Softening of water, exchanging calcium ions for sodium ions operates similar to nitrate removal.
The media within the filter is a "cation" resin whilst nitrate media is "anion". Using water, sodium in salt (NaCl) is swapped for the calcium in the water. In nitrate the chloride (Cl) is swapped for the nitrate in the water. Thus, in water softening sodium is added to the water. Sodium levels should be checked to ensure they do not exceed the maximum allowable value of 150mg/l. Current advice from the UK Department of Environment is that softened water should not be used for drinking and an unfiltered drinking supply should be taken off prior to a water softener.