Carbon filters

For problems such as bad taste or smell, an inexpensive carbon filter may be all you need.

These filters contain a disposable cartridge of activated carbon. Carbon is an extremely porous material: the tiny particles provide an immense surface area to attract and hold impurities. A single gram may have a surface area of more than 10 square metres!

Some grades of carbon will remove impurities such as chlorine by-products (trihalomethanes, or THMs), herbicides, pesticides and other organic chemical compounds. But most just make water taste and smell better.

Some cartridges will work for longer than others, or remove more chlorine from the water. Several factors affect their performance. A poorly designed cartridge may allow water to stream through too quickly. The longer water is in contact with the surface of the filter material, the more chance there is for chemicals to stick to it.

The carbon particle size also matters: the smaller the particle, the more outside surface area is available to attract impurities. That's why it's important to check that the filter you're buying complies with an accepted standard, such as the Australia/New Zealand performance standards for water filters AS/NZS 4348 or AS 3497:1998, or the relevant US National Sanitation Foundation (NSF) standard. See Our advice for more information on these standards.

Bacterial growth

Activated carbon can't remove disease-causing organisms (pathogens). They should be used only on water which has already been treated to make it safe.

If the cartridge is not changed regularly, you may end up making the water even more contaminated. This is because each time the filter is used some water remains in the carbon. Any bacteria in the water will also remain, along with organic substances that get trapped in the filter. Because there is no chlorine left to kill the bacteria, they can grow and multiply, feeding off the trapped organic material. Eventually they emerge in the water from your filter.

Some carbon filters claim to be "bacteriostatic". This does not mean they remove or kill bacteria; they simply contain a chemical, usually silver, to limit bacterial growth. The cartridges need to be replaced every six months or so. It's a good idea to write the installation date or expiry date on the cartridge before installing it. Ideally, there should be a device to warn you that the cartridge is no longer working: for example, an automatic shut-off or a reduction in flow.

For filters which don't provide such a warning, the standards require that the cartridge removes contaminants for twice the claimed capacity.

The performance of the cartridge alone may tell you when it's time for a new one. As impurities build up in the filter material, the filter slows or even stops. As the filter loses its ability to absorb any more chlorine, tastes and smells will probably reappear.

When replacing the cartridge look for one that is specified for the unit - ill-fitting cheaper substitutes may cause leaks.