There are four basic methods that can be used for hard water treatment. The most popular mechanism used in homes is known as sodium ion exchange, which removes the calcium and magnesium that cause hard water. Another method for treating small amounts of water is ultra-purification through reverse osmosis, although this is expensive and can take a long time. Chemicals such as detergents or lime can be used to remove the minerals from water, although this does not always produce water that can be consumed by humans. There also are some devices that claim to use magnets or electrical currents either to change the properties of the minerals in the water or to suspend them and remove them from the water, although there is little scientific evidence that these devices work.
In many homes, the most cost effective and efficient method of hard water treatment for the whole house is an ion exchange filter. This involves passing the water through a tank or device that contains a resin that holds either sodium or potassium. As the unwanted calcium and magnesium ions pass through the resin, they are extracted from the water and replaced in the water by sodium or potassium, which will not cause harm to pipes or machinery. The soft water created through an ion exchange filter can have a slightly higher concentration of sodium, but this amount is incredibly small and is not normally a medical concern.
Water purification is another form of hard water treatment. There are several methods that can be used, from reverse osmosis to physically capturing the unwanted minerals in a filter, all resulting in purified water. This method is often employed at the exact location where the water comes out of the system, such as a kitchen tap. The process can take some time, and the filters can be very expensive, so this method is best employed to remove the minerals from water used for drinking and cooking only and is not practical to service an entire household. Additionally, purification does not prevent the development of scale inside pipes and appliances.
Chemicals can be added as a form of hard water treatment. These chemicals can bond with the magnesium or calcium molecules and remove them from a solution, or they can break down the molecules. Using this type of hard water treatment is very common in industrial applications, but it is not used very often in homes because of the expense and because some chemicals can render the water unsafe for drinking.
There are a number of devices that use magnets, electrical coils or radio waves to change the chemical properties of the minerals in the water or to somehow remove them. There is no scientific evidence that these methods can be used as an effective hard water treatment. Some evidence exists that there is some type of interaction between electromagnetic fields and molecules in the water, but this interaction does not reduce the hardness of the water.