The brine tank is just what its name suggests, a plastic tank that contains a brine, water saturated with salt solution. The brine solution is typically made with salt or sodium. The system use the brine solution to “regenerate” the filter media after a set number of gallons has been reached. It depends on the capacity of the system. This filter regeneration takes place between 3-4 a.m, and does not leave or cause any salt to be placed in the water for use in the home. During the regeneration cycle the system simply by-passes itself and provides unfiltered water to the home.