Water softeners contain a specific filter media which is normally saturated with sodium ions. When water flows through this media, positively charged ions within the water trade places with the sodium ions. In this way hardness and certain heavy metals, which are positively charged, are removed from the water and stick to the media. Periodically, the softener is flushed with salty water. This replenishes the supply of sodium ions on the filter media and washes all the accumulated hardness and heavy metals down the drain.
Water softeners can be used to treat hardness as well as lead, manganese, and even iron up to certain concentrations. An important consideration is that a softener actually replaces these items in the drinking water with sodium. This means that the use of a softener will significantly increase the sodium intake of people who drink softened water. While this does not ordinarily cause a problem, it can interfere with low sodium diets which are sometimes prescribed for certain medical conditions.
Despite the fact that it can be regenerated, the filter media in water softeners does eventually become exhausted. Water softeners should be re-bedded about once every ten years. Salt consumption will depend on the amount of water which is used and the hardness of the water that is being treated.