The Brine Tank
The brine tank is just what its name suggests, a plastic tank that contains a brine, water saturated with salt. The brine solution is typically made with salt or sodium. However problems may result if you are on a restricted low sodium diet in that some of the sodium is transmitted into the drinking water. (This Mayo Clinic article is a nice overview of the issue).
Also, the bacteria in septic systems so necessary for the breakdown of waste may be adversely affected by high concentrations of sodium. Water softener sodium brine discharge into septic systems has been banned or limited in several states and municipalities including Fillmore, CA, Michigan, Texas and Connecticut for example.
So as an alternative, potassium may be used. Potassium is considered superior and environmentally friendly, albeit a bit more expensive, than salt and does not affect your health, watersheds or the environment or your septic system.