Water Softening - Water hardness is created by magnesium and calcium ions, usually present as bicarbonates. Water softening by water utility organizations is usually provided when the water supply has a hardness in excess of 150 parts per million. Water passage through ion exchangers which substitute sodium ions from the exchange medium for calcium and magnesium ions in the water. Sometimes aeration is used to remove dissolved iron. The iron ions combine with oxygen to form insoluble iron oxides, that are removed by filtration.
Aeration is usually a system of jets or fountains of water, or a system of riffles or waterfalls.
Compressed air is sometimes used, and air is injected into the bottom of a tank where it rises up through the water. Most large water utilities acquire water from several different sources, and blend their waters together to achieve the required EPA standards at the lowest cost. The above methods for treatment of public drinking water are a political compromise, based on cost-benefits tradeoffs that adjust allowable levels of selected contaminants to attain compliance. Performance is measured against minimum compliance standards, and not against health criteria. Keep in mind that several important contaminants are either ignored or even added to the water supply.