Ion exchange water softening

A softener does not remove hardness so much as trade it. Calcium and magnesium are swapped for sodium on a resin bed, which is then regenerated with brine.

What it does not do: Softening removes nothing but calcium and magnesium. Nitrate, chloride, sodium, pesticides, PFAS and micro-organisms all pass straight through.
The lead trade-off: Softer water is more plumbosolvent, so softening can increase lead pickup from old plumbing. This is one reason softened water is conventionally not plumbed to the kitchen tap.
Brine discharge: Regeneration waste raises the salinity of sewage, which is a growing constraint on wastewater reuse in areas with many softeners.

Text description

Hard water enters a bed of resin beads loaded with sodium ions. Calcium and magnesium ions bind to the resin more strongly and displace sodium, which enters the water. The output is soft water with additional sodium. When the resin is exhausted, concentrated brine is passed through, reversing the exchange, and the calcium-rich waste brine is discharged to the sewer.