Science - Water Filters Water filters: how do they work? by David Bradley
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The simplest way to filter water is to pass it through a material that catches any solid particles - a filter. Filter paper is the usual material used in a laboratory, but muslin and other types of cloth with a fine weave might be used in the kitchen for instance, for straining out all but the smallest of particles from a watery recipe. This kind of filtering only removes the solids; anything dissolved in the water that you might wish to remove simply passes through a filter paper or muslin cloth.

Simple filter
A light blue liquid with dark blue particles is poured through a filter paper cone. The liquid passes through the filter, while the solid bits are left behind on the paper.

Pure and simple
Simple cartridge filters that you might have in a water filter jug at home also use a paper or fibre block to trap particles. The material's porosity determines the size of the particles it can trap. But this kind of filter does not extract chlorine and other compounds that people worry about affecting taste and odour. A charcoal filter, composed mainly of highly porous activated carbon, can absorb some of these chemicals from water.

If you want to remove other dissolved substances from water, a more sophisticated approach is needed. But, why would you want to do this? The coffee and tea brewed with filtered water supposedly produces less scum on the surface. The calcium and magnesium ions that interact with organic compounds in these drinks, such as polyphenols, have nothing with which to form insoluble solids that float to the surface in your cup, and this results in a supposedly fresher, nicer tasting brew.

Hard and soft
Extracting the magnesium and calcium salts from water might also help save your hot-water plumbing. The salts of these elements form insoluble deposits of limescale in hot- water pipes, boilers and showers. These deposits cause blockages and make the system less energy-efficient, so using more fuel and wasting money.

Swapping ions
If the water supply is passed through a material known as an ion-exchange resin before it is fed into the hot-water system, the problem of limescale can be avoided.

An ion-exchange resin acts as a kind of filter for the dissolved calcium and magnesium salts. However, rather than simply removing these dissolved minerals, as the name suggests, it swaps them for another element, sodium, which does not form insoluble salts. With only sodium salts dissolved in the water, the pipes remain unclogged by limescale.

An additional benefit of softening the water with an ion-exchange resin is that you use less soap to get a lather in the bath because the calcium and magnesium salts that would otherwise react with soap molecules to form scum and use up the soap are not present in the water.

Water softener An ion exchanger is used to soften water and prevent limescale. Negatively charged resin is flooded with sodium ions from a brine tank. The sodium ions have a weak positive charge, so attach themselves to the resin. When hard water is run through the resin, the strongly positive calcium and magnesium ions are attracted to the resin, displacing the sodium ions, which replace them in the water.

The water supply to a house is pumped through a bed of small plastic resin beads, or a highly porous mineral called a zeolite. A zeolite's tiny pores provide a huge surface area on to which sodium ions (from salt) can stick. As the water flows past the sodium ions, they swap places with the calcium and magnesium ions, thereby softening the water.

Each Ca2+ or Mg2+ ion displaces two Na+ ions into the water.

The pores in the zeolite beads eventually become clogged with calcium and magnesium ions and they can no longer soften the water. At this point you have to regenerate the zeolite beads. This usually involves soaking the beads in a strong solution of sodium chloride, which is common salt. The sodium ions from the salt force out the calcium and magnesium ions, and replace them with sodium. The calcium and magnesium solution is then flushed out of the system down the drain.

An ion-exchange resin cannot be used to solve the problem of limescale in a kettle, however. Swapping the magnesium and calcium, which are good for you, for sodium, leads to a higher concentration of sodium in the water, making it a potential health hazard for some individuals. The actual amounts involved, however, are far smaller than the quantities of all these elements we get from food every day. A more important problem can be the accumulation of bacteria in the ion-exchange unit, which is perfectly harmless for warming the house or washing but is not fit for drinking.

Putting osmosis into reverse
Water can also be filtered using a process called reverse osmosis. Reverse-osmosis filters produce pure water by forcing untreated or tap water through a semi-permeable membrane. Osmosis itself involves the movement of a solvent through a layer of material - a semipermeable membrane - that has tiny pores, big enough to allow water through but too small for anything else to get through. The solvent passes through the membrane if the solution on the other side has a higher concentration of particles dissolved in it. The movement reaches equilibrium when the concentrations are the same on both sides of the membrane.

Gore-tex fabric is an example of a semi-permeable membrane. It has billions of tiny holes in it (pores) that allow water vapour molecules from your body sweat to pass through to the outside air but prevent bigger liquid drops of rainwater getting in.

In reverse osmosis, a semi-permeable membrane of thin plastic allows water molecules to pass through but not the bulky ions of salts. Pure water is put on one side of the membrane, with the impure, salt-filled water on the other. Then pressure is applied to push the salty water back the wrong way through the membrane so that, rather than equalising the concentrations, the water molecules in the salty solution pass through, adding to the pure water on the opposite side. The process is quite slow and expensive to run, but very effective.

Reverse osmosis
i) When an empty glass tube is half submerged in a beaker of water, the liquid level stays the same in the beaker and the tube.
ii) An empty tube is sealed with a semi-permeable membrane and half-filled with salt solution. At first the liquid level stays the same as in the beaker.
iii) Water from the beaker gradually squeezes through the semi-permeable membrane by osmosis and pushes up the liquid level in the tube.

Environmental impact While there are obvious benefits to having filtered water for drinking and softened water for a soapier shower, there are disadvantages in terms of cost - both financially and energywise. Cartridges for drinking water filters, for instance, have to be manufactured, transported and replaced periodically. The filtered water only lasts a short time before bacteria build up and it becomes unsuitable for drinking, so if you don't use all the water in a short time, you end up wasting the excess. So, is a clearer cup of tea worth the environmental effects?

The benefits in terms of a more efficient heating system also have to be offset against the amount of salt required, the water used to flush the system and the energy to run the pumps in a water softener. It is a difficult equation to balance, and you have to ask whether the fuel saved having clearer pipes outweighs the energy cost of purifying the salt, running the pump and, indeed, installing the softening unit in the first place.

About the author…
David Bradley is a freelance science writer specialising in chemistry. He can be reached through his Elemental Discoveries website at www.sciencebase.com

Further information about water filtration methods
www.madsci.org/posts/archives/mar97/853648369.Ot.r.html
How sodium works in water softeners

www.madsci.org/posts/archives/mar2000/953662016.Ch.r.html
Why household water filters need replacing

www.cranfield.ac.uk/sims/water/research.htm
School of Water Sciences at Cranfield.

www.cranfield.ac.uk/sims/water/magnets.htm
Research into magnetic water softeners