Water Works
The three basic types of water treatment systems used in homes are carbon filters, water softeners, and reverse-osmosis purifiers.

Filtration systems are categorized as either point-of-use or point-of-entry. The former mount under the sink and serve a single faucet; the latter, also called whole-house systems, filter all of the water coming into the house. Systems are also classified by the technology they use. The three basic types used in homes are carbon systems, softeners, and reverse-osmosis units.

Carbon filters come in point-of-use and point-of-entry models. Water is forced through a cartridge or series of cartridges loaded with carbon granules. Depending on the model, carbon filters can remove everything from odors to lead, mercury, and even bacteria. These filters are relatively inexpensive but must be changed periodically, otherwise they can clog. A clogged filter can reduce water flow, breed bacteria, and put contaminants back into the water.

In fact, the water coming out of an overloaded filter can be more contaminated than that going into it.

Water softeners, or ion-exchange units, are whole-house systems that remove the calcium, magnesium, lead and other minerals found in "hard" water.

As water flows through a bed of sodium-impregnated resin beads, mineral ions are drawn from the water to the beads, while sodium ions are drawn from the beads to the water.

The units periodically clean and recharge the beads by rinsing them with a brine solution. Recharging can take place daily or weekly, depending on softener capacity and water use.

One drawback to these systems is that in some areas, brine cannot legally be dumped into a private septic system. They also take up more space than carbon systems.

But perhaps the biggest potential problem is that a softener can raise the level of sodium in the home's water. To avoid this, some people connect softeners only to hot-water pipes, which aren't used for drinking.

Some models can also be loaded with potassium instead of sodium, or a reverse-osmosis system can be added to remove the sodium.

Reverse-osmosis, or RO, filters are a good complement to a water-softening system. These compact units are usually mounted under the kitchen sink. They use the same technology that submarines use to filter salt from seawater, making them perfect for removing the sodium that a softening system adds to drinking water.

In an RO filter, water is forced through a semipermeable membrane that filters out any contaminants. An RO unit will remove a number of common minerals, including iron, nitrates, and lead. It will also get rid of bad tastes. RO units can be quite slow, however.

Reverse osmosis filters such as the one pictured above force water through a semipermeable membrane that filters out both minerals and bad tastes.