You may be wondering why a water softener can’t reduce TDS when it is removing minerals like calcium and magnesium.
That’s because water softeners use an ion exchange process to replace hard minerals with sodium ions.
Since sodium ions are being exchanged for calcium and magnesium ions, the TDS of your water isn’t directly affected. For every calcium and magnesium ion taken out, a sodium ion is put in. The higher the mineral content in your water, the more sodium is exchanged to soften it. The sodium content of softened water completely depends on how hard it was to begin with.
Softened water is certainly better for cleaning and bathing, and will extend the life of appliances like your washing machine and water heater. However, the spotting you notice from soft water may actually be sodium spots.
When water evaporates from your clean dishes or after washing your car, a powdery sodium residue could be left behind.
The good news is, sodium spotting can be very easily wiped off with a towel. The same cannot be said for soap scum and limescale spotting. You can also avoid sodium spots by thoroughly hand drying your car or dishes instead of letting them air dry.