Why do we add alum to purify water?

(N/A) Water often contains negatively charged colloidal impurities. When alum $(K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O)$ is added to water,it dissociates to provide $Al^{3+}$ ions. These $Al^{3+}$ ions neutralize the charge on the colloidal particles,leading to their coagulation. The coagulated impurities settle down at the bottom and can be removed by filtration,resulting in purified water.