19th century illustration of Alum crystals. Alum is both a specific chemical compound and a class of chemical compounds. Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are also manufactured fro industrial uses. Alum has been used at least since Roman times for purification of drinking water and industrial process water. Between 30 - 40 ppm of alum for household wastewater, often more for industrial wastewater is added to the water so that the negatively charged colloidal particles clump together into "flocs", which then float to the top of the liquid, settle to the bottom of the liquid, or can be more easily filtered from the liquid, prior to further filtration and disinfection of the water. Pliny informs us that 'alumen' was found naturally in the earth. He calls it 'salsugoterrae'. Alums are soluble in water and have a sweetish taste