A water softener is a device used to remove calcium and magnesium ions. These minerals can cause a variety of problems, including staining dishes and plumbing fixtures and reducing the effectiveness of soap. A water softening system works by using a process called ion exchange, which involves replacing the hard metals in the water with softer metals, such as sodium ions. The water has then "softened".Water softening systems can be installed at the point of entry to a home or building, or they can be installed at the point of use, such as under the kitchen sink.

Water softeners are commonly used in areas where the water supply has a high level of hardness, which is measured in grains per gallon (GPG). Water with a hardness level of 1 to 3.5 GPG is considered soft, while water with a hardness level of 3.5 to 7 GPG is considered medium. Water with a level of 7 GPG or higher is considered hard.

In a water softening system, hard water is passed through a bed of resin beads, which are coated with sodium ions. As the hard water passes through the beads, the calcium and magnesium ions of the water are attracted to the sodium ions and exchange places with them. The sodium ions take the place of the calcium and magnesium ions in the water, while the calcium and magnesium ions remain in the resin beads.

After a certain amount of water has been treated, the resin beads are saturated with calcium and magnesium ions and must be regenerated. During the regeneration process, a brine solution is used to wash the calcium and magnesium ions out of the resin beads and replace them with sodium ions. The water softener system is then ready to treat more hard water.

Water softeners are generally effective in reducing the negative effects of hard water, such as soap scum and scale build-up. They can also help lather soap and shampoo and can improve the taste and smell of drinking water. However, they have some disadvantages, such as the need for regular maintenance and the fact that they can add sodium to the water supply.