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Water filter jug kettle. Photo by Lee J Haywood.

Water filters

by Chris Woodford. Last updated: September 17, 2013.

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: An electric kettle with a built-in water filter. Photo courtesy of Lee J Haywood published on Flickr under a Creative Commons Licence.

How water filters work

An early drinking water filtering system using reed beds, sand, charcoal, and gravel.

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? If you want to be sure, you can run it through a water filter.

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: Living filter: A classic wastewater filtering system outlined in a 1901 patent by Cleophas Monjeau of Middletown, Ohio. Dirty water drips down from the tank at the top (blue), passes through vegetation (probably a reed bed), which removes nutrients, organic matter, some kinds of pollution, and some bacteria, before dripping down through sand, charcoal, and gravel filters. The cleaner water is collected for reuse in another tank at the bottom. Reed beds are still widely used in purifying wastewater to this day, including in systems for cleaning up runoff from highways. Artwork from US Patent 681,884: Purifying water by Cleophas Monjeau, issued September 3, 1901, courtesy of US Patent and Trademark Office.

Four types of water filters

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

Activated carbon

Water treatment plant in Iraq

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 filter in our artwork up above, though there's no reed bed in this system. 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.

A closer look at reverse osmosis

If you've studied biology, you've probably heard of osmosis. When you have a concentrated solution separated from a less concentrated solution by a semi-permeable membrane (a kind of filter through which some things can pass, but others can't), the solutions try to rearrange themselves so they're both at the same concentration.

Wait, it's simpler than it sounds!

Suppose you have a sealed glass bottle full of very sugary water and you stand it inside a big glass jug full of less sugary water. Nothing will happen. But what if the bottle is actually a special kind of porous plastic through which water (but not sugar) can travel? What happens is that water moves from the outer jug through the plastic (effectively, a semi-permeable membrane) into the bottle until the sugar concentrations are equal. The water moves all by itself under what's called osmotic pressure.

Artwork explaining how reverse osmosis works

That's osmosis, so what about reverse osmosis? Suppose you take some contaminated water and force it through a membrane to make pure water. Effectively, you're making water go in the opposite direction to which osmosis would normally make it travel (not from a less-concentrated solution to a more-concentrated solution, as in osmosis, but from a more-concentrated solution to a less-concentrated solution).

Since you're making the water move against its natural inclination, reverse osmosis involves forcing contaminated water through a membrane under pressure—and that means you need to use energy. In other words, reverse-osmosis filters have to use electrically powered pumps that cost money to run. Like activated charcoal, reverse osmosis is good at removing some pollutants (salt, nitrates, or limescale), but less effective at removing others (bacteria, for example). Another drawback is that reverse osmosis systems produce quite a lot of waste-water—some waste four or five liters of water for every liter of clean water they produce.

A nanofiber water filter

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.

Animation showing how magnesium and calcium ions are exchanged for sodium in an ion exchange water filter.

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.)

Artwork: How ion exchange works: Magnesium and calcium ions (orange and red) flow into the water filter crystals (gray), which initially contain sodium ions (yellow). The magnesium and calcium ions become trapped and the sodium ions are released in their place.

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!

Should we stop drinking bottled water?

Many people buy water filters or bottled water in an often mistaken belief that tap water is dirty or harmful to drink. In fact, as the Environmental Protection Agency (EPA) reveals, over 90 percent of US water systems meet the federal standards for tap water quality. In England and Wales, for the year 2007, 99.96 percent of drinking water met national and European standards (involving some 40 different quality measurements). Those figures are pretty remarkable really, when you consider just how dirty we make water and some of the things (like pesticides and car oil) that people flush down their drains. Even so, the high quality of most drinking water doesn't stop people spending something like $35 billion, worldwide, each year, buying bottled water that's several thousand times more expensive than tap water.

?

Cost isn't the only drawback of bottled water. Most of it comes in disposable plastic bottles that are hard to recycle. Dumped in landfills, washed away in rivers, dropped on beaches, burned in incinerators—plastic bottles add to the pollution that's reducing the quality of Earth's natural water supply. How ironic: by buying "clean" bottled water to keep ourselves healthy, we're helping to make Earth a dirty place and making things worse overall.

Next time you buy a bottle of water, don't throw the bottle away: why not keep it—and refill it with tapwater? Providing you wash the bottle out properly, you can reuse the bottle any number of times. Chances are you'll get water that's just as healthy, but at a fraction of the cost both to your pocket and to the Earth. Alternatively, buy yourself a hygienic, refillable aluminum bottle.

Best of all, give the money you save on bottled water to WaterAid and help some of the people who genuinely lack clean water in developing countries. Let's count ourselves lucky we don't have to drink water straight from a dirty river, like many people still do.

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Water filters. Retrieved from http://www.explainthatstuff.com/howwaterfilterswork.html. [Accessed (Insert date here)]

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