Reverse Osmosis works by using a high pressure pump to increase the pressure on the salty side of the RO and force the water across the semipermeable RO membrane, leaving almost all (around 95% to 99%) of the dissolved salts behind in the reject stream.
Reverse Osmosis removes the total dissolved solids.
A hard water softener is an appliance that uses sodium chloride, also known as salt, to treat hard water. The resin beads chemically attract the unwanted 'hard' mineral ions and exchange them with sodium ions. The component of softener resin is mainly a strong acid called Cation resin.
The softener plant removed only the hardness of water and not total dissolved solids (TDS).