There are four main types of filtration and they employ a mixture of physical and chemical techniques.
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).
Photo: Charcoal 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.
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.
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.
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.
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.
Here's what a reverse osmosis filter unit looks like in practice, shown in cutaway. Unfiltered water (blue pipe) is pumped into a purification unit (gray) and passes through a plastic, semi-permeable membrane (yellow) made (in this case) of cellulose acetate. Clean water flows out through the red pipe; impurities flush away through the green pipe:
Artwork: A cutaway of a basic, reverse osmosis membrane filter. Artwork courtesy of US Patent and Trademark Office from US Patent 3,390,773: Water purification system by Ulrich Merten. Gulf General Atomic Inc, July 2, 1968.
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.
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.
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.)
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.
Artwork: Distillation involves heating water to kill contaminants and separate out impurities. Water boils at 100°C (212°F), so steam captured at exactly this temperature should, in theory, consist of nothing but water. In practice, it's not quite so easy!
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!
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