You'll be dead within days if your water supply is cut off. 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. 25% of the bones even if they look completely solid, is water. 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 On average), we need 2.4 litres (0.6 gals) of water each day. With water so important to our lives, it's hardly surprising we like it. 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?
How water filters work
An early drinking water filtering system using reed beds, sand, charcoal, and gravel.
Water filters use two different techniques to remove dirt. Physical filtration means straining water to remove larger impurities. In other words, a piece of thin gauze or a very fine textile membrane acts as a physical filter as a glorified sieve (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 is chemical filtration. Removing impurities chemically as they pass water through an active material.
Four types of water filters
There are four main types of filtration and they employ a mixture of physical and chemical techniques.
By passing dirty water from homes and factories through beds of charcoal and sand, a water treatment plant filters water for reuse. Activated carbon granules based on charcoal (a very porous form of carbon, made by burning something like wood in a reduced supply of oxygen) is the most common household water filters used. What is known as charcoal is like a cross between the graphite "lead" in a pencil and a sponge? Packed with nooks and crannies, it has a huge internal surface area. It attracts and traps chemical impurities through a process called adsorption (where liquids or gases become trapped by solids or liquids). It can't cope with "hardness" (limescale) and Heavy metals (unless a special type of activated carbon filter is used), sodium, nitrates, fluorine, or microbes etc.
In reverse osmosis, water passes through but the contaminants remain behind means forcing contaminated water through a membrane (effectively, a very fine filter) at pressure. If you've studied biology, you've probably heard of osmosis. The solutions try to rearrange themselves so they're both at the same concentration. 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),
You stand a big glass jug full of less sugary water in a sealed glass bottle full of very 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? That's osmosis, so what about reverse osmosis? Cutaway artwork showing the basic features of a reverse osmosis filter
A nano fibre water filter
For "softening" water (removing limescale), Ion-exchange filters are particularly good. They're designed to split apart atoms of a contaminating substance to make ions (electrically charged atoms with too many or too few electrons). They exchange "bad" ions for "good" ones as they trap those ions and release, instead, some different, less troublesome ions of their own.
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. These compounds of magnesium and calcium split apart to form ions. Hard water contains magnesium and calcium compounds and when you pour it into an ion-exchange filter softening occurs.