If I live in a major metropolitan area, is it legal for me to put a salt-based water softener on my household plumbing system?

A water softener reduces the dissolved calcium, magnesium, and to some degree manganese and ferrous iron ion concentration in hard water. These "hardness ions" cause three major kinds of undesired effects. Most visibly, metal ions react with soaps and calcium-sensitive detergents, hindering their ability to lather and forming a precipitate—the familiar "bathtub ring". Presence of "hardness ions" also inhibits the cleaning effect of detergent formulations.

Secondly, calcium and magnesium carbonates tend to adhere to the surfaces of pipes and heat exchanger surfaces. The resulting build-up of scale can restrict water flow in pipes. In boilers, the deposits act as an insulation that impairs the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this can lead to failure of the boiler. Thirdly, the presence of ions in an electrolyte, in this case, hard water, can also lead to galvanic corrosion, in which one metal will preferentially corrode when in contact with another type of metal, when both are in contact with an electrolyte.

Finally, it is unlikely you would be able to use a salt-based softener because salt-based water softeners are banned in many cities across the United States and elsewhere. Such units use backwashing and sodium chloride salt as a regenerative agent. In metropolitan areas with central sewer systems these chlorides are transferred to the wastewater treatment plant where they are generally discharged into local streams and rivers. Since excess chloride can have detrimental effects on the ecosystems of streams and rivers, the EPA and various states have established regulatory levels for chloride discharge from municipal and local wastewater treatment plants. The treatment plants can spend significant amounts of money to upgrade their equipment to handle chloride discharge requirements or they can ban salt-based water softeners, which are significant sources of chloride. Most have chosen the latter alternative.