All water softeners use a synthetic resin to soften the water.
During this process, sodium or potassium brine solution is rinsed through the resin. Calcium and magnesium ions are bumped off and go down the drain, while the sodium or potassium ions attach. Until recently, a major objection to water softeners was the sodium they added to the water. Now, potassium chloride salt substitute is widely available. Fruits and vegetables are naturally high in potassium.