Carbon is one of the oldest technologies available for drinking water filtration. Sanskrit texts from 2,000 B.C. speak about filtering water over charcoal and examples exist in Egyptian tombs as well.
Carbon was, and is, a pretty effective technology for filtering out turbidity, or unclear water, and will improve the taste and odor profile in water. In fact, most modern membrane technology will use carbon as a pre-filter, post-filter, or both.
How Carbon Filters Work
What carbon does well is largely a property of its physical structure. It is extremely porous. One pound of carbon will have up to 100 acres of surface area.
How it actually works is through a process of adsorption where something is getting trapped in that extremely porous surface of the carbon.
In addition to the sediment, taste and odor use written about over 4,000 years ago, carbon is also effective at the reduction of a few other contaminants:
Volatile Organic Compounds (VOCs)
The EPA houses these nasty chemicals under a list of 32 compounds. Many might be somewhat familiar and include benzene, carbon tetrachloride, dioxin, styrene, toluene, chloroform, and vinyl chloride. One of the other 32 items is total trihalomethanes (TTHMs). It’s the TTHMs list where things get weird, that “one thing” is actually contaminants numbering in the thousands of compounds and many are the byproducts of disinfection, primarily from the use of chlorine.
Carbon is actually great at removing chlorine. However, the Chemical Manufacturers Association and it’s lobbyists have absolutely promised us that it is not a health hazard, merely an aesthetic preference. Regardless, carbon is great at removing chlorine given a slow enough flow rate over the media bed.
In a related story, I probably should have someone else start my car for the next few weeks. I’d probably be safer double-crossing a crime family than dare upset the Chemical Manufacturers Association or say anything about the countless amount of research on chlorine and cancer rates.
What Carbon Isn’t Good At
Carbon is virtually useless when it comes to the removal of dissolved compounds. If you take the carbon gravel you’ll find in a pitcher-style filter or refrigerator filter, these aren’t going to significantly touch dissolved inorganic compounds or lead, copper, and mercury.
Similarly, these charcoal filters aren’t going to reduce the suspended salts or radionuclides. Probably three of the biggest hot-button issues that are not addressed at all by carbon are nitrates, flouride, and sodium.
In the EPA’s microbiological control side of things, there are four characteristics: turbidity (floating stuff), cysts (cryptosporidium & giardia), bacteria and viruses. Carbon is pretty good on the first.
For the rest, carbon is not only ineffective, but possibly counterproductive. That same porous property of carbon will catch some bacteria. And it’s dark, damp and temperature controlled. That’s also the recipe for growing some species of bacteria. Using your basic carbon filter on water with pathogens or simply using that filter for too long can be far more dangerous than one would think.
That “reset” button on the front of the refrigerator is removing the warning light, not the problem.
Selecting a Carbon Filter
Carbon is really, really basic. It is a matter of only a few characteristics.
Construction / Density
Cut open a Brita filter or your refrigerator filter. Most of those are the same gravel you’ll use in a fish tank. Water is extremely lazy. Given the choice between fighting through your intermittent gravel or just going around it, that water is going to go straight into your glass.
You’ll find other higher quality filters that are extremely dense, filtering down to as small as 0.5 microns. In general, high-density carbon blocks will outperform the gravels and many of the loose granulated activated carbon (GAC).
Flow Rate / Contact Time
Like all physical treatment, if water goes over the media bed too fast, virtually nothing is filtered. You can’t take a small filter, tie it in before the water softener, and expect to have any results going through the system. So the efficacy of an 18″ block carbon filter will be far, far greater than that of a 4″ long gravel Brita-style filter.
While carbon is not only ineffective, but counterproductive, in dealing with pathogens, some of the better filters will combine use silver for its bacteriostatic properties. You can actually get to pretty decent filtration with some carbon filters.
Keys on Carbon Filters
Like any water technology, look for the certifications. If they’re not presented front and center on the marketing material, there is probably a reason. Most are merely taste and odor filters making your filter about 4,000 years old.
Probably the other big issue is the replacement life. Some of these manufacturers will speak to 10,000 gallons of chlorine reduction capability, but if the VOC and TTHM life is diminishing at 1,000, when should you replace it?
As always, if you have any questions, let us know.