The name “water filter” is misleading. The word filtered literally means only that the water has passed through a medium containing tiny holes or fine passageways that screen out suspended particles.
When traveling in a country whose water supply is suspect and you ask a waiter whether the water is filtered, an affirmative reply may mean little more than that you can see through it.
Here at home, filter has become a generic word for a device that does more than clarify the water; it purifies it by removing tastes, odors, toxic chemicals, and pathogenic microorganisms. The idea is to make sure the water is safe and palatable.
Your nose and palate will tell you whether you want to remove odors and tastes. As far as toxic chemicals and pathogens are concerned, an analysis can be provided by many local water companies or independent labs. Depending on your degree of paranoia, you may feel like searching for a filter that will remove everything from the water but its wetness. Keep in mind, though, that it’s a waste of money to buy a device to remove things that aren’t there. Continually replacing the cartridges can be expensive.
What kinds of “bad stuff” can contaminate water? Industrial and agricultural chemicals; chlorine and its byproducts; metal ions; and cysts, which are tiny chlorine-resistant capsules of protozoan parasites such as cryptosporidium and giardia that can cause abdominal cramping, diarrhea, and even more serious symptoms in people with weakened immune systems.
Cryptosporidium and giardia cysts are generally bigger than one micron or 40 millionths of an inch, so any barrier with holes smaller than that will screen them out. But not all filter devices contain such particle filters, so if these contaminants are of concern to you, check the product’s literature to see if the performance claims include cyst reduction.
Commercial water filters, which may be either batch-at-a-time pitchers or attachments to faucets or supply lines, remove other contaminants in three ways: with charcoal, with ion exchange resins, and with actual particle filters.
The workhorse of most water filters is activated charcoal, a material that has a prodigious and indiscriminate appetite for chemicals in general and gases (including chlorine) in particular.
Charcoal is made by heating organic matter such as wood in a limited supply of air, so that it decomposes into porous carbon but doesn’t actually burn. Depending on how it is manufactured, the charcoal can contain an enormous amount of microscopic internal surface area. An ounce of so-called activated charcoal, the best kind is made from coconut shells, can contain some 2,000 square feet of surface area. That surface area makes a highly attractive landing field for wandering molecules of impurities in water or air, and when they land they stick.
Activated charcoal is used to adsorb colored impurities from sugar solutions and to adsorb poisonous gases in gas masks.
That wasn’t a misprint. Adsorption, with a “d,” is the sticking of individual molecules to a surface, while absorption, with a “b,” is the wholesale soaking up of a substance. Charcoal adsorbs; sponges absorb. In water filters, the charcoal removes chlorine and other odoriferous gases and a variety of chemicals such as herbicides and pesticides.
Now about those ion exchange resins. They’re little plastic-like granules that remove metals such as lead, copper, mercury, zinc, and cadmium. These are, of course, not present in the water as chunks of metal but as ions.
When a chemical compound of a metal dissolves in water, the metal goes into solution in the form of ions: positively charged atoms. We can’t just pluck these ions out of the water with charcoal, for example, because removing positive charges would leave the water with a surplus of negative charge, and Nature makes that a very costly operation in terms of expended energy; she vastly prefers that the world remain electrically neutral.
What we can do is exchange those positive ions for other, more harmless positive ions: sodium ions or hydrogen ions, for example. That’s what an ion exchange resin does. It contains loosely bound sodium or hydrogen ions that can swap places with metal ions in the water, leaving the metals effectively trapped in the resin.
The resin (as well as the charcoal) eventually becomes fully loaded with contaminants and must be replaced. How long it continues to work depends on how contaminated your water is. If your water is hard, the ion exchange resin will also remove calcium and magnesium ions, and you’ll have to replace it sooner.
Most domestic water filters contain both activated charcoal and an ion exchange resin, usually mixed together into a single cartridge. They therefore remove metals and other chemicals, but not necessarily pathogenic cysts. As I’ve said, check the claims about cysts in the product literature.
Do the purification filters remove fluoride? Generally, no. Fluoride is a negatively charged ion, not a positively charged one. So it is ignored by the ion exchange resin, which has only positive ions to swap. But when a filter cartridge is new, some fluoride may be removed from the first gallon or two, presumably by adsorption on the charcoal.
After that, however, the filter doesn’t remove fluoride.