Impurities can be removed from the liquid either by physical adsorption or by chemical adsorption. Various natural and synthetic adsorbents have been studied and developed. For example, activated carbon is one of the major adsorbents used that has also been used for a long time.  Adsorbents are used not only to remove organics but also to remove heavy metal ions, chlorine components, phosphates and others. 

Due to the limited adsorption capability, adsorbents need to be renewed from time to time. Used adsorbents can be recycled or are treated as wastes.