10 of one part per million.

For the Organics category, the primary treatment in all cases and the only recommended treatment in most cases, is activated carbon.

The EPA’s Pesticides category lists many familiar poisons such as Aldicarb, Chlordane, Heptachlor, and Lindane. In all cases, activated carbon is the only recommended treatment.

Of the Herbicides listed (2,4-D, Atrazine, etc.), activated carbon is the only treatment recommended.

In short, for Organics, Pesticides, and Herbicides, which together make up 90% of EPA's regulated substances, the standard treatment, and in many cases the only treatment recommended, is activated carbon.

What carbon filtration does not do can be seen in the remaining three categories of the EPA contaminant list.

Microbiological contaminants.  Carbon is listed as a treatment in only under one category--turbidity. It is not recommended for coliform removal or for cysts, though in reality some of the very tight solid carbon block filters now on the market remove bacteria (though manufacturers seldom make this claim), and many carbon block filters are now certified for removal of cysts (giardia and cryptosporidium). 

Inorganic contaminants.  Carbon appears in the EPA list only as a preferred treatment for mercury.  Carbon filters are also frequently engineered to remove lead (by the addition of ion exchange resins) and some carbon filter makers claim asbestos reduction, but by and large carbon is not a good treatment for most inorganic water contaminants.

The same is true in the final category, Radionuclides, where carbon is ineffective and reverse osmosis (RO) and ion exchange are definitely the treatments of choice.

Chloramines and Chlorine

Chlorine was not considered in the discussion above because the EPA does not consider it a water contaminant. Nevertheless,  chlorine removal is a top priority of water filter purchasers.  Chlorine removal is what carbon is best at, and nothing else equals carbon’s ability to remove chlorine. Carbon, especially the specially processed carbon known as "catalytic carbon," is also the best treatment by far for chloramines, the increasingly (un)popular chlorine substitute.

Fluoride, another EPA-unlisted additive that people frequently want removed from tap water, is not readily removed by carbon filtration.  (Although carbon can remove fluoride under the right circumstances, its performance is sporadic and unpredictable, so it's best not to count on it.) I should note, too, that a unique carbon called "bone char," which is made from animal bones, is widely used as a fluoride remover in some parts of the world, though rarely in the United States.

TDS, "total dissolved solids," the count of the total mineral content of water, is not affected by carbon filtration. The EPA suggests a non-enforceable upper limit of 500 parts per million (ppm) total dissolved solids. TDS can be reduced only by reverse osmosis or a distiller or by a very expensive process called deionization.  

Sodium, Fluoride, and Nitrates

Tap water ingredients that people most frequently want removed that are not readily removed by carbon filtration are fluoride, nitrates, and sodium. (Sodium is measured in water as part of the Total Dissolved Solids, discussed above.) Reverse osmosis and distillation remove all three. so either of them, combined with a high quality carbon filter, provides complete treatment. All three can also be removed by selective, non-carbon filters designed for the purpose. For example, you can obtain a double filter with one fluoride and one carbon cartridge if fluoride removal is desired. The fluoride cartridge does not contain carbon but a specialty medium called Activated Alumina.

When distiller sellers or zealous home marketers show you a chart that indicates that reverse osmosis (RO) units do not remove chlorine or certain chemicals, keep in mind that RO units contain one or more carbon filters. In fact, “thin film” RO units, the most common type, must remove chlorine from the water as the very first operation else the unit’s membrane will be destroyed. Such statements are simply advertising cheap shots that are technically true but in reality totally false and intentionally misleading. It's surprising and disappointing that some large companies actually do this in their promotional literature.

A reverse osmosis unit that has at least two high quality carbon filters is the best and most complete drinking water treatment for the home. When people say they want a water treatment system that "removes everything," reverse osmosis is as close as you can get.

Numerical water facts from B. Bee Sharper, the Pure Water Gazette's numerical wizard.