Natural and wastewater contain small particulates that are suspended in water forming a colloid. These particles carry the same charges, and repulsion prevents them from combining into larger particulates to settle. Historically, dirty water is cleaned by treating with alum, Al2(SO4)3 . 12H2O, and lime, Ca(OH)2. The phenomenon is known as coagulation: Al2(SO4)3 . 12H2O Al (aq)3+ + 3SO4(aq)2− + 12H2OSO4(aq)2− + H2OHSO4 (aq)- +OH− (causing pH change) Ca(OH)2Ca(aq)2+ + 2 OH− (causing pH change) The slightly basic water causes Al(OH)3,Fe(OH)3 and Fe(OH)2 to precipitate, bringing the small particulates with them and the water becomes clear.