Water filters

Last updated: September 2, 2008.

You can survive without food for several weeks, because your body will gradually switch to using stored fat and protein to make its energy. But cut off your water supply and you'll be dead within days. Water equals life: it's as simple as that. Around two thirds of your body (as much as 75 percent if you're a baby) is H2O. Even your bones, which you might think are completely solid, contain about 25 percent water. On average, we need 2.4 liters (0.6 gals) of water each day to keep ourselves healthy (though we don't have to drink anything like that much—we get a lot of our water from inside foods). With water so important to our lives, it's hardly surprising we like it clean, pure, and tasty. That's one reason people spend so much money on water filters that can remove any harmful impurities. How do they work—and do we really need them? Let's take a closer look!

Photo: A simple, household, water-filtering jug. This one has an activated carbon filter inside it.

How water filters work

Thanks largely to an unusual molecular structure, water is amazingly good at dissolving things. (We look at this in more detailed in our main article on water.) Sometimes that's helpful: if you want to bust the dirt from your jeans, simply throw them in your washing machine with some detergent and the water and soap will literally pull the muck away like a magnet. But there's clearly a downside to this too. All of our water constantly circulates through the environment in what's known as the water cycle. One minute it's rushing through a river or drifting high in a cloud, the next it's streaming from your faucet (tap), sitting in a glass on your table, or flushing down your toilet. How do you know the water you're about to drink—with its brilliant ability to attract and dissolve dirt—hasn't picked up all kinds of nasties on its journey through Earth and atmosphere?

Water filters use two different techniques to remove dirt. Physical filtration means straining water to remove larger impurities. In other words, a physical filter is a glorified sieve—maybe a piece of thin gauze or a very fine textile membrane. (If you have an electric kettle, you probably have a filter like this built into the spout to remove particles of limescale.) Another method of filtering, chemical filtration, involves passing water through an active material that removes impurities chemically as they pass through.

Photo: A simple physical filter (shown here in close-up) is little more than a sieve. This one is a piece of fine plastic mesh designed to stop limescale (the little white particles you can see trapped here) escaping from a kettle.

There are four main types of filtration and they employ a mixture of physical and chemical techniques.

Activated carbon

The most common household water filters use what are known as activated carbon granules (sometimes called active carbon or AC) based on charcoal (a very porous form of carbon, made by burning something like wood in a reduced supply of oxygen). Charcoal is like a cross between the graphite "lead" in a pencil and a sponge. It has a huge internal surface area, packed with nooks and crannies, that attract and trap chemical impurities through a process called adsorption (where liquids or gases become trapped by solids or liquids). But while charcoal is great for removing many common impurities (including chlorine-based chemicals introduced during waste-water purification, some pesticides, and industrial solvents), it can't cope with "hardness" (limescale), heavy metals (unless a special type of activated carbon filter is used), sodium, nitrates, fluorine, or microbes. The main disadvantage of activated carbon is that the filters eventually clog up with impurities and have to be replaced. That means there's an ongoing (and sometimes considerable) cost.

Photo: A water treatment plant filters water for reuse by passing dirty water from homes and factories through beds of charcoal and sand. It's like a giant version of the jug filter in our top photo. Photo by Betsy J. Weiner courtesy of US Army and Defense Imagery.

Reverse osmosis

Reverse osmosis means forcing contaminated water through a membrane (effectively, a very fine filter) at pressure, so the water passes through but the contaminants remain behind.

Photo: A NanoCeram Nanoalumina filter is a physical filter made from an alumina-based ceramic. It has nanoscale fibers— small enough to remove 99.99999 percent of viruses and bacteria from polluted water or air. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Ion exchange

Ion-exchange filters are particularly good at "softening" water (removing limescale). They're designed to split apart atoms of a contaminating substance to make ions (electrically charged atoms with too many or too few electrons). Then they trap those ions and release, instead, some different, less troublesome ions of their own—in other words, they exchange "bad" ions for "good" ones.

How do they work? Ion exchange filters are made from lots of zeolite beads containing sodium ions. Hard water contains magnesium and calcium compounds and, when you pour it into an ion-exchange filter, these compounds split apart to form magnesium and calcium ions. The filter beads find magnesium and calcium ions more attractive than sodium, so they trap the incoming magnesium and calcium ions and release their own sodium ions to replace them. Without the magnesium and calcium ions, the water tastes softer and (to many people) more pleasant. However, the sodium is simply a different form of contaminant, so you can't describe the end product of ion-exchange filtration as "pure water" (the added sodium can even be problematic for people on low-sodium diets). Another disadvantage of ion-exchange filtration is that you need to recharge the filters periodically with more sodium ions, typically by adding a special kind of salt. (This is why you have to add "salt" to dishwashers, from time to time: the salt recharges the dishwasher's water softener and helps to prevent a gradual build-up of limescale that can damage the machine.)

Distillation

One of the simplest ways to purify water is to boil it, but although the heat kills off many different bacteria, it doesn't remove chemicals, limescale, and other contaminants. Distillation goes a step further than ordinary boiling: you boil water to make steam, then capture the steam and condense (cool) it back into water in a separate container. Since water boils at a lower temperature than some of the contaminants it contains (such as toxic heavy metals), these remain behind as the steam separates away and boils off. Unfortunately, though, some contaminants (including volatile organic compounds or VOCs) boil at a lower temperature than water and that means they evaporate with the steam and aren't removed by the distillation process.

On balance

You can see that different types of filtration remove different pollutants—but there's no single technique that removes all the contaminants from water. That's why many home water-filter systems use two or more of these processes together. If you're looking for a home water filter, tread carefully. Bear in mind that you won't necessarily remove all the nasties. Remember, too, that most water filters require some kind of ongoing cost and, without regular maintenance to keep them working properly, can leave your water in worse shape than it was to begin with!