The technology of ion exchange is often employed for aesthetic purposes in the form of water softeners to remove minerals, and it is also useful for the removal of inorganic chemicals, heavy metals and dissolved solids when used as water polishing technology. Fluoride, radium, and to some degree nitrate are all effectively treated with ion exchange resins. The removal of these contaminants is accomplished through adsorption of the contaminant ions onto a resin exchange medium. In this case, one ion is substituted for another ion on the charged surface of the medium, usually a synthetic plastic resin. This resin surface is designed as either cationic (negatively charged) or anionic (positively charged). The medium is saturated with the exchangeable ion before treatment operations begin. During exchange, contaminant ions replace regenerant ions because they are preferred by the medium. After the medium reaches equilibrium with the contaminant ions, the medium is regenerated with a suitable solution, such as sodium chloride, potassium chloride, caustic soda or acid solution. Ion exchange waste is highly concentrated and requires careful disposal.