A coagulant (also known as flocculant) is used for aggregation or flocculation of the smallest colloidal particles of dirt in liquids. The substances are used in a variety of ways, in wastewater treatment plants, for discoloration of drinking water (removal of humic substances), in swimming pools, paper mills or in industrial processes for example. Trivalent metal salts such as iron (III) salts and aluminum salts, but also other hydroxide formers such as lime milk, soda or sodium aluminate are mostly used as flocculants. Flocculants form larger micro flocs from the colloids, which can be easily removed by filtration. If solids removal by sedimentation or flotation is desired, a flocculation aid is used in addition to the flocculant, which forms larger macro flocs from the micro flocs.