Alum is potassium aluminum sulfate and water to form a crystal, daily life used to precipitate magazines in the water, also known as alum, potassium alum, potassium aluminum alum, potassium alum. It is a compound salt of potassium sulfate and aluminum sulfate containing water of crystallization. Soluble in water, insoluble in ethanol. Alum is sour and astringent, cold and toxic. Therefore, it has antibacterial effect, astringent effect, etc., can be used as traditional Chinese medicine. Alum can also be used in the preparation of aluminum salt, baking powder, paint, tanning material, clarifying agent, mordant, paper, waterproofing agent. The first process of tap water production is to add "water purification agent" in the raw water - alkaline aluminum chloride (commonly known as alum), alkaline aluminum chloride in the raw water can produce a positive charge, so that the slight particles in the water by the electrostatic effect of the formation of larger particles in order to easily precipitate. The "pre-chlorination" process is a good way to make the water precipitate more easily. Before adding chlorine" can be chosen according to the original water, its role is mainly: ① coagulant, mainly oxidation of humus and colloid in the water, so that it can produce coagulation and precipitation; ② algaecide, according to the amount of algae in the original water to decide whether to add (algae in the water is too high content can produce a bad smell), in order to kill the algae. Alum contains aluminum, long-term excess will accelerate the aging of the human body, it is believed that may lead to an increase in the incidence of Alzheimer's disease, tap water in the alkaline aluminum chloride content should not be harmful to the human body.